Gayathri Vaidyanathan, E&E reporter
Published: Friday, June 5, 2015
The global warming “pause” does not exist, according to scientists at the National Oceanic and Atmospheric Administration.
Their finding refutes a theory that has dominated climate science in recent years. The Intergovernmental Panel on Climate Change (IPCC) in 2013 found that global temperatures in recent years have not risen as quickly as they did in the 20th century. That launched an academic hunt for the missing heat in the oceans, volcanoes and solar rays. Meanwhile, climate deniers triumphantly crowed that global warming has paused or gone on a “hiatus.”
But it now appears that the pause never was. NOAA scientists have fixed some small errors in global temperature data and found that temperatures over the past 15 years have been rising at a rate comparable to warming over the 20th century. The study was published yesterday inScience.
That a minor change to the analysis can switch the outcome from a hiatus to increased warming shows “how fragile a concept it [the hiatus] was in the first place,” said Gavin Schmidt, director of the NASA Goddard Institute for Space Studies, who was unaffiliated with the study.
According to the NOAA study, the world has warmed since 1998 by 0.11 degree Celsius per decade. Scientists had previously calculated that the trend was about half that.
The new rate is equal to the rate of warming seen between 1951 and 1999.
There has been no slowdown in the rate of global warming, said Thomas Karl, director of NOAA’s National Centers for Environmental Information and lead author of the study.
“Global warming is firmly entrenched on our planet, and it continues to progress and is likely to continue to do so in the future unless emissions of greenhouse gases are substantially altered,” he said.
Errors from weather stations, buoys and buckets
That NOAA has to adjust temperature readings is not unusual. Many factors can affect raw temperature measurements, according to a study by Karl in 1988.
For instance, a weather station may be situated beneath a tree, which would bias temperatures low. Measurements made near a parking lot would read warm due to the waves of heat emanating from asphalt surfaces. NOAA and other agencies adjust the raw temperature data to remove such biases.
It has become clear in recent years that some biases still persist in the data, particularly of ocean temperatures. The culprit: buckets.
Ships traverse the world, and, occasionally, workers onboard dip a bucket over the hull and bring up water that they measure using a thermometer. The method is old school and error prone — water in a bucket is usually cooler than the ocean.
For a long time, scientists had assumed that most ships no longer use buckets and instead measure water siphoned from the ocean to cool ship engines. The latter method is more robust. But data released last year showed otherwise and compelled NOAA to correct for this bias.
A second correction involved sensor-laden buoys interspersed across the oceans whose temperature readings are biased low. Karl and his colleagues corrected for this issue, as well.
The corrections “made a significant impact,” Karl said. “They added about 0.06 degrees C per decade additional warming since 2000.”
The ‘slowdown hasn’t gone away’
What that means for the global warming hiatus depends on whom you ask. The warming trend over the past 15 years is comparable to the trend between 1950 and 1998 (a 48-year stretch), which led Karl to say that global warming never slowed.
Other scientists were not fully convinced. For a truly apples-to-apples comparison, the past 15 years should be compared with other 15-year stretches, said Peter Stott, head of the climate monitoring and attribution team at the U.K. Met Office.
For instance, the globe warmed more slowly in the past 15 years than between 1983 and 1998 (the previous 15-year stretch), even with NOAA’s new data corrections, Stott said.
“The slowdown hasn’t gone away,” he said in an email. “While the Earth continues to accumulate energy as a result of increasing man-made greenhouse gas emissions … global temperatures have not increased smoothly.”
The disagreements arise because assigning trends — including the trend of a “hiatus” — to global warming depends on the time frame of reference.
“Trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends,” the IPCC stated in 2013, even as it discussed the pause.
Robert Kaufmann, an environment professor at Boston University who was unaffiliated with the study, called trends a “red herring.”
A trend implies that the planet will warm, decade after decade, at a steady clip. There is no reason why that should be the case, Kaufmann said. Many factors — human emissions of warming and cooling gases, natural variability, and external factors such as the sun — feed into Earth’s climate. The relative contributions of each factor can vary by year, decade, century or on even larger time scales.
“There is no scientific basis to assume that the climate is going to warm at the same rate year after year, decade after decade,” he said.
Copying the language of skeptics
Trends are a powerful weapon in the hands of climate deniers. As early as 2006, deniers used the slowdown of warming from 1998 onward to say that global warming had stopped or paused.
The idea of a “pause” seeped into academia, launching dozens of studies into what might have caused it. But there was a subtle difference between scientists’ understanding of the pause and that of the skeptics; scientists never believed that warming had stopped, only that it had slowed compared with the rapidly warming ’90s. They wanted to know why.
Over the years, scientists have unraveled the contributions of volcanoes to global cooling, the increased uptake of heat by the Pacific Ocean, the cooling role of La Niñas and other drivers of natural variability. Their understanding of our planet’s climate evolved rapidly.
As scientists wrote up their findings, they unwittingly adopted the skeptics’ language of the “pause,” said Stephan Lewandowsky, a psychologist at the University of Bristol who was unaffiliated with the NOAA study. That was problematic.
“That’s sort of a subtle semantic thing, but it is really important because it suggests that these [scientists] bought into the existence of the hiatus,” he said.
Then, in 2013, the IPCC wrote about the pause. The German government complained that the term implies that warming had stopped, which is inaccurate. The objection was ignored.
NOAA’s strong refutation of the hiatus is particularly weighty because it comes from a government lab, and the work was headed by Karl, a pioneer of temperature reanalysis studies.
NOAA will be using the data corrections to assess global temperatures from July onward, Karl said. NASA is discussing internally whether to apply the fixes suggested in the study, according to Schmidt of NASA.
The study was greeted by Democrats in Congress as proof that climate change is real. Sen. Barbara Boxer (D-Calif.), ranking member of the Environment and Public Works Committee, used it as an opportunity to chide her opponents.
“Climate change deniers in Congress need to stop ignoring the fact that the planet may be warming at an even faster rate than previously observed, and we must take action now to reduce dangerous carbon pollution,” she said in a statement.