How hot? ...

The consequences

of missing the boat

are severe & very

long-lasting, and

very unfair for our

grandkids, and

theirs, and theirs ...

The experts are worried

Not long ago (in September 2009) a conference was held in Oxford to discuss the significance of a 21st century warming of at least 4℃. Some of the best minds in the business were there, specifically to consider what, only a short time before, had been - well, unthinkable ... that we might be facing a near certainty of this much warming, with a high probability of even more. In private, many delegates were prepared to admit to fears of 6 or 7℃ - an awful prospect which has nevertheless become realistic. [You can examine the excellent presentations complete with graphics here:

If you were listening to the chatter during the last few years, specially in the year before Copenhagen, you’d have heard lots about the “2℃ safety barrier”, which was said to be associated with a peak CO2 of 400-450ppm. In fact, a lot of official talk still relies on these numbers, just as the better known government-sponsored enquiries - Stern, Garnaut, etc did. But, as far as I know, nobody who works closely and constantly on climate science problems thinks any more that we can stop at 2℃. That’s what the Oxford meeting was for - to re-start the public & political dialogue with numbers that more closely match what we now know about the responses of Earth’s climate system to greenhouse forcing. The trouble with 2℃ is that, in a way, we’re already there; in other words, if we turned off all the emissions tomorrow, the world would still continue to warm by that much.


The reason we can say this with confidence is that it’s now possible to assess the stored heat in the ocean rather precisely - and that is what the measurements confirm. You have to appreciate that virtually ALL the excess heat acquired by the planet goes into the ocean. The entire atmospheric heat content at any one time would fit into the top 3 metres of ocean water - it follows that we don’t get any decent idea of how much heat is being accumulated from the near-surface temperature (which is what we experience directly and record on our thermometers); and the idea that the ocean is holding back another degree right now seems bizarre ... but it is so.

This way of looking at global warming - thinking of ocean heat storage instead of temperature - helps to make sense of something else that seems a bit strange ... that is, the fact that raising the global mean temperature by a few degrees can be such a big deal. It’s the redistributed heat that causes all the unwanted effects, not thermometers.

Strictly, the way to understand our planet’s predicament is to consider the planetary energy imbalance - that is, the amount of energy (heat) entering the Earth system (land, sea, air & biosphere) in excess of the quantity leaving. This can now be directly measured, thanks to remote sensing instruments and satellites.

I want to tell you now about a study that says as clearly as anything what we CAN say about the eventual surface temperature on Earth. The authors, two researchers at the UK’s Tyndall Centre, are highly respected and competent scientists - not the sort that roams around on glaciers digging holes, but theoreticians of the first rank - Kevin Anderson, the Centre’s Director, and Alice Bows, an astrophysicist who works on climate change problems.

Anderson & Bows: “Reframing the climate change challenge in light of post-2000 emission trends”

Their paper appeared in Philosophical Transactions of the Royal Society A, 2008; 366, 3863-3882 You can find it using this link:

Their argument begins like this: if you want to know how hot it’s going to be, you need to know three things, all connected in different ways:

1. the total quantity of all the greenhouse gases (in CO2 equivalent gigatonnes, or GT) humans have put into the air since the industrial era began in ernest - say around 1850. This number comes to about 1.1 trillion tonnes (excluding emissions due to deforestation, which are harder to estimate).

2. What the peak CO2 concentration will be - and how soon it will arrive;

3. How fast CO2 falls after the peak.

From what we now know, the first number can be figured out from the other two, so the authors concentrate on these. Their starting point is the actual emissions trajectory for the first decade of the century (unlike other estimates which used hypothetical rates), which has turned out to be much higher than anyone expected - around 3% annually. [PNAS June 12, 2007 vol. 104 no. 24 10288-10293]

This is an important fact about the world - the way human societies are behaving, and the way the Earth system is responding - any projection must start here. The authors then imagine several scenarios: three of them attribute dates to the peak CO2 in 2015, 2020 & 2025 respectively; then they postulate three post-peak annual reduction rates from a 2020 peak of, respectively, 3%, 5% & 7%; and an hypothesised emissions pathway from deforestation which assumes a significant global reforestation program.

They do not include the effects of aviation on the upper atmosphere; nor recent findings on carbon sinks; nor the notoriously incalculable but potentially large positive feedbacks; and they allow agricultural emissions consistent with sustaining the world food supply - so their conclusions are, if anything, conservative. It’s easy to demonstrate that the later and higher the peak is, the steeper the post-peak decline must be for any particular target. So they ask, “is there any feasible pathway from the current emissions trend to a peak CO2 of 450ppm before 2025?” The answer, in their own words: “...The only meaningful opportunity for stabilizing at 450ppm ... occurs if the highest of the IPCC’s cumulative emissions range is used and if emissions peak by 2015”. That’s 5 years away ... and remember, 450ppm gives at best an even chance of avoiding more than 2℃.

