Climate Change News

Global Warming "Hiatus" Never Happened, Study Says

After digging into existing measurements of Earth surface temperatures, a team of scientists finds there was no "hiatus" in temperature rise, which was thought to have started in 1998.

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A presumed pause in the rise of Earth’s average global surface temperature might never have happened, according to new research published this week. Instead, the apparent hiatus, first reported by the Intergovernmental Panel on Climate Change (IPCC) in 2013, resulted from a shift during the last couple of decades to greater use of buoys for measuring sea surface temperatures. Buoys tend to give cooler readings than measurements taken from ships, explained Thomas Karl, director of the National Centers for Environmental Information of the National Oceanic and Atmospheric Administration (NOAA) and lead author on the research paper. Uncorrected, those discrepancies led to the apparent slowdown since 1998 in the long-observed rise of Earth’s average surface temperature.

“The biggest takeaway is that there is no slowdown in global warming,” Karl said. Global warming over the past 15 years is the strongest it’s been since the latter half of the 20th century, he added.

Measurement Bias

To measure how Earth’s average global temperature is changing, scientists combine hundreds of thousands of measurements from Earth’s surface, taken by land instruments, ships, buoys, and orbiting satellites. Scientists must comb through these data to eliminate random errors and correct for differences in how each type of instrument measures temperature.

In a new paper published Thursday in Science, Karl and his colleagues report that they dug into NOAA’s global surface temperature analysis to examine how sea surface temperatures (SSTs) were being measured. Scientists measure SSTs in several ways—by collecting ocean water in a bucket and measuring its temperature directly, measuring the temperature of water taken in by a ship’s engine as a coolant, or using floating buoys moored at locations scattered around the planet’s oceans.

Buoy-distribution_Web
Locations of NOAA’s ocean buoys as of 1 June. Credit: NOAA

Each technique records slightly different temperatures in the same region, so scientists have to adjust the data. In the last couple of decades, the number of buoys has increased, Karl explained, adding coverage to 15% more of the ocean. Because buoys tend to read colder temperatures than do ships in the same places, Karl and his team corrected for this bias by adding 0.12°C to each buoy measurement.

By combining the ocean data with improved calculations of air temperatures over land around the world, Karl and his colleagues found that overall global surface warming during 2000–2014 was 0.116°C per decade, more than twice the estimated 0.039°C per decade, starting in 1998, that IPCC had reported. Further, having reexamined global temperatures as far back as 1880, Karl and his team found that from 1950 to 1999, the rate of warming was 0.113°C per decade.

“The [new] data still show somewhat slower warming post-2000 than in the preceding decades, but the difference is no longer statistically significant, which means it is no longer justifiable to say that there was a ‘hiatus,’” said Steven Sherwood, director of the Climate Change Research Centre at the University of South Wales, Australia. He was not involved in the study.

“The fact that such small changes to the analysis make the difference between a hiatus or not merely underlines how fragile a concept [the hiatus] was in the first place,” said Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies in New York City.

Faux Pause?

Since the IPCC report came out, scientists have been investigating what was causing the supposed hiatus. Some studies found evidence that the leveling of global temperature rise resulted from absorption of the “missing” heat by the world’s oceans.

Other research indicated that although there was no pause in global temperature rise, there might have been a temporary slowdown of warming for the Northern Hemisphere. Michael Mann, director of the Earth System Science Center at Pennsylvania State University, and his colleagues found earlier this year that natural fluctuations in the Pacific and Atlantic oceans led to cooling of the tropical Pacific, driving down average temperatures for the northern half of the globe. Mann calls this phenomenon a “faux pause.”

“The slowdown, which appears to relate to internal oscillations with an origin on the tropical Pacific, primarily impacts Northern Hemisphere mean temperature and is barely evident in global mean temperature,” Mann told Eos.

Even as perceptions of a global warming pause remained widespread, some scientists were finding evidence to the contrary. A paper in Nature Climate Change last year reported that there was “no pause” in heat extremes around the world. What’s more, NOAA concluded that 2014 was the planet’s hottest year on record.

According to the new analysis, Karl said, global average temperature rise between the years 2000 and 2014 is virtually indistinguishable from that which occurred over the latter half of the 20th century.

—JoAnna Wendel, Staff Writer

Citation: Wendel, J. (2015), Global warming “hiatus” never happened, study says, Eos, 96, doi:10.1029/2015EO031147. Published on 5 June 2015.

© 2015. The authors. CC BY-NC 3.0
  • Renellys Perez

    “Scientists measure SSTs in several ways—by
    collecting ocean water in a bucket and measuring its temperature
    directly, measuring the temperature of water taken in by a ship’s engine
    as a coolant, or using floating buoys moored at locations scattered
    around the planet’s oceans.” Scientists may have historically measured temperature by collecting ocean water in a bucket, but this statement gives the false impression that scientists do so systematically.

  • Rick L

    “Scientists measure SSTs … using floating buoys moored at locations scattered around the planet’s oceans.” Most of the buoys are drifting, not moored (all of the buoys in the “Status of Global Drifting Array” plot are drifters). They measure temperature, ocean currents, and some measure air pressure and/or salinity.

  • Thomas Holmes

    The way EOS summarizes the article makes it seem that the 0.12 bias correction on the buoy data was the only adjustment in SST record compared to previous work and that it alone explains the difference in trend from +0.04 to +0.11 C/d. In fact the article mentions 11 improvements, and the buoy correction only added 0.012 C per decade.

    From the paper: “Of the 11 improvements in ERSST version 4 (13),
    the continuation of the ship correction had the largest impact on
    trends for the 2000-2014 time period, accounting for
    0.030°C of the 0.064°C trend difference with
    version 3b. (The buoy offset correction contributed 0.014°C dec−1 to the difference, and the additional weight given to the buoys because of their greater accuracy contributed 0.012°C dec−1. See supplementary materials for details.)

  • OFBG

    Of course, a denier might ask why it was argued that buoys record lower temps – maybe ships record
    higher temps. Perhaps buoys are the “gold standard,” and concern about warming has been skewed by less-accurate shipboard measurements.

    I’m one of those who never believed that the “hiatus” occurred, but for different reasons. My frustration with climate scientists has been that so long as we stay in “raising awareness” or “countering deniers” mode,
    nothing with get done to combat what is demonstrably going on. In this regard, I believe that the work reported in “Global warming “hiatus” never happened, study says” was a waste of time, effort, and funding. You don’t have to be a “true believer” in global warming to do something about it.

    Whether you accept that global warming is occurring or not, it would be good to take steps to make the environment less toxic. As has been shown in EOS, we have in the past several decades acted to clean up the air in our cities. This required some government intervention, but cooperation with regulations was largely due to the fact that people wanted cleaner air for themselves and their children.

    We need to stop bludgeoning “non-believers” with data and start encouraging actions to reduce air pollution. Cut back on the emissions that make for dirty air, and our “carbon footprint” will also be reduced. Not to mention that as an incentive, the promise of cleaner air in a decade or so trumps the possibility of “saving the planet” over the next century.

    • john

      We have water temperature data including thermocliine levels since the WWII. Surface temperatures alone may or may not indicate a warming oceans, but increased depths of ocean thermoclines would be clear evidence of warming oceans. However I have not seen aany analysis of changing thermoclines on a world wide basis. Why not?

      • OFBG

        Not sure why you replied to my comment, john, but your question is interesting.