The break confused scientists all over the world, who have long tried to explain why it happened.
But Rajaratnam says his work restores confidence in those climate projections. Scientists have found that the recent rate of global warming is the result of faulty statistical methods rather than any kind of pause.
Climate change skeptics had hailed the supposed hiatus as a sign that global warming had ended, or that the warming was just part of natural variances.
The latest claim comes for an inter-disciplinary team of scientists at Stanford University led by Bala Rajaratnam, an assistant professor of statistics and of Earth system science.
Diffenbaugh said that he was always skeptical regarding the global warming pause, and that he found it odd that the indicators showed so heavily fluctuating data only outside of the 1998-2012 time period. The scientific community was so convinced by the findings, that discussion of the so-called hiatus was a hot topic at the 2013 climate change assessment by the Intergovernmental Panel on Climate Change (IPCC).
To really confirm or refute a pause in global temperatures, the team decided to to use Stanford statistician Joseph Romano’s “subsampling” technique created in 1992.
It turns out that the global warming “hiatus” may not have actually occurred. Numerous ocean buoys measuring sea surface temperatures during the past couple of decades were faulty and gave cooler readings than measurements from ships.
Scientists at the National Oceanic and Atmospheric Administration (NOAA) reached the same conclusion this summer when they found that the hiatus was the result of biases and errors in temperature measurements worldwide. When these measurements were corrected, the hiatus signal actually disappeared.
More importantly, the Stanford group’s technique does not rely on strong assumptions to work. This stands in stark contrast to Stanford and NOAA scientists that say the hiatus in warming never even existed.
For their study, the team adopted a different approach involving methodical examination of temperature data and examination of the statistical tools scientists were using to analyze the data. “The underlying assumptions of these analyses often weren’t justified”, Rajaratnam said.
The paper, published Thursday in in the journal Climatic Change, looked at a comprehensive set of papers, statistics and historicalrecords to see if temperatures really stopped rising around 1998, for about 15 years-what many have dubbed the climate change “pause” or “hiatus”.
2014 went on to break global surface temperatures, beating 2010’s record.
To get around this, the authors used a technique invented by Romano in 1992, called “subsampling” which discerns whether a variable, such as surface temperature or even stock prices, has changed in the short term based on a limited amount of data.
“In order to study the hiatus, we took the basic idea of subsampling and then adapted it to cope with the small sample size of the alleged hiatus period”, Romano said.
The existing statistical methods weren’t appropriate to handle the data related to the purported hiatus, which was one of the problems with previous studies, says Rajaratnam.
The Stanford group’s technique also handled temporal dependency in a more sophisticated way than in past studies.
Furthermore, the team had to consider the relationship of one temperature data point to the ones that were taken leading up to that instant.
The researchers use the example of placing 50 coloured marbles representing a year into a jar to illustrate the technique.
“Observational and model estimates further suggest [Atlantic Multidecadal Oscillation] shifts have an effect on global mean near-surface temperatures of about 0.1˚C”, the Met Office wrote in its September climate outlook.
“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”, co-author Michael Tsiang said.