Shindell's comment focuses on the impacts of action to mitigate the effects of black carbon, tropospheric ozone and other non-carbon dioxide human climate forcings, and comes from his perspective as lead author of an excellent UNEP report on the subject that is just out (here in PDF and the Economist has an excellent article here). (Shindell's comment was apparently in response to an earlier Dot Earth comment by Raymond Pierrehumbert.)
In contrast, Caldeira invokes long-term climate change to defend the importance of focusing on carbon dioxide:
If carbon dioxide and other long-lived greenhouse gases were not building up in the atmosphere, we would not be particularly worried about the climate effect from the short-lived gases and aerosols. We are concerned about the effect of methane and black carbon primarily because they are exacerbating the threats posed by carbon dioxide.Presumably by "climate effect" Caldeira means the long-term consequences of human actions on the global climate system -- that is, climate change. Going unmentioned by Caldeira is the fact that there are also short-term climate effects, and among those, the direct health effects of non-carbon dioxide emissions on human health and agriculture. For instance, the UNEP report estimates that:
If we eliminated emissions of methane and black carbon, but did nothing about carbon dioxide we would have delayed but not significantly reduce long-term threats posed by climate change. In contrast, if we eliminated carbon dioxide emissions but did nothing about methane and black carbon emissions, threats posed by long-term climate change would be markedly reduced.
[F]ull implementation of the measures identified in the Assessment would substantially improve air quality and reduce premature deaths globally due to significant reductions in indoor and outdoor air pollution. The reductions in PM2.5 concentrations resulting from the BC measures would, by 2030, avoid an estimated 0.7–4.6 million annual premature deaths due to outdoor air pollution.There are a host of reasons to worry about the climatic effects of non-CO2 forcings beyond long-term climate change. Shindell explains this point:
There is also a value judgement inherent in any suggestion that CO2 is the only real forcer that matters or that steps to reduce soot and ozone are ‘almost meaningless’. Based on CO2’s long residence time in the atmosphere, it dominates long-term committed forcing. However, climate changes are already happening and those alive today are feeling the effects now and will continue to feel them during the next few decades, but they will not be around in the 22nd century. These climate changes have significant impacts. When rainfall patterns shift, livelihoods in developing countries can be especially hard hit. I suspect that virtually all farmers in Africa and Asia are more concerned with climate change over the next 40 years than with those after 2050. Of course they worry about the future of their children and their children’s children, but providing for their families now is a higher priority. . .The UNEP report notes that action on carbon dioxide is not going to have a discernible influence on the climate system until perhaps mid-century (see the figure at the top of this post). Consequently, action on non-carbon dioxide forcings is very much independent of action on carbon dioxide -- they address climatic causes and consequences on very different timescales, and thus probably should not even be conflated to begin with. UNEP writes:
However, saying CO2 is the only thing that matters implies that the near-term climate impacts I’ve just outlined have no value at all, which I don’t agree with. What’s really meant in a comment like “if one’s goal is to limit climate change, one would always be better off spending the money on immediate reduction of CO2 emissions’ is ‘if one’s goal is limiting LONG-TERM climate change”. That’s a worthwhile goal, but not the only goal.
In essence, the near-term CH4 and BC measures examined in this Assessment are effectively decoupled from the CO2 measures both in that they target different source sectors and in that their impacts on climate change take place over different timescales.Advocates for action on carbon dioxide are quick to frame discussions narrowly in terms of long-term climate change and the primary role of carbon dioxide. Indeed, accumulating carbon dioxide is a very important issue (consider that my focus in The Climate Fix is carbon dioxide, but I also emphasize that the carbon dioxide issue is not the same thing as climate change), but it is not the only issue.
In the end, perhaps the difference in opinions on this subject expressed by Shindell and Caldeira is nothing more than an academic turf battle over what it means for policy makers to focus on "climate" -- with one wanting the term (and justifications for action invoking that term) to be reserved for long-term climate issues centered on carbon dioxide and the other focused on a broader definition of climate and its impacts. If so, then it is important to realize that such turf battles have practical consequences.
Shindell's breath of fresh air gets the last word with his explanation why it is that we must consider long- and short- term climate impacts at the same time, and how we balance them will reflect a host of non-scientific considerations:
So rather than set one against the other, I’d view this as analogous to research on childhood leukemia versus Alzheimer’s. If you’re an advocate for child’s health, you may care more about the former, and if you’re a retiree you might care more about the latter. One could argue about which is most worthy based on number of cases, years of life lost, etc., but in the end it’s clear that both diseases are worth combating and any ranking of one over the other is a value judgement. Similarly, there is no scientific basis on which to decide which impacts of climate change are most important, and we can only conclude that both controls are worthwhile. The UNEP/WMO Assessment provides clear information on the benefits of short-lived forcer reductions so that decision-makers, and society at large, can decide how best to use limited resources.