By PAUL C. “CHIP” KNAPPENBERGER and PATRICK J. MICHAELS
Currently, details are few, but apparently the results of a major scientific study on the effects of anthropogenic aerosols on clouds are going to have large implications for climate change projections—substantially lowering future temperature rise expectations.
In a blog post from the Department of Meteorology of the University of Reading, Dr. Nicolas Bellouin describes some preliminary results from a research study he leads investigating the influence of aerosols on cloud properties. The behavior of clouds, including how they are formed, how long they last, how bright they are, etc., plays a very large role in the earth’s climate system, and is considered the weakest part of global climate models. The climate model cloud deficiency results from a combination of scientific uncertainty about cloud behavior, as well as the modeling challenges that come from simulating the small spatial and temporal scales over which the important processes take place.
When it comes to the influence of human aerosol emissions on cloud properties, the scientific mainstream view is that aerosols modify clouds in such a way as to result in an enhanced cooling of the earth’s surface—a cooling influence which has acted to offset some portion of the warming influence resulting from human emissions of greenhouse gases (primarily from the burning of fossil fuels, like coal, oil, and natural gas to produce energy). In the absence of this presumed aerosol cooling effect, climate models predict that the earth should warm at a much faster rate than has been observed. A large cooling effect from aerosols was thus introduced in the early 1990s as a way to “fix” the climate models and bring them closer in line with the modest pace of observed warming. Despite that “fix,”climate models continue to overpredict the observed warming rate—which is bad enough news for climate models already.
But the new results, reported by Bellouin, make things much worse for them. His team’s investigations show that the anthropogenic cooling impact from clouds is much less than “assessed” by the U.N.’s Intergovernmental Panel on Climate Change (IPCC) and also much less than employed by climate models. Less enhanced cloud cooling means that greenhouse gases have produced less warming than the climate models have determined. Another way to put it is that this new finding implies that the earth’s climate sensitivity—how much the earth’s surface will warm from a doubling of the pre-industrial atmospheric carbon dioxide concentration—is much below that of the average climate model (3.2°C) and near the low end of the IPCC’s 1.5°C to 4.5°C assessed range. This result comports with the concept of “lukewarming” (which you can read more about here).
Bellouin summarizes his findings:
Radiative forcing is a measure of the imbalance in the Earth’s energy budget caused by perturbations external to the natural climate system, such as the emission of aerosols into the atmosphere by human activities. Our preliminary [research] estimate of radiative forcing due to aerosol-cloud interactions, based on satellite observations of aerosol amounts and cloud reflectivity, is –0.6 W m−2. The negative sign indicates a loss of energy for the climate system. The estimate of climate models for the same radiative forcing is stronger, typically larger than –1 W m−2. What causes that discrepancy? Over the past few months, I have discussed with experts in aerosol-cloud interactions, and there are reasons to expect that aerosol-cloud interactions are weaker than simulated by climate models – and perhaps even weaker than the preliminary [research] estimate.
Bellouin promises a more formal and detailed release of his team’s findings in August.
As they stand, the results of this new study seem to confirm the results of an analysis published last year by Bjorn Stevens of the Max-Planck Institute for Meteorology which also showed a much smaller anthropogenic enhancement of the cooling property of clouds.
When the Stevens results were incorporated into a determination of the earth’s climate sensitivity made by Nic Lewis, the result was a best estimate of the earth’s climate sensitivity of 1.5°C with a narrow range of 1.2°C to 1.8°C. This is a significant lowering and narrowing of the IPCC’s assessed range (again, 1.5°C to 4.5°C). The lower the climate sensitivity, the less future warming will result from our greenhouse gas emissions, the smaller any resultant impact, and the less the “need” to “do something” about it. Also, Lewis’ narrow range of uncertainty increases our confidence that climate change will not be catastrophic—that is, will not proceed at a rate that exceeds our ability to keep up.
At the time, we wrote:
If this Stevens/Lewis result holds up, it is the death blow to global warming hysteria.
The findings being reported by Nicolas Bellouin show, in fact, the Stevens/Lewis result to be holding up quite nicely.
Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”