It has been a bad week for global warming skeptics.
On top of the National Oceanic and Atmospheric Administration's announcement that 2015 will almost certainly beat 2014 as the hottest year on record (not to mention the U.K. Met Office’s projection that 2016 will smash heat records), new research out of Stanford University is trashing climate skeptics' “hiatus” theory, which claims there was a 15-year lull in rising global temperatures between 1998 and 2013.
The paper, published Thursday in the journal Climatic Change, looked at a comprehensive set of papers, statistics, and historical records to see if temperatures really stopped rising around 1998, for about 15 years—what many have dubbed the climate change “pause” or “hiatus.”
Turns out it was just a statistical error, said Noah Diffenbaugh, climate scientist at Stanford and coauthor of the study.
“Our results clearly show that, in terms of the statistics of the long-term global temperature data, there never was a hiatus, a pause or a slowdown in global warming,” Diffenbaugh said in a statement.
So, what were other scientists getting wrong?
First, the data points were misleading. In a study released in June, NOAA officials reexamined their ocean temperature points from buoy measurements. They found that for the past couple of decades, their buoys had been returning cooler temperature readings than the measurements gathered by ships in the same area. When they reworked the data, they found that the rate of warming (about 0.116 degrees Celsius per decade) was nearly identical to that of the past five decades.
To cover their bases, lead researcher Bala Rajaratnam, an assistant professor of statistics and earth system science at Stanford, said the team combined NOAA’s old and corrected measurements to see if a climate pause could be detected.
“By using both datasets, nobody can claim that we made up a new statistical technique in order to get a certain result,” Rajaratnam said in a statement.
On top of the potentially inaccurate data points from previous NOAA reports, previous climate studies relied on statistic-gathering techniques the team says weren’t able to handle the small set of data in a 15-year period.
“We saw that there was a debate in the scientific community about the global warming hiatus, and we realized that the assumptions of the classical statistical tools being used were not appropriate and thus could not give reliable answers," said Rajaratnam.
To really confirm or refute a pause in global temperatures, the team decided to use Stanford statistician Joseph Romano’s “subsampling” technique created in 1992. The team pointed to the differences between the classical technique and subsampling with a marble analogy:
Imagine placing 50 colored marbles, each one representing a particular year, into a jar. The marbles range from blue to red, signifying different average global surface temperatures.
"If you wanted to determine the likelihood of getting 15 marbles of a certain color pattern, you could repeatedly pull out 15 marbles at a time, plot their average color on a graph, and see where your original marble arrangement falls in that distribution," said Michael Tsiang, a Stanford graduate student and study coauthor. "This approach is analogous to how many climate scientists had previously approached the hiatus problem.
In contrast, the new strategy is akin to stringing the marbles together before placing them into the jar. “Stringing the marbles together preserves their relationships to one another, and that's what our subsampling technique does,” Tsiang said. “If you ignore these dependencies, you can alter the strength of your conclusions or even arrive at the opposite conclusion.”
With their technique, the team found that global warming did not slow down, stall, or pause.
“When we compared the results from our technique with those calculated using classical methods, we found that the statistical confidence obtained using our framework is 100 times stronger than what was reported by the NOAA group,” Romano said.
The temperature fluctuations we see from month to month and year to year should be compared with the stock market, Diffenbaugh says.
“What is clear from analyzing the long-term data in a rigorous statistical framework is that, even though climate varies from year-to-year and decade-to-decade, global temperature has increased in the long term, and the recent period does not stand out as being abnormal,” he said.
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