What if we took +4C as inevitable?

The range of possible responses to climate change is typically broken down into two main strategies -- "adaptation" -- learning to live with it -- and "mitigation" -- slowing the rate at which we are warming the planet, primarily but not exclusively by reducing the emission of greenhouse gases.

Lukewarmers, as one would expect, don't like mitigation much. That's kind of the point of being a lukewarmer, accepting the main bullet points of the science whilst opposing limits on greenhouse gas emissions. (If you are a lukewarmer who accepts the basic theory of AGW and supports curbs on greenhouse gases -- well, I'm sorry to have to tell you that this is the point in the movie when the tough guy realizes he was bitten in that last skirmish and has began the inevitable transformation into one of them.)

The lukewarmer argument -- which I explored in some detail here and here -- has traditionally been that there's nothing much to worry about. It's a little warming, big whoop. It was a lot warmer a couple hundred million years ago, and the dinosaurs didn't have any problems with it at all.

But more recently it's become clear problems are coming -- big problems -- and they can't be ignored. And the discussion has turned to adaptation, as opposed to mitigation. This is Judith Curry's main line of argument: local adaptation to problems as they emerge.

I was confronting this argument, and I had a revelation: the lukewarmers are right (no, really). Adaptation is the name of the game. Why? Because, in all probability, a +4C world is coming. The only question is when.

The SRES temperature projections are well known:

We passed year 2000 concentrations a decade ago without slowing down, so that ship has pretty much sailed, but no problem; it looks like the B1 scenario limits us to just under +2C. That's great! The earth was warmer than that a mere 120,000 years ago! Should be fine. Onward mitigation!

Here's a little Easter egg of bad news, lurking in the fine print of the AR4 like an unpaid bill:

For the B1 constant composition commitment run, the additional warming after 100 years is also about 0.5°C, and roughly the same for the A1B constant composition commitment (Supplementary Material, Figure S10.3). These new results quantify what was postulated in the TAR in that the warming commitment after stabilising concentrations is about 0.5°C for the first century, and considerably smaller after that, with most of the warming commitment occurring in the first several decades of the 22nd century.

So it's going to continue warming for a hundred years past the right edge of the graph. But then it stops . . . right?

Yeah . . . no. It keeps warming. The deep ocean warming sees to that. The ice-albedo feedback sees to that, too, but it isn't modeled in the AR4. These also aren't the carbon-cycle models we have coming in the AR5, but the expectation is that permafrost melting and other carbon-cycle feedbacks will contribute to warming as well.

How long can this go on? Recent research ("The contribution of timescales to the temperature response of climate models," Jarvis & Li (2011)) is not reassuring:

Both the magnitude and timescale of climate change in response to anthropogenic forcing are important consideration in climate change decision making. . . . As a result of this emergent property of climate models, it is possible to offer credible partitioning of the full equilibrium gain of these models, and hence their equilibrium climate sensitivity, between two discrete timescales; one decadal associated with near surface ocean heat equilibration; and one centennial associated with deep ocean heat equilibration. Timescales of approximately 20 and 700 years with a 60:40 partitioning of the equilibrium gain are found for the models analysed here. A re-analysis of the emulation results of 19 AOGCMs presented by Meinshausen et al. (Atmos Chem Phys Discuss 8:6153–6272, 2008) indicates timescales of 20 and 580 years with an approximate 50:50 partition of the equilibrium gain between the two. This suggests near equal importance of both short and long timescales in determining equilibrium climate sensitivity.

So when does it stop?

After the year 3000, the global mean surface temperature is predicted to be almost constant due to the compensating effects of decreasing atmospheric CO₂ concentrations due to oceanic uptake and delayed response to increasing atmospheric CO₂ concentrations before. 

That's a comfort.

So here's the sharp end of the argument, and the point where I turn it back (you knew I would) on the adapt-niks. +4C is coming. Even if we follow a B1 (low population, intensive emissions cuts) pathway, and hold concentrations fixed from 2100, we still hit +4C eventually (with +2C in short term warming, Jarvis & Li tells us another +2C is in the pipeline.)

So we have to adapt to a +4C world -- sea levels tens of meters higher, 50% of animals extinct, agricultural production decimated, climate refugees in the hundreds of millions. To adapt we need money -- vast amounts of economic resources to resettle refugees, relocate coastal cities, cope with almost unimaginable heat waves, mega-droughts, and huge storms. We need scientific and technological progress -- a lot of progress, before we can have any confidence in feeding billions of people in a +4C world, before we can cope with the mass extinctions and radical rearrangement of floral and fauna in a +4C world.

Hey, we don't know what you're talking about; warming is great

There is only one thing that will give us vastly more money, scientific knowledge and technical know-how. Time. We need time to adapt. Buying time has got to be the cornerstone of any adaptation strategy. People like Bjorn Lomborg advocate deferring serious mitigation to the far future, when we have more money and know-how. Adaptation is then presented as the default alternative. But accepting the reality of a +4C world shows that logic to be backwards. If we are going to be richer and smarter in the future, we should defer adaptation to our future selves by pursuing mitigation now.

We know a +4C world in coming. What we don't know is when. If we do nothing by way of mitigation, smart money says it will be before the end of this century. Fifty years -- 2060 -- if we're really unlucky. And without mitigation, we will rapidly pass +4C on the road to something worse.

Fifty years is not a very long time. Fifty years ago people already flew across the country on jets, watched television, got antibiotics when they were sick. They had cars and rockets and fusion bombs. They were like us. 

With intensive mitigation -- holding the short-term warming to under +2C -- +4C is still waiting for us, but in six hundred years. Six hundred years is a very long time. Six hundred years ago the scientific revolution had not been conceived, democracy had not been seen since Athens; there was no internal combustion -- steam engines belonged to the distant future at that point. Medicine was little better than the shaman who drilled holes in heads thousands of years before. Shakespeare was as far in their future as the 23rd century is from us.

In six hundred years, if we don't destroy ourselves, our technical and scientific abilities will be unrecognizably superior to those we have today. As well they should be, before we rely on them to cope with temperatures warmer than the earth has seen in millions of years.

+4C is coming. We're not ready yet. So if you truly believe in human progress, in continual growth and the expansion of human knowledge, the conclusion is clear: mitigation is the best adaptation.