Making Waves: Microwaves in Climate Change

Climate change is arguably the single most important global challenge of the 21st century. It is appropriate that in this, our first installment of our new editorial series Making Waves, we explore some of the ways that microwave technologies have been involved in this critical research field. Skipping over the obvious but essential roles of microwave communications and tracking in space-based resource management, we instead focus on techniques and data that these systems provide. We also look at some of the more unusual applications and ideas that are being enabled by both current and future microwave component advances and the impacts that they might have on local, and perhaps someday on more global scales. The intent of the article is to provide the reader with links between, and references to microwave technologies and climate science. The authors want to encourage more researchers to seek out and collaborate on ideas and proposals that directly or indirectly bridge these often disparate areas of development. They also look forward to responses from the engineering and science communities via this editorial series as a means of bringing together engineers and scientists interested in sharing new ideas and forging collaborations that might otherwise not form without such a stimulus. Future articles in this series will emphasize other potentially high impact fields with particular cross-over applications to microwave theory and technology.


I. INTRODUCTION
Climate change is arguably the single most important global challenge of the 21 st century. Energy consumption is rising rapidly, especially in developing countries, and the continued use of fossil fuels is bringing the world closer and closer to the brink of an irreversible climate catastrophe attributed to greenhouse gas production and its role in induced global temperature rise. There has never been a time when global resource and climate monitoring were more necessary, and when every direct or indirect method for reducing fossil fuel dependence and/or the release of additional greenhouse gases was essential. In this article we take a broad look at some of the microwave techniques that have traditionally played a major role in the field of resource management and in activities related to documenting and understanding climate change or mitigating its impacts.
Skipping over the obvious but essential roles of microwave communications and tracking in space-based 1 and groundbased wireless technologies and transportation management, we instead focus on techniques and data that these systems use and provide. We also survey some of the more unusual applications and ideas that are being enabled by both current and future microwave component and systems advances and 1 Note that all satellites utilize radio frequency (generally microwave) uplinks and down links for control as well as data transmission back to earth. Although technically these applications of microwave signaling are at the very heart of all satellite (and most ground based communication and tracking) systems, we are purposely not including this as a science application involving climate change or global resource monitoring for this tutorial article. Rather we count this as enabling technology, since it is ubiquitous in almost every application where communications between a sensor and a data receiving station are involved, both on the ground and in airborne or space applications. the impacts that they might have on processes that can assist in mitigating climate change. The intent of the article is to provide the reader with links and references to microwave technologies related to climate science, and to encourage more researchers to seek out and collaborate on ideas and proposals that directly or indirectly bridge these often disparate areas of development. The tutorial is broken up into sections that highlight particular characteristics and general applications of microwaves: sensing, heating, power generation, power beaming, and propagation.
We begin with the "elephant in the room" that is satellite systems that utilize microwave instruments and techniques to gather information on a global scale. These instruments help to assess and track various ground resources and atmospheric constituents over time, as well as furthering our understanding of processes and models that help shape climate policy. This is followed by a series of brief sojourns into applications where microwaves play some key role that may help mitigate our steadily advancing path towards global temperature rise, or coping with the consequences. These take the forms of reducing dependencies on fossil fuels; helping to reduce, transform, or capture emitted CO 2 or other catalytic chemicals; alternative or more efficient means of producing necessary chemicals or fuels; enhancing nutrients and crop yields; energy generation and transmission; and ways in which microwave measurements can help track and monitor enhanced weather related phenomena caused by global warming.

