Modelling the Apocalypse

Let’s all put on our science-fiction hats and imagine that humans get wiped off the face of the Earth tomorrow. Perhaps a mysterious superbug kills us all overnight, or maybe we organize a mass migration to live on the moon. In a matter of a day, we’re gone without a trace.

If your first response to this scenario is “What would happen to the climate now that fossil fuel burning has stopped?” then you may be afflicted with Climate Science. (I find myself reacting like this all the time now. I can’t watch The Lord of the Rings without imagining how one would model the climate of Middle Earth.)

A handful of researchers, particularly in Canada, recently became so interested in this question that they started modelling it. Their motive was more than just morbid fascination – in fact, the global temperature change that occurs in such a scenario is a very useful metric. It represents the amount of warming that we’ve already guaranteed, and a lower bound for the amount of warming we can expect.

Initial results were hopeful. Damon Matthews and Andrew Weaver ran the experiment on the UVic ESCM and published the results. In their simulations, global average temperature stabilized almost immediately after CO2 emissions dropped to zero, and stayed approximately constant for centuries. The climate didn’t recover from the changes we inflicted, but at least it didn’t get any worse. The “zero-emissions commitment” was more or less nothing. See the dark blue line in the graph below:

However, this experiment didn’t take anthropogenic impacts other than CO2 into account. In particular, the impacts of sulfate aerosols and additional (non-CO2) greenhouse gases currently cancel out, so it was assumed that they would keep cancelling and could therefore be ignored.

But is this a safe assumption? Sulfate aerosols have a very short atmospheric lifetime – as soon as it rains, they wash right out. Non-CO2 greenhouse gases last much longer (although, in most cases, not as long as CO2). Consequently, you would expect a transition period in which the cooling influence of aerosols had disappeared but the warming influence of additional greenhouse gases was still present. The two forcings would no longer cancel, and the net effect would be one of warming.

Damon Matthews recently repeated his experiment, this time with Kirsten Zickfeld, and took aerosols and additional greenhouse gases into account. The long-term picture was still the same – global temperature remaining at present-day levels for centuries – but the short-term response was different. For about the first decade after human influences disappeared, the temperature rose very quickly (as aerosols were eliminated from the atmosphere) but then dropped back down (as additional greenhouse gases were eliminated). This transition period wouldn’t be fun, but at least it would be short. See the light blue line in the graph below:

We’re still making an implicit assumption, though. By looking at the graphs of constant global average temperature and saying “Look, the problem doesn’t get any worse!”, we’re assuming that regional temperatures are also constant for every area on the planet. In fact, half of the world could be warming rapidly and the other half could be cooling rapidly, a bad scenario indeed. From a single global metric, you can’t just tell.

A team of researchers led by Nathan Gillett recently modelled regional changes to a sudden cessation of CO2 emissions (other gases were ignored). They used a more complex climate model from Environment Canada, which is better for regional projections than the UVic ESCM.

The results were disturbing: even though the average global temperature stayed basically constant after CO2 emissions (following the A2 scenario) disappeared in 2100, regional temperatures continued to change. Most of the world cooled slightly, but Antarctica and the surrounding ocean warmed significantly. By the year 3000, the coasts of Antarctica were 9°C above preindustrial temperatures. This might easily be enough for the West Antarctic Ice Sheet to collapse.

Why didn’t this continued warming happen in the Arctic? Remember that the Arctic is an ocean surrounded by land, and temperatures over land change relatively quickly in response to a radiative forcing. Furthermore, the Arctic Ocean is small enough that it’s heavily influenced by temperatures on the land around it. In this simulation, the Arctic sea ice actually recovered.

On the other hand, Antarctica is land surrounded by a large ocean that mixes heat particularly well. As a result, it has an extraordinarily high heat capacity, and takes a very long time to fully respond to changes in temperature. So, even by the year 3000, it was still reacting to the radiative forcing of the 21st century. The warming ocean surrounded the land and caused it to warm as well.

As a result of the cooling Arctic and warming Antarctic, the Intertropical Convergence Zone (an important wind current) shifted southward in the simulation. As a result, precipitation over North Africa continued to decrease – a situation that was already bad by 2100. Counterintuitively, even though global warming had ceased, some of the impacts of warming continued to worsen.

These experiments, assuming an overnight apocalypse, are purely hypothetical. By definition, we’ll never be able to test their accuracy in the real world. However, as a lower bound for the expected impacts of our actions, the results are sobering.

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10 thoughts on “Modelling the Apocalypse

  1. My first thought would be what happens to all nuclear power plants and chemical factories that store all kinds of nasty stuff. Climate change may be a rather minor worry if we go away without turning off the lights.

  2. Interesting stuff Kate. Given the global energy imbalance which should leave us with ~0.6°C unrealized warming at present, why is there no additional warming in the apocalypse scenario? Does natural carbon sequestration kick in without human emissions in play, perhaps?

    Also, mind if we re-post this on SkS?

    • Feel free to repost this at SkS!

      Many of the “committed warming” scenarios are actually for constant composition: so rather than emissions dropping to zero, emissions stay at a low level to maintain the current CO2 concentrations. To my knowledge, this is where the 0.6 C figure comes from.

  3. Hopefully, anyone having been given ideas by watching Twelve Monkeys (hey, you asked for a ‘science fiction hat’!) will, upon reading this article, realise that eradicating humanity doesn’t actually solve the problem.

    Humanity got us into this fine mess, and is needed to get us out of it again — and without making the problem still worse in the process (not something that we’ve got a particularly good track record in…).

  4. For a well written, intensely interesting, intelligent account of how the Earth would respond to our sudden disappearance, I strongly recommend “World Without Us”, by Alan Weisman, published in 2007. As I recall (I read the book four or five years ago, before the issue of climate change had reached its current position of primacy in my consciousness), the book deals with just about every aspect of the consequences of sudden human departure from Earth except climate change, so in that sense it forms a perfect complement to the studies described in Kate’s article above.

  5. thumbs up for Lord of the Rings climate model :-), to get Mirkwood growing one almost has to assume huge snows to Ered Mithrin… This would mean the Misty Mountains are lower in the far north near Gundabad. There is no mention of the East Emnet (Rohan) tornados at all in the book but maybe they passed there off season. Eregion seems to be quite ideally located wrt to this ;-) But really, we have no good maps of the eastern parts so it’s a bit haphazard to guess how the weather systems in Rhûn would have affected the gathering of evil forces there.

    On subject, I’d say the reforestation of many fields would diminish the CO2 forcing quite quickly (100-200 years) though the animals escaped from the farms & pastures would try to prevent this somewhat. The bound carbon is that much more in dense forests than in the fields.

  6. Good post Kate,

    This thinking about how global temperature will evolve with emissions rather than composition has provided an interesting perspective for the research community, and also because it may be a useful metric for policy-makers.

    Regarding the question above about committed climate warming: it’s true that the traditional idea of “warming in the pipeline” comes from constant CO2 concentration scenarios, i.e., one where the forcing stays the same. It’s a bit unfair, however, to call this “committed” since stabilizing CO2 at a constant concentration requires some future emissions (though on the order of ~80 reduction, see the National Academies report in 2010 on stabilizing targets). It can’t really be described as “committed” if it requires future emissions to make sense of.

    The zero emission scenario, in contrast, is obviously hypothetical but calls for a reduction in CO2 concentration. This decreases the magnitude of the forcing such that the warming leftover is near zero.

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