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Study: Global Warming Hiatus Attributed to Redistribution

Courtesy of NOAA

Rather than experiencing a “global warming hiatus” from 1998 to 2013, scientists delineate in a new journal article a more complex picture of Earth’s temperature. They support redefining the hiatus as a redistribution of excess heat, primarily through the oceans.   

Researchers from NCEI, NASA, the National Center for Atmospheric Research, and major academic institutions suggest the term hiatus could be a mischaracterization of the period from 1998 to 2013 despite the term’s adoption by the scientific community. Their work was published November 22, 2016, in the new open access American Geophysical Union journal Earth’s Future.

Hiatus implies a pause in warming conditions, when in fact, the heat energy likely went elsewhere, says co-leading author Tim Boyer, an NCEI scientist. The article “Global Warming Hiatus: Slowdown or Redistribution?” supports the idea that rather than a hiatus a redistribution has occurred. The global warming hiatus was defined as a slowdown in the increase of Global Mean Surface Temperature (GMST).

“While this is a very important measure because it is what we experience as inhabitants of the Earth's surface, it is not a full measure of heat accumulation in the Earth's system,” Boyer says. “More than 90% of excess heat in the Earth's system enters the ocean, where it is moved away from the surface.”

Hiatus Study Identifies Key Points

The research team emphasizes three key points in the article:

  • From 1998 to 2013, the rate of global mean surface warming slowed.
  • Natural, decadal variability plays a crucial role in the rate of global surface warming.
  • Improved understanding of ocean distribution and redistribution of heat, including better systems for collecting deep-ocean temperatures, will help researchers better understand and monitor the mechanisms which lead to a slowdown in the increase of GMST.
Graph of yearly ocean and atmosphere heat content anomalies
Yearly ocean (black) and atmosphere (blue) heat content anomaly from ​IPCC AR5 report (Rhein et al. 2013). Ocean heat content combines upper and deep ocean heat content values for each year from the report. Microwave Sounding Unit (MSU) temperatures were used instead of GMST. MSU temperatures cover the troposphere and stratosphere, indicating change over a larger portion of the atmosphere.

The paper dovetails an article last year in the journal Science that also questioned the veracity of the hiatus, contending that limitations in past data collections hampered scientists’ understanding of the true forces at work. The authors, led by NOAA/NCEI’s Thomas Karl, concluded that the rate of warming in the early 21st century had not stopped but had been as fast or faster.

The latest article continues to articulate the complex nature of calculating the status of Earth’s temperature. Scientists derive global temperature from many types of data about solar energy—what happens to it, how much is stored, when, and where.

According to the new paper, several uncertainties and knowledge gaps exist that challenge an understanding of the dynamics affecting the planet. The article reviews those issues, positing insights about what scientists do and do not know.

Gaining a Clearer Picture of Heat Trends

The authors recommend that the slower trend in GMST for the 15-year span beginning in 1998 should be more critically and thoroughly studied as a redistribution. Targeted temperature data, particularly from the deep oceans, could tell scientists the extent of redistribution.

“The amount of heat absent from the atmosphere, which caused the global warming ‘hiatus,’ was extremely small compared to the amount of heat that was added to the ocean in the same time period,” says Boyer.

In fact, according to Boyer, the amount of heat not tallied due to insufficient observations in some ocean areas was probably larger than the amount of heat apparently lost in calculations from the global mean surface temperature.

More data from the oceans will help, but collecting broader measurements is not easy. This is difficult because better indicators require instrumentation to be deployed to the remotest regions of the ocean, under ice, and to greater ocean depths.

Because the global mean surface temperature is a surface temperature calculation, the authors recommend the most appropriate counter-indicator is the ocean heat content measurement. However, the ocean heat record does not go back nearly as far historically as the surface temperature record. This complicates analyzing long-term trends.

Studying Warming Rates in the Future

Currently, 3,500 drifting floats in the ocean collect temperature data through the Argo Program, but the fleet of instruments has its limitations. The authors advocate advancements in Argo. Temperatures from the upper tier of the ocean can be collected, but gathering crucial deep-ocean temperatures below 2,000 meters still poses a challenge. A recent Deep Argo program is promising.

Peer-reviewed studies operate under the consensus that warming continues, indicative of the overall data. Short-term fluctuations come into play, from seasonal and interannual to decadal variations. Regional conditions and weather patterns such as El Niño can affect research results as well. Understanding these variables and acknowledging uncertainties are broached in the article. The authors conclude that the scientific community would be well served to vigorously expand and strengthen their scientific assessments.