Where’s the heat?

Photo credit: NASA

Global warming may have slowed, but that doesn’t make it any less real

By Travis Metcalfe

In the 2004 action film The Day After Tomorrow, global warming melts enough polar ice to disrupt natural ocean currents, plunging the northern hemisphere into a new ice age. While the story is clearly in the realm of science fiction, the premise of the movie is based on real predictions of the possible impacts of climate change that would actually play out more slowly. Boulder is home to some of the world’s leading experts on these kinds of complex interactions between the atmosphere, polar ice and the ocean.

One of the inconvenient truths about climate change is that warming in the atmosphere has slowed down in the past 15 years or so. Make no mistake, the planet is still getting warmer. According to a report issued by NASA and the National Oceanic Atmospheric Administration (NOAA) earlier this year, 2014 was the warmest year on record, and the top 10 hottest years have all occurred since 1998. But the Earth has been getting warmer more slowly than in previous decades, something that climate change deniers are quick to point out. Emissions of heat-trapping pollution have continued to rise, so why did the warming slow down?

Climate scientists like Kevin Trenberth at the National Center for Atmospheric Research (NCAR) acknowledge that surface warming has slowed since the late 1990s, but he notes several sources of natural variability that may help explain the slowdown. Regular magnetic cycles in the sun change its brightness slightly over decades, and our star went through a relatively quiet period in the mid-2000s that may account for up to 10 percent of the recent slowdown in warming. Another small contribution came from a series of volcanic eruptions that blasted tiny particles into the upper atmosphere where they temporarily blocked sunlight and had a cooling effect. But Trenberth thinks most of the slowdown can be blamed on natural cycles in the oceans. We’ll come back to this idea later.

In contrast to the recent slowdown of atmospheric warming, polar melting has accelerated during roughly the same time period. Scientists at the National Snow and Ice Data Center (NSIDC) in Boulder have been using satellites to track the extent of Arctic sea ice since 1979. This March, when ice normally covers the largest area for the year, NSIDC recorded the lowest extent of Arctic sea ice in 35 years. If the trend continues, the expected area at the annual minimum this September could fall below the record melting seen in 2012. Whatever happens, keep in mind that the top 10 years with the lowest sea ice extent have all occurred since 2002.

Sea ice doesn’t tell the full story, because it only measures the area covered by ice regardless of its thickness. A study published last month in the journal Science claimed that some regions of Antarctica have lost up to 18 percent of their thickness in less than two decades. The researchers determined the thickness of the ice by bouncing a laser beam off the surface while simultaneously using satellites with ice-penetrating radar to find the depth of the underlying rock at the same location. The biggest losses were found in regions of Antarctica where the ice sits directly on the sea floor, suggesting a possible connection to deep ocean currents, according to the authors.

This brings us back to Trenberth’s assertion that the ocean may be the main culprit for the recent slowdown in atmospheric warming. Although some details remain to be understood, there are many long-term interactions between the atmosphere and the ocean. The best-known example is probably the “El Niño” phenomenon, a band of warm water that develops in the tropical Pacific and leads to periodic changes in global temperatures and rainfall. These circulation patterns in the ocean change over decades, sometimes absorbing extra heat from the atmosphere and other times releasing it.

Trenberth notes that the Pacific has mostly been absorbing heat from the atmosphere since 1998, just when atmospheric warming began to slow down and polar melting started to accelerate. The previous episode of heat uptake by the Pacific Ocean ended in 1976, when it switched to a state of mostly releasing heat and adding to the atmospheric warming caused by heat-trapping pollution. These cycles are not perfectly regular, but if the pattern continues we might expect that atmospheric warming will again accelerate and polar melting could slow down sometime in the next decade.

Humanity is currently running an uncontrolled experiment with our home planet. Scientists in Boulder and around the world have made great strides in understanding the complex interactions that drive climate change, and even the small natural variations that influence its pace over time. The recent slowdown in atmospheric warming and the acceleration of polar melting may not have developed as quickly as the plot-line of a Hollywood film, but the future promises to be no less dramatic.

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