Mother Nature can't take the blame for this century's string of record-breaking heat waves, a new study finds.
Fourteen of the 15 warmest years in recorded history occurred between 2000 and 2015 (and it was recently announced that 2015 was Earth's hottest year since record keeping began, in 1880). The odds are between 1 in 10,000 and 1 in 170,000 that natural climate swings caused the sweltering-high temperatures around the world, researchers reported Monday (Jan. 25) in the journal Nature Scientific Reports (opens in new tab). For 2014 alone, there's a one-in-a-million chance that the monster heat record occurred only from natural climate variability.
"The risk of heat extremes has been multiplied due to human greenhouse-gas emissions, as our data analysis shows," study co-author Stefan Rahmstorf said in a statement. "The anomalous warmth has led to unprecedented local heat waves across the world, sadly resulting in loss of life and aggravating droughts and wildfires," said Rahmstorf, a professor at the Potsdam Institute for Climate Impact Research in Germany. [Watch Earth Get Hotter — 135 Years of Temperature Changes Visualized]
The scientists tested the influence of global warming by combining real-world climate data, such as global and Northern Hemisphere temperatures, with state-of-the-art climate models. The researchers' statistical analysis separated natural climate variability, such as El Niño-caused ocean warming, from climate change brought about by human activities. The scientists ran the analysis with different data sources and statistical approaches and found that, in all cases, the record-high temperatures required human intervention.
The new analysis was spurred by news stories published soon after 2014, the researchers said. In those reports, scientists said the odds were extremely low that so many record-breaking hot years could occur without global warming.
"The press reports last year about the unlikely nature of recent global temperature records raised some very interesting questions, but the scientists quoted hadn't done a rigorous calculation," said lead study author Michael Mann, director of the Earth System Science Center at Pennsylvania State University in State College. "As a result, the probabilities reported for observing the recent runs of record temperature by chance alone were far lower than what we suspected the true probabilities are."
The record temperatures seen since 2000 are roughly 600 to 130,000 times more likely to have occurred under human-caused conditions than in the absence of such conditions, the researchers report. "We can, in this sense, attribute the record warmth to human-caused climate change at a high level of confidence," Mann wrote in an op-ed published Monday at Live Science.