No, global warming hasn’t “paused.” It hasn’t stopped. And it certainly hasn’t reversed. It would be wonderful if that were the case. But it’s not. Demonstrably, provably, it’s not. In fact, global warming has accelerated over the period of the alleged “pause” (http://bit.ly/1qW3B0P). When one understands the physics of the accumulating energy imbalance in the climate system on the whole, he shouldn’t expect global warming to stop (http://bit.ly/1uNO5Dj). Not with atmospheric CO2 concentration currently hovering at 400 parts per million, 43% higher than the pre-Industrial 280ppm (http://bit.ly/1qOKzJf). Not with atmospheric CO2 concentration rising 2-3ppm per year; the 2.9ppm increase in 2013 was the largest year-to-year change in three decades (http://bit.ly/YJ9inZ; http://bit.ly/1oLWnaM). Not with 2.3% of warming accumulating in the atmosphere and 93.4% of excess heat going into the oceans (http://bit.ly/1qB1CQm). (The excess heat accumulating in the system is equivalent to the energy of 4 Hiroshima bombs per second (http://bit.ly/1sSaGje). The increase in global ocean heat content is measured at around 20x10^22 joules (http://bit.ly/1ww41xO)). Not with the continued massive decline in Arctic sea ice area and volume, even with two years of uptick from the 2012 record low (http://1.usa.gov/1qUMQS9). Not with land ice sheets continuing to lose gigatons of mass annually (http://bit.ly/1qUMRp7; http://1.usa.gov/1qW49Un). Not with global sea level rising without surcease (http://1.usa.gov/1uxV3Om). Not with parts of the West Antarctic ice sheet already on an irreversible course to complete collapse (http://bit.ly/1uNOkOE; http://1.usa.gov/YJ9OCC). Not with multiple other lines of evidence all indicating the continued warming of the climate system due to the energy imbalance caused by the man-made buildup in the atmosphere of CO2 and other greenhouse gases.
Beyond understanding the basic physics of why we shouldn’t expect the energy imbalance in the global climate system to slow or stop, the so-called “pause” is susceptible to scientific scrutiny. It can be analyzed to determine whether it’s true, whether it’s a data artifact, what internal and external factors may be causing it, and whether climate models are skillful at simulating it. In a number of recent studies, scientists have done just that.
First, one should understand the “pause” derives from a “double cherry-pick.” It reflects the choice of a short-term trend beginning with 1998 and focuses only on the atmospheric component of the climate system where less than 3% of total warming accumulates. A short-term interval of 10 to 15 years is unreliable for making determinations about long-term climate patterns. Variations in internal and external forcings over short time scales can create noise that obscures the underlying signal. Multi-decadal observations are more reliable, as short-term noise tends to recede against the long-term signal. Further, focusing on the atmosphere where less than 3% of warming accumulates yields an incomplete, even misleading, picture. While some have pointed to the apparent pause in surface temperature warming to claim global warming has slowed or stopped, the evidence shows global warming continuing apace in the climate system on whole (http://bit.ly/1qW3B0P).
Apart from its cherry-picked derivation, what does science tell us about the apparent pause in surface temperature rise over the period since 1997?
Particularly strong El Niño conditions prevailed in the Pacific in 1997-98. It was, in fact, a record El Niño year, surpassed only by the 2010 El Niño. The warmer sea surface temperature (SST) associated with El Niño conditions contributes to the warming of atmospheric surface temperature. Consequently, 1998 is the hottest year on record outside the 21st century. Cherry-picking 1998 as the first year of the short-term trend stacks the deck with an anomalously high starting point. Further, between 1998 and 2012, La Niña conditions were more prevalent than El Niño. The cooler SST under La Niña conditions has a slight cooling effect on atmospheric surface temperature. In addition, the increased gradient between La Niña’s slightly cooler SST and the overlying air in contact with the ocean surface causes more atmospheric heat to be absorbed in the ocean. Recent research reveals unprecedented ocean warming since 1998, particularly in the deep ocean (http://bit.ly/1uxViJq). During much of the same period, the Sun was in a prolonged solar minimum, producing a slight cooling effect (http://bit.ly/1y6LpGx;http://1.usa.gov/1y341ao). With the unusually warm 1998 as the starting year, coupled with the solar minimum, accelerated uptake of heat in the ocean, and La Niña conditions predominating in the Pacific, global average atmospheric surface temperature appears to rise more slowly over the ensuing 15-year period relative to the long-term trend -- hence, the “pause.”
