Year in review: Global warming continues apace

A supposed pause in global warming that has been fodder for climate change doubters never really existed, researchers reported in 2015.

The fuss began when studies showed that decades of warming appeared to have leveled off in 1998. From that year through 2012, Earth’s yearly average surface temperature increased at one-third to one-half the average rate from 1951 through 2012. This warming “hiatus,” as it came to be known, had climate scientists scratching their heads and climate doubters gloating.

In June, scientists at the National Oceanic and Atmospheric Administration announced that they had finally found the cause of the hiatus, and it wasn’t shifting winds or pint-size volcanic eruptions as some scientists proposed. Instead, small biases and gaps in temperature data had created an artificial plateau. The slowdown never existed (SN: 6/27/15, p. 6). The biggest culprit, the scientists found, concerned measurements of ocean surface temperatures.

Before World War II, sailors took temperature measurements from water hauled over the sides of ships in buckets. Later, sailors relied on water pumped in to cool ship engines. Today, researchers take more accurate measurements from scientific buoys. On average, buoys record a temperature reading 0.12 degrees Celsius cooler than those from ships. As methods changed, the temperature readings created a false ocean cooling trend that partially canceled out the global trend.

Correcting for these biases and adding more extensive data, including from the Arctic, the researchers found that the Earth’s average surface temperature warmed by 0.116 degrees per decade between 2000 and 2014. That’s roughly in line with the warming rate recorded over the second half of the 20th century.

Even without the bias correction, the warming hiatus was on flimsy footing, says Stanford statistical climatologist Bala Rajaratnam. In November, Rajaratnam and colleagues argued in Climatic Change that the supposed stalling could have been explained by natural variability and didn’t negate the long-term trend anyway. The global changes, which threaten to inundate coastal towns and cities as sea levels rise, introduce more extreme weather and alter ecosystems across the globe, are not abating. 

“What we’re doing right now to the climate is unprecedented,” says Richard Zeebe, a paleoclimatologist at the University of Hawaii at Manoa.

Carbon dioxide levels in Earth’s atmosphere — calculated as a monthly average for the entire globe — reached above 400 parts per million this year, higher than at any point in recorded history. Researchers also announced this year that glaciers along the Southern Antarctic Peninsula that had been mostly stable are now in decline (SN Online: 5/21/15), and one of the continent’s largest ice shelves, Larsen C, has a rapidly spreading crack (SN: 7/25/15, p. 8). Ocean warming and meltwater from vanishing glaciers are contributing to sea level rise, a total of 80 millimeters since 1993. As reflective ice melts into heat-absorbing dark open ocean, warming increases further.

In news from the opposite pole, scientists now predict that the Arctic Ocean will have its first ice-free summer sometime around 2052, almost a decade sooner than previously projected (SN Online: 8/3/15). The rapid warming in the Arctic may also cause more deadly heat waves across the Northern Hemisphere (SN: 4/18/15, p. 13), and a warming Pacific is expected to boost typhoon intensities 14 percent by 2100 (SN: 6/27/15, p. 9).

To put the pace of change in perspective, Zeebe and colleagues studied a time about 56 million years ago when carbon dioxide levels increased from about 1,000 ppm to between 1,700 and 2,000 ppm. The period’s rapid CO₂ rise, and its roughly 5 degrees of warming, seemed like the best contender for matching or exceeding today’s CO₂ increase. During that time, little to no ice covered Earth’s surface and forests reached from pole to pole.

Using climate simulations and ocean sediment data, Zeebe and colleagues calculated that the period during which carbon dioxide levels increased lasted at least 4,000 years, and the carbon release rate during that time had been at most 1.1 billion tons a year. That rate is just about a tenth of the approximately 10 billion tons of carbon released via fossil fuel burning in 2013 (SN: 5/30/15, p. 15). The finding suggests that modern climate change has no close historical analog, says Zeebe.

Even airlines could end up feeling the heat. In January, climate scientists predicted that warming air, which is less dense than cold air, will force airlines to reduce carrying capacity over the coming decades (SN: 2/7/15, p. 15).