Perhaps the most pervasive meme in the climate crank blogosphere is that the Earth hasn’t warmed for the last 10 years (or since 1998). You’ve not doubt heard this many times, or variants thereof (e.g. that the world has cooled since 2002, etc.). Flourishes on this theme include claims that the last century of global warming was wiped out in January 2008, or that we are in dire risk of plunging into a new ice age. There has been more refutations of this silly notion than I could possibly cite, but some good ones can be found here, here, here and here. I’ve even devoted a whole lecture to it in my Climate Change Q&A series and written a brief about it for AusSMC.
Despite these many careful and logical explanations as to why this meme is fatally flawed, it persists, and indeed remains a favourite recycled talking point among the sceptical elements of the mass media (I guess because it something so simple to throw out there, and yet requires some science or stats to show why it is unscientific tosh). But say we, being generous folks or simply for the sake of argument, decide to give people like Bob Carter and Andrew Bolt the benefit of the doubt and accept that they really do believe that the Earth’s air temperatures haven’t warmed for a decade (or so). What would this mean for global warming?
Well, not a lot, as it turns out.
The exponentially increasing activities of modern civilisation is causing a build-up in the atmosphere of long-lived greenhouse gases, such as carbon dioxide produced by industrial emissions from coal and oil burning, methane belched out from livestock, and nitrous oxide emitted from soils after fertiliser application. (This simple fact is disputed only by the most weird fringe of denialism). Furthermore, fundamental atmospheric physics tells us that this will cause a planetary energy imbalance, which can only be ‘corrected’ (brought back into radiative equilibrium) by a raising of the global temperature.
Now, following this expectation, air temperatures have risen by about 0.5C over the last few decades. But that is not where the real action is. You see, most of the extra solar energy trapped by the Earth’s slightly thicker blanket of greenhouse gases has not gone into raising air temperatures. It’s poured into the vast oceans (which contain about hundreds of times the volume of the atmosphere), and been ‘used up’ in causing the phase change required to turn polar and mountain ice into water. This has lead to rising sea levels from thermal expansion of the water as it gains heat, as well as contributions from melting glaciers and mountain ice caps, sea ice albedo changes, and mass loss from major ice sheets (Greenland and West Antarctica).
Indeed, it has been shown that about 90% of this additional energy has be used to heat water and about 7% to melt ice. Only about 3% is left over to warm the air. So we shouldn’t be at all surprised if air temperatures show the weakest response to the enhanced greenhouse effect – at least in the short term.
Fred Pearce explains it very nicely in a New Scientist article:
Water stores an immense amount of heat compared with air. It takes more than 1000 times as much energy to heat a cubic metre of water by 1 degree Centigrade as it does the same volume of air. Since the 1960s, over 90% of the excess heat due to higher greenhouse gas levels has gone into the oceans, and just 3% into warming the atmosphere (see figure 5.4 in the IPCC report (PDF)).
Globally, this means that if the oceans soak up a bit more heat energy than normal, surface air temperatures can fall even though the total heat content of the planet is rising. Conversely, if the oceans soak up less heat than usual, surface temperatures will rise rapidly.
This is why surface temperatures do not necessarily rise steadily year after year, even though the planet as a whole is heating up a bit more every year. Most of the year-to-year variability in surface temperatures is due to heat sloshing back and forth between the oceans and atmosphere, rather than to the planet as a whole gaining or losing heat.
The record warmth of 1998 was not due to a sudden spurt in global warming but to a very strong El Niño (see figure, right). In normal years, trade winds keep hot water piled up on the western side of the tropical Pacific.
During an El Niño, the winds weaken and the hot water spreads out across the Pacific in a shallow layer, which increases heat transfer to the atmosphere. (During a La Niña, by contrast, as occurred during the early part of 2008, the process is reversed and upwelling cold water in the eastern Pacific soaks up heat from the atmosphere.)
A temporary fall in the heat content of the oceans at this time may have been due to the extra strong El Niño.
Since 1999, however, the heat content of the oceans has steadily increased again (despite claims to the contrary). Global warming has certainly not stopped, even if average surface temperatures really have fallen slightly as the Hadley figures suggest.
In the long term, some of the heat being soaked up by the oceans will inevitably spill back into the atmosphere, raising surface temperatures. Warmer oceans also mean rising sea levels, due to both thermal expansion and the melting of the floating ice shelves that slow down glaciers sliding off land into the sea. The West Antarctic Ice Sheet, which rests on the seabed rather than on land, is also highly vulnerable to rising sea temperatures.
So, next time a climate sceptic turns to you and says ‘Global warming is nonsense ’cause the Earth hasn’t warmed in the last 10 years’, you can simply reply ‘Errr – why are you ignoring 97% of the problem?’.