II. SATELLITE-BASED MICROWAVE SENSING
Satellite remote sensing spans almost the entire electromagnetic spectrum from the ultra violet to long radio waves. The particular wavelength range that is utilized depends on the specific function of the sensor (imaging and mapping, spectroscopy, tracking geophysical processes etc.), the desired resolution, and the transmission through the atmosphere (Fig. 1). The advantages of working in the microwave spectrum (1-300 GHz) are primarily: (1) the ability to penetrate through the atmosphere, even in the presence of clouds, rain, or dust; (2) emission and absorption features that are unique to this portion of the electromagnetic spectrum (vibrational and rotational spectral line transitions that can be used for fingerprinting particular gaseous molecules and atoms); (3) spectral radiance measurements that can be converted into accurate altitudinal-based temperature and pressure profiles on a global scale; (4) absolute ground and water temperatures; (5) 2D and 3D terrain mapping, emphasizing particular geological constituents, properties, or spectral signatures, including polarization (ice or snow, rock or vegetation, wet or dry soil, etc.); (6) highly accurate altimetry, even over water, and including snow and ice thickness and wave height; (7) wind and wave speeds and direction; and (8) the possibility for variable resolutions based on fixed and synthetic aperture antenna techniques.
Other remote sensing wavelengths and technologies can also perform some of these functions, and multi-spectral systems offer the most comprehensive measurement coverage. Many satellites carry a broad range of individual instruments that cover wide swaths of the available remote sensing spectrum. These also have the advantage of cross correlating and cross calibrating distinct measurement products where data sets overlap. Since this is a microwave journal targeted at a microwave engineering audience, we will focus only on the microwave regime and the associated sensor systems that are most often being deployed and used.
Microwave remote sensing from space-borne and high altitude aircraft or balloon platforms is generally classified into two major categories: active sounding using radar techniques and passive sensing using radiometric methods.
Active microwave remote sensing has been at the forefront of the field of climate science since the development of radar (radio detection and ranging) and its very early use in measuring the height of the Earth's ionosphere at 4.28 MHz by Breit and Tuve in 1925 [1], [2].
Radar techniques employed on aircraft, balloon, or spaceborne platforms are typically deployed for three common measurement categories: imaging, scattering (scatterometers), and ranging (altimeters). Within these categories information on object size, position, composition, distance, speed, and direction of motion are all possible. In addition, the resolution (for imaging purposes) can be either fixed by the actual transmitting/receiving antenna dimensions and the distance to the scene: d = Rθ = R * 1.22λ/D, with d being the spot diameter, R the distance from the antenna or range, θ the antenna beamwidth, and D the antenna diameter, or it can be computationally enhanced in at least one dimension (along the satellite track, for example) using synthetic aperture techniques -increasing D by using the motion of the transmitter and coherently processing the signals from sequential pulses.
As the acronym implies, radar systems must generate and transmit a signal that is bounced, or scattered off the object being targeted and then detected after a generally known time delay (Fig. 2). The time delay is a function of the round trip distance to the target and back to the detector and yields the range through the velocity of radio wave propagation in the intervening media, 2 typically the velocity of light, c. In the simplest rendition the received power at the transmitting antenna over time P r (t), is related to the transmitted power P t , by [4]: where R is the range, t is the round trip time, and α is a fraction that represents the very small amount of transmitted power that makes it back to the transceiver.
The transmitted power can be a continuous wave signal, a pulse, a frequency swept signal, an amplitude or phase modulated signal, or a combination, like a chirped pulse. The received signal will appear as a time delayed and modified version of the original transmitted wave and carries information about the scene itself or an object within the scene. If, as in almost all cases, the scene is large compared to a wavelength, return signals from different regions in the scene will arrive at different times due to differences in their ranges, and the above relationship becomes an integral over the various distances encountered [4]: where B(R) is the return from a series of small regions in the scene separated by a distance R, and the integration limits involve the time of flight to all portions of the scene and the length of time the transmitted signal is deployed, typically the duration of the pulse, the modulation frequency sweep time, or the chirp period. From the point of view of power, when the transmit and receive antennas are the same, and have an effective area A, P r = (P t A 2 σ )/(4πλ 2 R 4 ), where σ is the scattering cross section of the scene and is a function of the area, geometry, and material composition. This is the familiar radar equation for wavelengths much smaller than the scene, and shows that the power drops off as the distance to the fourth power. Note also that A 2 typically contains the antenna radius to the fourth power, so increasing antenna size has a dramatic effect on the return signal through the antenna gain. Again, in a satellite application where the scene is a large area s, and we are receiving signals from its individual parts, the received power is a sum over the area of the scene that the main antenna beam is illuminating [6]: where σ 0 is the average scattering coefficient across the scene. For a continuous wave (CW) signal transmission of frequency f, radial motion in the scene towards or away from the source is directly translated into a frequency shift f± f through the Doppler effect and produces speed information [5]: velocity of an object in the scene, v = c f/(2f) for values much less than the speed of light.
With a pulsed radar, the amplitude variation of the return signal over the time window translates into information on the scattering properties of various points within the scene and can be used to construct images that reflect the characteristics of the individual objects within the field of view of a single pulsed waveform.
If a continuous wave transmitted signal is frequency modulated, for instance as a linear sawtooth-style sweep (FMCW), the received power spectrum can be converted to both range and amplitude. In this configuration the detected frequency sweep contains a frequency shift that relates directly to the distance [4]: f = K t = 2KR/c, with a range resolution of: where K is the slope of the linear frequency sweep, ( f/ t), and B w is the bandwidth of the frequency sweep. This form of radar can be used for terrain mapping (3D imaging) as well as Doppler. The form of modulation can also be sinusoidal, square wave or keyed, a stepped staircase, triangular, or other variants to enhance specific object or scene parameters.
There are many dozens of excellent texts on various forms of radar techniques and instruments going back to the MIT Radiation Lab Series and series Editor Louis Ridenour's opening volume on Radar System Engineering [5]. For the most common satellite applications, where the distance to the scene is always much much greater than the antenna scale (focal length/diameter >> 1) the authors suggest the excellent series of three pioneering texts by Fawwaz Ulaby, et al. [4], [6], [7] from the 1980's, the single volume update by Ulaby and Long [8] in 2015, and the more recent text by Charles Elachi [9] and Jacob van Zyl, released in 2021 [10].
Passive microwave radiometry (extracting the equivalent temperature of an object by recording its electromagnetic power emission in the microwave frequency range) has been utilized for determining properties of the Earth since the invention of the "Dicke-switched radiometer" first described by renowned physicist and astronomer, Robert H. Dicke in 1946 [11]. The methodology uses a calibration load of known absolute blackbody temperature that is part of the same optical path, and observed via the same electronics detector package, that is employed for direct scene observations. The calibration source is sampled frequently enough, and over a long enough observation time, that random or systematic gain variations through the entire optical path (or at least out to the load position) are averaged out. In 1945, Dicke, and then MIT Radiation Lab colleagues R. Beringer, R. Kyhl and A. G. Vane, put the new methodology to good use in what is likely the first remote sensing and radiative transfer application in the microwave range: measuring atmospheric water vapor absorption at 20, 24, and 30 GHz using the background of cold space at different antenna tilt angles (varying atmospheric path lengths) with direct comparison to in-situ aircraft data [12].
In radiometric sounding the key measurement parameter is the brightness temperature of the object(s) in the observed scene comprising the beam area of the antenna on the ground as viewed through the distance to the receive antenna. The Rayleigh-Jeans approximation to Planck's Law (accurate to around 1% for wavelengths below 100 GHz and temperatures near 300 K) relates the brightness temperature of a perfect blackbody (material that absorbs all the radiation incident upon it) to its physical temperature T [13]: B bb ( f ) = 2kT f 2 /c 2 W/m 2 /Hz/sr, where B bb is the blackbody brightness temperature in frequency units, k is Boltzmann's constant, and f is the frequency.
The power that would be received by a perfect antenna immersed in a perfect blackbody emitter (or receiving a blackbody signal that fills the entire antenna beam) is simply [12], [14]: P r = kTB w , watts, where B w is the bandwidth of the receiver in Hz. However, typical objects are not blackbodies and they absorb (or radiate) only a portion of the energy available from their physical surroundings. Their brightness temperature T b can be expressed as the temperature at which a blackbody would produce the same power in the receiver: T b =εT, where ε is defined as the emissivity of the object or the average over the scene within the antenna beam. Perfect absorbers have an ε of 1, and perfect reflectors have an ε of 0. Emissivity is a function of the geometric and physical properties of an object (or scene) and varies with frequency and polarization as well as the surface roughness relative to a wavelength (scattering properties) and the direction of the received signal relative to the orientation of the antenna (tilt angle). Loosely quoting from [13], for frequencies around 10 GHz, sea water viewed directly from above has an emissivity of 0.37, loamy soil with 25 percent moisture content comes in at 0.6, and dry soil at 0.9. The corresponding brightness temperatures (assuming a physical temperature of 300 K for all three terrains) would then be [13] 111 K, 180 K, and 270 K, respectively.
When a real antenna is looking at a scene through a directed beam and an intervening medium, the brightness temperature emitted from the scene has to be processed through the loss of the medium, and extracted from the additional signals coming from (a) the medium itself (Earth's atmosphere, in this case), (b) directions other than those the primary antenna beam is pointing towards (pick up from back lobes and sidelobes), (c) reflections from the sky bouncing off the scene and entering the antenna via the main beam and also modified by the intervening medium, and (d) the antenna temperature itself ( Fig. 3). Somewhat abridging [13]: Here, T R is the signal received at the antenna from a particular direction and with a particular footprint determined by the beam width and distance. T b is contained within the T ap term and is the actual brightness temperature we want to determine and record. η represents the antenna radiation efficiency (how much power goes into all the beams vs. what might be reflected or lost), and α represents the ratio of power in the main beam versus the side and back lobes. T ap contains the scene brightness temperature we are trying to measure reduced by the loss L of the intervening atmosphere T b /L, plus the reflected sky temperature T sc , also reduced by L. T up is the combined temperature of the atmosphere below the antenna (spacecraft) which also enters the main beam. T sl is the received signal that enters through the sidelobes and back lobes of the antenna, and T 0 is the contribution of the antenna itself, assuming it is not a perfect receiver.
To make matters even more complex the loss in the atmosphere L, is a strong function of the frequency because many gaseous molecules are prevailing absorbers in the microwave region. Polar molecules such as water and oxygen are particularly problematic with dominant pressure broadened rotational transitions at 22.3, 183.3 and 323.8 GHz (water vapor), and 60 and 118.7 GHz (oxygen). These absorption resonances and others are in fact primary radiometer products for monitoring atmospheric processes and extracting vertical temperature and pressure profiles -oxygen is especially useful because it is well mixed in the atmosphere [14]. In order to accurately characterize the atmospheric path loss at different frequencies and across varying weather and cloud 866 VOLUME 3, NO. 3, JULY 2023 conditions complex radiative transfer models are required and these have been given a lot of attention in the atmospheric science community [8], [10], [14].
Since thermal measurements of the Earth typically involve a temperature range between 3 K (cold space) and 320 K (ground) the temperature sensitivity (minimum detectable temperature difference) is a key performance metric of a radiometric receiver. Typically, heterodyne receiver systems are utilized with customized intermediate frequency bandwidth B IF and integration time τ , and the minimum detectable temperature is [15]: where T sys is the equivalent system noise temperature and includes contributions from the receiver, the antenna, and the scene. T min can generally be arranged to be 1K or less for integration times shorter than a second and bandwidths in the MHz region using room temperature heterodyne receivers operating anywhere in the range from 1 to 300 GHz. Unlike active radar remote sensing instruments, passive radiometers have more limited operating modes and somewhat less applicability. However, they are essential for making direct measurements of specific atmospheric constituents via the recording of unique spectral absorption/emission signatures (spectral lines) associated with the calculable quantum mechanical vibrational and rotational motions of polar molecules. Radiometers can also be used for imaging temperature variations in a scene associated with the brightness temperature difference within an observed terrain of mixed materials (i.e., water, soil, vegetation etc.). Most importantly, by utilizing accurate models for spectral line broadening by pressure and temperature, radiometers can measure these parameters (T and p) at altitude with well sampled vertical profiling, either from the ground or from space. Finally, by scanning through the atmosphere from an orbiting platform against the background of cold space, rather than aiming down towards the ground (the technique known as atmospheric limb scanning), direct measurements of the distribution and abundance of dozens of molecules comprising the stratosphere and upper troposphere can be measured across the globe on a daily basis providing essential data on water vapor, oxygen, carbon monoxide, sulfur dioxide, hydrochloric acid, OH, and many other gases associated with important global processes, including ozone depletion, acidification, and global warming [16].
In surveying the uses of these two techniques for the many hundreds of subsequent global studies related to climate change, we begin with some applications for satellite-based microwave radiometry and then follow with the more versatile active radar systems.

