Still No Support for Global Warming 'Slowdown'

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A new set of studies from the British government’s Meteorological Office has addressed the claims by climate change skeptics that global warming has “stopped” or “paused” or is “slowing down.”

The claims generally rest on two assertions: That global temperatures have either been stagnant, or have increased very little, since roughly 1998; and that new studies suggest that ‘climate sensitivity’ – the amount that average temperatures are expected to increase in response to a doubling of carbon dioxide in the atmosphere – may be lower than expected.

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The Met Office’s response is a trio of reports that, in style and length, resembles a set of ‘Climate Cliff Notes,’ and which, taken in order, provide a step-by-step assessment of the skeptical claims.

The first is a 25-page equivalent of the view from 30,000 feet, outlining the many different observations of the global climate and what they tell us. It underlines that an apparent slowdown in surface temperature by itself does not show that global warming is easing to a halt, because many other observations show that it isn’t.

In fact, the reason so many scientists accept that Earth is warming is not solely down to changes in surface air temperatures. There is actually a multitude of measurements: of atmospheric concentrations of carbon dioxide and other greenhouse gases; of atmospheric water vapor; of air temperature near the ground, in the lower atmosphere, and in the stratosphere; of changes in glacier mass balance and polar sea ice; of sea level rise and, importantly, changes in the heat content of the ocean. The accumulation of evidence from all these observations points unhesitatingly to a warming planet.

As the first Met Office report states, “a wide range of climate quantities continue to show changes. For instance, we have observed a continued decline in Arctic sea ice and a rise in global sea level. These changes are consistent with our understanding of how the climate system responds to increasing atmospheric greenhouse gases.”

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The second installment in the trilogy specifically addresses possible causes for the supposed slowdown in near-surface temperature warming. One is that the total amount of heat that the planet is receiving as a result of greenhouse warming is somehow decreasing – which is demonstrably untrue. (In fact, the planet’s total heat budget is growing, and greenhouse gases are rising at an increasing rate.) So, absent an unexpectedly strong negative feedback that is counteracting the heating, the other likely cause is that the heat is, at least for now, going somewhere else. And as several recent studies have pointed out, that ‘somewhere else’ at least for now  appears to be the ocean.

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The last of the three reports asks what all this means for future projections of planetary warming. Importantly, it underlines that ‘climate sensitivity’ is a complex notion. It can mean, for example, ‘transient climate response’ (TCR), which is the increase in global mean surface temperature on reaching a doubling in atmospheric CO2; or equilibrium climate sensitivity (ECS), which is the global surface temperature should atmospheric CO2 concentrations stabilize at a doubling of their pre-industrial levels (i.e. at around 560 ppm). That might seem like semantics, but the latter tends to be higher than the former, and the calculation of each varies according to the types of feedbacks (e.g. increases in atmospheric water vapor, decreases in sea ice) that are included. (And, as climate blogger Joe Romm has noted, in practical terms both are limited because at present rates we are headed for atmospheric CO2 levels that are “way, way past 560 ppm.”)

The Met Office study concluded that it is indeed possible that, under some revisions of climate sensitivity, the upper limits of temperature increase under a doubling of atmospheric CO2 may be a little lower than previously believed. But that’s no cause for celebration; in practical terms, what that means is that “the most likely warming is reduced by only 10%, indicating that the warming that we might previously have expected by 2050 would be delayed by only a few years.”

Factor in the existence of similar “plateaus” in surface temperature increase, mitigating factors such as strong, cooling La Nina conditions for much of the last decade or so, and the fact that some changes – notably, reductions in Arctic sea ice – are anyway rapidly outpacing models, the Met Office concludes that even if surface temperature increases have slowed down recently, that “does not, in itself, materially alter the risks of substantial warming of the Earth by the end of this century.”

IMAGE: The circumpolar current swirling ice as viewed from 30,000 feet high. (Seth Resnick/Corbis)