It is one of climate contrarians' most widely repeated arguments against mainstream climate science: Global warming slowed down — or even stopped — in 1998.

Even the United Nations Intergovernmental Panel on Climate Change (IPCC), which is the world's most credible mainstream climate-science authority, acknowledged in its 2013 report that warming slowed down during this period.

The IPCC said the temperature trend from 1998 to 2012 was about one-third to one-half of the warming trend during the period from 1951 to 2012.

However, a new study published Thursday in the journal Science by top climate researchers at the National Oceanic and Atmospheric Administration (NOAA) aims a kill shot at the hiatus once and for all. Led by Thomas R. Karl, who directs NOAA's National Centers for Environmental Information (NCEI) in Asheville, North Carolina, scientists say that the so-called pause in warming during that period was an artifact of improperly adjusted surface-temperature data.

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By adding thousands of surface-temperature stations (for a total of about 30,000, compared to 7,000 in older studies) to correct for biases in temperature measurements from ships and buoys, as well as including the period through 2014, which was Earth's warmest year on record, the new study says the planet has continued to warm at a rapid rate comparable to prior periods.

Figure released by NOAA showing the temperature trend from revised surface data. Image: NOAA/NCEI

"The rate of temperature increase over the last 15 years is virtually identical to what we've seen over the last half of the 20th century," Karl said during a media call on Thursday.

Using improved surface-temperature data, the new study is the first paper to assert that the hiatus is an illusion caused by "artifacts of data biases." Dozens of previous studies sought to explain why the hiatus occurred.

Mashable sought out the views of about a dozen top climate scientists not involved in the new study. Remarkably, they were nearly unanimous in saying that while it improves the accuracy of surface-temperature records, the study does not support the authors' conclusion that the so-called warming pause never happened. Instead, they said it simply proves that changing the start and end dates used for analyzing temperature trends has a big influence on those measurements, a fact that was already widely known.

To understand why the start and end dates are so crucial, it's first necessary to see the study's main results.

Study finds greater warming if you include the past two years

The study's authors made relatively small adjustments to surface-temperature data by, for example, more than doubling the number of land surface stations in their analysis, and correcting for biases caused by non-climate-related differences in temperature readings taken from the expanding network of drifting surface buoys, compared to older methods of taking temperatures from ships.

Since NOAA's temperature record dates back to the late-19th century, it contains many transitions from one temperature-sensing method to another, and each method tends to have warm or cold biases that scientists need to correct for in order to arrive at an accurate longterm temperature timeline.

For example, ocean-temperature data from thermometers located at a ship's engine intake tend to be milder than data obtained by thermometers attached to drifting buoys. The new study used an updated record of sea-surface temperatures known as the "Extended Reconstructed Sea Surface Temperature dataset version 4,"or ERSST4, which factors in more buoy data and reduces uncertainties associated with their measurements. This data shows more warming than previous iterations did.

The improved ocean data narrowed the temperature gap by nearly half between temperature trends from 1998 to 2012 compared to the 1951 to 2012 period used in the IPCC report, the study found. The difference was just 0.043 degrees Celsius, compared to 0.078 degrees Celsius, using the IPCC's older methods.

Graphic showing previous temperature trend measurements and revised calculations. Image: NOAA/NCEI

The study looked at temperature data from 1998 to 2014, rather than 1998 through 2012 as most hiatus studies had, and found that the warming trend was 0.020 degrees Celsius higher compared to the trend ending in 2012.

In addition, it found that changing the beginning and end dates by two years also eliminates the hiatus, with the temperature increase 0.030 degrees Celsius higher between 2000 and 2014 than between 1998 to 2012.

Finally, the study found that the trend from 1950 to 1999, when manmade global warming first emerged from the background noise of natural climate variability, is "virtually indistinguishable" from the warming magnitude seen between 2000 to 2014.

Pushback from other researchers

Scientists who have investigated the warming hiatus or are otherwise involved in assessing climate change on various timescales said the study's key shortcoming is that it does what mainstream climate scientists have long criticized climate contrarians — often now referred to as "climate denialists" — of doing: cherry-picking start and end dates to arrive at a particular conclusion.

Gerald Meehl, a climate researcher at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, told Mashable in an email that while he finds the new study laudable for improving temperature measurements, there are flaws in how the researchers interpreted the data. For example, Meehl said there is still a lower warming trend from 1998 to 2012, compared to the previous base period of 1950 to 1999, "thus there is still a hiatus defined in that way."