A. SATELLITE-BASED RADIOMETRY
The first successful satellite-borne microwave radiometer was flown on NASA's Mariner-2 spacecraft in 1962. The two channel radiometer operating at 15.8 and 22.2 GHz, and based on Dicke's pioneering load-switched microwave measurement techniques [11], returned early observations of the temperature of the Venusian atmosphere. Results showed that the atmosphere was optically thick (the microwave signals did not penetrate to the surface) and the atmosphere had an average temperature of around 500 K. The microwave instrument was developed at the NASA Jet Propulsion Laboratory by former mentor and dear friend, Frank Barath and his colleagues [17], [18] and was the start of the microwave remote sensing revolution that would help change the way we monitor our global environment.
The first use of radiometry for Earth observations from space came in 1968, with the launch of the USSR's Cosmos 243 [19]. The satellite carried four radiometers covering 3.5 to 37 GHz and measured water vapor, including cloud content, ocean surface temperature, sea ice borders surrounding Antarctica, and brightness temperatures on the ground and on sea water and ice. The US responded with two types of microwave radiometers on Nimbus 5 launched in 1972 and spearheaded by another former mentor and dear colleague at JPL -Joe W. Waters [20], who would go on to pioneer the use of microwave limb scanning up to THz frequencies [21]. The Nimbus E microwave spectrometer contained five nadir pointing radiometers covering the 22.2 GHz water vapor line for measuring humidity and cloud water content, and extended up to 58.8 GHz to cover the oxygen bands for deducing temperature profiles [22]. The temperature measurements agreed almost perfectly with in-situ balloon-borne radiosonde data [23] and established this technique as a principal method for long-term microwave observations of the atmospheric temperature from space. Nimbus 5 also carried an electronically steered array at 19 GHz to measure ground emissivity, and record snow and ice coverage.
By 1980, only eight years after Nimbus 5, more than a dozen satellite instruments had been deployed for Earth remote sensing observations using microwave radiometers [8].
In the past four decades this number has increased dramatically, as have the numbers of receiver channels, the frequency range, and the sensitivity (minimum detectable temperature differences). The development and deployment of the scanning multichannel microwave radiometer (SMMR) [24] on both Nimbus 7 and Seasat, both launched in 1978, set the stage for a series of core climatological measurement instruments that have flown continuously to this day, and are planned to fly well into the future. SMMR carried five radiometer channels from 6.6 to 37 GHz, but subsequent missions quickly increased the frequency to the 183 GHz water vapor line, with the advantage of much higher spatial resolution from the same size antenna. A parallel program in the US Air Force -DMSP (Defense Meteorological Satellite Program) launched a similar instrument -SSM/T (Special Sensor Microwave Temperature sounder) around the same time, with a 50-60 GHz receiver complement targeting oxygen, that also began a continuous measurement capability for meteorological and climate data monitoring for the military [25], [26].
Today substantially evolved versions of these instruments are being flown by the US National Oceanic and Atmospheric Administration (NOAA) under the designation JPSS (Joint Polar Satellite System) [27] and by the US Air Force Space Command under the Defense Weather Satellite System [28]. Much of the data generated from the early instruments just enumerated have been recently reprocessed and are available in user friendly form on NASA's EARTHDATA web pages [29]. There are also many other countries that have since fielded passive microwave sounders for both meteorological and climate monitoring purposes including (the top six): Russia, China, U.K., Japan, India, and the European Union. More recently, commercial companies have gotten involved [30], and the push for smaller and less expensive platforms like CubeSat [31] have dramatically expanded the available suite of satellites fielding radiometric instruments.
The passive microwave sounders are mainly targeted at extremely accurate temperature profiling -day and night, in rain or fog, through clouds and dust. Beginning with the deployment of SMMR on Seasat, radiometric instruments have added measurements of [10]: water content in clouds, rain rates, wind velocity, sea temperature, sea ice concentrations, snow cover, and even soil moisture measurements using emissivity variations at different frequencies to extract surface or underground values [32]. As the retrieval algorithms improve, the data from passive microwave sounding gets better and better, and the half century of global monitoring data is now an essential constituent of the Earth's climate record [33].
Before leaving passive instruments it is worth mentioning one more style of microwave remote sensing that has had, and is still having, a very significant impact on climate monitoring, as well as in understanding important environmentally destructive processes such as ozone depletion [34]. This is the observing technique we mentioned earlier known as microwave limb scanning. Rather than looking down at the Earth, the satellite antenna scans through the atmospheric limb as it traverses its orbital path, nodding from the top of the stratosphere down to the troposphere, and collecting the emission spectra from discrete molecular line transitions as a function of altitude and position. By observing the thermal emission against cold space and arranging the antenna beam so that it collects data from a limited vertical profile beginning at the top of the atmosphere and dropping down in consistent steps, the spectral line signatures from each specific altitude within the beam path can be separated out. This results in a complete vertical and horizontal profile of the position and abundance of any of a large number of molecular species that can be continuously tracked and monitored. The technique was demonstrated with two major Earth remote sensing instruments developed at JPL under principal investigator Joe Waters: Upper Atmospheric Research Satellite Microwave Limb Sounder, launched in 1991, and Aura Microwave Limb Sounder launched in 2004 [21].

