On CO2 lifetime and the incompleteness of short-term mitigation options

Quite frequently in discussions on climate change, projections are given out to some time period like “by 2100” or by some useful metric like “a doubling of CO2.”


Global Warming Art

These are perfectly acceptable for talking about how much warming we’d get in a specified amount of time, or at a certain level of CO2, etc. but often people lose sight of the fact that all of these lines are still going up. The world does not end when we hit the first doubling of 560 ppm, or when we hit 2100.  Even in discusing climate sensitivity, if we suppose that the sensitivity is on the low end of the IPCC projections, there is still nothing to stop us from tripling or quadrupling CO2 if continue on business as usual for centuries.  The real issue here is coal, which is much larger than what is left from oil and gas.  I can imagine how year 2200 or 2300 seems like such a long time away, but imagine if the ancient Greeks or Romans were faced with the issues we have today– how would they have left the world, and would we like it?

Geo-engineering has been following me a bit this week. Earlier today I heard someone talking about injecting sulphate aerosols into the atmosphere to offset the warming effect due to rising CO2 concentrations. It doesn’t seem like one of the more practical options out there, and is not exactly at the forefront of “solutions” to the climate problem. Still, it is brought up quite a bit in “what if” thought experiments, it’s probably something we should know out there, and a lot of people still take it seriously. I have my concerns over playing games with an incompletely understood climate system by trying to balance the energy balance of the planet with a higher albedo. I have concerns with short-term mitigation options as a whole though.

Climate Change is both a short-term and a long-term problem. When we inject a pulse of CO2 in the atmosphere, it has a different variety of “fates.” About three-quarters of the drawdown of the excess CO2 takes place in centuries by dissolving in the oceans. There is still some left over CO2 that becomes becomes neutralized by reactions with calcium carbonate and then igneous rock, but these processes take much longer. If there was just air and water and no rocks this tail end would basically stay there forever. So essentially our excess CO2 will effect climate for thousands to tens of thousands of years. In fact, using up the entire coal reservoir could prevent another glaciation for half a million years or longer. So if we offset rising CO2 with sulphate aerosols, we’d have to commit to that for thousands of years, which is absurd. If we were to stop, then the aerosol effect would disappear rather quickly, while CO2 was still very high. Talk about an abrupt climate change!

It is still feasible to keep atmospheric CO2 from exceeding about 450 ppmv by 2100 (which has been argued in the peer-review to be a threshold for “dangerous” interference with the climate system), if emissions from coal and unconventional fossil fuels are constrained. Coal-fired plants need to be phased out within decades, raising prices on carbon emissions helps, and a stronger backbone for alternative energy development needs to be put in place. None of these things are impossible nor impractical with technology or economics; we simply need the will. Continued discussion of short-term “offsetting global warming solutions” could work in sync with longer term strategies, but by themselves they are pointless.

6 responses to “On CO2 lifetime and the incompleteness of short-term mitigation options

  1. On your first point:

    The world also won’t end in 2100. Not enough model runs, and not nearly enough graphs and images shown to the public, extend into the next century. This is misleading about the extent of warming that can be expected when the (climate reaches equilibrium). For example, in some of the future emissions scenarios used by the IPCC, atmospheric concentrations reach a plateau (550 ppm) in 2100. People make the mistake of looking at that scenario and assuming the warming ends in 2100 too. However, the lag in the climate system – the same lag that would result in ~0.5 C warming over the next several decades even if we froze atmospheric concentrations today – would lead to further warming well into the 22nd century and possibly beyond. warming does not stop in 2100 due to the lag in the climate system.

  2. Nice blog, Chris.

    One problem with the sulfate aerosols idea — it only solves part of the CO2 problem (the radiative transfer part). We’d still see the transformation of ocean chemistry (“ocean acidification”) …

    People need to recognize the difference between two broad categories of geoengineering methods. One type focuses on albedo, and only ameliorates the radiative transfer effects of CO2. The other type (sequestration) actually removes CO2 from the atmosphere, and thus helps with both climate and ocean chemistry.

    This is not intended as an “endorsement” of any geoengineering approach, by the way ….

  3. Are these “Possibly related posts: (automatically generated)” mandatory for your blog’s page format, or are they an option you control? e.g. “At Witz’ End – Global Warming Pro” — why bother?

    Response– I could probably get rid of them somehow, but I’m not sure. They weren’t my choice to put them there

  4. Even if the geo-engineering is technically OK, then there are possible follow on effects likely. I’ve just read a blog on that on http://www.climatechangetriage.net quoting Professor Lovelock (of Gaia fame) worrying about a techno-fix that might lead to another, as yet unidentified climate change problem, requiring another techno-fix and so on. So the message is that we need to be VERY careful with techological fixes

  5. Are there charts like the one out to 2100 you used above, going through several more centuries? I thought I’d seen them.

    Response– IPCC AR4 Chapter 10 has discussion on long term commitments (pg. 822 onward) but that’s the best I’ve seen. Don’t know how much confidence I have in them.– chris

  6. Thanks; here’s some info on the longer term, taken from the IPCC:
    http://www.manicore.com/anglais/documentation_a/greenhouse/temp_increase.html

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