I watched the much-hyped BBC4 show climate change by numbers and thought it was pretty good. It covered 3 basic numbers; why we think global surface temperatures have risen by 0.85oC since 1880, why we’re 95% (or more) sure that more 50% of the warming since 1950 has been anthropogenic, and why – if we want to have a reasonable chance of keeping warming below 2oC – we can only emit a total of 1 trillion tonnes of Carbon (1000GtC).
The reason that we think we’ve warmed by 0.85oC since 1880 is because that’s what the data suggests. The programme did, however, do a very good job of describing how there are problems with the data and how these are corrected. I hadn’t realised that 1880 was the time when most sea surface temperature measurements were done with wooden, rather than canvas, buckets. It discussed Kalman filtering – more commonly known as homogenization – which is a method for dealing with errors in measurements taken over some time interval, and discussed Kriging, the method used by Cowtan & Way (2013) to account for coverage in the HadCRUT4 temperature dataset. What I hadn’t realised was that Kriging was pioneered in South Africa to estimate the distribution of gold in the Witwatersrand.That we’re at least 95% certain that more than 50% of the warming since 1950 was anthropogenic is probably best illustrated by the figure on the right, taken from this RealClimate post. The figure shows the distribution of the fractional contribution of the warming since 1950 that can be attributed to anthropogenic influences. The most likely contribution is about 110%, and there is only a very small chance that it could be as low as 50%. As the programme explained, this was an attribution study; you run models will all possible factors included and then remove certain influences to determine how much each of the different factors influenced the warming. In fact, the programme claimed that the actual analysis suggested that we should be more than 99% certain that more than 50% since 1950 was anthropogenic, but downgraded it to more than 95% because of other uncertainties. The 1000 GtC that we can emit overall if we want a reasonable chance of warming staying below 2oC is best illustrated by the figure on the left. It shows cumulative emissions (since 1870) with Carbon on the bottom axis and CO2 on the top. As is clear, anything more than 1000 GtC would probably result in more than 2oC of warming. Of course, you could change to some different climate sensitivity numbers if you wish (Nic Lewis’s, for example) but this would only change things by maybe 30% or so (so, maybe 1500GtC if some of the most optimistic estimates for climate sensitivity are right – which I suspect they are not).
I think this whole cumulative emissions issue is quite important and not necessarily well understood, so I thought I would stress a few things that maybe aren’t stressed often enough.
- How much we warm by some point in the future depends primarily on our total emissions, not on how fast or slow we emit. If we increase our emissions, we’ll simply warm faster than if we keep them constant or reduce them.
- Given the above, if there is some level of warming that will do extreme damage (and I think there obviously is) the faster we emit now, the more drastic our actions will need to be in future if we wish to avoid reaching that level. In other words, if there is a level of warming that will be damaging, then that sets the cumulative total anthropogenic emissions that we need to avoid in order to avoid this level of warming. Therefore, the more we emit now, the less we can emit in the future – assuming we want to avoid this level of warming.
- It’s largely irreversible. Once we’ve reached a certain level of warming, it will stay there even if we halt all emissions (which is clearly virtually impossible). In fact, if we were to reach the point where warming is clearly damaging, we can almost certainly not avoid continuing to warm. We could, presumably, start considering geo-engineering, but that carries it’s own extreme risks. Maybe we can find a way to remove CO2 from the atmosphere, but is that going to be easier than avoiding getting there in the first place?
So, I thought it was a good programme. Explained some really quite complex concepts very clearly. I’m assuming that people outside the UK may not have been able to watch it yet, but I believe there is an international version being produced (although, I don’t know for sure). I discovered that one of the presenters, Norman Fenton, has written his own post with some thoughts about the programme. Some of the final points may illustrate that even someone who can present a programme like this can give some of the controversies a bit more credence than many may regard as reasonable. In fact, that was maybe the one criticism of the programme – a slight focus on controversies that probably exist more in the public sphere, than in the scientific domain.