B. SATELLITE-BASED RADARS
Perhaps the first airborne radar microwave remote sensing measurement to look at atmospheric properties took place in the 1940s and consisted of a somewhat crude determination of the excess water vapor absorption on humid days from signals sent between two operational aircraft 3 [91], [92]. Rain radars based on particle-size-dependent backscattering soon emerged [93]. Synthetic aperture techniques were developed and demonstrated in the early 1960's [94], [95], [96]. NASA's Skylab, which launched in 1973, carried a 13.9 GHz pulsed radar altimeter which was primarily used for oceanographic sounding [97]. Ulaby and Moore [98] advocated combining radiometer and radar instruments to help with calibration, especially for ocean wind speed measurements made by radar scatterometers that correlated wave crests with velocity. By the time Charles Elachi and Walter Brown turned their L-band (1275 MHz) airborne synthetic aperture radar imager (designed for measuring snow and ice sheet thickness) towards the surface of the ocean in 1974 [99], remote sensing in the microwave regime was already well established. However, the new information -direct tracking of wave crests and subsequent retrieval of ocean wind speed-bought them an instrument slot on the upcoming NASA Seasat satellite program [9]. The extremely successful radar instrument deployment made microwave synthetic aperture radar a key component of many future space-based environmental monitoring platforms.
Seasat launched in June 1978, and along with the SAR instrument, carried a radar altimeter, a microwave scatterometer, and microwave radiometers for ocean and wave height, wind speed and direction, and emissivity and temperature, respectively. The 1275 MHz SAR radar used a 10.7x2.2 meter antenna and could resolve 25 meters on the ground [100], Fig. 4. The SAR system also imaged dry land. The long wavelength deep penetrating microwaves made some amazing pictures of subsurface features [101], as well as traversing through foliage covered regions and yielding a wide range of new geologic and environmental data that would play a 3 J. H. van Fleck [91] and also Robert Dicke [12]   significant role in resource management and human activities affecting the climate [102], [103]. Elachi and Brown also were able to use the SAR technique for rainfall monitoring [104] and many other valuable environmental applications [105], including 3D terrain mapping [106]. The SAR system flew on three additional NASA Shuttle missions and two major planetary missions (Magellan and Cassini), while Elachi was still involved. The technique continues to evolve and expand an already significant suite of applicable measurements. Many US SAR instruments, plus SAR systems from the European Space Agency, Japan, Germany, Canada, and even Argentina and Finland, have all been launched since Seasat [10].
Radar scatterometers have also played a significant role in Earth monitoring and mapping, and have accompanied SAR systems on many satellite platforms. Scatterometers are excellent at measuring wind speeds [85], [107], precipitation [108], cloud density [109], surface roughness and foliage coverage [110], [111], as well as other geologic features.
One last recent remote sensing innovation that has gotten a lot of attention is NASA JPL's Grace (gravity recovery and climate experiment) and its follow-on mission Grace-FO, launched in 2002 and 2018 respectively. The Grace platforms detect very subtle changes in gravitational pull on two synchronously orbiting satellites, originating from localized mass changes that occur directly below the orbital path. The differential gravitational pull affects the speed and position of one satellite relative to the other, and this very small change is monitored and recorded by 24 and 32 GHz radar ranging instruments that are accurate to ten microns [141]! Surprisingly, these minute motion shifts can be correlated with small mass changes under the satellites and can be used to very accurately track changes due to climate affected water reservoirs, underground water aquifers, glaciers, ice and snow pack, deep ocean pressure and height, and other relevant geological variables impacted by a changing climate.
New microwave radar applications include higher frequency instruments [142] that can measure water vapor profiles, cloud density and structure, and derive radiance and pressure profiles in the atmosphere with smaller antennas. The use of active interferometric techniques are now being deployed to get unprecedented resolution [143] without the need to have massive antennas or continuous synthetic aperture superposition.

