Two degrees of warming

Now that it looks as if some sort of agreement may come out of Copenhagen, its natural to ask what sort of agreement we need. The current targets being proposed suggest that warming should be limited to 2 degrees over the next century. That implies stabilising atmospheric C2 concentrations at 450 ppm, and an agreement to cut developed country emissions by 20-25 per cent by 2020, with convergence to a level 90 per cent below current developed country levels by 2050 would be adequate (note that this part of the post is based on my reading of Garnaut, Stern & IIPCC, not my own expertise).

At least some discussion in Australia suggests that these targets are hopelessly weak and by implication that it would be better to oppose any action than to lock ourselves into an agreement of this kind. I disagree, and I will try to spell out why.

First up, as in other debates about climate change, it is important to pay attention to the science, rather than to rely on prejudice or on supposed authorities who are either unqualified or whose qualifications aren:t relevant. Thats why I have refused to debate climate science delusionists here, instead pointing them to the results of scientific research, summarised by the IPCC and other bodies.

But the case here is a bit different. The big impacts of climate change will be on agricultural production and natural environments and the relevant experts are ag scientists/economists and ecologists. The views of climate scientists like James Hansen, while very important in projecting the climatic effects of CO2 emissions, have no particular standing when it comes to assessing the damage associated with any particular climatic change.

As regards the ag economics, I am an expert, and am therefore happy to explain my position and discuss it with readers. I will add some links later, but for the moment Ill ask you to take statements of fact on trust that Ive done the work to verify them

What matters for agriculture is not so much the ultimate change in average temperature and rainfall, but the pace of change. Agriculture is undertaken in a wide range of climates, so there are very few places where an extra couple of degrees will make farming impossible. What changes in temperature and even more changes in rainfall will do is change the kind of crops that can be grown in any given location. Some areas will be more productive, and some less so, but, in the long run these effects will mostly cancel out. For a change of more than 2 degrees, the negatives predominate and they become overwhelming after about 4 degrees.

As regards the pace of change, 2 degrees warming over a century implies 0.2 degrees per decade on average which is like shifting the climate about 100 km closer to the equator each decade. That involves some costs, but they are probably manageable.

If agriculture can handle 2 degrees of warming, it seems likely that most human activities will do so. So, as far as human activity is concerned, it makes sense to target 2 degrees of warming as a reasonably conservative choice.

If you want to justify a target lower than 2 degrees, it has to rely on concerns about ecological damage and loss of biodiversity. There is no doubt that 2 degrees of warming will do a lot of damage, for example to coral reefs. But quite a few of the ecologists I talk to are at least as concerned about the immediate threats to biodiversity (in the case of coral reefs, these include overfishing, destructive fishing methods and nutrient runoff) as about the current rate of climate change. As the IPCC shows, business as usual would be disastrous. But the difference between 1 and 2 degrees of warming is probably less than the difference between sustainable and unsustainable choices in industries like fishing and forestry, and we could make a lot of progress on the latter issues at very modest costs.

Obviously, you can always push for a more ambitious target at higher cost. But the costs accelerate pretty quickly once you aim below 450 ppm. So, people advocating more ambitious targets ought to say how much they would be willing to pay and what they would give up.

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