- Joint Program Report
To gauge the overall effect of complex interactions among emissions, atmospheric chemistry, and climate, we have conducted a series of simulations on 120-year time scales, using a global coupled chemistry-climate model along with various predictions of anthropogenic emissions and different assumptions for chemistry and climate model parameters. To specifically identify the impacts of chemical species on climate change, we have also carried out sensitivity runs which include or exclude radiative effects due to increasing concentrations of greenhouse gases, and sulfate aerosols produced from anthropogenic sulfur emissions.
Interactions among emissions, atmospheric chemistry, and climate are known to be complex. Based on current predictions of increasing future anthropogenic emissions, the climate will likely warm by some uncertain amount in the next century. The cooling effect caused by sulfate aerosols can offset a certain amount of this warming (about 20%, in our tests), but not the whole amount. It has been found that emissions of CH4, CO, NOx, and SO2 significantly influence the tropospheric concentrations of several important chemical species. Specifically, our simulations indicate that the average tropospheric OH concentration in the year 2100 will be 16-31% lower than its current level. As a result of this reduction, the predicted lifetimes of certain chemical species increase as we move into the next century. For example, in the year 2100 the lifetimes of CO and CH4 increase by 2 months and 2.5 years respectively from their current estimates. Also, we find that the overall influence of climatic variations on chemistry is less than that due to the increasing trends of emissions, especially for CH4.
Climatic variations can still subtly impact many chemical patterns in the troposphere through changes in water vapor, temperature, rainfall, and cloud-cover patterns. However, predicted changes in climate do not reverse the overall trends of changing chemistry represented by the reduction in tropospheric OH concentration and related increases in the lifetimes of chemical species, which are driven primarily by the increasing emissions.