Perhaps the biggest question about global warming is what environmental economists refer to as ‘the shape of the damage curve.’ I would say that the scientific evidence that global warming is taking place is essentially ironclad (though the relationships of some events, such as more severe hurricanes, to it are rather more tenuous). Equally true is the fact that human beings are contributing to the warming of the planet. What the damage curve represents is the amount of harm caused by each additional unit of global warming, expressed in terms of the cost that would be required to mitigate it. Mitigation costs include everything from relocating people to dealing with larger malarial areas, making agricultural changes, and increased building heating and cooling costs in different areas. The costs are net of any benefits that global warming provides: such as being able to grow certain crops farther north, or having a longer growing season overall.
The most benign possibility would look something like this:
The damage increases steadily with the amount of mean global temperature change. This is helpful because it allows us to predict the degree of future damage quite effectively and make reasonably good choices with regard to how much reduction in greenhouse gas (GHG) emissions we should undertake.
A worse option looks like this:
The damage increases at an increasing rate, as temperature does. This seems intuitively more likely than the first option, since bigger increases are likely to unbalance more and more complex biological and climatological systems.
An even worse option looks like this:
It is possible that climate change would involve a big jump that we wouldn’t see coming until it was too late. An example would be the much talked about possibility that the Gulf Stream, which warms Western Europe, could be disrupted. The biggest reason this is problematic is because we might believe we were in a scenario like the one in chart one, only to be proved spectacularly wrong.
The trillion dollar question, of course, is which of these approximations we should adopt as the basis for policymaking, until such a time as compelling evidence for one of the possibilities or another emerges (hopefully not by means of humanity actually following one of those curves too far). The most cautious option is to assume that the progression would be like chart 3, with the break at an unknown location. The prudent policy, then, would be to try and stabilize GHG levels at their present positions. Of course, that could involve massive reductions in possibilities for economic growth in the rich world and poverty reduction in the poor world. A tricky decision to make, in the face of such important considerations on both sides.
Personally, I don’t think any serious action will be taken until some very real evidence of the harm that can be caused by global warming has manifest itself. As for the question of what should be done, in the ideal circumstance, I am profoundly uncertain. What do other people think?