It’s all about me (thane)!

In the standard ‘forcings bar chart’ such as seen in Hansen’s papers, or in TAR, or AR4 (figure 2.20), each change in atmospheric composition is given a separate column. Thus ozone and aerosol effects are denoted separately. Starting off with a paper we wrote in 2005, though, a different approach that is perhaps more useful to policy makers has also been adopted. This ‘emissions-based’ viewpoint attributes the forcings to the actual emissions, rather than to the eventual concentration. Thus since some of the ozone increase is related to CH4 emissions, you get to include that under CH4. The other ozone precursors (carbon monoxide and volatile organic compounds) can also now be blamed for a portion of the ozone impact.

This was incorporated into figure 2.21 in AR4, where it is clear that the impact of methane (once some indirect effects are included) is greater than you would have thought based on the ‘abundance’ viewpoint. Note the changes basically only affect the reactive species. When thinking about the various metrics, the emissions-based view is more closely tied to GWP than the traditional abundance-based approach. A big difference is that GWP is looking forward in time, while emission-based forcings are looking back at historical events.

The increasing sophistication when it comes to attribution and GWP is strongly connected to the development of more comprehensive Earth System Models (ESM) in recent years. These are the descendants of the General Circulation Models of the climate that have been developed over the last 30 years, but that now include interactive atmospheric chemistry, aerosols (natural and anthropogenic) and sometimes full carbon cycles in the ocean and land surface. This extra machinery allows for new kinds of experiments to be done. Traditionally, in a GCM, one would impose atmospheric composition forcings by changing the concentrations of the species in the atmosphere e.g. the CO2 level could be increased, you could add more sulphate, or adjust the ozone in the stratosphere etc. However, with an ESM you can directly input the emissions (of all of the relevant precursors) and then see what ozone levels or aerosol concentrations you end up with. This allows you to ask more policy-relevant questions regarding the net effects of a particular sector’s emissions or the impact of a specific policy on climate forcing and air pollution (see here for a discussion).

Our new Science paper (Shindell et al, 2009) expands on some of the earlier work (as was discussed here) and extended consideration of the indirect effects of CH4 and CO (carbon monoxide) to aerosols as well. This is necessary since SO2 requires oxidants to transform to sulphates (and so is affected by the perturbation of the chemistry by other emissions), and it takes into account the competition between nitrates and sulphates for ammonia (which means that there is a small anti-phasing effect – increasing sulphates tends to decrease nitrates and vice versa). When we did this, we found that methane’s impacts increased even further since increasing methane lowers OH and so slows the formation of sulphate aerosol and, since sulphates are cooling, having less of them is an additional warming effect. This leads to an increase in the historical attribution to methane (by a small amount), but actually makes a much bigger difference to the GWP of methane (which increases to about 33 – though with large error bars).

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