Global Temperature Trends

THIS POST FIRST APPEARED 9 DEC 2009. SEE THIS FOR MORE.

In an earlier post I made the case that one needs to know only two things about the science of climate change to begin asking whether accelerating decarbonization of the economy might be worth doing:

• Carbon dioxide has an influence on the climate system.
• This influence might well be negative for things many people care about.

That is it. An actual decision to accelerate decarbonization and at what rate will depend on many other things, like costs and benefits of particular actions unrelated to climate and technological alternatives. In this post I am going to further explain my views, based on an interesting question posed in that earlier thread. What would my position be if it were to be shown, hypothetically, that the global average surface temperature was not warming at all, or in fact even cooling (over any relevant time period)? Would I then change my views on the importance of decarbonizing the global energy system?

And the answer is ... no!

My concern about the potential effects of human influences on the climate system are not a function of global average warming over a long-period of time or of predictions of continued warming into the future. A point that my father often makes, and I think that he is absolutely right, is that what maters are the effects of human influences on the climate system on human and ecological scales, not at the global scale. No one experiences global average temperature and it is very poorly correlated with things that we do care about in specific places at specific times.

Consider the following thought experiment. Divide the world up into 1,000 grid boxes of equal area. Now imagine that the temperature in each of 500 of those boxes goes up by 20 degrees while the temperature in the other 500 goes down by 20 degrees. The net global change is exactly zero (because I made it so). However, the impacts would be enormous. Let's further say that the changes prescribed in my thought experiment are the direct consequence of human activity. Would we want to address those changes? Or would we say, ho hum, it all averages out globally, so no problem? The answer is obvious and is not a function of what happens at some global average scale, but what happens at human and ecological scales.

In the real world, the effects of increasing carbon dioxide on human and ecological scales are well established, and they include a biogechemical effect on land ecosystems with subsequent effects on water and climate, as well as changes to the chemistry of the oceans. Is it possible that these effects are benign? Sure. Is it also possible that these effects have some negatives? Sure. These two factors alone would be sufficient for one to begin to ask questions about the worth of decarbonizing the global energy system. But greenhouse gas emissions also have a radiative effect that, in the real world, is thought to be a net warming, all else equal and over a global scale. However, if this effect were to be a net cooling, or even, no net effect at the global scale, it would not change my views about a need to consider decarbonizing the energy system one bit. There is an effect -- or effects to be more accurate -- and these effects could be negative.

Of course, not mentioned yet is that action to improve adaptation to climate doesn't depend at all on a human influence on the climate system, warming or cooling or whatever. Adaptation makes good sense regardless. So clearly my policy views on adaptation are largely insensitive to any issues related to global average temperature change.

The debate over climate change has many people on both sides of the issue wrapped up in discussing global average temperature trends. I understand this as it is an icon with great political symbolism. It has proved a convenient political battleground, but the reality is that it should matter little to the policy case for decarbonization. What matters is that there is a human effect on the climate system and it could be negative with respect to things people care about. That is enough to begin asking whether we want to think about accelerating decarbonization of the global economy.

To fully assess whether accelerated decarbonization makes sense would require us to ask, are there any other good reasons why accelerated decarbonization might make sense? And it turns out, there are many.

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