C. ATMOSPHERIC SENSING WITHOUT SATELLITES
Many of the measurements that satellites can make with microwave radiometers and radars can generally be done from the ground with higher resolution (shorter range, and therefore smaller beam diameter), but more limited spatial coverage. Molecular line spectroscopy is one exception because the absorption in the lower atmosphere obscures the signals that are emitted higher up and pressure broadening obscures the fingerprinting available from individual emission signatures. For these measurements high altitude aircraft or balloon platforms are needed.
Ground based and airborne platforms often provide direct comparison with in-situ sampling to assist in calibrating remote sensing instrument products. NASA and NOAA in the US, and government agencies in many other countries as well, support a wide range of platforms and continuous campaigns for monitoring particular climate relevant atmospheric and geologic parameters [144]. These platforms (NASA has more than a dozen aircraft and over fifty specific ongoing science and observation programs [145]) provide the infrastructure to test new measurement techniques and technologies as well as to hone particular global change models or understand particular processes. The formation of hurricanes and cyclones, the distribution of clouds and aerosols, ice and snow coverage, water resource management, the impacts of wildfires, ozone levels, CO 2 studies, the convection of water vapor from the tropics into other latitudes, soil moisture and biomass studies, and many more climate related activities are being monitored [145]. The number of technical papers covering these microwave-based measurements and instruments is also voluminous: 7200 radar and 2150 radiometry remote sensing papers in Web of Science at last look, and 13,600 listings on IEEEXplore. It is not feasible to mention more than a few of these in this short tutorial.
Ground based radar has been used extensively for weather and climate monitoring since the 1940s when it was deployed to track free floating airborne radiosonde instruments in order to provide location information for in-situ weather measurements [146]. Extensive radar (and radiometer) observations were applied to the oceans (mostly from aircraft) in the 1960s [147]. Ocean temperature and surface roughness were key parameters. By the 1970s, airborne and ground based radar systems were being deployed for geological and geomorphological measurements [148], snow and ice coverage, thickness, and water content [149], land surface roughness and wind speeds [150], soil moisture and vegetation coverage [151], and rain and weather monitoring [152]. The 1980s saw the rise of synthetic aperture techniques -both SAR and inverse SAR, and increased use of polarization information [153]. Ubiquitous Doppler radars measured local cloud cover and rain content [154], and uniquely identified clear air turbulence [155]. SAR interferometry and high resolution measurements were introduced in the 1980s [156]. From the 1990s onwards, the radar remote sensing field simply exploded with greatly improved retrieval algorithms, hundreds of new instruments, and a massive effort to increase existing, as well as to add, new resource monitoring programs, both to support climate science and the greatly increased satellite data that was pouring in [8]. Today, non-satellite based radar remote sensing goes from MHz [157] to THz [157], [159], and from monitoring birds [160], to finding deep underground coal fires [161], to locating plastic accumulations on the ocean [162].
On the radiometric front, ground based observations of the atmosphere to retrieve temperature and humidity profiles, based on the techniques of Dicke [12] and the calculations of van Vleck [91], have been ongoing since the 1950s [14], [163]. Aircraft and supporting ground measurements helped Ulaby et al. add passive emissivity characterizations for distinguishing soil moisture [164] and monitoring vegetation [165]. Staelin used radiometric imaging from aircraft for atmospheric profiling [166], and rain and storm monitoring [167]. Joe Waters [16] added molecular and atomic emission line measurements [168] reaching well into the THz spectrum [169] with his limb scanning technique from aircraft and balloon platforms [170]. High resolution ground terrain mapping in fog and through clouds was demonstrated by JPL's Bill Wilson in the 1980s using a first-of-its-kind helicopter-borne 98 GHz imaging radiometer [172]. Wilson also pioneered a passive radiometric wind sounder for aircraft measurements over the ocean [173]. Accurate snow and ice retrievals were developed in the 1990s [174], [175]. Today, ground-based microwave radiometers in a range of frequencies are actively deployed around the world for atmospheric monitoring [176], [177] and for local weather events [178], [179].

III. MICROWAVE HEATING
We now turn away from microwave radiometry and radar sensing techniques to take a look at a much more familiar characteristic of this region of the electromagnetic spectrum: microwave heating, and how this feature might be deployed in climate change studies or mitigation.
Using microwaves for heating foods and other substances -including people [180] -has been a widespread application since the realization of high power microwave resonance tubes (klystrons and magnetrons) in the 1920s [181] and 1930s [182]. Progress was enhanced after the design and successful commercial production of the microwave multiple-cavity magnetron by British physicists Sir John Randall and Henry Boot during World War II [183]. The magnetron was originally developed as a radar transmitter, but was soon commercialized for the food industry when Raytheon's Percy Spencer and Tappan appliances came together in the early 1960s to introduce the Radarange TM [184]. By 1967, the Amana Radarange was available for the homeowner for less than $500, and boasted kW output power in the microwave ISM (industrial, science and medical) bands at 914 MHz at 2.45 GHz [185].
Quoting from [186], "RF heating is based on the complex permittivity [185]:ε = ε 0 (ε r + jε i ), where ε 0 is the free space permittivity in Farads/meter, ε r is the real part of the dielectric constant, and ε i = σ /ωε 0 is the imaginary part, with σ the conductivity in Siemens/meter and ω = 2π f the applied frequency. The RF absorption comes from σ via excitation of free carriers or electric polarizability (dipole absorption) and is conveniently expressed by the ratio of ε i /ε r ࣕ tan δ = σ /(ωε 0 ε r ). The power dissipated, or the heating of the material at a given internal location, is then given by P i (x, y, z) = σ |E i (x, y, z)| 2 = ωε 0 ε r tan δ|E i | 2 , where E i is the field inside [185]. For a sample that is small compared to the wavelength there is very inefficient power absorption. When the sample is on the order of the wavelength there can be significant or even resonant absorption, and when the sample is large compared to the wavelength there will be an exponential penetration of the power P inside = P sur f ace e −αx , where x is the depth in cm, and α is the absorption coefficient in cm −1 . The field drops to 1/e of its surface value at a depth D, [185]: where λ 0 is the wavelength in free space. When tan δ <<1, D ≈ 0.318λ 0 /( √ ε r tan δ)." For gases at low pressure, the microwave excitation goes directly into resonant absorption modes (rotational and vibrational motions) that characterize, and are unique to, particular polar molecules, and the energy can cause both heating and mode changes. Water vapor is the most ubiquitous target, and hydrolysis to generate hydrogen gas, the most obvious climate-aligned energy application.
Today, the magnetron and its higher power electron streaming succedents (gyrotrons and orbitrons) are still the most efficient way to generate high power microwaves, despite the recent development of high power solid-state semiconductor materials and amplifiers 4 [187]. Even in the 1960s, power levels in the kilowatt range could be generated with fist-sized components and efficiencies approaching 80% [188]. Scaling up in power to the gigawatt range [189] or in frequency to the THz regime [190] appear feasible. This has opened up a range of potential heating applications that can have both a direct (through replacement of fossil fuel 4 914 and 2540 MHz magnetrons in commercial microwave ovens have advertised efficiencies (plug-to-RF) between 70 and 88% at power levels approaching 1kW. New GaN and SiC power amps are getting closer with power added efficiencies of over 60% at these frequencies and similar power levels, at least at 914 MHz [187]. sources), and a peripheral (through chemical processes) impact on climate change. We examine a few of these connections and potential applications next.