Yet despite the predominance of La Niña conditions, the prolonged solar minimum, and increased ocean heat uptake, 2000-2009 was the warmest decade on record (http://bit.ly/1wtBx82); it likely was the warmest in thousands of years (http://bit.ly/1qB2Jzj). Each of the 12 years from 2001-2012 features one of the 14 warmest years on record. Interestingly, if one looks at the 15-year period 1996-2010 instead of the period 1998-2012, not only is there no apparent pause, but the trend is slightly greater than the long-term trend (http://bit.ly/1msUbql). Moreover, the apparent slowing in the rate of surface temperature rise between 1998-2012 does not break the long-term trend, which is unambiguously upward.
In The Guardian a couple weeks ago Dana Nuccitelli summarized several recent studies that looked at the apparent slowdown in atmospheric surface temperature (http://bit.ly/Xma0X1). As noted already, the science shows some of the apparent slowdown is attributable to the slight decrease in solar irradiance during the solar minimum, to the predominance of La Niña conditions (otherwise known as the negative phase of the El Niño Southern Oscillation (ENSO)), and to increased deep ocean heating. Volcanic activity since 1998 also was elevated. The sulfurous aerosols volcanoes spew into the atmosphere reflect incoming solar radiation back to space, producing a cooling effect. Changes in solar output, volcanic activity, and ocean cycles can’t be predicted accurately over short periods. One should not expect climate models that are designed to simulate climate patterns over long timescales to provide precise short-term forecasts given the unpredictability over short timespans of variations in certain internal (ocean cycles) and external (volcanism, solar radiation) forcings. Over the long term, however, these factors tend to be stable and make little difference on the climate change human activities are causing. When scientists include the observed short-term changes in ENSO, solar output, and volcanic activity, the climate models accurately simulate about half the observed surface temperature change over the period of the “pause.”
Another contributor to the apparent “pause” is a data artifact. Gaps in the weather station network used to collect temperature data exist in the polar regions and parts of Africa. We know surface temperature in the Arctic is rising faster than the rest of the world (http://bit.ly/YJa8kO). The under-representation of polar data produces a cooling bias in the calculation of global average surface temperature. Gavin Schmidt explains how a team of scientists filled in the gaps using statistical techniques and satellite data (http://bit.ly/1lAHwjX). The scientists found filling in the data gaps accounted for nearly half the difference between the observed temperature and modeled simulations.
When the data gaps and solar, volcanic, and ocean data are all accounted for, the models accurately simulate the observed global average surface temperature rise over the period since 1997.
In sum, science reveals there has been no pause in the energy imbalance accumulating in the climate system on whole since 1997. Short-term, unpredictable variations in internal and external forcings account for a portion of the apparent slowdown in surface temperature rise. Gaps in weather station data, particularly from the rapidly warming Arctic, account for the remainder. The majority of excess heat has been taken up by the oceans. When the models are updated with the gap data and short-term forcing variations, they accurately simulate the observed surface temperature change since 1997. Borrowing Gavin Schmidt’s terminology, the models are in fact skillful (http://bit.ly/1wlnyy4; see also “Well-estimated global warming by climate models” by Stephen Lewandowsky: http://bit.ly/1uD6ljx).
This exercise in examining the “pause” is emblematic of how science works. We started with the curious observation of an apparent slowing in global surface temperature rise. Scientists undertook to discover an explanation for it, and they succeeded. Now, we have a fuller understanding of the climate picture over the past decade and a half. We know that global warming has not slowed or stopped. Understanding the physics of the global energy imbalance, we can reasonably expect it not only to continue, but when the Pacific ENSO switches from negative to positive phase and the current Atlantic heat uptake cycle switches (http://bit.ly/1sUG2FP), as they invariably will at some point, we can expect surface temperature rise to accelerate.
A key aspect of the enterprise of science is that we remain open-minded to revise our thinking based on new information. The recent research on the “pause” allows us to discard as myth the idea of a global warming hiatus. Richard Feynman famously said, “Science is what we do to keep from lying to ourselves.” With the emergent scientific evidence showing no pause in global warming, we ought to expect reputable media outlets to stop treating those who stubbornly refuse to revise their thinking in the face of the evidence and continue to assert claims that have been debunked as legitimate, earnest participants in the climate conversation. For decades “merchants of doubt” have succeeded in preventing or delaying actions to address climate change by spreading misinformation and promulgating false claims. We cannot afford another decade of dithering (http://bit.ly/1wuQUNq;http://bit.ly/1oLWnaM). So let's put the talk of a global warming "pause" on permanent hiatus and move on to discussing solutions.
Further reading:
“Is Global Warming Really Slowing Down?”by Chris Mooney: http://bit.ly/1qB1MHd
“Who Created the Global Warming Pause?” by Chris Mooney: http://bit.ly/1msUbql
“Yeah, About That Global Warming ‘Pause’…” by Phil Plait:http://slate.me/1wmsoLN
“Still No Support for Global Warming ‘Slowdown’” by Kieran Mulvaney:http://bit.ly/YLClav
