Whenever the subject turns to climate-change policy, one question constantly pops up: Wouldn't it be easier to adapt to a warmer world than to spend money shrinking greenhouse-gas emissions now? David Orr has an essay tackling this query over at Environment360, giving four big reasons why mitigation is "easier, cheaper, and more ethical" than depending solely on adaptation:

1. First, the record shows that climate change is occurring much faster than previously thought, will affect virtually every aspect of life in every corner of Earth, and will last far longer than we’d once believed. ...

2. Second, the implications of the choice between adaptation and mitigation do not fall just on those able, perhaps, to temporarily adapt to climatic destabilization, but rather on those who lack the resources to adapt, and to future generations who will have to live with the effects of whatever atmospheric chemistry we leave behind. ...

3. Stabilizing climate now will be expensive and fraught with difficulties, but it will be much cheaper and easier to do it sooner rather than later under much more economically difficult and ecologically harrowing conditions....

4. Fourth, efforts to adapt to climate change will run into institutional barriers, established regulations, building codes, and a human tendency to react to--rather than anticipate--events. There are, in economist Robert Repetto’s words, "many reasons to doubt whether adaptive measures will be timely and efficient, even in the U.S. where the capabilities exist." ...

Read Orr's piece for the longer version. Point number one deserves some elaboration. Some economists, such as William Nordhaus, have tried to tally up the costs and benefits of cutting emissions to avoid drastic climate change, like so: Estimate how much it would cost to reduce greenhouse-gas emissions now. Figure out how much damage climate change will do in the future. Select an appropriate discount rate to reflect the fact that humans will presumably be richer in 2050 or 2100. Then, rack 'em up! (It's also worth tossing in the secondary benefits of cutting emissions—cleaner air, better health, less dependence on Middle Eastern oil regimes...)

Now, this approach leaves ample room for disagreement. For one, the estimated costs of shrinking emissions depend on all sorts of unknown variables (the pace of technological innovation, say). Second, there's contention over what discount rate is appropriate to use (see John Quiggin for a good discussion—part of the choice comes down to ethics, specifically how much you value the welfare of future generations). Then there's Marty Weitzman's point—there's some uncertainty as to how bad, exactly, climate change could get. But the "long-tail" scenarios (in which we've completely destabilized the climate) are incredibly bad, and need to be factored in.

Here's an example: The MIT Joint Program on the Science and Policy of Climate Change recently released the results of its climate modeling, and found that the median projections for average global surface warming by 2100 are 5.2C (9.4F). That's quite shocking, and a rise that high would produce plenty of devastating, unmanageable effects. (And warming won't be uniform—an average rise of 9.4F means a roughly 20F rise in the Arctic. Guess how Greenland's ice sheets fare?) But note this is only the median projection. On our current course, MIT calculates there's still a 9 percent chance that average global warming could be greater than 7C (12.6F) by 2100, as feedback cycles kick in. You can see all the probabilities outlined in this graph:

           

It's difficult to conceive of how humanity would "adapt" to a 12.6F average rise in temperature. One way to think of climate mitigation is to think of it like insurance. The McKinsey Global Institute did a study on the costs of de-carbonizing the economy to keep us below 450 ppm (that is, to glide toward the right-hand chart in that MIT graphic). They write: "The macroeconomic costs of this carbon revolution are likely to be manageable, being in the order of 0.6–1.4 percent of global GDP by 2030. To put this figure in perspective, if one were to view this spending as a form of insurance against potential damage due to climate change, it might be relevant to compare it to global spending on insurance, which was 3.3 percent of GDP in 2005." That seems a useful way to frame this question.

--Bradford Plumer