A. DRYING
The drying of materials and foodstuffs is as old a technology as one might imagine, and involves every conceivable form of heat transfer from direct burning of wood, to wind and solar, to modern electrical induction. From the point of view of energy efficiency (total kW hours needed to evaporate 1 kg of water from a material) microwave heating falls into the middle of the pack of available non-fossil fuel burning drying solutions with a typical value of 5 kWh/kg and efficiencies ranging wildly from 8 to 62% [191]. There is a strong dependence on the dielectric properties of the material being heated, the physical arrangement, and the coupling efficiency of the RF, which is frequency, mode, and geometry dependent.
Induction, solar, and vacuum drying are all somewhat more efficient than microwave techniques [191], but each has its limitations. For particular applications, such as food and grain drying via a fluidized bed (hot air levitating particulates in a sealed chamber), microwave assisted heating improves the efficiency by 95% over hot air alone [192]. In pyrolysis, heating is done in the absence of oxygen and leads to irreversible chemical decomposition. Gaseous, liquid, and solid products are all possible. Microwaves are used in speeding up the reactions, increasing the heating efficiency, and maintaining lower pyrolysis temperatures (in the range of 200-300 C) [193].
Other microwave and microwave-assisted heating applications are widespread and include everything from clothes dryers [194] to leaf (herb and tea) drying [195], and plant-based human and animal food preservation [196]. Industrial waste pre-processing is also a common microwave application [197]. There is even some research involving positive impacts of microwave heating on enhancing plant nutrition [198], vegetable disease mitigation [199], and extraction of pollutants [200].
Microwave heating based on current RF source technology is a fairly efficient electrical technique and avoids the use of direct fossil fuel consumption. For most applications (including those described in the next several sections), one can visualize the source as a form of microwave oven consisting of a magnetron and a cavity (or waveguide) containing the material to be heated. For heating gases, a sealed chamber can be used. As mentioned earlier there is also a growing potential for the use of all solid-state sources with high efficiencies [187], [201].

B. ALTERNATIVE FUEL PRODUCTION
Perhaps the most prolific use of microwave and microwaveassisted heating as related to climate change at the current time, is in the conversion of biomass (from deceased plants and animals), municipal solid waste (sewage), and industrial waste (plastics and oil) into sustainable fuels. There are literally dozens of pyrolysis and catalyst assisted processes being looked at, both in the laboratory and on an industrial scale, that utilize microwaves at one or more stages of the conversion process. For biomass, typically the end product is either ethanol (C 2 H 5 OH) or sometimes syngas (generally methanol CH 3 OH); biodiesel (fatty acid esters with an alcohol group generated from vegetable oils and animal fats); or biogas (mostly methane CH 4 and carbon dioxide CO 2 ). The methane and CO 2 can also be further processed to make hydrogen gas H 2 and carbon monoxide CO. Another end product, biochar (charcoal composed of biomass products) is discussed in the next section. Typical heating temperatures are between 200 and 600 C depending on process and biomass material, but temperatures as high as 900 C are not uncommon for some processes.
Biomass production exceeds 120 billion tons per year worldwide (10 billion tons dry weight) with an energy capacity of around 2.2 × 10 21 J [202]. According to United Nations statistics, the average global energy use in 2022 was approximately 6.2 × 10 20 J [203], well below the potentially available biomass energy resources. However, biomass output must also provide the needed food for the world's population. Current output hovers around 10 billion tons of vegetable crops and 330 million tons of animal products, not yet sufficient to eliminate worldwide hunger [204]. This means the search for additional biomass sources is ongoing, and includes waste cotton [205], water cultivated algal sources [206], discarded animal fats [207], and other biowaste materials [208].
Making biofuels [209], [210] and other useful chemical commodities involves multiple steps and many intermediary products [202] too complex and numerous to go into here, but many of these require pyrolysis at some stage [211], [212]. Microwave heating offers the potential for deep penetration into the biomass material and effective temperature control. Microwave assisted pyrolysis also has rapid turn on and turn off, no requirement for agitation, and the potential for uniform internal heating, assuming the material has uniform volume dielectric properties. In this issue of IEEE JOURNAL OF MICROWAVES, Robinson et al. [201] show that single frequency solid-state microwave sources can allow for some input tuning and subsequent match into the biomass material for much higher coupling efficiency and much more efficient internal heating.

C. BIOCHAR
Biochar (Fig. 5) is an indispensable recycled material with an incredibly wide range of applications that relate directly and indirectly to climate change mitigation [213]. Biochar can be an end product of biomass pyrolysis and it can be an important fuel source in and of itself. Essentially it is solid carbon (charcoal) but produced from (amongst other treatments and catalysts) slow or fast heating of some initial biomass material in an anaerobic process. If derived from plants, it is often classified as carbon neutral because its production from biomass involves initial carbon capture through photosynthesis [202]. The biochar can also be used in the aforementioned fuel extraction process as an enhanced microwave absorber to increase the microwave coupling efficiency and speed up heating [214]. Biochar is also an extremely important ingredient for improving water retention in soils, filtering out noxious chemicals, improving crop yield as part of a fertilization regimen, sequestering carbon for extended periods, and dealing with an ever increasing accumulation of solid waste [215].
The heat required to create biochar is between 300 and 900 C depending on the process used and the starting materials [214], Table I. As with most microwave heating applications, the ISM bands at 914 and 2450 MHz are typically employed. With a fast heating process (and higher temperatures -up to 900 C) more biochar is generally produced than in a slow pyrolysis reaction (temperature typically below 500 C) [214]. The particular choice of process depends on many factors including the production of associated gases (slow process) or bio-oil (fast pyrolysis). As with any carbonized product, the absorption properties of the material are also useful in a wide range of applications from water filtering to serving as a catalyst in a variety of chemical production processes. Besides its use as a heating agent (production value of 29-30 kJ/gm [193]), biochar is also a CO 2 sink and its ultimate composition can be controlled by appropriate choice of initial biomass properties [214]. It has been proposed as a carbon capture and cleansing agent for large scale agricultural use (as a fertilizer) and many other applications that can help ameliorate the impacts of climate change [213], [214], [215].

D. CHEMICAL CONVERSIONS
In a final application for microwave-assisted heating related to mitigating the impacts of climate change we take a brief look at gas conversion -both creating cleaner burning fuels and capturing or converting CO 2 . Both of these chemical processes can be assisted with the application of microwave heating.
Converting CO 2 into other products involves breaking two highly stable covalent bonds (O=C=O) and generally requires significant energy. A promising technique is to use a non-thermal plasma discharge, and microwave sources can play a role in this process [216]. The microwave energy is generally delivered to the gas chamber through a waveguide using a high power pulse in the several hundred MHz to few GHz frequency range, or the gas can flow directly through the source waveguide. Many common CO 2 conversion reactions are possible but one of the most common is the production of ethanol (CH 3 OH). A commercial system mentioned in [216] can produce 1.5 kilotons of ethanol per year from 3.3 kilotons of biomass.
Conversion of atmospheric CO 2 into solids or liquids for long term storage is also an application of interest, and microwave magnetrons can be efficiently employed for this process [217], [218]. Plasma temperatures can reach 6000 K and energy conversion efficiencies are reported to be between 40 and 90% [217], Table I. Finally, CO 2 char can be converted to CO (C + CO 2 ↔ 2CO) for gaseous fuel storage and reuse employing microwave heating (gasification) [219]. Extremely high efficiency can be achieved (97%) with gas temperatures in the 900 C range. H 2 can also be produced through: H 2 + CO 2 ↔ CO + H 2 O with excellent efficiency [219]. Other researchers have reported similar gasification advantages using microwave heating [220], and the conversion of CO 2 into many useful chemical products [217].
Other types of carbon conversions are possible with the introduction of catalysts and additional gaseous compounds, and many can benefit from the very efficient microwave assisted heating of polar gases at their resonant frequencies, or using broadly applicable microwave plasma discharge. Expansion of the technologies to large commercial scale plants is still a challenge, but our next topic involves the scaling up of microwave sources to enormously high output power. These may someday be applied to the carbon capture and carbon gasification processes just described.

