- Hermann Harde, Helmut-Schmidt-University, Holstenhofweg 85, 22043 Hamburg, Germany
The Intergovernmental Panel on Climate Change assigns the human influence on our climate with very high confidence (95%) to be responsible for the actual climate change, and particularly the emission of greenhouse- (GH)-gases are made responsible for the observed global warming over the last 140 years.
However, anthropogenic contributions to this warming are still quite contradictorily discussed, and because of the far reaching consequences for future climate predictions above all it is important to scrutinize, how far such an assertion of a pure anthropogenic climate change can really be confirmed by the increasing GH-gas concentrations, or how far also native effects like variations of the solar activity can explain the observed rising temperature. Also the impact of thermally and solar induced cloud cover changes, which affect our climate, but which are not always well understood, have to be considered with its implications on the observed temperature changes.
In this contribution we compare composed rural land and sea surface temperature measurements of the Northern Hemisphere with simulations performed by an advanced 2-Layer-Climate-Model (2LCM), which allows to calculate the global temperature trend under the simultaneous impact of increasing CO2 concentrations and solar variability. This model with its main features is briefly presented together with the CO2 radiative forcing, the solar radiative forcing and their specific feedbacks.
Our studies cover simulations under quite contrasting conditions, on the one hand based on the model means of the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6), characterized by Equilibrium Climate Sensitivities (temperature increase at doubled CO2 concentration) of ECS = 3.2°C for CMIP5 and ECS = 3.78°C for CMIP6, on the other hand based on our own calculations of CO2 radiative forcing with an ECS = 0.68°C. For the solar radiative forcing we consider six different Total Solar Irradiance (TSI) time series with significantly different trends, which with respect to their impact on global warming are subject of a further amplification by thermally induced feedbacks as well as solar induced cloud feedbacks. Together these amplifications are denoted by the Equilibrium Solar Sensitivity ESS (temperature change at ΔTSI = 0.1%) with values varying between 0.19°C and 0.9°C depending on the prevailing feedbacks. The amplification due to cloud changes was derived from observations within the International Satellite Cloud Climatology Project over the 1980s and 1990s.
From these simulations we see that under CMIP5/6 conditions with large thermal feedbacks but very flat solar variability the calculated temperature increase over the Industrial Era is distinctly larger than found from observations. Even CO2 forcing alone contributes to a too large warming, in particular the observed dominant temperature variations over the last century with a broader dip over the 50s to 80s cannot be traced back only to CO2, which was only monotonically increasing over the considered period and mistakenly is assumed to be only of anthropogenic origin.
On the other hand calculations relying on our own CO2 radiative forcing data with significantly smaller thermal feedback but larger solar variability show excellent agreement with the land-ocean-temperature composite. So a simulation with an ECS = 0.68°C, an ESS = 0.32°C and based on the TSI time series of Hoyt & Schatten reproduces the stronger temperature drop over the 50s till 80s and also the total warming of about 0.9°C over the Industrial Era with a correlation factor of r = 0.95.
Consideration of additional forcings like the Atlantic Multi-Decadal Oscillation or other native oscillations can further improve this agreement, but as long as their size and origin cannot better be identified, is solar radiative forcing and its amplification by induced cloud changes the most plausible explanation for the observed temperature changes, all the more also the other forcings are more or less controlled by the solar wind and superposed planetary gravitational impacts (Mörner et al.).
From our calculations we derive a CO2 affected portion to global warming over the Industrial Era of not more than 0.34°C and over the last century of only 0.24°C, which is 30% of the total warming, while apparently two thirds are caused by the solar impact. As human CO2 emissions should not have contributed more than 15% to the increase over the Industrial Era, the anthropogenic fraction to global warming is expected to be only 0.05°C.