Even Three Scorching Years Don't Make a Trend
The planet has broken global heat records for three years in a row -- a finding that, paradoxically, may raise public concern about global warming, but won’t change long-term climate forecasts.
First, the good news: Scientists say the run of record-breaking temperatures doesn’t mean that we should expect every year to be relentlessly hotter than the last. From year to year, temperatures may cycle from hotter to cooler and back again. The bad news, however, is that without serious cuts in greenhouse-gas emissions, the world will get much warmer over the long term. There’s widespread consensus that over the course of the 21st century, unchecked global warming will disrupt agriculture, flood cities and drive species to extinction.
Focusing on record-breaking years alone can be misleading. It means throwing away the bulk of your data, said statistical physicist Sidney Redner of the Santa Fe Institute. Looking at systematic long-term trends, he said, “is the more kosher way to do things.”
It’s easy to fool yourself or others by zeroing in on a small piece of data. In the early 2000s, some people isolated the relatively cool years following big spikes in 1997 and 1998 and wrongly concluded that global warming had stopped. That’s like a person who gains 50 pounds over a decade claiming the trend has reversed because he lost a pound between Feb. 3 and Feb. 4. Unsurprisingly, claims about “the pause” were more popular among politicians than among scientists. Taken as a whole, the data show a clear warming trend over the last few decades.
Still, an impressive number of record years have been piling up in the 21st century, with 16 of the 17 hottest years on record having occurred since 2000. What, if anything, does this mean? Two years ago, the Associated Press generated some controversy by quoting a statistician who claimed that the odds were only one in 650 million that without man-made global warming we’d have observed what was then nine of the 10 hottest years on record having occurred since 2000. Climatologists say the odds such a streak are not quite that low, because yearly variations in global temperature aren’t independent, like coin tosses, but tend to cluster. A hot year is more likely to be followed by another hot year than a cool one.
One reason hot years tend to clump together is a semi-periodic oscillation of ocean currents known as El Niño, and its cooler counterpart, La Niña. This cycle happens because heat from the sun doesn’t warm the earth’s oceans uniformly. Trade winds tend to allow a blob of warm water to build up in the tropical Pacific every few years. Once the heat builds past a certain point, it breaks free, sending ocean currents into a different pattern. Warmer water moves from the deep ocean toward the surface, where it releases heat into the atmosphere. Rainfall patterns change and global atmospheric temperatures spike. (Fishermen in Peru named the phenomenon El Niño -- after the Christ child -- because they noticed that unusually warm currents would periodically come around Christmas.)
The cycle normally produces an El Niño condition every three to seven years, said Kevin Trenberth, a climate researcher at the National Center for Atmospheric Research in Colorado. But every once in a while there’s a more dramatic oscillation -- what he calls a “super El Niño.” One occurred in the early 1980s, then again from 1997 through 1998, and most recently in the period spanning 2014, 2015 and 2016 -- our recent triple record-breaker. Whether these super El Niños are natural phenomena or are connected to man-made global warming remains an area of investigation. The result is that instead of a steady increase in temperatures, we see periods of relatively little warming followed by spikes -- but with a long-term upward trend.
That still leaves the question of what’s causing the trend. Variations in solar output and volcanic eruptions can cause the global climate to heat and cool, but volcanologists and solar physicists report no changes that would account for rising temperatures measured over recent decades.
Several lines of evidence bolster the case that human-generated greenhouse gases are to blame, said Benjamin Santer, a climatologist at the Lawrence Livermore National Laboratory in California. Back in the 1960s, Princeton University meteorologist Syukuro Manabe predicted that global warming caused by changes in the sun would create different patterns of heat in the atmosphere than would global warming from greenhouse-gas buildup. If the sun were the culprit, Santer explained, it would heat the atmosphere from the top down, distributing the extra heat uniformly. If greenhouse gases were to blame, the lower part of the atmosphere would absorb more heat, leaving the upper atmosphere cooler.
That wasn’t something scientists could test in the 1960s, Santer said, but with data from satellites and modern weather balloons, he and colleagues recently observed the signature of greenhouse gas warming that Manabe predicted.
The theory of carbon dioxide warming is also backed by basic physics. Tabletop experiments show that carbon dioxide absorbs infrared radiation, while oxygen and nitrogen do not. While carbon dioxide makes up just a trace of the atmosphere, it’s responsible for keeping the planet from freezing into a giant snowball.
The current spate of records, said Santer, is exactly what the scientists expected to see, given what they know about greenhouse-gas warming and the El Niño cycle. Last year, climatologist Michael Mann decided to calculate the odds of getting 13 of the 15 hottest years on record occurring since 2000 if natural forces were acting alone. His conclusion: around one in 10,000 -- not as slim as what AP reported, but not exactly odds you’d want to bet the future on.
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