IV. MICROWAVES IN FUSION
One of our best hopes for solving the energy production bottleneck has always been the potential to develop commercially viable fusion reactors. Despite the decades long (first controlled fusion reactor using magnetic confinement was Los Alamos National Laboratory's Scylla I in 1958 [221]) investment in this potentially infinite source of clean energy, there is still no solution in hand. However, there are many major research programs ongoing, and significant progress continues for both laser assisted (heated fuel pellet) and traditional tokamak style (magnetic torus plasma confinement) approaches. In the tokamak world microwaves play a key part in heating up the plasma. We expect to publish an excellent review and detailed description of the microwave techniques employed in the plasma fusion community in an upcoming article [222] from which we have generated the summary which follows.
Plasma temperatures for achieving the conditions for fusion are in excess of 150 million degrees. The strong toroidal magnetic field and the presence of a high density plasma causes electrons to spiral around the field lines in a path that resonates with a frequency that is determined by the strength and direction of the field, the electron cyclotron resonance ECR: f ecr = eB/(2π m e ), where e is the electron charge, B is the magnetic field, and m e is the electron mass. For very high fields the electron mass has to be replaced by its relativistic form with the rest mass m o,e increased by the usual velocity modification: Since tokamak magnetic fields tend to be in the 1-10 Tesla range, the ECR frequency varies between a few GHz to over 150 GHz. Sometimes both the fundamental and second harmonic ECR frequencies are targeted for plasma heating.
The required microwave power levels to produce the high temperatures required for fusion are typically in the several hundred kW to 10 MW range. The largest demonstration tokamak system now under construction, the International Thermonuclear Experimental Reactor (ITER) will use both inductive heating and heat generated by the fusion reaction itself for its primary source of energy. However, it will also deliver more than 20 megawatts of microwave power at 170 GHz to its magnetically confined plasma as an ECR assist to its primary heating system.
There are two other resonant frequencies that plasma scientists make use of for heating in their tokamak reactors, the ion cyclotron resonance (ICR) frequency, and the lower hybrid (LH) oscillation frequency. The ICR is similar to the ECR, but falls at a much lower frequency due to the higher mass of the ions in the plasma (typically protons) and is generally in the MHz region. The LH excitation is a longitudinal oscillation in the direction of the magnetic field and involves both electrons and ions. It falls in the middle of the ECR and ICR frequencies and is typically in the 2-10 GHz range. The LH mode is particularly useful for driving currents in the plasma that can get as high as 100 kA!
The microwave sources needed for generating the extremely high powers used in tokamaks (from MHz to >100 GHz) vary in design and implementation, and for the ICR mode, solid-state devices using GaN amplifier technology can satisfy some fusion programs. However, most fusion applications employ high power electron tube devices such as traveling wave tube amplifiers [223], klystrons [224], and gyrotrons [225] to excite the three common plasma heating modes.
Even though plasma heating can be demonstrated using pulsed microwave power, continuous wave operation is eventually required. The desired energy is so high that it usually requires several individual sources to be efficiently power combined, or at least synchronized to fall onto the plasma from multiple directions and locations. In the more powerful reactors such as ITER, twenty-four individual 1 MW gyrotrons with a pulse length greater than 500 seconds will be eventually employed for the ECR heating alone [222]!

