Albedo Changes Drive 4.9 to 9.4C Global Warming by 2400

This study ties increasing climate feedbacks to projected warming consistent with temperatures when Earth last had this much CO₂ in the air. The relationship between CO₂ and temperature in a Vostok ice core is used to extrapolate temperature effects of today's CO₂ levels. The results suggest long-run equilibrium global surface temperatures (GSTs) 5 degrees Celsius warmer than immediately "pre-industrial" (1880). The relationship derived holds well for warmer conditions 4 and 14 million years ago (Mya). Adding CH₄ data from Vostok yields 8.5 degrees Celsius warming due to today's CO₂ and CH₄ levels. Long-run climate sensitivity to doubled CO₂, given Earth's current ice state, is estimated to be 8.2 degrees Celsius: 1.8 directly from CO₂ and 6.4 from albedo effects. Based on the Vostok equation using CO₂ only, holding change in GST to 2 degrees Celsius requires 318 ppm CO₂. This means Earth's remaining carbon budget is estimated to be negative 313 billion tonnes. Meeting this target will require very large-scale CO₂ removal. Lagged warming of 4.0 degrees Celsius (or 7.4 when CH₄ is included), starting from today's 1.1 degree Celsius change in GST, comes mostly from albedo changes. Their effects are estimated here for ice, snow, sulfates, and cloud cover. This study estimates magnitudes for sulfates and for future snow changes. Magnitudes for ice, cloud cover, and past snow changes are drawn from the literature. Albedo changes, plus their water vapor multiplier, caused an estimated 39 percent of observed GST warming over 1975-2016. Estimated warming effects on GST by water vapor; ocean heat; and net natural carbon emissions (from permafrost, etc.), all drawn from the literature, are included in projections alongside ice, snow, sulfates, and clouds. Six scenarios embody these effects. Projected change in GSTs on land by 2400 range from 2.4 to 9.4 degrees Celsius. Phasing out fossil fuels by 2050 yields 7.1. Ending fossil fuel use immediately yields 4.9, similar to the 5.1 inferred from paleoclimate studies for current CO₂ levels. Fossil fuel phase-out by 2050 coupled with removing 71 percent of CO₂ emitted to date yields 2.4 degrees Celsius. At the other extreme, postponing peak fossil fuel use to 2035 yields 9.4 degrees Celsius GST, with more warming after 2400.