A local environmental association (the ARPE in Chantonnay) invited me to speak about climate change after their AGM last weekend. Needless to say, I leapt at the opportunity and set about updating my slides. However, I soon realised that I would have to start all over again in the light of the IPCC’s AR5, published last September. For better or for worse, that report set the agenda for the next seven years and must now be the starting point for anyone wanting to talk about climate change.
Putting a new slideshow together proved to be a lengthy business — and a fascinating one. I had to grapple with the nuts and bolts of the IPCC report, for one thing, but I also had to fit it into the patchwork of my existing knowledge. The first part was hard work; the second led to a couple of interesting insights, as I realised that the IPCC report is at least as important for what it doesn’t say as for what it does.
While I was still getting my ideas sorted out, I went to listen to a guy called Cédric Ringenbach talk about climate change and Third World sustainable development. It was much too early in the morning for me and I was struggling to stay awake as he took his audience through the parallel between atmospheric temperature and CO2 during successive ice-ages, when suddenly — ping! And I was wide awake. It was almost a throw-away remark, in brackets: “…atmospheric CO2 was driven by temperature”. Well, yes, of course, it’s obvious when you think about it. But that changes everything!
I’ll come back to that one later. First I want to look at the IPCC position on Greenland.
The Greenland ice sheet comprises 3 million cubic kilometres of ice. If it all melted, sea level would rise by 7 or 8m. AR5 says there’s no danger of that happening for a thousand years or more. In fact AR5 says very little about Greenland. This is because, despite records going back 20 years, there is, according to the IPCC’s admirably strict statistical methodology, insufficient data.
This deserves some explanation.
Consider these two graphs. They both represent the same data, before and after re-treatment by the IPCC: Earth temperatures from 1850 to 2012. Above, each year is plotted and the resulting curve is confusing to read and subject to misinterpretation (e.g. “Global warming stopped in 1998”!). Below, only the average per decade is plotted: this eliminates the anomalies and provides a clearer picture of the long-term tendency. It has to be said that, generally speaking, this is one of the IPCC’s strong points. But it has its limits and Greenland is one of them.
Greenland is different. Because if Greenland were to go seriously wrong then everything else could go tits up very quickly. So quickly that a further wait of 10 or 20 years to obtain “sufficient” data could leave us with virtually no time to react. Suspecting that might be so, I got Excel to do some calculations for me.
AR5 tells us only that Greenland lost a yearly average of 34 Gt of ice during the decade to 2001, and a yearly average of 215 Gt in the decade to 2011. For the IPCC there are only two points on the graph and to all intents and purposes this “insufficient data” is of no use when calculating, for example, long-term sea level rise. Thus the main cause of sea level rise is taken to be thermal expansion and the top end of the worst case scenario runs to no more than a 98cm rise by the year 2100. Moreover:
Based on current understanding, only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century. However, there is medium confidence that this additional contribution would not exceed several tenths of a meter of sea level rise during the 21st century.
Now, it seems to me reasonable to suppose that Greenland ice melt is not going to stop tomorrow. I say that because (i) AR5 tells me that ocean temperatures will continue to rise for the next several centuries, (ii) AR5 also tells me that surface temperatures will rise more quickly in the Arctic than elsewhere, and (iii) a recently published study by a team from Melbourne university indicates that the range of possible global warming is not 1.5 to 5° as was thought when AR5 went to press, but 3 to 5°. So Greenland ice loss clearly has a future. That being so, what hypotheses can we make about the rate of loss?
It might continue at the present rate: i.e. the decade on decade yearly average would continue to grow by increments of 181 Gt. That straight-line graph would stop in about 550 years because there’d be no ice left. Let’s be generous and admit that that falls within the IPCC’s 1000-year ballpark.
However, it also seems reasonable to consider the possibility that the rate of loss might increase and that, for instance, 215 = not (34+181) but (34*6). If that were the case, and if ice loss were to continue at that rate, the Greenland ice sheet would be gone by the middle of the century — this century!
I have to admit, though, that a factor of 6 is enormous, so I asked Excel to reduce it progressively. The results were scarcely more comforting. Here is the file:
And here is the picture:
Unless I’ve made an elementary mistake somewhere, we have to draw the conclusion that any exponential degree of ice loss would be catastrophic.
It seems to me that the IPCC have ducked this issue in a manner that is less than ingenuous, and in doing so they may well have prepared a rod for their own back. If they found themselves having to backtrack radically on sea level in their next report, they’d look very silly indeed. The sceptics would have a field day and that would not be good for the public’s perception of the science.
That being the case, the IPCC might want to consider a trade-off between the extreme rigour of their statistics and the possibility of issuing a timely warning. I would suggest they rework the numbers on the basis of 5-year periods. Alternatively, they might stick to 10-year periods but calculate them every five years, with a point on the graph for 1991-2001, another for 1996-2006, and so on. I shall do this myself as soon as the numbers become available, but for the time being they are still under wraps.
Now, to get back to that ping…
Many of you will recognise this still from Al Gore’s An Inconvenient Truth showing the parallel fluctuations of temperature and atmospheric CO2 over a period of 650,000 years.
How many people, I wonder, have looked at that graph and thought, as I did, “Yep, I’ll buy that — the more CO2 there was in the atmosphere, the warmer it was”? We “know” that because — clever-clogs, all genned up on climate change — we know that CO2 is the villain of the piece. But, for a very long time, it wasn’t. I love that quote from Mark Twain that Gore uses: It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.
The key here is that the stately rhythms of those succeeding ice-ages were part of a closed system. For hundreds of thousands of years the default state of planet Earth was what we call an ice-age. Every 100,000 years or so the combined effect of the three Milankovitch cycles provided just enough extra energy to pull the Earth out of the ice for a brief interglacial period of 10-15,000 years, after which the temperature dropped back to “normal”. Energy was a variable; but the quantity of “negotiable” CO2 was constant. Gases dissolve more easily in cold water, so at the beginning of each Milankovitch cycle, as the water warmed, the ocean freed up CO2. And then took it back as the temperature dropped. Thus atmospheric CO2 varied between 200 and 300ppm but never went any higher.
Then we came along; and by injecting vast quantities of “new” CO2 into the atmosphere, very quickly, we knocked the whole system out of kilter. The greenhouse effect kicked in and the temperature started rising when it should have been sliding down to a nice comfortable ice-age. In other words, atmospheric CO2 now drives temperature — and it looks set to drive it a long way.
The IPCC are not in the business of dreaming up marketing slogans, but by failing to point out that Homo sapiens has brought about such a fundamental shift, they’ve missed out on a wonderful opportunity to communicate. In its plodding, pedantic way, AR5 issues a stern warning:
Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.
But that, frankly, is unlikely to make anyone sit up and take notice.
Atmospheric CO2 is currently at 400ppm. The IPCC’s best case scenario envisages a levelling off at 450… but don’t hold your breath. The worst case scenario shoots up to 1,200 by the end of this century. As far as I can make out, that’s where we’re heading if we insist on business as usual.
For present purposes the numbers don’t matter. The important thing is to realise what we’ve done: in just 250 years, we’ve managed to reverse the basic dynamic of a system which had been ticking over nicely for the best part of a million years. We have brought about the end of an epoch on a geological scale.
Now, contrary to what I said at the beginning of this article, knowing this doesn’t change the numbers. But as a lever to understanding it sure as hell might move something.