The most likely explanation for the lack of significant warming at the Earth’s surface in the past decade or so is that natural climate cycles—a series of La Niña events and a negative phase of the lesser-known Pacific Decadal Oscillation—caused shifts in ocean circulation patterns that moved some excess heat into the deep ocean. Even so, recent years have been some of the warmest on record, and scientists expect temperatures will swing back up soon.
The “pause” in global warming observed since 2000 followed a period of rapid acceleration in the late 20th century. Starting in the mid-1970s, global temperatures rose 0.5 °C over a period of 25 years. Since the turn of the century, however, the change in Earth’s global mean surface temperature has been close to zero. Yet despite the halt in acceleration, each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.
The long-term trend—change over the course of a century or more—is what defines “global warming,” not the change from year to year or even decade to decade. Rising emissions of carbon dioxide and other greenhouse gases since the Industrial Revolution explain most of the overall warming trend over the past century, and the rate of emissions has not slowed significantly in the recent past. So what else has been going on in the climate system over the past decade that could account for the pause in Earth’s surface warming trend?
During the last decade, a longer than usual solar minimum cycle, several volcanic eruptions, and relatively low amounts of water vapor in the stratosphere may have helped cool the atmosphere temporarily. But recent research suggests that the Earth’s natural climate variability—natural, short-term fluctuations in the climate system that occur on a year-to-year basis or longer—may have played the most pivotal role of all by transferring excess heat from the Earth’s surface into the deep ocean.
One of the most well-known natural climate oscillations—the El Niño-Southern Oscillation (ENSO) cycle—causes swings in sea surface temperatures in the tropical Pacific. Although ENSO originates in the equatorial Pacific Ocean, a strong El Niño or La Niña event is capable of bumping global temperatures upward or downward for a year or two. Since the last major El Niño event in 1997-1998, a series of La Niña events have dominated the tropical Pacific, resulting in a prolonged cooling of sea surface temperatures that has also likely stalled the rise in global temperatures.
Evidence for the global influence of these La Niñas comes from an innovative model experiment by a team of scientists from Scripps Institution of Oceanography. When they forced a climate model to closely follow observed temperatures in the tropical Pacific—mirroring the repeated La Niña events—the model simulated no significant trend in global warming since 2000. This led the group to believe that global temperatures would have continued to rise throughout the last decade if not for the prolonged cooling in the Pacific.
Just because the global surface temperature has not risen significantly in the past decade doesn't mean the Earth's heat energy imbalance has vanished, though. Excess heat energy trapped by greenhouses gases can have more than one fate in the Earth system; among other things, it can cause water to evaporate, it can melt ice, and it can be mixed into the deep ocean by overturning currents.
That mixing coupled with water's naturally large heat capacity makes the global ocean the Earth’s biggest absorber of heat; scientists estimate the ocean absorbs more than 90 percent of the excess heat trapped in the atmosphere by greenhouse gases. When analyzing temperature patterns at different depths of the ocean, scientists observed that deep ocean temperatures—measured more than a half-mile down from the surface—began to rise significantly around 2000, while shallower waters warmed more slowly. This divergence took place at the same time that a natural climate cycle called the Pacific Decadal Oscillation, or PDO, was shifting to a negative phase.
Unlike the ENSO cycle, which affects the climate on a year-to-year basis, the PDO affects the climate on decadal timescales. Since the late 1990s, the negative phase of the PDO cycle has contributed to cooler sea temperatures at the surface of the tropical (similar to La Niña) and northeastern Pacific. Strong prevailing winds during the negative phase of the PDO also stir up the ocean and mix surface waters down into the deep ocean, allowing heat to penetrate to greater depths.
The deep ocean may have been able to "hide" excess heat trapped in the Earth system by greenhouse gases, contributing to the warming “pause” in the last decade, but scientists know that heat energy doesn't just disappear. Eventually, natural ocean circulation may bring some of the extra heat stored in the deep ocean back to the surface, which can happen during an El Niño event, for example.
Meanwhile, other environmental indicators of climate change—melting ice in Greenland, the retreat of Arctic sea ice, global sea level rise—continue to send a clear signal that Earth is still warming. Over the coming century, human-caused warming will continue, with natural variability periodically speeding up or slowing down the pace from decade to decade.
*Editor's Note: Since this article was written, researchers have continued to try to quantify where exactly the deep ocean heat has gone and how it got there. The theory remains that the Pacific Decadal Oscillation is the main mode of variability regulating temperature in the Pacific Ocean, contributing to the global warming hiatus and driving other climate trends elsewhere. But in the meantime, recent studies have investigated the cause of recent warming trends in the Atlantic and Southern Oceans. Some questions remain for scientists, including: What physical mechanisms drive variability in the ocean basins? How deep is Earth’s accumulating heat penetrating the depths of the ocean? This is a fast-moving area of research, so stay tuned for future posts on this topic.
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