May 18, 2015

What Difference does Mitigation Make?



One of the common talking points of those who oppose U.S. climate measures is that they will have a miniscule impact on global temperatures, even if successful. We have been struck by the fact that those environmentalists, who are usually strident critics of statements from the “unconvinced,” have been almost totally silent on this issue. We thought it was time that we explained why we think skeptics are mostly correct, but draw a misleading conclusion, and also why the greens don’t seem to want to talk about it.

The argument is perhaps best put in a 2013 Cato piece featuring a handy calculator that shows that, even if the U.S. reduced its CO2 emissions to zero, it would have an effect on global temperatures measured in hundredths of a degree in 2050 or 2100. Therefore, they conclude, any efforts to reduce emissions are not worth it. You can also run the calculator for all industrialized countries as a group, but not, tellingly, for action by all major emitters. The calculator is based on an EPA sponsored model and uses the IPCC’s “A1B” scenario. And the results it produces are, in our view, probably correct, for what that is worth.

The A1B scenario was first generated in the late 1990s, and is based on an economic and social “storyline” which leads to an emissions pathway of rapid emissions growth in developing countries in the first half of the 21st century, and a slow global decline thereafter.

IPCC uses its scenarios to generate probabilities of temperature outcome ranges for particular emission stocks in the atmosphere (not annual emissions, which are best thought of as “flows”). No change in annual emissions—especially from one country—can be directly translated into a single temperature increase at a single future year. There are too many intervening uncertainties over how much of the emissions are absorbed in the oceans or forests, how sensitive the temperature is to changes in atmospheric GHG stocks, and how quickly the impact occurs. The A1B scenario produces warming chances of between just under 2 and just over 4 degrees C (roughly 3 to 7 degrees F) compared to the 1980-1999 period, as shown in the chart below from the IPCC’s 5th Assessment Report.

Screen Shot 2015-05-18 at 10.06.22 AM

Now we have to take step back from Cato’s framing of this as an exclusively U.S. issue, and look at what is going on in the developing world. Every credible energy forecast shows that most energy and emissions growth is happening outside the industrial countries. MIT’s Joint Program on Global Change’s Outlook 2014 (close to the IPCC’s A1B in several respects and therefore helpful in breaking the IPCC’s high-level picture into more usable factors) is one of the very few energy forecasts that goes out to 2100. It suggests that by that date, greenhouse gas emissions from today’s developing world will be five times those then being emitted by today’s industrialized countries. The perhaps predictable assumption—that it is all China’s fault—is wrong: In 2100 emissions from countries excluding India, China, and the currently developed countries will be some 45 percent of the total, or almost three times the predicted emissions from today’s industrialized countries.

This demonstrates the first hard truth of the climate problem: It cannot be solved by any one country, and in fact, it cannot even be collectively solved by all the industrialized countries. Then comes the second hard truth: The climate outcome is the same for free riders as it is for those who act, but unless the larger free riders act to reduce their emissions, it is unlikely that anyone will do so.

The vast majority of the greenhouse gases now in the atmosphere come from past emissions by the industrialized countries. If those countries do nothing, why would poorer countries risk derailing their own development by unilaterally curtailing their emissions? Without the industrialized countries acting to—as the developing nations would say—put their own houses in order, it is impossible to believe that developing countries will act on their own. Action by the industrialized nations is thus necessary in order to secure the required collective action, while being insufficient on its own.

It’s uncomfortable for both sides to put this as clearly as we need to: action by the U.S. on its own will not—as Presidential and green rhetoric sometimes suggests—come anywhere near to solving the climate problem, but if a industrialized economy as big as the United States decides to be a free rider, there is no reason why anyone else would take action.