The aerosol factor

In the climate as in so many other parts of our lives, it’s not the things that matter, but the way they all interact. So often, basic characteristics of one part of a system are amplified or partially offset by other parts, leading to a non-obvious emergent properties of a system. This is why economic projections are so insanely hard to get right, and it’s also why I constantly get on my soap box about linear extrapolations–in almost any system that’s important enough to be interesting, as soon as you start changing one part of the system, even “just a little”, you wind up with cascading effects that trigger results “no one could have seen coming”.

One example of this perverse complexity in climate science, and one that I’m surprised hasn’t received a lot more attention, is the issue of aerosols from burning coal. There’s a new study out from the NOAA on aerosols in general (NOAA: Climate Effects of Atmospheric Haze a Little Less… Hazy) (emphasis added):

Scientists have used a new approach to sharpen the understanding of one of the most uncertain of mankind’s influences on climate-the effects of atmospheric “haze,” the tiny airborne particles from pollution, biomass burning, and other sources.

The new observations-based study led by NOAA confirms that the particles (“aerosols”) have the net effect of cooling the planet-in agreement with previous understanding-but arrives at the answer in a completely new way that is more straightforward, and has narrowed the uncertainties of the estimate. The findings appear in this week’s Journal of Geophysical Research – Atmospheres.

…

In balancing the budget for the processes perturbing the heating and cooling of the Earth, Murphy and colleagues found that since 1950, the planet released about 20 percent of the warming influence of heat-trapping greenhouse gases to outer space as infrared energy. Volcanic emissions lingering in the stratosphere offset about 20 percent of the heating by bouncing solar radiation back to space before it reached the surface. Cooling from the lower-atmosphere aerosols produced by humans balanced 50 percent of the heating. Only the remaining 10 percent of greenhouse-gas warming actually went into heating the Earth, and almost all of it went into the ocean.

The new study tackled what the IPCC has identified as one of the most uncertain aspects of the human impacts on climate. Aerosols, which can be either solid or liquid, have complex effects on climate. Sulfate particles formed from pollution can cool the Earth directly by reflecting sunlight. Soot from biomass burning absorbs sunlight and warms the Earth. Aerosols can also affect the formation and properties of clouds, altering their influence on climate. The net effect of all these direct and indirect factors is a cooling by aerosols, which has partially offset the warming by greenhouse gases.

OK, that’s interesting, in a geeky, climate scientist wannabe way, but what does it matter here in the Real World?

Consider this: A lot of the aerosols in our atmosphere come from burning fossil fuels and biomass. (For much more detail on this, see Chapter 2 of the IPCC’s Working Group I report, The Physical Science Basis [large PDF].) While some aerosols have a warming effect and others have a cooling effect, they combine for a net cooling effect of 1.1 or 1.2 Watts per square meter, as mentioned above. (One component, sulfate aerosols, come almost exclusively from coal burning, the occasional volcano aside, and account for 0.4 W/sq. meter of that 1.2 cooling effect.) So, what happens when we do what science tells us we must to serve our own long-term interests, namely burn much less coal?[1] The level of aerosols drops very quickly, in some cases starting in a matter of days. But all that CO2 we’ve been building up the atmosphere for 250 years is still up there and still forcing the planet to retain more of the sun’s heat, and CO2 has such a long lifetime in the atmosphere that reducing fossil fuel burning won’t bring its level down for a very long time. That means the more successful we are in cutting back on coal use worldwide, the more of a short-term surge we’ll see in planetary warming, with its own knock-on effects.

If we reduce our burning of fossil fuels enough to trim even 50% of the effect of their aerosol emissions (and we likely have to cut far more than that), we’ll add 0.6 W/sq. meter to the net effect of human-induced warming. That would push our net “contribution” from all human activities from about 1.6 W/sq. meter to 2.2 W/sq. meter, an increase of 37.5%. (Again, see the IPCC link I gave above, and in particular the bar graph on page 136.)

Look around at the rampant signs of climate change, from defrosting permafrost to changing plant and animal behavior to much greater melting of Arctic ice, etc., and imagine humanity’s climate forcing increased by a third or more.

This is one of the reasons why I think we’ve effectively forced ourselves to rely on geoengineering in the near future. Trying to reduce CO2 emissions “enough” will be incredibly difficult, but when you also take into account this extra kick to planetary warming from a drop in human-emitted aerosols (assuming we’re successful in weaning ourselves from coal), it seems clear that we’re locked into a much worse situation than many people realize.

[1] If you think I just totally ignored any hope of CCS (carbon capture and sequestration) being a major contributor in this issue, you get a gold star.