Probably the most frustrating argument on climate science in the public discourse is about the hiatus in surface temperature rise, and the failure of the models to predict it. The persistent hyperbole, the seeming logic validated largely by a disregard for some basic laws of physics, is merely a prelude to the frenzy that the next IPCC report (AR5) is likely to fuel. There are trying times ahead.
To better gird ourselves before the onslaught, it seems like a good time to review another important report, which speaks very clearly to the current arguments about the rate of global warming and the accuracy of the climate models. Here’s a taster from the foreword:
“If carbon dioxide continues to increase, the study group finds no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible. The conclusions of prior studies have been generally reaffirmed. However, the study group points out that the ocean, the great and ponderous flywheel of the global climate system, may be expected to slow the course of observable climatic change. A wait-and-see policy may mean waiting until it is too late”.
Sage words then. Anyway, delving into the detail, we find a clear statement about likely temperature increases for a doubling of CO2 – or climate sensitivity – based on global circulation models (GCMs):
“When it is assumed that the CO2 content of the atmosphere is doubled and statistical thermal equilibrium is achieved, the more realistic of the modelling efforts predict a global surface warming of between 2°C and 3.5°C, with greater increases at high latitudes”.
And then we come to the part so relevant to topical argument:
“One of the major uncertainties has to do with the transfer of the increased heat into the oceans. It is well known that the oceans are a thermal regulator, warming the air in winter and cooling it in summer. The standard assumption has been that, while heat is transferred rapidly into a relatively thin, well- mixed surface layer of the ocean (averaging about 70 m in depth), the transfer into the deeper waters is so slow that the atmospheric temperature reaches effective equilibrium with the mixed layer in a decade or so…It seems to us quite possible that the capacity of the deeper oceans to absorb heat has been seriously underestimated, especially that of the intermediate waters of the subtropical gyres lying below the mixed layer and above the main thermocline. If this is so, warming will proceed at a slower rate until these intermediate waters are brought to a temperature at which they can no longer absorb heat.
“Our estimates of the rates of vertical exchange of mass between the mixed and intermediate layers and the volumes of water involved give a delay of the order of decades in the time at which thermal equilibrium will be reached. This delay implies that the actual warming at any given time will be appreciably less than that calculated on the assumption that thermal equilibrium is reached quickly. One consequence may be that perceptible temperature changes may not become apparent nearly so soon as has been anticipated. We may not be given a warning until the CO2 loading is such that an appreciable climate change is inevitable. The equilibrium warming will eventually occur; it will merely have been postponed”.
The capacity for taking up heat is also discussed:
“…the upper-thermocline reservoir communicates effectively with the mixed layer on time scales of several decades. Therefore, the effective thermal capacity of the ocean for absorbing heat on these time scales is nearly an order of magnitude greater than that of the mixed layer alone”.
Damn, that sounds like a lot of heat storage to me. A lot of energy down there, idling around with nothing to do. Yet.
Anyway, it’s a good report addressing topical issues clearly. What it doesn’t address is why the oceans might have started taking up more heat recently, and there’s a good reason for that.
It was published in 1979.
It’s the Charney Report; “Carbon Dioxide and Climate: A Scientific Assessment”, drawn up by a National Research Council study group led by Jule Charney, at the behest of the US National Academy of Sciences. They also took advice from experts in the field, James Hansen and Richard Lindzen among them.
It is clear that even back then, climate science was aware of the role the oceans could play in the suppression of surface temperatures. Reading the report is fascinating, not least because these guys made quite a few educated guesses that have proved remarkably robust. Of course, the models they referred to were very crude, and temperature estimates were agreed more by committee than science, but the take-home points must not be lost: nearly a decade before the foundation of the IPCC, scientists were warning us very clearly of the potential danger in which we were placing ourselves. And they understood that the oceans could disguise the warming – which makes a mockery of claims that the hiatus was not anticipated.
If the potential role of the oceans was so clear, what then of climate sensitivity? Right now, in anticipation of AR5, there’s a lot of fevered speculation about the IPCC reducing the lower bounds of the range of temperature increase expected for a doubling of CO2. (This bonfire of the vanities fanned by several recent papers saying much the same thing. Haven’t these people heard of hubris?)
This argument may rather spectacularly miss the point. Claiming some comprehensive conquest of the ‘alarmists’ if the sensitivity is thought to be lower is a Pyrrhic victory over an academic issue. The question that must concern us is one science can’t really answer; for every degree of warming, how much will climate change damage us and the ecosystem we depend on, and how dear will be the cost, not just in money but in lives?
We’re short of incontrovertible evidence, but all over the world people are peering at the sky and ground, the oceans and the ice, and declaring that something seems amiss. We are not yet so estranged from our atavistic connections to nature that we can’t pick up on this unease, this tension in the natural world, in each other.
It doesn’t help when we read about so many other disquieting things happening elsewhere. Any one phenomenon can be adequately dismissed by the complacent as ‘natural variation’, but focussing on one issue at a time is a form of tunnel vision. It’s so many simultaneous things; together, they paint a much more compelling, truthful and dangerous picture. And so far, we’ve only experienced 0.8 degrees C of warming. Who in his right mind would dare suggest that 1.5 degrees of warming is safe?
In the end, we don’t yet know how the ocean’s heat content has changed, but we know it has. We don’t know exactly how surface temperatures are being suppressed, but we know warming can’t have stopped. We don’t know for sure what’s causing the ice to melt so fast, although we have a pretty good idea that temperature might have something to do with it.
What we do know is that the same energy trapped and re-radiated by greenhouse gases last century is obliged to be trapped and re-radiated in this century too – and we’re still pumping out greenhouse gases like there’s no tomorrow (and that’s not something you’d want to make into a self-fulfilling prophesy).
Climate change hasn’t stopped. True to its name, it’s changed – just like Charney knew it would back when he wrote his report. He posted it in 1979, so we’ll get it eventually, right?
Graph via Kevin Tremberth at the Royal Meteorological Society website .