(Warning: this post contains liberal worrying and extreme use of graphs. Reader discretion is advised.)

Until recently, scientists spoke of carbon concentrations – and temperatures – peaking and then falling back. But a recent paper in the Proceedings of the National Academy of Sciences shows that “climate change … is largely irreversible for 1,000 years after emissions stop”. Even if we were to cut carbon emissions to zero today, by the year 3000 our contribution to atmospheric concentrations would decline by just 40%. High temperatures would remain more or less constant until then. If we produce it, we’re stuck with it.

This seemingly innocuous passage came from Gorgeous George Monbiot’s comment piece in yesterday’s Guardian. Read it again and try and stop your mind boggling at this fact: even if carbon emissions ceased today, concentrations will only have halved in a thousand years time? Like most science it seems unlikely but is very much a sincere possibility. It also highlights one of the major facts that policymakers tend to overlook; carbon emissions have an obscene shelf life. What we emit now will stay with us for a long, long time.

Basic theory on global warming runs as follows; the sun’s rays enter the atmosphere and warm the Earth. Some of this heat gets reflected back from the surface but gets trapped by the atmosphere, thus keeping the surface at a reasonable temperature. However the emission of carbon dioxide (amongst other gases) accumulate and trap more of the sun’s heat then before. This is the foundation of the famous ‘greenhouse effect’ which is the main cause of global warming.

Naively, one would think that by shutting off carbon emissions it will stop its accumulation and stop the greenhouse effect. This does not consider the already existing gases which still has an impact on warming. It is these existing gases that Solomon et. al. investigate in the paper that was mentioned above. (Even better, the paper is currently open access so anyone can download it and check it, as should be the case with all scientific literature.)

Using existing and established climate models, they simulated the case where carbon emissions were allowed to accumulate to a series of set values, before stopping any output. They then measured how the existing emissions decayed and what their ‘steady-state’ levels were; that is, what the values would be like when decaying forces ceased to act.

The results, as you can guess, do not inspire confidence. Instead of carbon dioxide going away, it only slowly decays and will still stay at high levels after 1000 years. As George Monbiot pointed out above, only 40% will dissipate on average. Why is this? The researchers note that around 50% of atmospheric carbon dioxide can be retained by the atmosphere and so will have a hard time going away. Also, the vast oceans of the world absorb such gases and emit it at later times, which replenish any pollution which may otherwise have gone away.

Sexy graphs #1: the top one shows how carbon dioxide dissapates slowly over time, with the bottom one describes what effect this may have on surface temperature.

Sexy graphs: the top one shows how carbon dioxide dissipates slowly over time, with the bottom one describes what effect this may have on surface temperature.

Obviously as carbon dioxide will still be present, there will still be a greenhouse effect. Again, this does not bode well for humanity, with a temperature rise of two to four degrees predicted. Note that this is assuming that no further emissions occur after a cut-off point; what will be the actual rise with some emissions still in place, as will happen in reality? I shudder to think at that one.

Solomon et. al. then investigates how this affects precipitation and sea levels. If there is a mere two degree increase in temperatures, North Africa and Southern Europe will be worst hit, they predict, as they will suffer the most obvious reductions in precipitation. Mexico will also be badly affected. Mull on this for a moment; these are mainly poorer areas of the world which will suffer a heavy blow which will affect water supplies, agriculture and desertification. Sea rise is also inevitable due to a combination of icecaps melting and thermal expansion (that is, hot things which naturally expand). The rise will be around half a meter to a meter high, and could be as big as two meters. Doesn’t sound like much, but this is still plenty of water which can submerge many coastal features and towns.

This post and the paper may be depressing; it might say to you that no matter what one does about reducing their carbon footprint, dangerous climate change is inevitable. This should not be the case. The main effects depends on how much we output now, as well as reducing any emissions in the future. If this is stemmed immediatly, then the changes will be slight and the damage minimal; hopefully it can also be negated by human ingeniousness. Start these reductions today to save tomorrow.

(Reference; Solomon et. al. “Irreversible climate change due to carbon dioxide emissions”, Proceedings of the National Academy of Sciences of the United States of America, 106(6):1704 – 1709,2009.)

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