Researchers led by Shang-Ping Xie, a climate scientist at Scripps Institution of Oceanography, University of California San Diego, find that climate responds differently to natural variability than it does to human-caused global warming and that this distinction needs to be addressed by scientists who evaluate the “hiatus” in the rise of global surface temperatures observed since 1998.
Xie and colleagues reported in the Nov. 9 issue of the journal Nature Geoscience that though the deep oceans continue to warm, ocean heat uptake should be expected to slow down rather than accelerate during the hiatus, a contrast to widely held assumptions about the nature of the hiatus. Xie, whose research is supported by the National Science Foundation and NOAA, said that his prediction is supported by observations made by satellites and climate model simulations.
The increase in greenhouse gases caused by burning fossil fuels traps heat in the climate system. The earth warms in response, radiating part of the energy surplus back to space. The rate at which this mechanism works is called a climate feedback parameter, which determines how fast the earth warms in response to greenhouse gas increase. Previous theories assume that the same climate feedback works for both anthropogenic warming and natural variability, but the new study shows that the feedback differs fundamentally between two types of climate anomalies.
“State-of-the-art climate models make the distinction,” Xie said. “Our results call for long-term observations to estimate climate feedback parameters, at least a few decades long, to average out natural cycles.”
The researchers called for more direct measurements of deep-ocean temperatures to validate the prediction. Such measurements have only been available for the top 2,000 meters (6,500 feet) of the oceans from the international Argo network, an array of floats spread through the world oceans, for a decade. A new generation of so-called Deep Argo floats that can measure ocean “vital signs” at even greater depths have only recently been deployed.
The hiatus refers to the slowing trend of surface air temperatures since 1998, the year of the last major global El Niño event, which spiked global mean temperatures. The trend is still rising enough to set new global warming records but has slowed considerably compared to previous decades of the 1970s-1990s, even as greenhouse gas emissions from human activities have continued to rise during recent years. Scientists have come up with various interpretations of the reasons behind the hiatus in recent years. A common explanation, initiated by Xie in a 2013 paper in Nature, is that cooling of the tropical Pacific Ocean is the prime force behind the tapering-off of temperatures.
Natural variability comes with different timescales. The slow tropical Pacific cooling that causes the hiatus since 1998 is part of the Interdecadal Pacific Oscillation (IPO) while El Niño and La Niña are the fast cycle that repeats every few years.
“A super El Niño is growing in the tropical Pacific and will set new records for global mean temperature for 2015 and 2016,” Xie said. “The decadal outlook is harder because historically the slow IPO behaves differently from fast-changing El Niños.”
Because of natural cycles of various timescales, global temperature rises in a staircase-like manner instead of shooting up straight, said Xie.
“With the current hiatus we are on a flat step of the staircase, but the greenhouse gas increase drives the staircase going upward,” added Xie, the first Roger Revelle Chair in Environmental Science at Scripps. “Our new results show that we can tell apart the greenhouse warming from natural cycles if we have sufficiently accurate measurements of the Earth’s energy budget.”
Paper co-authors are Yu Kosaka of University of Tokyo and Yuko Okumura of University of Texas at Austin.