Global climate change: Analyzing anthropogenic warming and causality

Here, we analyze recent measured data on global mean surface air temperature anomalies (GMTA) and various external forcing's covering the last 160 years using newly developed techniques that allow discrimination between correlation and causality. This evaluation is based on a new concept for calculating the information flow between time series. Our result demonstrates one-way causality in the sense that the recent CO
2
increase is causing the temperature increase and not the other way around. The positive value of the information flow indicates further that CO
2
has a positive feedback and therefore a destabilizing effect on GMTA; more CO
2
would lead to a stronger increase in GMTA. The results of investigating the information flow between the major radiative forcing's and the GMTA time series clearly show that total Green House Gases (GHG), dominated in particular by CO
2
forcing, is the main driver of changing global surface air temperature. Radiative forcing caused by aerosols and clouds is still important, but significantly smaller. Neither forcing by solar irradiance nor volcanic forcing contributes in a significant manner to the GMTA development. Finally, we applied the same causality analysis to the globally-gridded GMTA product in order to assess regional “sensitivity” to anthropogenic forcing's versus natural modes of variability. This analysis reveals a surprising causal pattern: the increasing anthropogenic forcing mainly originated in the northern hemisphere has especially strong warming effects in the southern hemisphere.