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Risky Business: Gambling on Climate Sensitivity

September 19, 2010

There are some things about our climate we are pretty certain about. Unfortunately, climate sensitivity isn’t one of them. Climate sensitivity is the estimate of how much the earth’s climate will warm if carbon dioxide equivalents are doubled. This is very important because if it is low, as some sceptics argue, then the planet isn’t going to warm up very much. If sensitivity is high, then we could be in for a very bad time indeed.
There are two ways of working out what climate sensitivity is (a third way – waiting a century – isn’t an option, but we’ll come to that in a moment). The first method is by modelling:

Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).

The second method calculates climate sensitivity directly from physical evidence:

These calculations use data from sources like ice cores, paleoclimate records, ocean heat uptake and solar cycles, to work out how much additional heat the doubling of greenhouse gases will produce. The lowest estimate of warming is close to the models – 1.8°C (3.24°F ) on average – but the upper estimate is a little more consistent, at an average of around 3.5°C (6.3°F).

It’s all a matter of degree

To the lay person, the arguments are obscure and complicated by other factors, like the time the climate takes to respond. But climate sensitivity is not just an abstract exchange of statistics relevant only to scientists. It also tells us about the likely changes to the climate that today’s children will inherit.

Consider a rise in sea levels, for example. Predictions range from centimetres to many metres, and the actual increase will be governed by climate sensitivity. The 2007 IPCC report proposed a range of sea level rises based on different increases in temperature, but we now know they underestimated sea level rise, perhaps by a factor of three, in part because of a lack of data about the behaviour of Greenland and Antarctic ice-sheets.

Current estimates of sea level rise alone, as a result of a two degree rise in temperature, are very worrying. More worrying is that the current projections do not account for recently accelerated melting of polar regions. There are also many other possible effects of a 2°C rise (3.6°F) that would be very disruptive.

All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4.5°C or even more. Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against mitigation because of climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being we don’t need to do anything much about it. Others suggest that because we can’t tell for sure, we should wait and see.

In truth, nobody knows for sure quite how much the temperature will rise, but rise it will. Inaction or complacency heightens risk, gambling with the entire ecology of the planet, and the welfare of everyone on it.

10 Comments leave one →
  1. September 20, 2010 1:47 am

    When others have argued with me in threads, stating that because we’re not sure how sensitive climate is to increases of GHG concentration in the atmosphere, Anthropogenic climate change could be trivial, I use the following metaphor.
    Just say, there is a freeway you frequently use that is notorious for deer crossing – it’s caused many accidents and you’ve even been brought to a sudden halt on a few occasions. Now, you might feel confident driving at a certain speed because you know your vehicle’s history well and know the brakes are new, the tires are good etc.
    However, you travel home this evening in a mates car which you know little history about, but knowing this particular mate, it could quite potentially be neglected (it even sounds a little too noisy to be healthy). You know also, that if you did damage the vehicle, repairs would come out of your pocket – something you certainly cannot afford currently. Do you risk travelling as fast as you would in your car?
    Of course, some people would regardless – but most, if not all people, can see the increased risk in this situation. That’s the important part. We know enough to see the danger in the situation and addressing ACC is simply sensible risk management when not all, but many factors are well known.

  2. G. Thomas Farmer, Ph.D. permalink
    September 20, 2010 2:25 am

    Climate sensitivity is very important and there are many scientific facts that indicate the Earth’s climate will continue to warm even if we stop all carbon pollution to the atmosphere. The main thing we as humans have to worry about is the methane being released from permafrost in Siberia, Canada, and Alaska. Methane has more than 20 times the warming potential of CO2. It is possible that methane release has caused faunal extinctions in the geologic past. We were introduced to methane hydrates with the BP oil well fiasco during April 2010. These methane hydrates are very unstable if the environment under which they exist is disturbed. The methane just bubbles to the surface. Extensive deposits of methane underlie continental shelves throughout the world. It is time to re-focus our attention on methane and not so much on CO2, although we know the latter is causing the rise in temperature. The non-scientists and illiterates out there might understand methane,. They obviously don’t understand CO2.

  3. Graham Wayne permalink*
    September 20, 2010 6:33 am

    Moth: the risk management argument is the one that baffles me most when sceptics oppose – and sometimes ridicule it. I find this particularly weird when you consider that virtually every step we would take to mitigate climate change (by reducing CO2 output), we need to take anyway because of peak oil, energy security, global resource management and population growth. Deniers seem so intent on fighting the battle, they can’t see the extent of the war we are waging on nature itself, and the speed at which we are losing.

