Consequences of Climate Change

short vs long run

My first post here was a sketch of my view of the consequences of climate change, that there are both good and bad effects and we do not know enough to say whether the net effects is positive or negative. It now occurs to me that I can say a little more.

From that post:

There are two approaches to answering that question. The first is to ask whether there are general reasons to expect climate change along the predicted lines, a gradual increase in average temperatures due mainly to increased CO₂ in the atmosphere, to have net negative effects. The second is to look at specific externalities, make some rough estimate of their size, and add them up.

There is one a priori reason to expect net negative effects from change — that current human activity is optimized against current conditions, making change in either direction presumptively bad.

That is a short run effect, relevant over decades but not centuries. Over centuries people can adjust what crops they grow and how they grow them, where they live, how they heat and insulate their houses.

There are three a priori reasons to expect positive effects. One from that post:

More warmth is generally a good thing when you are cold, a bad thing when you are hot. Due to the physics of the greenhouse effect, it warms cold times and places more than hot, raises the temperature of winter more than summer, of the polar regions more than the equator.

Another comes from comparing an average temperature map of Earth to a population density map: Human habitation is limited by cold not by heat. Some of the hottest regions are densely populated, the coldest are empty. The same is true if we replace the average temperature map with a maximum temperature map. It follows that warming, at least moderate warming, increases the amount of habitable land. In a later post I estimated the size of the effect, for global warming of three degrees Celsius, as an increase in land warm enough to live in by about the area of the United States.

Sea level rise reduces the area of habitable land but that effect is trivial in comparison, smaller by two or three orders of magnitude.

The third positive effect is CO₂ fertilization. CO₂ is an input to photosynthesis; increasing its concentration in the atmosphere increases yield for most crops, decreases the need of all crops for water, both unambiguous benefits.

The bad effect exists in the short run, the good effects in both short run and long.

How Much Warming?

Obviously there are amounts of warming that could make Earth uninhabitable. That raises two questions: How much warming could be produced by burning fossil fuel? How much is consistent with human life continuing?

The answer to the first question is provided by an article on the effect of burning all the fossil fuel currently believed to exist. The authors estimate is that global temperature would increase, over a period of about 2000 years, by a little less than 12 degrees C, then gradually decline.

A lower bound on the global temperature at which life on Earth becomes impossible is provided by the highest temperature at which organisms similar to us are known to have survived.

An increase of 12 degrees would get global temperature back to where it was about fifty million years ago, a time at which mammals much like us already existed. It would, however, get substantial parts of the world too hot for humans, which is why my claim about increased habitable area was limited to moderate warming, the next two or three centuries but not the next millennium or two.

Burning all fossil fuel would melt Antarctica and raise sea level by about 50 meters, but it would not cook us, judging at least by the best evidence currently available. Nor, as far as we can tell, would it set off some future change that would produce catastrophic warming; we have been there before and it didn’t. If it happened tomorrow it would be a catastrophe, but not with a thousand years to adjust.

The world would not even look all that different.

The world with 50 m of sea level rise.

Conclusion

In the short run, anthropogenic climate change might make us worse off. In the long run it will make us better off.

Probably.¹

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Past posts, sorted by topic. Some go into consequences of climate change in much greater detail.

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1. Only probably because the world is a very complicated place and I might have missed something; feel free to make suggestions. Also, this post is about the a priori approach; I have discussed many of the detailed arguments in past posts. Some, such as the effect of temperature on mortality, disappear over a time long enough for populations to shift, but not all.