Cutting through the haze: Nailing down the role of aerosols in climate change

ResearchBlogging.orgClimatologists have long known that aerosol "haze" plays an important role in determining just hot things get on Earth. But figuring out just what the role is has proven frustrating. The tiny, airborne particles that fly out of smokestacks, tailpipes and ordinary biomass fires can either help cool the earth by reflecting sunlight, for example. But they can also settle on snow in the form of soot and absorb heat. Now comes a paper that claims to have calculated the net effect. If the researchers are right, they've identified an important piece of the climate change puzzle.

Just as important, however, is their use of observations, rather than computer models, to produce an accurate "heat budget" of the planet. To anyone who hasn't already made up their mind that climatologists are universally misguided, this should help boost confidence in the science behind anthropogenic global warming.

In "An observationally based energy balance for the Earth since 1950" (Journal of Geophysical Research, doi:10.1029/2009JD012105, subs req'd) Daniel Murphy and his colleagues at the Earth System Research Laboratory in Colorado, the University of Leeds and NASA's Langley Research Center in Virginia, narrow down the uncertainty factors associated with aerosols. The most recent IPCC estimates had total aerosol forcing somewhere between -0.2 to -2.3 watts/sq. meter (In other words, the net effect is a cooling). But Murphy's team has done better, squeezing that rangedown to between -0.3 and -1.9, with -1.1 as the best guess, which is pretty much in line with the IPCC midpoint.

That may not sounds like much, but it's a significant improvement, and allows climatologists to derive all sorts of other numbers, including estimates of just how much of the warming effects of carbon dioxide and other greenhouse gases are being offset by aerosols.

A striking result of the Earth energy budget analysis presented here is the small fraction of greenhouse gas forcing that has gone into heating the Earth. Since 1950, only about 10 ± 7% of the forcing by greenhouse gases and solar radiation has gone into heating the Earth, primarily the oceans. About 20 ± 9% has been balanced by increased outgoing radiation. About 20% of the forcing by greenhouse gases and solar radiation has been offset by volcanic aerosols. The remainder, about 50%, has been balanced by the direct and indirect effects of anthropogenic aerosols.

In other words, if we were to cut sharply back on aerosol emissions -- which most everyone concerned about pollution would like to continue doing -- the result would be a lot more warming -- as much as double. I asked Murphy to comment on this possibility. He replied "All else being equal, if you remove some cooling, the Earth would get warmer."

It also means that all those geoengineering proposals for injecting aerosols in the atmosphere to counter global warming might actually be effective, even cost-effective. (Although that would do nothing about ocean acidification and other non-warming consequences of fossil-fuel combustion. It would also almost certainly produce myriad unintended consequences, but that's another post.)

This kind of information will likely prove critical in the years ahead, and it's hard to exaggerate the value of such research. Murphy made a point of making sure I understood the other major use of his work, though::

One of the bottom lines of the study is that it is possible to construct a reasonable budget of how the Earth has responded to the warming induced by greenhouse gases using observations rather than global climate models. That adds confidence to statements such as "anthropogenic aerosols have a net cooling effect."

How true. Computer simulations are wonderful things, but like Monty Python's Camelot, they're just models, and require reliance on a lot of assumptions. Observational data are always going to be preferred, if you have a choice. And given how well the observations from Murphy et al match some the models' predictions, the climate change pseudoskeptics are going to find they have even fewer legs on which to perch.

Murphy et al even take a little stab at the denial contingent toward the end of their paper:

Questions have been raised in the popular media about the reduced rate of warming since 1998 (e.g., http://transcripts.cnn.com/TRANSCRIPTS/0901/13/ldt.01.html; http://www.realclimate.org/index.php/archives/2009/01/cnn-is-spun-right-round-baby-right-round/langswitch_lang/in#more-640). Figure 5 shows that the recent change in the atmospheric warming rate is not unusual compared to the decadal variability in the rate of warming over the past 50 years [Easterling and Wehner, 2009]. Further, unless there have been surprisingly large contributions other than thermal expansion to sea level since 1998 the continued rise (Figure 5c) suggests that a significant amount of energy has gone into the oceans rather than warming the surface.

That's yet another reminder (following last month's report from the NCDC that the oceans are warmer than ever) that just because you had a lousy summer, that's no reason to conclude the entire planet has stopped warming.
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Murphy, D., Solomon, S., Portmann, R., Rosenlof, K., Forster, P., & Wong, T. (2009). An observationally based energy balance for the Earth since 1950 Journal of Geophysical Research, 114 (D17) DOI: 10.1029/2009JD012105

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Goodness gracious, no wonder there are "skeptics." I am an educated, literate person and have to read this stuff several times to get to the root. If the lynchpin is communicating this to the general population...we're doomed.

Amazing how the alarmists still leap onto any article that seems to prove their point, and trumpet it as compelling "evidence". If they behaved like proper scientists and watched both sides of the "debate", they would realize that their science is cold, and their theory is dead in the water.
They need an overall and complete statement of how the theory of AGW is purported to work. IPCC has avoided such a document, and nobody else has even tried.
The shame is that the alarmist camp does not even seem to think such a statement is worthwhile.

By Jock Shockley (not verified) on 10 Sep 2009 #permalink

I have become intrigued by the debate of which to emphasize, environment vs. climate change, in order to move this process of fixing this planet along.

In some ways it seems it is all the same thing, humans piling up pollutants and befouling the earth, whether it's CO2 warming the earth, or particulate matter causing cancer. What is the most compelling argument to people who prefer to remain in denial?

On another matter, empirical evidence, I have written before that the trees on the Eastern Seaboard are showing symptoms of severe distress. At this point, every single vegetative form of life is exhibiting signs of toxicity, and since I'm not a scientist, I can only speculate that they are being poisoned by atmospheric gasses. Ozone produced from gasoline and coal emissions is known to kill plants but since that has been around for decades and the decline is relatively dramatic and sudden, I suspect that the more recently government-mandated addition of ethanol to gasoline is the primary causative agent.

Ethanol emits acetaldehyde, which produces peroxyacetyl nitrates when mixed with UV radiation, and PANs are extremely deadly to plant life (and animal).

I've tried in vain to get information from the EPA, NOAA, and other government-funded agencies about just what the levels of PANs are. Anybody who wants to weigh in, feel free. Trees are the foundation of our ecosystem on land, as coral reefs are in the sea. Without them every other species faces mass extinction.