Next they ask, “What would be the peak CO2 if emissions peak in 2020, and then reduce by 3%, 5%, or 7%?”. The answer, again using some conservative assumptions, is between 550 & 650ppm - most likely closer to the higher figure.

In summary, the circumstance that just might make possible a 450ppm peak are:

• 21st century cumulative emissions of 858 GT CO2

• Peak emissions date 2015;

• Zero net emissions by 2060-2075;

• Post-peak emission reduction rates in the energy & process sectors of 6-8%.

PLUS a number of pretty optimistic assumptions about the responses of the climate system.

The authors put it like this:

“While this analysis suggests stabilizing at 450ppm is theoretically possible, in the absence of an unprecedented step change in the global economic model and the rapid deployment of successful CO2 scrubbing technologies, 450ppm is no longer a viable stabilization concentration.”

To appreciate how far we are from the resolute actions here implied, you only have to check out what’s happening to tar sands in the Canadian province of Alberta, the coal deposits in Queensland, the NIger Delta oil fields, the ports of China, and plenty of other places where it is perfectly clear that the economic system is not even close to reducing its dependence on fossil fuels. So these authors are by no means remiss concluding that 2℃ is “an inappropriate and dangerously misleading mitigation and adaptation target.

The authors point out that there has never been a planned economic contraction of anything like 6% - the only near-example we have is the old Soviet Union in the early 1990’s, when untold misery followed an involuntary economic collapse. Their stark conclusion is that we will not even make it to a peak of 650ppm, because even this “demands the majority of OECD nations begin to make draconian emission reductions within a decade.”

So what can we learn from this study? First, it is not contentious. Nothing whatsoever has been postulated here which is not conservative rather than speculative. Second, the conclusions are nothing more than an extrapolation of current trends - in other words, we are shown some of the consequences of what we are doing right now. Third, the implications of a peak CO2 well over 650ppm (some experts are prepared for 1000ppm) are staggering. We can’t begin to imagine how different a world that hot would eventually be; we can’t even say how hot because such an enormous greenhouse forcing would set off many system feedbacks we have no way of estimating. Even climate history can’t tell us because we don’t know exactly when the world had that much carbon in the air before.

Some things are clear though. We know that 650ppm would be more than enough to make the world ice-free, sooner or later. The two vulnerable ice sheets, the ones that are melting right now, would be the first to go - perhaps in a century or two or three, maybe quicker - then the great East Antarctic ice sheet would melt. No one has any idea how long that would take. Altogether, this process would make the sea rise over 70 metres. In the last year or so, we were given an important new insight into the link between CO2 and temperature in the distant past. [You can read about this study here] A new method of assessing atmospheric CO2 back to 20 million years was described and, as far as possible, validated.

The last time CO2 was in the range 400 -425 ppmv (where it will be in the next decade) was 15 million years ago, when the global mean temperature was 3-6℃ warmer, and the sea 25-40m higher.

“There was little glacial ice on land or sea ice in the Arctic, and a marine-based ice mass [the West Antarctic ice sheet] on Antarctica was not viable.”

If this study is confirmed, it means that, even if things were arrested today just as they are, Greenland and the WAIS would go and the sea would continue to rise to a total between 25 & 40 metres. It also means that the climate is more sensitive to greenhouse forcing than we’d thought - something James Hansen has suggested for some time. And it means that in the long run, we bequeath to our descendants an unrecognizable planet, centuries of unbearable destruction and loss, and no certainty at all that the civilized structures and institutions we have built so laboriously can survive.

I would not wish you to think such sobering conclusions come easily, or that I am unaffected by them. I cannot imagine any grandparent, or anyone who thinks about the future receiving this news without distress. I’m convinced the only reason we are not all of us demanding action from our leaders is that the problem is what it is - difficult, global, and utterly unlike anything we’ve had to imagine before. Surely there is no one, not the grumpiest politician nor the hardest of oilmen, nor even the most bigotted "skeptic", who wants their grandkids to suffer. It is not bad intentions but bad ideas that hold us back.

If Anderson & Bows' insight were available to everyone, surely we would find it in us to make the sacrifices and spend the effort required to manage this looming catastrophe.

Ross Gelbspan has written eloquently about the climate problem for many years.

Here is a link to a talk on his excellent website, and another to one of his essays.

Kevin Anderson published an update in a non-technical essay in 2012 here