V. MICROWAVE POWER BEAMING
One of the authors remembers with great interest reading William Brown's 1974 PROCEEDINGS OF THE IEEE article on microwave power beaming [226] while an undergraduate studying physics. Having had some fun as a secondary school student building and flying gasoline powered model airplanes, he was impressed both by the space-age properties of this invisible beam of guided energy, as well as its use in powering a model helicopter carrying a relatively quiet electric motor. (He was also influenced by Brown's more personalized writing style for technical publications)! Although he can't say that this article alone pushed him into electrical engineering, by the time Brown's MTT Transactions article on space-power beaming came out in 1992 [227], he was already fully entrenched in the microwave community.
At first glance, the idea of sending power through space, instead of high cost and generally more lossy transmission lines or waveguides, seems both extremely titillating, as it solves a major problem in energy distribution, and at the same time fatally flawed, because it necessarily carves out an invisible region in space through which objects cross at their own peril. The concept depends on the efficient conversion of electrical energy (DC or plug power) into microwave frequencies, very effective illumination and focusing of the microwave power into, and out of, a large antenna, and then collecting as much of the RF energy as possible at a specific distance, while simultaneously converting it back to DC using a rectenna (rectifying device embedded into a receiving antenna).
The issues of safety and electromagnetic interference aside, when applied to climate change, the power beaming concept serves two major purposes: (1) it allows efficient distribution of energy over large distances on the ground, so as to bring green energy from a distant or difficult to reach point of origin to the end users, and (2) it might possibly solve the intermittent solar energy generation problem (both weather related and lack of night time operation) if the solar panels were placed above the atmosphere and the power was beamed down to the ground. Both of these applications have recently been reviewed in three excellent papers from this journal by Chris Rodenbeck et al., [228], [229], and Naoki Shinohara [230]. We summarize a few of the more salient features from these papers here. Note that power beaming can also be accomplished with optical wavelengths but suffers much more from atmospheric propagation losses (rain, dust, and clouds) when employed for the two circumstances relevant to climate change described here.
In spite of what one might expect from the large number and span of the involved frequency conversions, the overall efficiency in going from DC to microwaves and back is actually not too terrible, with a record value of 54% set in the laboratory by William Brown himself in 1975 [227]. High power microwave tubes and solid-state power devices can have plugto-RF efficiencies of 70-90% and rectifying antennas that directly convert the received RF to DC have demonstrated efficiencies of 85% or more [227]. If the frequency employed is below 10 GHz, the atmospheric attenuation is minimal over large distances, and only very modestly impacted by heavy rain or clouds, see Fig. 7 in ref. [228]. The most lossy portion of the link is the aperture-to-aperture coupling which, due to diffraction, follows a coupling metric τ that varies from 0 to approximately 2.5 for beam transmit and collection efficiencies spanning 0 to 100% [227], [228]: where A T and A R are the transmit and receive aperture areas, λ is the wavelength and d is the separation of the two antennas. For small d, the antenna-to-antenna coupling efficiency can approach 100%, but for distances approximating far-field values: 2D 2 /λ, D being the antenna diameter, the efficiency is generally below 15% [228]. Note that at large distances, the receive aperture is typically much larger than the transmit aperture due to significant beam diffraction and the desire to intercept as much transmitted power as possible. Surprisingly, the A T to A R coupling is strongest when the distance between the apertures falls into the Fresnel region (roughly 0.6 D 2 /λ < d < 2D 2 /λ), and when the transmitting aperture field distribution is optimized in amplitude and phase for the specific collection distance, see Fig. 6 in ref. [228]. The record for the highest received power over the longest path is from a test conducted at NASA's Goldstone antenna by Brown and JPL colleagues in the 1990s and stands at 35 kW over 1.55 km using a frequency of 2.4 GHz [228]. The issue of safety is dealt with by requiring that the power density anywhere within the propagating beam falls below the ICNIRP (International Commission on Non-Ionizing Radiation Protection) recommended limits for unrestricted full body exposure of RF below 6 GHz of 1 mW/cm 2 for 30 minutes [231]. For low microwave frequencies and large apertures this is a workable restriction, although some concepts for very high power systems seem to exceed this limit under conditions that the beam can be defocused, turned off, dimmed, or redirected, if breached by people, animals and birds, or reflective objects [228]. At least one large scale demonstration program to transmit 10 GHz power at km distances (1.1 km in this instance) was recently described by Rodenbeck et al. [229] and yielded 1.6 kW of received DC power with 91.2 kW transmitted (1.8% RF-DC efficiency), but projected improvements show efficiency gains up to 44%. Whether this technology can replace power transmission lines is still to be determined, but the potential for at least some niche uses seems plausible.
The second part of the power beaming dream is to collect solar energy continuously using an orbital platform and beam the power to the ground where it is picked up by an enormous rectifying antenna ground array and converted to DC. There is actually a NASA concept dubbed the Space Solar Power Satellite that was proposed in 1968 by Peter Glaser [232]. The initial design used a satellite in geosynchronous orbit (35,000 km from the Earth) with a solar array spanning 10 km 2 ! The proposal has undergone several iterations since, but at least two recent designs are still on the books and parameterized in [228]. One would work at 2.45 GHz and the other at 5.8 GHz (also an ISM band). Both would hope to provide gigawatts of DC power through a solar array that is four times more efficient in space than in the desert and bigger than Godzilla! The lower frequency concept transmits from a 1 km diameter antenna to a 10 km diameter ground rectenna array, while the higher frequency system requires only a half-km transmitter. Both concepts rely on an ambitious photovoltaic-to-ground DC conversion efficiency of close to 50%. Details can be found in [233], but don't count on implementation anytime soon. 5 5 In a post-review development, Caltech's Ali Hajimiri and team have successfully demonstrated the first wireless power beaming in space, between a transmit antenna and LED array, on the Space Solar Power Demonstrator (SSPD-1) satellite. The instrument, MAPLE (microwave array power-transfer low-orbit experiment), was launched on January 3, 2023 and is partly funded through a private foundation. A portion of the space-based power transmission was also detected on the ground using a radio telescope on the Caltech campus [234].

VI. MICROWAVE PROPAGATION AND OTHER IMPACTS
In this last short section, we mention a few interesting additional topics where microwaves and climate science intersect.
These days, we all depend so heavily on wireless communications that anything disrupting or influencing atmospheric propagation has widespread and significant consequences. As the climate changes, forecasts as well as demonstrated evidence for more severe weather events continue to amass. Kevin Paulson in Australia [235] and Péter Kántor and János Bitó in Hungary [236] have been examining the impact of severe weather, especially rainfall, on wireless links. This is certainly a topic that will rise in interest levels as storms increase in frequency and scale. There may be additional weather related variations in the quality and required density and power levels of our microwave communications networks as temperatures in some places increase, and atmospheric humidity levels continue to change.
Along the same vein, windshear is a nagging hazard for civilian and military aircraft and there is growing evidence that climate change is making things worse [237]. Both microwave Doppler radar [155], [238] and active microwave interferometry [239] can help. Microwave radar can also track bird migrations [160] which helps our understanding of both adaptation of species to climate change as well as global climate conditions. There are other localized weather events and patterns from tracking snow and hail, to severe winds in cyclones and hurricanes, and quantifying atmospheric water vapor during droughts, that are all aided by microwave techniques that have been mentioned in prior sections, but that will become more important as the climate evolves.
Turning to even more exotic couplings between climate and microwaves, we could not pass up the article by Takashi Ohira of Toyohashi University of Technology in Japan who wants to change the way we intend to use our electric vehicle technology by adding high frequency rectification into our roadway systems and eliminate batteries in our electric cars and trucks [240]! Finally, we want to end on a positive note with a very recent result from the NASA JPL microwave limb sounding team. They reported in [71] that the impacts of climate change on atmospheric circulation have shortened the expected lifetime for nitrous oxide to remain in the stratosphere and they predict 27% less N 2 O by the end of the 21 st century than had been estimated previously. This decrease of N 2 O would have a positive effect in slightly reducing global warming and ozone depletion.
With that, we think we have covered about as much of this intersecting technology as we have space for!

VII. SUMMARY
In this broad tutorial we have tried to cover some of the science and applications that bring together the study and observations of climate change, with microwave technology and instrumentation. On the global scale, nothing competes with the ingenious and evolving technological achievements that have filled our skies with complex, multispectral satellite instruments that can make incredibly accurate and sophisticated observations of physical parameters as diverse as highly accurate temperature profiling from space and backscattering from deep underground snow-ice and water-ice boundaries. Even more impressive is our ever increasingly erudite toolkit for pulling out shrouded or completely hidden physical attributes and resources from observed data when these are coupled with highly developed and vetted mathematical models -using slight changes in the orbital motion of a small object circling the Earth to infer the volume and depth of a buried water reservoir 2000 km below! We truly need to show more respect and homage to our world's scientists and engineers than most of our fellow Earthlings are willing to admit.
As we look closer to home, there are still many impacts of climate change that can be either helped directly, or indirectly, through the application of techniques and instruments that are centered around the microwave spectrum from (as the IEEE Microwave Theory and Technology society likes to advertise) MHz to THz. These take the general form of efficient or specialized heating, chemical conversion for green fuel production and carbon capture, clean power generation through fusion, and perhaps even wireless power transport. Many less impactful cross-connections can be found, and these authors hope the readers of this article will respond with additional collaborative work that brings the microwave engineering community and the climate science community into closer synergy.