The last decade was the hottest on record, and 2010 was the warmest year on record, continuing the long-term trend driven by human emissions (see above graph).
Still, in the past ten years, surface temperatures didn’t appear to warm as fast as many had expected — although the oceans kept warming rapidly, and Arctic sea ice melted faster than expected, as did the great ice sheets in Greenland and Antarctica (see here).
Now new research finds that the slowdown in the rate of surface warming is because trade winds have sped up in an unprecedented fashion, mixing more heat deeper into the oceans, while bringing cooler water up to the surface. Remember, more than 90 percent of human induced planetary warming goes into the oceans, while only 2 percent goes into the atmosphere, so small changes in ocean uptake can have huge impact on surface temperatures.
Lead author Prof. Matthew England explained in a news release:
“Scientists have long suspected that extra ocean heat uptake has slowed the rise of global average temperatures, but the mechanism behind the hiatus remained unclear…. But the heat uptake is by no means permanent: when the trade wind strength returns to normal –- as it inevitably will –- our research suggests heat will quickly accumulate in the atmosphere. So global temperatures look set to rise rapidly out of the hiatus, returning to the levels projected within as little as a decade.”
This is worrisome since the notion that there’s been a slowdown comes in large part from focusing on the HadCRUT4 data, rather than the NASA data. Since we don’t have permanent weather stations in the Arctic Ocean — the place where global warming has been the greatest — the decision by the UK’s Hadley Centre to exclude this area has meant they have lowballed recent warming. A recent reanalysis using satellite data to fill the gaps finds little slowdown in warming (see figure on right).
NASA, at least, has assumed the surface temperature in the Arctic is equal to that of the closest land-based stations. So NASA’s data is more accurate, and yet as you can see from the top chart, there has been no effective slowdown in surface temperatures, compared to the long-term trend. We have had a couple of La Niñas since 2010, and La Niñas temporarily reduce surface temps relative to the human-caused warming trend, much as El Niños increase surface temps relative to the trend.
When we see our next El Niño, which could be as soon as this summer, we can expect to see another global temperature record shortly thereafter, in 2014 or 2015. What the new study finds is that temperatures are likely to jump even more in the coming years since “the net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2°C.”