Is Climate Change Causing The Recent Streak Of Record-Setting Warm Years?

Odds are, yes

2015

2015

Last year was the warmest on record. Red and orange show areas that were above the average temperature last year. Blue areas show places where temperatures were cooler than average.Scientific Visualization Studio/Goddard Space Flight Center

We're on a roll. 2015 was the hottest year on record, just like 2014 was the hottest year on record, and before that, 2012 was the hottest year on record... In fact, 15 of the 16 hottest years on record have happened in the last 15 years. What are the odds?

In a paper released in Scientific Reports today, researchers found that the odds of a string of record-setting years like this are very slim, somewhere between 1 in 5,000 and 1 in 170,000. And that's after taking into account natural variations in the climate cycle, like El Niño, La Niña and warming due to volcanic eruptions.

Earlier predictions overstated the improbability of the record-setting streak, setting the likelihood of our current streak somewhere between 1 in 27 million and 1 in 650 million, worse odds than your chances of winning the Powerball (1 in 292 million). But those calculations assumed that each year was completely independent of the years around it, something that doesn't make sense in the real world.

"Natural climate variability causes temperatures to wax and wane over a period of several years, rather than varying erratically from one year to the next," said Penn State meteorologist Michael Mann, one of the authors of the paper. To get a sense of the actual probability, Mann and colleagues ran simulations using climate data from the real world, with one notable difference. They took out the part of the temperature data that was related to human factors. Then they used this all-natural data to create a million surrogate climate histories, looking for the odds of something like our record streak happening naturally.

Still, 1 in 5,000 and 1 in 170,000 aren't great odds. So what's going on? We are, of course.

Based on the astronomical odds of it happening by natural chance, we can be fairly confident that there's an external factor causing the string of hot years. But Mann took his analysis further. In addition to looking at the odds of this occuring only with natural climate variations, he re-did the calculations, this time adding back in the warming that scientists attribute to human activity (things like greenhouse gas emissions). They found that the likelihood of having 9 of 10 record-setting years occur since 2000 jumped up to 88 percent, and the likelihood of 13 of the top 15 years occuring since 2000 was 83 percent.

Put another way, the chances of our record-setting streak happening is somewhere between 600 and 130,000 times more likely to happen with human-caused climate change than without it. Those odds are too big to ignore.