Some Basic Calculations, Data and Questions

[WeatherRusty]

China started installing SO2 scrubbers in 2006 and 2007, which seems to have had the effect of reversing the trend of SO2 emissions from China lately. It's unknown when full compliance will take hold. India on the other hand, has not implemented this kind of program (as far as I can tell). Either way, the level of SO2 is still definitely higher than it was in the late 90s and early 2000s, but it does not seem to be increasing like it was from 1998-2006. This makes sense, since Chinese coal consumption tripled since 1998 (and the world about double).

As a personal anecdote, the "Asian Brown Cloud" was something I had to deal with frequently with forecasting flights around Asia. It's thick, nasty and reduces sunlight substantially, especially through cloud and widespread smog formation. The effects are mostly regional, but regional forcing can easily reach -20 to -30 w/m2 (not a typo). It's understandably more diffuse as it spreads out and is affected by natural precipitation scrubbing, but some of that pollution spreads out over a much larger area and the indirect effect on clouds seems substantial. We use global averages for this forcing, but it is typically very regionally concentrated and irregular.

In the end, SO2 vs CO2 is a Faustian bargain, because SO2 aerosols have a lifetime of days to weeks (on average), whilst CO2 emissions last decades to millennia. The irony is that while we try to clean up SO2 (for good reasons), it "unmasks" CO2's forcing. That's the big worry in climate sensitivity research. If that negative forcing number from aerosols happens to be fairly high (say -1.6 or more), then this implies a very large climate sensitivity (>4C) almost no matter which way you dice it. The lower sensitivities all require values on the very optimistic side of the forcing uncertainty ranges for aerosols. At the moment, it appears possible, but not likely. You'll notice the long, fat tail towards the high range of estimates. The central 3C estimate is bad enough, anything past 4C (a "tail event") is disastrous. Much higher values (>6C) can't be ruled out, but I think Hansen's argument from paleoclimate shunts those extreme values off to "unlikely".

When speaking of climate sensitivity it is important to note that what is being referred to specifically relates to a single doubling of CO2 alone. In other words, it estimates the total warming to be expected from a natural CO2 concentration of 280ppmv increased to 560ppmv once thermal equilibrium is reached with the forcing and all short term feedbacks have played out.

However, this is not meant to say CO2 concentration will stop rising at 560ppm, our mitigation response if any will impact greatly on that. This also does NOT include carbon feedback from the melting of the northern permafrost, the melting of arctic methane clathrates (Vergent's area of concern), tropical deforestation or any of the other anthropogenic related greenhouse gases or black carbon etc. All these additional contributions are on top of a single doubling of CO2 to produce a total anthropogenic forcing greater than 3.7Wm^2 to which global temperature should respond proportionately. This additional forcing is what drives the potential for temps at 4C and above.

With an increase of 2-3C the Earth would be in a temperature regime not experience for at least 15 million years. We can expect that much warming due to the single doubling of CO2 given the published estimates of climate sensitivity. How this triggers the aforementioned feedback issues potentially places the world in uncharted water, but we can be quite sure of one thing....the Earth will no longer possess a permanent northern ice cap at those sustained temperatures.