Is the world still getting warmer? If so, how fast? Has there been a global warming ‘pause’ or ‘hiatus’ or not? Or has the recent warming rate been as fast, or even faster, than that measured in the 1990s?
These are fundamental questions. They depend not on the complex computer models on which scientists base their projections of the future, but on simple measurement, on readings from thermometers sited in thousands of locations across the world, on land, on buoys in the oceans and in balloons and satellites.
Yet their answers are far from simple, and like many other areas of critical importance in climate science, subject to uncertainty and fierce debate – not that you would know this if you relied on almost all media reports.
Last month, the academic journal Science, the prestigious monthly organ published by the American Association for the Advancement of Science, published the first of two recent contributions to this discussion. The work of a team led by Thomas Karl from the National Oceanic and Atmospheric Administration (NOAA), it claimed, in the words of the headline on NOAA’s press release: ‘Data show no recent slowdown in global warming.’
This was a biggish deal. The hiatus, which until now scientists have assumed to have existed since around 1998, has been a significant weapon in the armoury of climate change sceptics, because it was not predicted by the vaunted computer models. The level of carbon dioxide in the atmosphere has continued to rise unabated, but average world temperatures have apparently either ceased to rise, or (depending on which of the several sets of temperature data one chooses), risen much more slowly than they did before.
A look back at the 2007 Fourth Assessment Report of the UN’s Intergovernmental Panel on Climate Change illustrates the point. It stated that in the near term, temperatures could be expected to rise by 0.2C per decade. The Fifth Assessment Report six years later admitted this has not been happening: since 1998, it revealed, the increase, at 0.05C per decade, has only been a quarter as great.
However, the accepted margin of error for aggregating readings from all those thermometers is about 0.1C. In other words, the measured rise lately has been only half as big as the error range. Balloon and satellite measurements from the upper atmosphere show no increase at all. Hence one can reasonably argue, as climate sceptics do, that there has been no statistically significant rising trend for well over a decade. Even the IPCC thought this important: the Fifth Report devoted many pages to the hiatus, and to the competing explanations as to why it may have occurred.
Except that NOAA now says it didn’t. According to Karl et al, the pause was merely an ‘artefact of the data,’ which disappeared once they made certain technical changes, mainly to the way they measured and ‘adjusted’ readings from the seas. These changes have the effect of revising the values from earlier years downwards, and increasing them since 2008.
Bingo! ‘The rate of global warming during the last 15 years has been as fast as or faster than that seen during the latter half of the 20th Century,’ says NOAA. ‘The study refutes the notion that there has been a slowdown or “hiatus” in the rate of global warming in recent years.’
Predictably enough, the new paper was widely and enthusiastically reported by media around the world –shorn of the several caveats that Karl et al included with their work. At last, here was proof that the sceptics and ‘deniers’ were wrong. The BBC website set the tone with its headline: ‘US Scientists: Global Warming Pause No Longer Valid.’ According to the Guardian’s John Abraham, the Karl paper should ‘end the discussion of the so-called pause, which never existed in the first place.’
Oddly enough, in a field where one is told that the science is ‘settled,’ there has been disagreement from several eminent scientists. Dr Ed Hawkins, a principal research fellow at Reading University, who no one could ever call a sceptic, wrote in his blog that even if one accepts NOAA’s data revisions, ‘there has clearly been a slowdown in the rate of warming when compared to other periods’.
Another one not buying the Karl paper’s message was Prof. Judith Curry of Georgia Tech. ‘Uncertainties in global surface temperature anomalies are substantially understated,’ she wrote. ‘This short paper is not adequate to explain the very large changes that have been made to the NOAA data set… while I’m sure this latest analysis from NOAA will be regarded as politically useful for the Obama administration [which is currently bent on using executive action to set unilateral emissions limits against the will of Congress], I don’t regard it as a particularly useful contribution to our scientific understanding of what is going on.’
I can reveal that the US House of Representatives science committee, led by the Texas Republican Lamar Smith, also has doubts. At the end of last month, committee staff sent emails to several experts in Britain, saying Mr Smith ‘is making climate change data within NOAA a priority’. The committee, they added, was seeking outside help to ‘analyse’ NOAA’s claims – apparently, it would seem, because some members do not trust NOAA’s ‘input’ alone.
A committee aide told me: ‘NOAA released a conclusion it claims is based on scientific analysis. It has provided the Committee with documents to show their methodology and we’re seeking to confirm that their conclusions are accurate.’
Among those submitting evidence that challenges NOAA’s assertion is the UK sceptic think-tank, which is chaired by Lord Lawson, the Global Warming Policy Foundation. In a study published today by the GWPF, Dr David Whitehouse has analysed the raw temperature measurements behind the NOAA paper’s claim.
His key finding is that the difference between the old and revised data is ‘much smaller’ than the margin of error which NOAA admits affects all its temperature readings.
Moreover, Dr Whitehouse says, NOAA exaggerated its supposed recent warming trend by cherry-picking its start and end dates, choosing 2000, an unusually cold year, as its starting point, and 2014, a very warm one, as its end. Therefore, he writes, there is ‘no robust evidence that the hiatus does not exist’.
The graph shown here, produced by Dr Whitehouse, shows how wide those error margins are. Each data point is a revised NOAA world average annual temperature – with the error bars added. (The temperatures shown are measured in thousandths of a degree above 14C.)
The same GWPF pamphlet contains an analysis by Gordon Hughes, Professor of Economics at Edinburgh. He finds that climate scientists have for years been ignoring statistical techniques designed to weed out random ‘noise’. These techniques have long been accepted as standard tools by researchers in fields such as econometrics and epidemiology, and Karl et al’s failure to deploy them means ‘the paper does not provide well-founded statistical evidence to draw any reliable conclusions about the rate at which global temperatures have been increasing’.
However, the most striking challenge to the paper comes from unexpected source – the July issue of the same journal that published Karl, Science. A new paper by Dr Veronica Nieves of the California Institute of Technology finds that the pause is real after all. Crucially, Nieves used NOAA’s own data – but drew very different conclusions. Needless to say, this paper has received no mainstream media publicity at all. The article you are reading now is its first mention outside the specialist literature and a handful of climate blogs. But in Prof Curry’s view, it shows ‘the hiatus lives’.