Generally, the biggest (recognized) sources of uncertainty in climate projections involve cloud processes. Compared to climate model resolution, clouds are small on both spatial and temporal scales. Further, there is much we don’t understand about clouds themselves, and especially the human influences on them.
One particularly vexing aspect of cloud processes which is neither well understood nor well modeled involves how anthropogenic aerosol emissions (i.e., particulate matter or its precursors such as sulfate and black carbon) alters them. The ways in which aerosols influence cloud properties are collectively known as the aerosol indirect effects on climate. The direct effect of aerosols, which includes blocking and scattering radiation from the sun (and the earth) is another forest of unknowns.
Consider how the climate impact of aerosols has been handled by the United Nations’ Intergovernmental Panel on Climate Change (IPCC). Up until their most‐recent (2007) “Fourth Assessment Report” (aka, “AR4”), the IPCC offered no “best estimate” for the magnitude of climate change resulting from the direct and indirect aerosol effects. Instead, a large range of possibilities was presented. According to the IPCC’s 2001 “Third Assessment Report” (TAR), when considered together, the net impact from aerosols was causing climate cooling, but the range for the estimated magnitude of the cooling was so large as to include the possibility of entirely offsetting the warming from the total of all anthropogenic greenhouse gas (GHG) emissions. Needless to say the IPCC assessed the “level of scientific understanding” of the climate change from aerosols as “low” to “very low.”
Climate scientists anticipate the climate impacts from aerosols by assessing the pressure that aerosol emissions put on global temperature through a quantity called “radiative forcing.” Radiative forcing is a measure of the change in earth‐directed radiation as a result of the presence of aerosols in the atmosphere. It is measured in Watts/square meter. If the radiative forcing is negative, then, everything else being equal, aerosols will cool lower atmospheric temperatures. Positive forcing changes create warming. It is generally assumed that the surface temperature response is about 0.75 degrees Celsius per additional Watt per square meter of positive radiative forcing. The current “forcing” from all the anthropogenerated greenhouse gases is estimated by the IPCC to be around 3 Watts/square meter, which, at equilibrium, would yield about 2.25°C of warming, something that obviously has not happened. And so, for nearly twenty years, the search has been on to try to determine why warming has been so pokey. The debate usually centers about the role of aerosols and whether or not increasing carbon dioxide will create more or less cloud‐related warming.
For what it is worth (and after you read on, you may conclude, “not much”), the AR4 IPCC report gives a “best estimate” for the aerosol direct effect of -0.50 Watts/square meter, and an aerosol indirect effect of -0.70 W/m2. The actual ranges were -0.1 to -0.9 W/m2, and +0/4 to -1.1 W/m2, respectively. The “best estimate” is that overall cooling from aerosols should be about 0.9°C.
Recent research has only made things murkier. In a recent issue of the Journal of Climate, a team led by Dr. Steven Ghan of the Department of Energy’s Pacific Northwest Laboratory (PNNL}, plugged an aerosol module into the widely used “Community Atmosphere Model” (version 5), from the U.S. National Center for Atmospheric Research (NCAR) in Boulder. They compared model results using a background (“control”) level of aerosols to the current atmospheric aerosol load.
[Note here the peculiar process of “model‐to‐model” comparisons. This is only valid if the models themselves comport properly with reality, which readers of The Current Wisdom know to be highly debatable]
What they found was rather interesting.
As for the direct effect from aerosols (again, that is from the direct reflection or absorption of incoming solar or outgoing terrestrial radiation by aerosols particles), Ghan et al. find that the net global climate forcing is very near zero. That is, they neither warm nor cool the lower atmosphere.
However, while Ghan et al. found that the direct climate forcing from aerosols was more positive (warmer) than the IPCC AR4 “best estimate”, Ghan et al. found that the indirect climate forcing from aerosols was more negative (cooler) than the IPCC AR4 best estimate. It total, in Ghan’s et al.’s model, changes to cloud characteristics induced by aerosol emissions led to a net climate forcing of -1.47 W/m2 +/- 0.11 W/m2—a value which is substantially more negative than the IPCC AR4’s best guess of -0.7 W/m2 with a range of +0.4 to -1.1 W/m2. As was the case for the direct climate forcing, the indirect climate forcing in the newest NCAR model lies outside the 5 to 95% confidence range of the IPCC AR4 estimates.
Think about that the next time you hear about the IPCC and the “scientific consensus”!
Before we get lost in climate models, perhaps a bit of reality should intrude, as can be found in a paper in press at Journal of Geophysical Research, by Minghuai Wang and another team, also from PNNL.
Wang used data collected from the satellites that make up the A‐train satellite constellation (http://en.wikipedia.org/wiki/A-train_(satellite_constellation)) to compare observed cloud and precipitation properties with those from cloud models, including the CAM5 model investigated by Ghan et al.
The A‐train constellation of satellites provides many different types of observations of a location taken from different satellites at virtually the same time. From this array of data, the researchers determined how often it rained from warm marine clouds and what effect the concentration of aerosols had on the chance of rain. They found that the probability of precipitation declined as the aerosol loading in the clouds increased. In other words, high aerosol loadings suppressed precipitation which in turn leads to a longer lived cloud which thus reflects away a greater amount of incoming solar radiation than it would otherwise (producing a negative climate forcing, i.e., cooling). This finding in and of itself is not new, as this “cloud lifetime effect” has been known to for some time.
However, what it new and important, is that the observed degree to which aerosols suppress precipitation was found by Wang et al. to be much less than that which is calculated by the cloud/aerosol schemes in the climate model. This means that the widely‐used Community Atmospheric model is producing a greater indirect aerosol effect (i.e., more cooling) than the observations show.
Wang also looked at two other models and found similar results.
Factoring this lower cooling effect from aerosols into the model results reported by Ghan et al., reduces the total (direct + indirect) climate forcing from aerosols to about -1 W/m2—a value that is about 15% lower in magnitude than the IPCC AR4 best estimate of -1.2W/m2. This would suggest that the net cooling effect of anthropogenic aerosol emissions is less than expected and thus so too must be the net warming effect from anthropogenic GHG emissions (in order for everything to balance out)—in other words, the climate sensitivity is lower than the models portray.
Whew! We finally get to our repeating meme here at The Current Wisdom—that global warming, while real, is being overestimated because of systematic errors that are propagating through climate science.
But, we want to caution about jumping to any firm conclusions here. The biggest thing evident in these new research papers is that the level of scientific understanding of aerosols and clouds is still quite low.
Bear this in mind next time you hear about how anthropogenic GHG emissions are impacting (or expected to impact) some aspect of the weather/climate.
Ghan, S., et al., 2012. Toward a Minimal Representation of Aerosols in Climate Models: Comparative Decomposition of Aerosol Direct, Semi‐Direct and Indirect Radiative Forcing. Journal of Climate, doi:10.1175/JCLI-D-11–00650.1, in press.
Wang, M., et al., 2012. Constraining cloud lifetime effect of aerosols using A‐train satellite observations. Journal of Geophysical Research, in press.