A new multi-institutional study of the so-called global warming "hiatus" phenomenon — the possible temporary slowdown of the global mean surface temperature (GMST) trend said to have occurred from 1998 to 2013 — concludes the hiatus simply represents a redistribution of energy within the Earth system, which includes the land, atmosphere and the ocean.
In particular, the researcher's point to the prominent role played by the global ocean in absorbing the extra heat from the atmosphere by acting as a "heat sink" as an explanation for the observed decrease in GMST, which is considered a key indicator of climate change.
In a study published in Earth's Future, a journal of the American Geophysical Union, lead author Xiao-Hai Yan of the University of Delaware, along with scientists from the National Aeronautics & Space Administration's (NASA) Jet Propulsion Laboratory (JPL) in Pasadena, Cal., and several other institutions discuss new understanding of the phenomenon.
"The hiatus period gives scientists an opportunity to understand uncertainties in how climate systems are measured, as well as to fill in the gap in what scientists know," Yan said.
"NASA's examination of ocean observations has provided its own unique contribution to our knowledge of decadal climate trends and global warming," said Veronica Nieves, a researcher at JPL and the University of California-Los Angeles and co-author of the new study. "Scientists have more confidence now that Earth's ocean has continued to warm continuously through time, but the rate of global surface warming can fluctuate due to natural variations in the climate system over periods of a decade or so."
Co-author Kevin Trenberth of the National Center for Atmospheric Research added, "The hiatus in the rise of global surface temperature is over, but understanding the processes involved helps us with future predictions."
Where's the missing heat?
While Yan said it's difficult to reach complete consensus on such a complex topic, a thorough review of the literature and much discussion and debate revealed a number of key points on which these leading scientists concur:
* From 1998 to 2013, the rate of global mean surface warming, which some call the "global warming hiatus," slowed.
* Natural variability plays a large role in the rate of global mean surface warming on decadal time scales.
* Improved understanding of how the ocean distributes and redistributes heat will help the scientific community better monitor Earth's energy budget. Earth's energy budget is a complex calculation of how much energy enters the climate system from the sun and what happens to it: how much is stored by the land, ocean or atmosphere.
"To better monitor Earth's energy budget and its consequences, the ocean is most important to consider because the amount of heat it can store is extremely large when compared to the land or atmospheric capacity," Yan said.
According to the paper, "arguably, ocean heat content — from the surface to the seafloor — might be a more appropriate measure of how much our planet is warming."
Charting future research
In the near term, the researchers hope this paper will lay the foundation for future research in the global change field. To begin, they suggest the climate community replace the term "global warming hiatus" with "global surface warming slowdown" to eliminate confusion.
"This terminology more accurately describes the slowdown in global mean surface temperature rise in the late 20th century," Yan said.
The scientists also called for continued support of current and future technologies for ocean monitoring to reduce observation errors in sea surface temperature and ocean heat content. This includes maintaining Argo, the main system for monitoring ocean heat content, and the development of Deep Argo to monitor the lower half of the ocean; the use of ship-based subsurface ocean temperature monitoring programs; advancements in robotic technologies such as autonomous underwater vehicles to monitor waters adjacent to land (like islands or coastal regions), and further development of real- or near-real-time deep ocean remote sensing methods.
Variability and heat sequestration over specific regions (e.g., Pacific, Atlantic, Indian, Southern Oceans, etc.) require further investigation, the researchers concluded. However, there is broad agreement among the scientists and in the literature that the slowdown in the global mean surface temperature increase from 1998 to 2013 was due to increased uptake of heat energy by the global ocean.
This research article is open access. A PDF copy of the article can be downloaded at the following link: http://onlinelibrary.wiley.com/doi/10.1002/2016EF000417/pdf.