Environmentalists have been getting very excited by a paper by England et al. which claims to have unearthed the reasons for the pause. Anthony Watts covered it a few days ago.
The paper is published in Nature Climate Change:
Despite ongoing increases in atmospheric greenhouse gases, the Earth’s global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing. Here we show that a pronounced strengthening in Pacific trade winds over the past two decades—unprecedented in observations/reanalysis data and not captured by climate models—is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake. The extra uptake has come about through increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline. At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering sea surface temperature there, which drives further cooling in other regions. The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2 °C, which can account for much of the hiatus in surface warming observed since 2001. This hiatus could persist for much of the present decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate.
Now, as Carl Wunsch points out, just because it's published by Nature, doesn't mean a paper is wrong, but in this case it turns out that there are indeed one or two problems with the results. Following a Guardian article hyping the story, Nic Lewis wrote to explain what was wrong. His contribution never appeared, so I'm reproducing it here.
Sir
May I point out an units error in your article "Global warming 'pause' caused by spike in speed of trade winds" (Guardian 10 February 2014) and a fundamental flaw in the research by Matthew England et al. that it highlights? A cooling effect of between 0.1C and 0.2C equates to 0.2F to 0.4F, not 32.2F and 32.4F – it is temperature changes that are referred to, not actual temperatures. [BH note: the article has now been corrected]
Matthew England's paper claims to show that the hiatus in global surface temperature since around 2001 is due to strengthening Pacific trade winds causing increased heat uptake by the global ocean, concentrated in the top 300 m and occurring mainly in the Pacific and Indian Oceans. But his study uses model-based ocean temperature "reanalyses", not measurements. A recent study by Lyman and Johnson of the US Pacific Marine Environmental Laboratory shows, using actual measurements of sub-surface ocean temperatures (infilling data gaps using a representative mean), that ocean heat uptake has actually fallen heavily from around 2002, whether measured down to 100 m, 300 m, 700 m or 1800 m. Indeed, they show an exceptionally large 90% fall in the heat content trend for the top 300 m between the decades 1993–2002 and 2002–2011. Several other observational datasets for the more often cited top 700 m ocean heat content also show a substantial reduction in heat uptake between those periods. So, unfortunately, ocean temperature measurements completely contradict Matthew England's neat explanation for the warming hiatus.