Climate researchers have published a new paper this week in the journal Nature Climate Change that acknowledges there has been a global warming slowdown from 2000-2014. Their research shows a hiatus did indeed occur and continued into the 21st century, contradicting another study last June that said the hiatus was just an artifact that “vanishes when biases in temperature data are corrected.” This is not the first time activists have tried to hide the hiatus by using dodgy methods. –Thomas Richard, The Examiner, 24 February 2016

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An apparent slowing in the rise of global temperatures at the beginning of the twenty-first century, which is not explained by climate models, was referred to as a “hiatus” or a “pause” when first observed several years ago. Climate-change sceptics have used this as evidence that global warming has stopped. But in June last year, a study in Science claimed that the hiatus was just an artefact which vanishes when biases in temperature data are corrected. Now a prominent group of researchers is countering that claim, arguing in Nature Climate Change that even after correcting these biases the slowdown was real.

“There is this mismatch between what the climate models are producing and what the observations are showing,” says lead author John Fyfe, a climate modeller at the Canadian Centre for Climate Modelling and Analysis in Victoria, British Columbia. “We can’t ignore it.”

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Ups and downs

The debate revolves in part around statistics on temperature trends. The study1 that questioned the existence of the slowdown corrected known biases in the surface temperature record maintained by the US National Oceanic and Atmospheric Administration (NOAA), such as differences in temperature readings from ships and buoys. This effectively increased the warming recorded, and the researchers also extended the record to include 2014, which set a new record high for average temperatures.

That work, led by Thomas Karl, director of NOAA’s National Centers for Environmental Information in Asheville, North Carolina, calculated the rate of global warming between 1950 and 1999 as being 0.113 °C per decade, similar to the 0.116 °C a decade calculated for 2000–14. This, Karl said, meant that an assessment done by the influential Intergovernmental Panel on Climate Change in 20133 showing that warming had slowed was no longer valid.

Fyfe and his colleagues argue2 that Karl’s approach was biased by a period of relatively flat temperatures that extended from the 1950s into the early 1970s. Greenhouse-gas emissions were lower then, and emissions of industrial pollutants such as sulphate aerosols were cooling the planet by reflecting sunlight back into space. Fyfe says that his calculations show that the planet warmed at 0.170 °C per decade from 1972 to 2001, which is significantly higher than the warming of 0.113 °C per decade he calculates for 2000–14.

Fyfe says that the advantage of this approach is that it takes account of events that affect decadal temperature trends. For instance, researchers have found that climate models underestimated the cooling effect of volcanic eruption and overestimated the heating from solar radiation at the beginning of the twenty-first century4. Other researchers are investigating variability in the Pacific Ocean, including a measure of sea surface temperatures known as the Pacific Decadal Oscillation (PDO)5. All these things can affect the climate, and mask the longer-term warming trend.

Bumps and wiggles

Susan Solomon, a climatologist at the Massachusetts Institute of Technology in Cambridge, says that Fyfe’s framework helps to put twenty-first-century trends into perspective, and clearly indicates that the rate of warming slowed down at a time when greenhouse-gas emissions were rising dramatically.

–Jeff Tollefson, Nature, 24 February 2016

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