    Dr. Tom (is it OK to call you that?): I didn’t know until recently that clathrates were also trapped in glaciers. Turns out the melting of the ice caps may have knock-on effects on methane release. However, I did read compelling science not long ago to suggest the methane cannot become the dramatic problem I and others have been writing about because the half-life of methane is too short. It converts to CO2 so quickly its residence time may be too short for there to be a build up. Massive releases may cause local heating, serving as a positive feedback, but it now appears that effect may not be sustained.

  4. September 20, 2010 6:45 am

    You couldn’t be more correct – I started this blogging with that very argument – that anthropogenic cause of climate change is irrelevant when you look at all the related issues (oil peaking, deforestation and acidification of oceans being that main points). All roads lead to a future where fossil fuels are an unessential component of human activity, whether we like it or not. So deep are the heels are dug into the denial pit that you’d swear that they would deny the price increase of petrol, even when it doubles.

  5. September 20, 2010 12:37 pm

    Thanks for another good post, however, this sentence baffled me.

    Probably the most obvious, and equally distributed, environmental change will be a rise in sea levels.

    I’m not sure I agree with either claim.

    (a) Most obvious: sea level rise will probably lag behind a number of other ill-effects. And there are plenty of ways of obfuscating over sea level rise (continental rebound, differing rates of rising and falling land, as well as see (b). I think the most obvious (and earliest) environmental change is already and will continue be Arctic sea ice melt.

    (b) Equally distributed. I’m not quite what you mean by this. If you mean that everyone will be equally affected by it, then I don’t see how someone living in New Orleans will be affected in the same way as someone living in Nepal. And even if you simply meant that everyone who lives near an ocean will face the same rise in sea level, then that is an oversimplification too. Not only are land heights also moving (in both directions in different parts of the world), but the local gravitational effects of massive ice caps losing mass will also mean a variety global distribution of rises. See more on this here.

  6. Graham Wayne permalink*
    September 20, 2010 1:57 pm

    Byron – I’m afraid your criticisms are too well made for me to even bother to discuss – it’s a crap sentence and it will have to go. I’ll have to change the text on the SkS site and here too – thanks very much for your comment.

  7. September 20, 2010 4:36 pm

    No problem – the effect of glacial gravity on the distribution of sea level rise was one of those “wow” moments for me. Something that makes so much sense that I really had never had an inkling of it before reading the articles I link to in that post.

  8. Dr. Tom permalink
    September 20, 2010 6:43 pm

    I didn’t mean to imply that glaciers held clathrates. They are tied up in organic remains in permafrost but some may be held in glaciers as the result of organic material in the ice. However, I doubt that this is much of a source. The major deposits of methane clathrates are on continental shelves throughout the world. I agree about the short half-life of methane but the danger is releasing a great deal of this CH4 to the atmosphere continuously over a relatively short period of time, then its being converted to CO2, so there would be a long term effect of the release.
    Dr. Tom is OK with me. I’ve been called worse, especially by deniers, skeptics, and others.

  9. ELSA permalink
    September 21, 2010 12:13 pm

    A number of points arise from this.

    1. Climate models: I keep promising you that I will write a piece on these and I will do so one of these days. In the meantime I am not sure how you take such widely divergent forecasts seriously. In effect you concede what I keep telling you, that the models are not reliable.

    2. “These calculations use data from sources like ice cores, paleoclimate records, ocean heat uptake and solar cycles, to work out how much additional heat the doubling of greenhouse gases will produce.” That seems to me a bit like saying you will use a thermometer to forecast future temperature. How on earth could you use these things to forecast the future unless you used the past information in conjunction with some type of model? But if that is what has happened how can you say that they are in any way independent of the models?

    3. For goodness sake all of you, cheer up. These comments seem to be that the outlook is terrible and where there is uncertainty things are likely to be even worse. The trouble with doing that is once you get a few cold winters people are just not going to take you seriously, even if you are right. Cheer yourselves up and look at the maximum increase forecasts: at least these have come down year by year since 2002. Who knows by 2020 they might hit zero.

    4. I know you will not agree but I think you all need a few more sceptics or denialists on this site. The above posts read like a CRU/IPCC “peer review” process where you each confirm the others point of view with a large pat on the back. Remember there is more joy in heaven over one sinner that repents than over 99 sinners who do not need to repent (not that I’m thinking of repenting any time soon!).

  10. Dr. Tom permalink
    September 24, 2010 10:53 pm

    I’m not a modeler nor do I have complete “faith” in models. However, they are tools available to climate scientists just like thermometers are tools. I don’t mean to equate the two but we need to have tools that allow us to interpret natural phenomena and to allow humans to attempt to predict the future based on past history. Thus, climate models. The climate modelers need to speak to this question about whether the models incorporate ocean data or the biosphere. My guess is they do. One of my next tasks is to re-read what the IPCC has to say about the models they use.

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