Aug 01 2007
Warming trends in Asia amplified by brown cloud solar absorption
Ramanathan, V., Ramana, M. V., Roberts, G., Kim, D., Corrigan, C., Chung, C., and Winker, D. Warming trends in Asia amplified by brown cloud solar absorption, Nature, 448, 575-578. DOI: 10.1038/nature06019 [Full text requires subscription.] See also: Brown clouds boost global warming: Aerosols over Asia incriminated in Himalayan glacial melting.
Abstract: Atmospheric brown clouds are mostly the result of biomass burning and fossil fuel consumption. They consist of a mixture of light-absorbing and light-scattering aerosols and therefore contribute to atmospheric solar heating and surface cooling. The sum of the two climate forcing terms—the net aerosol forcing effect—is thought to be negative and may have masked as much as half of the global warming attributed to the recent rapid rise in greenhouse gases. There is, however, at least a fourfold uncertainty in the aerosol forcing effect. Atmospheric solar heating is a significant source of the uncertainty, because current estimates are largely derived from model studies. Here we use three lightweight unmanned aerial vehicles that were vertically stacked between 0.5 and 3 km over the polluted Indian Ocean. These unmanned aerial vehicles deployed miniaturized instruments measuring aerosol concentrations, soot amount and solar fluxes. During 18 flight missions the three unmanned aerial vehicles were flown with a horizontal separation of tens of metres or less and a temporal separation of less than ten seconds, which made it possible to measure the atmospheric solar heating rates directly. We found that atmospheric brown clouds enhanced lower atmospheric solar heating by about 50 per cent. 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 0.25 K per decade may be sufficient to account for the observed retreat of the Himalayan glaciers.
The fact that “brown” aerosols warm the extreme lower atmosphere and surface is not surprising. [Menon et al., 2002] Dark particles, in the visible spectrum, will absorb incoming solar radiation, heat the particle, and heat the surrounding atmosphere. Light colored particles will not absorb the visible radiation; they reflect it. This is called the direct effect of aerosols on climate. [Ramanathan, 2001A] The fact that there are these brown aerosols over the Indian Ocean region is also known. [Ramanathan, 2001B]
What this paper adds is that the proposed warming from the aerosols is enough to melt the Himalayan glaciers. They use extensive aircraft data to probe these brown clouds. They then use a radiative transfer model to calculate the heating rates from both the brown clouds and greenhouse gases.
They conclude that “[t]he ratio of the ABC [atmospheric brown cloud] warming and the 1950–2000 greenhouse gas warming is about 1 over the Himalayan-Hindu-Kush region (Supplementary Fig. 7). The combined simulated warming of greenhouse gases and ABCs at the higher levels (3–5 km a.s.l.) is about 1.2 K, or about 0.25 K per decade during the period from 1950 to present…”
References:
Menon, M., et al., Climate Effects of Black Carbon Aerosols in China and India, Science 297, 2250 (2002). DOI: 10.1126/science.1075159
Ramanathan, V., et al. (A), Aerosols, Climate, and the Hydrological Cycle, Science 294, 2119 (2001). DOI: 10.1126/science.1064034
Ramanathan, V., et al. (B), Indian Ocean experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze. J. Geophys. Res. 106, DOI:10.1029/2001JD900133 (2001).
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I am a regular reader of your article. And I am very impress with your blog upon Global Warming. Now I am also write a blog upon effects and causes of Global Warming. This blog is collection of news & reviews like the study found that global warming since 1985 has been caused neither by an increase in solar radiation nor by a decrease in the flux of galactic cosmic rays. Some researchers had also suggested that the latter might influence global warming because the rays trigger cloud formation.
Let’s not forget about the effect of global warming on Hollywood, as detailed by this article:
http://www.bountyfishing.com/blog/2007/07/04/global-warming-effects-on-hollywood-sea-creatures
Ramanathan’s team modeled the effect of airborne soot across the Pacific and found a 40 percent heating heretofore ascribed to only CO2. That’s 30% of the Earth’s surface. Most of the sootfall in the Western USA, for instance, is from Asia (China…). That may then account for roughly 12% of all global warming.
The mid-tropospheric heating of the brown clouds, coupled with the snow-darkening effect of soot, makes brown clouds a very likely culprit in glacial recessions in the Himalayas. The Himalayan glacier fields lie at exactly the same altitude as the brown cloud layers, so directly vulnerable to both the atmospheric heating & soot deposition. It could well be that many tropical and subtropical glaciers are suffering from manifold assaults from human activity. Coupled with deforestation decimating arboreal microclimates then perhaps these very same high-altitude mountaintop glaciers are also suffering from loss of recharge precipitation.
- Hansen (2003, Hansen & Nazarenko) found that soot deposition is responsible for *most* (90%) of the Arctic melt-off due to the snow-darkening heat-absorbent effect of soot (lowering ice & snow albedo in the Arctic & sub-Arctic tundra, taiga, etc.). Hansen states that although the Arctic melt-off represents 25 percent (yes, 25%) of all observed global warming, he was most concerned about the other 75 percent (attributed to CO2). Most of the sootfall in the Arctic is from Asia & Siberian oil fields, etc.
- Other global sources of airborne soot include diesel emissions elsewhere in the world, slash & burn itinerant agriculture and cook fires. I can only speculate whether these sources might account for yet more global warming, but it’d be consistent with Ramanathan’s findings. Another 5 - 15 percent perhaps?
- Ramanathan has also commented that the heating effect of soot seems to inhibit formation of lower altitude clouds, causing yet additional net heating (loss of high cloud-top albedo & surface shading, both).
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Tallying the Artic soot-driven effect (~ 22%, since 90% of the Arctic melt-off appears to be soot-driven), and the Pacific aerosol soot-driven warming alone & that tallies to ~ 33%.
That tally doesn’t include the net soot-driven warming over Asia & the Indian Ocean, or other global seasonal soot-driven anomalies. AFAIK, the research isn’t there, so any number I ballpark is purely a WAG proffered only for thought & discussion.
I’m additionally ballparking anywhere between 33 percent (Pacific airborne + Arctic ice loss) to 48 percent (WAG another 15% for other global soot sources).
Accounting for only atmospheric effects, could soot account for a third of all observed atmospheric warming (75% atmospheric heating div. by 25% airborne soot)?
Ramanathan himself states that soot abatement could well offer a way out of the current conundrum, so don’t take it just from me, look to a real expert and leader in the field.
see my blog:
http://www.scientificblogging.com/the_soot_files
/lee
Oh, I meant to add… This is not to exculpate CO2, but perhaps abating industrial soot would be a good, relatively inexpensive place to start that’ll produce quick, tangible results (Arctic & tundral reclamation, normalized monsoon patterns in S. Asia…). Soot clears from the air quickly, as opposed to excess CO2 which will take nearly a century to self-remediate. CO2 is far-more insidious b/c of it’s long-term accumulation and effect, but the soot will have to be abated, regardless & as a first serious abatement project would serve well to obtain manifold benefits that’ll help societies commit to global progress on CO2 mitigation.
/lee
http://www.scientificblogging.com/the_soot_files