Where is the uncertainty in climate change?

It occurs to me that it might be useful to summarise just where, to the best of my knowledge at least, the scientific uncertainty is, given that PR organisations being paid to misinform the public by large corporations and loony right-wing millionaires, often like to exploit it.

First thing to say is that some things are well established. The science predicting the ‘greenhouse effect’ has been around a long time and is not in doubt (I’m going to take the phrase ‘among qualified scientists writing in relevant and respected peer reviewed journals … ‘ etc. as read here to save repeating it every paragraph) That we humans have significantly changed the quantities of greenhouse gasses in the atmosphere is not in doubt. We also understand pretty well the additional amount of solar energy these gasses (see this diagram ) trap in the climate system. We know that the Earth’s surface temperature has warmed significantly since we started burning fossil fuels and destroying forests etc.

When we put all these things together, by building mathematical models of what the science says should happen and what the data says did happen, they correspond closely enough that we can now conclude with reasonable certainty that the steady increase in average temperature is mainly due to greenhouse gases we caused to be emitted. Here are some graphs showing the predictions of the models against the data for real temperature changes.

In order to test this conclusion, more and more detailed models have been built and their more detailed predictions have been tested against real data.Increasingly, they show that the models accurately predict the data. So increasingly, science concludes that human-induced warming is for real. As we are continuing to increase the levels of greenhouse gasses to the point where the models predict very alarming things There are grounds for grave concern and hence increasingly strong calls from the scientific community for action.

Now if you follow that ‘very alarming’ link above, you’ll see the uncertainty ranges within the various different climate models shown as I-shaped bars. Each one shows the predictions for a different set of assumptions about the next 100 years. You can find details here basically they are different assumptions about how the global economy and population develop to 2100.

There are some consequences that are fairly straightforward to predict, for a given rise in temperature. For example, that 0.5-1.5m rise in sea level is pretty straightforward to predict and hence can be considered high-probability. Other changes are triggered at some hard to determine threshold though. For example the irreversable melting of major ice sheets and the switching off of the Atlantic Thermohaline Circulation are predicted to happen at some point if greenhouse gas levels continue to rise, but we’re not really sure a) what that point is and b) how much more emissions will rise given that we don’t yet know the outcome of the struggle between those who want to mitigate the effects now and those who want to keep emitting (and who are currently paying PR companies to lie about all this stuff so they can)

This brings us to the realm of value judgements, another area of uncertainty. In general, the PR people push some combination of three propaganda lines.

1) The science doesn’t prove this is happening (it does, see links above)
2) It’ll be fine anyway, we like warm weather (150million refugees is nice?)
3) It’ll be too expensive to fix (define “too expensive” you corporate leech)

The thing is, the science does prove this is happening, but it’s harder to show exactly what the effects will be. Some of them are regional for a start. So it’s not just a matter of uniform warming. It’s also a matter of changes in weather patterns and knock on effects like the extinction of species providing valuable ecosystem services or the wider spread of disease causing species.

So there is a sort of cascade of uncertainty. The basic climate models have some uncertainty in them but we pretty much understand how much. They can’t predict local weather in detail though, because limitations in computing power mean they have to work in units rather larger than Belgium and you need to model much smaller units to get an accurate idea of what effect a given mountain or forest has on local weather conditions. Once you get down to knock-on effects, like where and when crop failures will occur, or just when Wales becomes malarial, the uncertainty is also magnified by the inherent uncertainty of predicting behaviour in complex natural systems.

In addition, we have some potential problems with very grave impacts where the problem occurs at some undetermined threshold. One might use smoking as a metaphor here. We know it can cause cancer, but we can’t say with confidence just exactly how many cigarettes one needs to smoke to get it.

There are well established techniques for dealing with these kinds of uncertainty in science though, and they’ve been used as far as possible, but in the end, value judgements must be made and the uncertainty becomes polticial. The people you find regurgitating Exxon-sponsored disinformation online may be propaganda-spewing drones with damaged critical faculties, but it’s extremely unlikely that the CEO of Exxon is such a credulous dimwit.

Behind the PR, some calculations are being made and primarily I think, the last of the three points above is the one that counts. A value judgement has been made, by many political and business leaders that their interests will be better served by continuing economic growth and business as usual, no matter what the consequences for e.g. people in places like Bangladesh and especially, for people in Africa where the effects are likely to be severe even under some of the more moderate scenarios.

Mitigating action centres on limiting the concentration of greenhouse gasses in the atmosphere. Usually this is expressed as parts per million of CO2, the main greenhouse gas at present. In pre-industrial times, the level was stable in the range 180-300ppm, it’s now heading for 430ppm CO2 equivalent and rising fast.

One of the papers from the recent Met Office conference Parry (pdf!) suggests the following relationship between stabilisation levels and dangerous consequences.

Indications are:
– Stabilisation at 750ppm does not avoid most dangerous effects and very possibly triggers runaway climate change
– Stabilisation at 550ppm probably does avoid most, but at considerable human cost (this is the target suggested in the Stern Review)
– Stabilisation at 400ppm avoids dangerous effects.

The problem here is that we’re almost at 450ppm already and even the actions to stablise around 550ppm are being violently resisted by any number of wealthy interests. This is primarily because they imply significant costs (~1-3% of GDP) and distruption to the business models of some major companies. It’s also probably not unconnected to the high probability that the worst of the suffering associated with scenarios in the over-500ppm range are likely to be experienced by people in third-world countries who a) get the worst of the effects anyway in many cases and b) are much less able to mobilise economic and technical resources to mitigate the damage effects. (Although the Stern Report argues for using the IMF to make them accept adaptation on credit)

Between 550 and 750ppm though, the science increasingly strongly suggests that things start to get very bad for all of us. This is the level where the West Antarctic Ice Sheet and Greenland starting an irreversable disintegration and adding several metres to sea levels becomes a significant probability and numerous major cities are under threat from rising seas. Where billions experience water shortage (pdf!), food shortage (pdf!) and disease (pdf!) because weather effects, changes in rainfall patterns and so on are likely to get so extreme that they’re highly damaging to ecological systems on a global scale. This is the level where our remaining tropical forests are likely to become a net carbon source (pdf!) rather than a carbon sink and various other threshold effects and postive feedback loops may come into play leading to a significant likelihood of rapidly accelerating climate change, taking us into apocalyptic territory. Some of these effects, for example ~20m sea-level rises, even if emissions then stablised, are irreversable over thousands of years.

Even if we somehow stabilised at 450ppm right now, various other factors like population growth, soil erosion and so on are very likely to cause significant food shortages through much of the developing world this century. On the higher emission scenarios though, if business as usual (unconstrained CO2 emissions, unconstrained deforestation and so on, taking us beyond 750ppm) continues to 2100, we’re probably facing mass starvation for hundreds of millions in the third world and significant shortages even in much of the developed world, along with wars over water and hundreds of millions of starving refugees trying to escape to less damaged environments. As far as I can tell from the science, this is the world that corporate interests are foisting on us to protect their “prosperity”