Scientists have analysed millions of readings from across Earth’s oceans between 2000 and 2015, finding that sea surface temperature is rising at 1.17C per century compared with the 1.34C per century of previous estimates.
The difference is tiny in everyday terms but is important because the oceans are so large that even a warming by a tenth of a degree represents a big increase in the energy they store — and the potential impact on climate.
It is also politically potent, especially in America where an increasingly climate-sceptic Republican party will see it as confirmation of a suspected slowdown in global warming and evidence that previous warnings were exaggerated.
“The reduced warming emerging in the latest analysis is due to several separate factors,” said Professor Peter Thorne, chairman of the International Surface Temperature Initiative and co-author of the latest research. “This includes corrections to historic data collected from ships and the inclusion of new data about areas covered by ice.”
The research was led by Boyin Huang of the National Oceanographic and Atmospheric Administration (NOAA), America’s leading climate research agency. The paper has been discussed at conferences and is under peer review.
In any other field, such detailed findings might matter only to scientists. In America, however, the election of Donald Trump, who has described global warming as “a hoax”, has made climate science a political battleground.
The Republican-dominated congressional committee on science, space and technology has taken a keen interest in the “climate slowdown” between 1998 and 2012, when the rise in global temperatures appeared to fall from 0.12C a decade to 0.07C.
Its chairman, Lamar Smith, used this to claim that climate scientists had “greatly overestimated” global warming and was infuriated when, in June 2015, the NOAA published research suggesting that the apparent slowdown was down to data glitches — and the world had warmed as fast as ever.
This was controversial with scientists too — even Britain’s Met Office disagreed. Tom Karl, author of the NOAA paper, has been under attack ever since. Last week the pressure increased when John Bates, a former colleague of Karl’s who was a data manager at the NOAA until his retirement, wrote a widely reported blog post saying Karl “had his thumb on the scale . . . in an effort to discredit the notion of a global warming hiatus”.
Huang and Thorne’s research suggests the reality is more complex. “There was a bit of a slowdown but it was smaller than we thought and explained largely by natural variability,” said Thorne. “The underlying trend for the world to get warmer is still strongly present in our research.”
Peter Stott, head of climate monitoring and attribution at the Met Office climate centre, said: “The slowdown hasn’t gone away . . . However, our confidence in a warming world doesn’t just depend on surface temperatures. It is seen in a wealth of indicators, including melting snow and ice, and rising sea levels.”
The acceptance by the NOAA that there was even a small “climate slowdown” may please sceptics. However, Bates is unlikely to be among them. He told The Sunday Times: “I do not believe Tom Karl was cheating and did not mean to imply he was. I believe the evidence supports climate change, and do believe the world is warming — and this could be a threat. The details are the difficulty.”
The changes made by Karl et al. were influential but small and temporary and are turning out to be irrelevant anyway because of what the data is actually doing. Should anyone still think that the higher trends that include the years 2015 and 2016 are due to long-term global warming take a look at HadCRUT4, Fig 3, where you can see the deviation caused by the recent strong El Nino. One can see the decline to 2014 temperatures and the return of the “pause.”