Meehl said adding two years to the time period by including 2013 and 2014, which was a record-warm year, makes the warming trend appear to be 38% larger than previous studies that did not include them.

"My conclusion is that even with the new data adjustments, there still was a nominal hiatus period that lasted until 2013 with a lower rate of global warming than the warming rate of the last 50 years of the 20th century," he said, "and a factor of two slower warming than the previous 20 years from the 1970s to 1990s."

Lisa Goddard, director of the International Research Institute for Climate and Society (IRI) at Columbia University, told Mashable that the study does not support the conclusion that global warming didn't slow down for a relatively short time period.

"It is clear that Karl et al. have put a lot of careful work into updating these global products," Goddard said in an email. "However, they go too far when they conclude that there was no decadal-scale slowdown in the rate of warming globally. This argument seems to rely on choosing the right period — such as including the recent record-breaking 2014."

Another senior climate researcher, Kevin Trenberth of NCAR, said the hiatus depends on your definition of the term. To him, global warming never stopped, as climate skeptics argue, because most of the extra heat from manmade greenhouse gases (e.g. carbon dioxide) was redirected deep into the oceans from 1998 to 2012. However, surface temperatures did warm more slowly during this time.

"I think the article does emphasize that the kind of variation is now much more within the realm of expectations from natural variability, but it is a bit misleading in trying to say there is no hiatus," he said in an email.

In response to such criticisms, Karl said even the 1998-to-2012 period that climate skeptics have long focused on looks twice as warm with the revised data set — at 0.086 degrees Celsius of warming, compared to the previously calculated rate of just 0.039 degrees Celsius.

Using the new data, the 1998-to-2014 period shows warming that is "significantly positive," Karl said, of 0.106 degrees Celsius, up from 0.059 degrees Celsius using an older data set for the same period.

In light of the new data, other researchers recommend discounting the short-term fluctuations in favor of focusing on longterm warming.

Michael Mann, director of the Earth System Science Center at Pennsylvania State University, said the study helps drive home the point that "global warming continues unabated, as we continue to burn fossil fuels and warm the planet."

"Cherry-picking a warm start date like 1998, as contrarians are fond of doing in an attempt to downplay global warming, was never scientifically defensible to start with, and this article once again confirms that," Mann said in an email.

Why focus on a 15-year period anyway?

Computer simulation of ocean currents and sea-surface temperatures across the tropical Pacific on June 4, 2015. Image: Earth Simulator

Regardless of whether the hiatus was really a hiatus, two things are clear. First, that slowdown is over anyway, given the record-warm 2014 and indications that 2015 may be a repeat of that. Second, focusing on relatively short timescales may be distracting from longterm global warming; however, it is important, since governments and businesses make decisions on shorter timescales. Decade-to-decade fluctuations in warming can affect everything from the productivity of agriculture in India to the likelihood that a U.N. climate treaty will be enacted, as a record-warm year can put pressure on politicians to act.

The IRI's Goddard, who has published extensively on the challenge of improving predictions of climate on decadal timescales, said she is puzzled as to why the new study discounts the importance of such short-term climate zigs and zags.

"All one has to do is to look at the time series to appreciate that the climate varies on all timescales, even when averaged over the whole globe. Global temperatures do NOT present a monotonic time series in which each year is warmer than the year before," she said. "I think that societally, it is important to realize that there will be periods of slowdown, as well as periods of acceleration."

Moving toward 'more sane' observing networks

One uncomfortable truth in climate science is that even at a time when we can wear computers on our wrists, we still don't have a truly global, extremely reliable network of climate-observing stations, which will prevent the need for additional data corrections in the future.

The new study reveals yet again that surface-temperature data has many flaws, according to Peter Thorne, a climate researcher at Maynooth University in Ireland. In an interview, Thorne said critics of climate science are incorrect in charging that global warming is an artifact of urban heat islands and other influences on thermometers; but at the same time, our approach to taking the Earth's temperature needs to be rethought.

Thorne said more investments should go toward establishing redundant, carefully calibrated temperature-observing networks where data is currently sparse, such as the Arctic, much of Africa and especially the oceans.

"The uncertainty in the marine records is far greater than in the land records," he said. "If we put enough good quality, traceable, redundant observations around the globe, we can make sense of all the other observations that aren’t so good."

"There is no need to bequeath onto researchers in 50 years time a similar mess."