The roles of aerosols and clouds as radiative forcing is the least well known aspect of the climate change problem. According the the IPCC 2007 Summary for Policymakers, aerosols represent a radiative forcing of approximately -1.2 W/m2, combining the direct effect of aerosols and the cloud albedo effect (also known as the first indirect effect or the Twomey effect). The estimate error in this value is +/- 1.2 W/m2. Other radiative forcings, such as CO2, have a much higher level of scientific understanding. The result is that the total net anthropogenic forcing is estimated at 1.6 W/m2 +/- 0.9 W/m2. The large uncertainty in the net forcing is almost all the result of unknown, or not-well understood, effects from clouds and aerosols.

Kim and Ramanathan (2008) use multiple satellite observing systems, along with ground-based measurements to compare the radiative forcing from aerosols and clouds with model results.

Partial Abstract:

This study integrates global data sets for aerosols, cloud physical properties, and shortwave radiation fluxes with a Monte Carlo Aerosol-Cloud-Radiation (MACR) model to estimate both the surface and the top-of-atmosphere (TOA) solar radiation budget as well as atmospheric column solar absorption. The study also quantifies the radiative forcing of aerosols and that of clouds… The model was validated against instantaneous, daily and monthly solar fluxes from the ground-based Baseline Surface Radiation Network (BSRN) network, the Global Energy Balance Archive (GEBA) surface solar flux data, and CERES TOA measurements. The agreement between simulated and observed values are within experimental errors, for all of the cases considered here: instantaneous fluxes and monthly mean fluxes at stations around the world and TOA fluxes and cloud forcing for global annual mean and zonal mean fluxes; in addition the estimated aerosol forcing at TOA also agrees with other observationally derived estimates. Overall, such agreements suggest that global data sets of aerosols and cloud parameters released by recent satellite experiments (MISR, MODIS and CERES) meet the required accuracy to use them as input to simulate the radiative fluxes within instrumental errors… More

This paper attempts to validate a Monte Carlo radiative transfer code with observations from both space- and surface-based observing systems. In that regard, I agree with the authors: this paper is an improvement over previous papers. That is, the paper simply takes better observations and a better understanding of how radiation interacts with aerosols/clouds to give a more accurate and precise values of their radiative forcing. It is primarily a evolution of our understanding of how aerosols interact with radiation, not a revolution. And in my view, it’s a fairly small step. There are some interesting things in this paper for those interested in aerosols, but for most people, I doubt this will gather much interest.

There is one aspect of this paper that I find highly intriguing. Near the very end of their paper, they talk about the regional importance of aerosol as a radiative forcing.

This figure shows the total aerosol radiative forcing including clouds. The top panel shows the outgoing radiative forcing at the top of atmosphere (TOA). There are several important things to note about this figure. Aerosol forcing is not always negative. For instance, over areas of high albedo, such as Greenland, Antarctica, and highly-reflective deserts, the TOA forcing is positive. The TOA forcing over deserts in this image is much closer to zero than over the poles due to the fact that ice has a much higher albedo than desert sand. This is a well-understood property of aerosols, but does not usually make it into the debate in the public sphere. Usually aerosols are either cooling (”white” sulphates) or warming (”black” carbon). The reality is that sulphates can equally provide a positive forcing and that black carbon can provide a negative forcing (although extrememly unlikely) given the right conditions based upon the combination of single scattering albedo of the aerosol and the underlying surface albedo.

Another important aspect of this figure is that the forcings due to aerosols are extremely regionalized. There is little aerosol forcing over the Pacific Ocean, while the radiative forcing over Western India and the Central Atlantic are high. This is because aerosols in the troposphere have a residence time of approximately 7-10 days. Larger aersols, such as sea salts, are large enough that the effects of Brownian motion are small compared to gravitational settling and thus they naturally fall out of the atmosphere fairly rapidly. For smaller particles, such as sulphates which undergo gas to particle conversion, the effects of Brownian motion are of the same magnitude as gravitational settling, and thus these particles do not fall from the sky as a basketball would. Their removal mechanism is called wet deposition, and they basically get rained out of the atmosphere. Since water has a residence time of 7-10 days, so do these small aerosols.

This is in contrast to other radiative forcing elements identified by the IPCC. Carbon dioxide has an extremely long residence time. In fact, carbon dioxide does not have just one residence time, it has several depending upon the removal mechanism. But for the most part, once carbon dioxide is inserted into the atmosphere, it stays there for a long time. Therefore, it is said to be well mixed - in the troposphere at least. As can be seen in the figure above, aerosols are not globally well mixed.

This figure shows the annual cloud mean radiative forcing from the Monte Carlo radiative transfer model (MACR, panel A), CERES derived observations (panel B), and the difference between the model and observations (panel C). Qualitiatively, the top two panels look very similar; the spatial pattern are the same. We can see increased clouds in the Intertropical Convergence Zone (ITCZ), and decreased cloudiness in the Horse Latitudes (30-35 deg N and S).

When the two panels are compared quantitatively, interesting results begin to show. For instance, the region over the Amazon has a higher negative radiative forcing in the model than in the observations. Similarly, it over-estimates the negative radiative forcing over Southeast Asia. In both of these regions there is extensive biomass burning and deforestation, perhaps suggesting that the model does not do as good of job with carbon aerosols.

The other area where the model diverges from the observations is in the mid-latitudes over the Pacific, but especially in the Northern Hemisphere. I have no hypothesis why this would be, but it certainly needs to be investigated further.

All in all, this is a good, but long paper. I with the aspects dealing with the regional forcing of climate were more fully discussed. As is, this is an important paper which shows that aerosols are an important aspect of the climate system, and that they are regional in nature. Therefore, in order to make skillful regional climate predictions, aerosols and clouds must be included in model parameterizations, and the parameterizations must reflect the physical processes involved. Also, if the areas of wide disagreement between the models and observations over the Amazon and Southeast Asia are because of biomass burning, this could represent an important radiative forcing due to land cover and land use change.

References:
Kim, D., Ramanathan, V. (2008). Solar radiation budget and radiative forcing due to aerosols and clouds. Journal of Geophysical Research, 113(D2) DOI: 10.1029/2007JD008434

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