In a stunning article entitled “Improved Attribution of Climate Forcing to Emissions,” a group of researchers from NASA's Goddard Institute for Space Studies and Columbia University in New York, led by Drew T. Shindell, have called into question the values used to calculate the “forcing” due to various greenhouse gases. “We calculated atmospheric composition changes, historical radiative forcing, and forcing per unit of emission due to aerosol and tropospheric ozone precursor emissions in a coupled composition-climate model,” states the paper's abstract. “We found that gas-aerosol interactions substantially alter the relative importance of the various emissions. In particular, methane emissions have a larger impact than that used in current carbon-trading schemes or in the Kyoto Protocol.”
According to the study, emissions of NOx, CO, and methane have substantial impacts on aerosols by altering the abundance of oxidants, especially hydroxyl, which convert SO2 into sulfate. Global abundance of hydroxyl and sulfate changes by 18% and 13% for increased NOx by significant amounts. By –13% and –9% for CO, and by –26% and –11% for methane. Coupling in the other direction is very weak because reactions of gases with aerosols only have a small effect on the amounts of radiatively active ozone and methane. For example, SO2 emissions enhance the removal of NOx through reactions on particulate surfaces, causing ozone to decrease, but the radiant forcing is only –0.004 W/m2. Increased SO2 leads to substantially reduced nitrate aerosol, however, owing to greater ammonium sulfate formation at the expense of ammonium nitrate.
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