Saturday, April 13, 2013

The better question is what should the global average temperature revert to after stabilizing

An argument I've seen more than once from climate inactivists sometimes comes in the form of a question, "what is the ideal average global temperature," as if the question has a deep implication. In mid-gallop from "there's no warming; the warming is all natural; humans have little contribution," this is the step, "the warming gets us to a better temperature anyway," before they move on to "the overall negative effect isn't that bad; it's too soon to take action; it's too late to take action."

The first naive thought would be that places like Alaska should welcome some warmth, and a lot of the world's land mass is polar. What they miss is how melting permafrost results in sinking roads and buildings, forests die because insect pests survive mild winters more easily, and coastlines disappear with the loss of sea-ice protection from waves. If you put Hawaii's climate in Alaska, then Alaska would suffer. Both the biological and human environments are adapted for the climates they have.

So here's my hypothetical alternative:  assume, very optimistically, that in the year 2050, gross CO2 and equivalent emissions have been reduced 95% from present through a variety of technological and behavioral changes, and that carbon-negative technologies like biochar and biomass-plus-sequestration balance out the remaining 5%. What do you do next year and the following years?

Simplest answer is do even better, if you can. The rule that when you find yourself in a hole, the first thing to do is stop digging, hasn't yet been satisfied. The oceans will be transferring back latent heat for decades after 2050, so even zeroing out emissions won't be enough to stop further warming. If you can get an increase in carbon-negative activities so that effect, plus annual ocean absorption of CO2, means the reduced atmospheric CO2 warming balances out the latent heat release from oceans, then at that point we'll have stopped digging deeper. And then, what next?

A further increase in carbon-negative actions will mean anthropogenic forcing is slightly net negative compared to the previous year. Continuing that year after year would start to raise the question, when do we stop? What average temperature are we aiming for? I don't think it's the 1850 average - neither we humans nor many ecosystems will function most naturally at that level.

I don't really have the answer; I just think it's an interesting question. Maybe more of a science fiction question, but our children will (hopefully) have to deal with it someday. As a policy question, the most recent, highest temperature will not be the one that people or ecologies are most adapted to, and neither will a temperature from a century or two earlier. People probably adapt faster than ecosystems, so if we choose a human-biased priority then the aimed-for cooling will be less pronounced than one prioritizing ecosystem recovery. Different societies and different ecosystems will have different ideal stabilizing temperatures, but unless they're really good with geo-engineering, then we only get one level of net forcing.

Maybe there won't still be millions of subsistence farmers on the edge of malnutrition 50 years from now, but I wouldn't count on that. Stabilizing their precipitation patterns probably should rank in the highest priority, but we'll have to see how much political pull they'll have to make that happen.

UPDATE:  Good comments, esp from Tom Curtis who says it's not correct to call the heat transfer back from oceans "latent heat". I'm not sure I agree though that the optimal temperature for humans or nature would be a pre-industrial temperature, either the 1850 temp I discuss above or Tom's reference to typical (warmer) Holocene temps. Natural ecosystems will have spent the previous 150 years moving in response to climate change - trying to get them to move again when 9 billion people are in the way could be a recipe for even further losses. Humans will be even more adapted to the existing climate.

A chosen temperature would eventually have to be low enough to stabilize the Greenland and West Antarctic ice sheets, although I assume we've got many additional decades or longer to do that.

I think in the very long term we would want to return to something like Tom's preferred temperature level.