By Sebastian Lüning, Fritz Vahrenholt
[Translated, edited by P Gosselin]
In our book “The Neglected Sun” we wondered a lot about the cooling effect of aerosols that was assigned in the climate models. Aerosols are tiny dust particles and droplets that act to diffuse sunlight and thus as a rule act to cool the earth. But by how much? In Chapter 5 of our book we wrote:
According to the IPCC, the cooling effect of aerosols offsets about two thirds of the power of CO2. In the IPCC’s view, aerosols reduce the warming generated by all greenhouse gases by 45 percent. But the uncertainty is large – it could be 15 percent, or even 85%, because we have only modest to low level of scientific “understanding of the relationships.”
Today very few are aware that the climate models generate far more warming than what we really produced over the last 100 years. The IPCC strategy: All the surplus heat is cancelled by aerosols until the models “fit”. The cooling joker is thus badly needed in order to maintain CO2’s high climate sensitivity.
In March 2015 we saw some progress in the aerosol discussion. One of the authors of the latest IPCC report claimed that the range of uncertainty concerning the effect of aerosols on climate had been greatly reduced thanks to new research findings, and in the meantime there’s been a lot of talk that the cooling potential of aerosols indeed had been significantly exaggerated in the past. The real cooling value is actually at the lower limits of the range assumed up to now by the IPCC.
The most important and boldest claims come from Bjorn Stevens, one of the three directors at the Hamburg-based Max Planck Institute for Meteorology (MPIM). That paper appeared in the Journal of Climate. What follows is the paper’s abstract:
Rethinking the lower bound on aerosol radiative forcing
Based on research showing that in the case of a strong aerosol forcing, this forcing establishes itself early in the historical record, a simple model is constructed to explore the implications of a strongly negative aerosol forcing on the early (pre 1950) part of the instrumental record. This model, which contains terms representing both aerosol-radiation and aerosol-cloud interactions well represents the known time history of aerosol radiative forcing, as well as the effect of the natural state on the strength of aerosol forcing. Model parameters, randomly drawn to represent uncertainty in understanding, demonstrates that a forcing more negative than −1.0 W m−2 is implausible, as it implies that none of the approximately 0.3 K temperature rise between 1850 and 1950 can be attributed to northern-hemispheric forcing. The individual terms of the model are interpreted in light of comprehensive modeling, constraints from observations, and physical understanding, to provide further support for the less negative ( −1.0 W m−2 ) lower bound. These findings suggest that aerosol radiative forcing is less negative and more certain than is commonly believed.
In general one should be careful not to overuse the word “sensational”. But here the word is most suitable. Surprisingly the German media has been deadly quiet on this. A Google news search reveals that there has not been a single article written about the paper. Undesirable news that the media prefer not to make public?
The implications of the paper were immediately recognized within the scientific community. On March 19, 2015, Nic Lewis explained the paper’s far-reaching implications at Steve McIntyre’s Climate Audit and Judith Curry’s Climate Etc.: Also the climate sensitivity gets further limited, and most likely is near the lower limit of the IPCC’s given range. Lewis’s calculations using the new Stevens value yield a most probable mean value for CO2 climate sensitivity (and indeed for the long-term “ECS”) of 1.45°C of warming for each doubling of CO2. The new total range suggested by Lewis ranges from 0.9 to 1.65°C per doubling of CO2. This is far below the IPCC’s latest range of 1.5 to 4.5°C per doubling of CO2.
Figure 1: Range of CO2 climate sensitivity according to calculations by Nic Lewis using the latest Stevens 2015 values. Source.
Bjorn Stevens was fully aware of the avalanche of reactions this would unleash. It is going to take awhile before his IPCC colleagues get over their indigestion and allow the new findings to flow into their modeling work. Until that happens, it is best to avoid any media storm. The MPIM intentionally did not issue a press release to announce the paper. As the English-language media busily discussed the logical consequences of the paper, the MPIM in Hamburg eventually found it necessary to put out a statement. On April 2, 2015, Stevens put out a statement saying that his paper only addressed aerosols and would not be appropriate for speculation on CO2 sensitivity. With it he buys himself a little public peace – for the time being. However the scientific community will not be able dodge the consequences of the paper over the mid to long-term.