May 12, 2017, by Tim Radford
Findings from new research on a ‘hiatus’ support the argument for anthropogenic climate change. Image: NASA Goddard Space Flight Center via Flickr
Swiss climatologists dismiss the long-term effect of a ‘hiatus’ in the latest contribution to a long-running debate about a slowdown in global warming.
LONDON, 12 May, 2017 – Just as one group of European scientists has defined the “so-called global warming hiatus” as an illusion, a second group has brought it back to tenuous reality – this time as a problem of definition.
Last year was the hottest ever recorded: the previous records were set in 2014, and then again in 2015. The latest exercise in scientific head-scratching might seem silly after a procession of increasingly hot record temperature years. But it offers a nice demonstration of the scientific drive: committed researchers do not like unresolved questions.
If the climate is a machine, then why did it not seem to run – at least for a few years – as the manual said it should run?
The latest entrants to the hiatus argument are from the Swiss Federal Institute of Technology, known as ETH Zurich, and they define the years of global mean surface air temperatures between 1998 and 2012 as a “hiatus”, a period when the Earth “seemed hardly to warm”. They recognise that some climate sceptics then claimed that global warming “stopped in 1998”.
But the Swiss scientists don’t see it that way. They argue in Nature journal that a combination of mechanisms contributed to the hiatus.
And they conclude: “When we take these into consideration, what we are left with from the apparent hiatus is not inconsistent with the understanding of human influence on global climate. In fact, it increases the confidence in the dominant role of humans in global warming.”
But first, the story so far. In the 1980s, some scientists suggested that the greenhouse effect might already be doing what theorists had warned of: increasing planetary temperatures.
From the 1980s to 1998, global temperatures rose by an average of 0.17°C per decade, and 1998 was a year of unprecedented temperatures. Thereafter, compared with long-term averages, temperatures still continued to rise to 2012, but by a rate of 0.04°C per decade. So 1998-2012 became known as the years of a pause, a slowdown or a hiatus.
Some researchers took the news calmly: in a global climate that varied daily, seasonally, annually and on a decadal basis, there was no guarantee of an inexorably steady climb in global averages, they said.
“A few years of additional data are unlikely
to overturn the vast body of evidence that
supports anthropogenic climate change”
Others looked for a direct influence: one group reasoned that a pattern of volcanic eruptions might have cooled the atmosphere over the decade in question. Other climatologists suggested that the heat that might have been expected in the atmosphere had been transferred to the oceans.
Yet another team looked at all the scientific reports that discussed a notional slowdown, pause or hiatus, and found no widely accepted single definition of the puzzle scientists had set themselves.
Another group argued that even if the global averages had only crept up, the levels recorded during heat extremes had certainly got much higher, which nullified the idea of a slowdown.
One group decided to take a longer view, and see the so-called hiatus years in the context of the whole, and then pronounced the slowdown a statistical illusion. Yet another looked at how the data had been gathered, and where from, and calibrated it all again, and once more the hiatus became invisible: there had been no slowdown.
Increasingly, this became a consensus. The hiatus was only there if you looked at climate change in the short term.
Need for clarity
But even this conclusion wasn’t good enough for Iselin Medhaug, professor of climate physics at ETH Zurich. She and her research partners still wanted an explanation of why people even thought there was a hiatus and why one research team’s conclusion was soon contradicted by another, and then another.
They identified at least three different definitions of any so-called hiatus, different judgments of the extent of the period under discussion and different starting points, and of course the problem of different datasets of global air surface temperatures.
Add to all these the peculiar case of 1998 – a year of a powerful El Niño temperature oscillation in the Pacific that took temperatures to a level with no precedent in history – and the contradictions could be reconciled.
“Not every year will be warmer than the previous year, so depending on the climate scenario there is no reason why future trends could not be different from those in the past,” they write.
“The hiatus no doubt was, and still is, an exciting opportunity to learn for many research fields. Social sciences might find this an interesting period for studying how science interacts with the public, media and policy.
“In a time coinciding with high-level political negotiations on preventing climate change, sceptical media and politicians were using the apparent lack of warming to downplay the importance of climate change. It is easy to paint a controversial picture, but as often the devil is in the detail.
“A few years of additional data are unlikely to overturn the vast body of evidence that supports anthropogenic climate change.”
So that’s it. Climate change is real. The thermometer is rising. There may have been a hiatus, depending on your definition and timescale, but it did not and does not change the big picture of a world of greenhouse warming driven by carbon dioxide emissions in the atmosphere, as a consequence of fossil fuel combustion worldwide.
The last words from the Swiss scientists are these: “The hiatus has not changed our projections of the overall magnitude of climate change or the emission reductions that are required to address it.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.