August 8, 2007

Clouding Asian Warming

In 1998, Balling et al. published an article in Climate Research dealing with summer and winter warming rates in several widely-used gridded temperature time series. As seen in Figure 1 below, the Balling crew (which includes several World Climate Report team members) found that winters were warming far more than summers, based on near-surface thermometer records, for a large part of northern and central Asia over the period 1946-1995. We repeated the analyses for the satellite-based lower-tropospheric temperature measurements over the period 1979-1995 and found the same red blob (wamer temperatures) over northern and central Asia. We suggested in the article that the build-up of greenhouse gases would most impact the coldest and driest air masses of the world, which just happen to be the air masses that cover northern and central Asia in the winter. Elevated greenhouse gas concentrations in warm and moist air masses would have less of an effect given the overwhelming greenhouse effect of naturally occurring water vapor. We had produced what appeared to be a smoking gun – the greenhouse “fingerprint” looked rather obvious in our analyses. Of course, finding that the coldest and driest air masses of the planet were warming slightly is seen by some as a blessing and not a great cause for concern – are residents of northern Siberia really worried about their winters being a bit warmer?


Figure 1. Seasonal difference (winter minus summer) in temperature trends (°C per decade) for the thermometer-based near-surface data over the period 1946-1995 (from Balling et al., 1998).

Well, the smoking gun has become a bit cloudier given a recent article in Nature entitled “Warming trends in Asia amplified by brown cloud solar absorption.”

Brown clouds come from biomass burning and fossil fuel consumption, and throughout the climate literature, the brown clouds were thought to have a cooling effect at the surface. Brown clouds absorb and scatter incoming solar radiation and were assumed to warm the low atmosphere where they reside but cool the surface. Much of the literature on brown clouds is based on numerical modeling studies and large uncertainties exist in many critical estimates.

A team of scientists from the Center for Clouds at Scripps Institution of Oceanography and the NASA Langley Research Center decided to have some real fun flying unmanned aerial vehicles carrying miniature instruments that measure aerosol concentrations, soot amount, and solar fluxes. The Ramanathan et al. group flew the vehicles around in a polluted area of the Indian Ocean, and over 18 different missions they achieved a vertical resolution of tens of meters and a temporal resolution of less than ten seconds. They basically gathered the best data set ever on brown clouds and their effect on heating rates in the low atmosphere and surface. They discovered that solar absorption, largely by soot, was causing a lot of warming (near 1°C in a day) around 2 km in height, but they found that the brown cloud of the Indian Ocean was causing a warming, not cooling, at the surface of around 0.25°C per day.

A new satellite called CALIPSO actually measures the concentrations and vertical extent of brown clouds, and as presented by Ramanathan et al., central Asia has a significant brown cloud found in all swaths (note below that the data are from November to March). The team next ran a numerical model with greenhouse forcing and the brown cloud concentrations found by CALIPSO, and they concluded “Our general circulation model simulations, which take into account the recently observed widespread occurrence of vertically extended atmospheric brown clouds over the Indian Ocean and Asia, suggest that atmospheric brown clouds contribute as much as the recent increase in anthropogenic greenhouse gases to regional lower atmospheric warming trends. We propose that the combined warming trend of 0.25K per decade may be sufficient to account for the observed retreat of the Himalayan glaciers.”


Figure 2. Color-coded profiles of the backscatter return signal from the CALIPSO lidar showing the vertical distribution of brown clouds (from Ramanathan et al., 2007).

So we discover here that the warming of central Asia, which many thought could be the result of the build-up of greenhouse gases, may now be caused by a brown cloud problem as well. This suggests that the sensitivity of surface temperatures to greenhouse gas concentrations must be substantially less than previously thought, after all, prior to this study, we were willing to ascribe all observed warming in recent decades in Asia to the increase in greenhouse gas concentrations. Now we find out that half of the observed warming is associated with the brown cloud, not greenhouse gases.

If nothing else comes from this recent work, the Ramanathan et al. article reminds us all of the enormous uncertainties associated with our understanding of the climate system.

References:

Balling, R. C., Jr., P. J. Michaels, and P. C. Knappenberger, 1998. Analysis of winter and summer warming rates in gridded temperature time series. Climate Research, 9, 175-181.

Ramanathan, V., M.V. Ramana, G. Roberts, D. Kim, C. Corrigan, C. Chung, and D. Winker, 2007. Warming trends in Asia amplified by brown cloud solar absorption. Nature, 448, 575-578.




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