Since 1998, human activity has trapped more than two billion atomic bombs’ worth of extra heat in our planet’s climatic system—primarily by raising greenhouse-gas concentrations to their highest levels in 800,000 years, at the fastest rate Earth has seen since the age of dinosaurs.
That extra heat can’t accrue without consequence. Two new reports confirm that 2016 was the hottest year on record—and demonstrate that human activity made the freakish heat far likelier than it otherwise would have been.
On August 10, the U.S. National Oceanic and Atmospheric Administration (NOAA) released its annual State of the Climate report, published in concert with the American Meteorological Society. The 298-page report, prepared by a team of scientists led by NOAA’s National Centers for Environmental Information, affirms that 2016 was the hottest year in 137 years of recordkeeping.
The peer-reviewed report states that with a global annual average CO2 concentration of 402.9 parts per million, 2016’s greenhouse gas concentrations are the highest on record—and the first year in more than 800,000 where CO2 levels exceeded 400 parts per million.
It also says that in 2016, global averages for sea surface temperature, sea level, and the temperature of the lower atmosphere all reached record highs, while Antarctic sea-ice extent hit record lows. Surface temperatures also reached record warmth, aided by a strong El Niño in early 2016.
A release accompanying the report notes that these “major indicators of climate change continued to reflect trends consistent with a warming planet.”
Loading the Climate Dice
While 2016—and 2015 and 2014, the two previous record-holders for hottest year—may have been unusual, it’s worth asking: How likely or unlikely would this extreme weather be, without humans’ influence on climate?
One new study, published in Geophysical Review Letters on August 10, concludes that recent heat records would be vanishingly unlikely without humans loading the “climate dice” by artificially adding heat.
The study finds that the 2014-2016 streak of record-breaking heat had less than a 1-in-3,000 chance of occurring in the absence of human-caused warming. Human influence increased the odds in the heat streak’s favor, making it a 1-in-100 event.
In particular, the unusual warmth of 2016 would’ve been a one-in-a-million event in a world without human-caused warming. With humans loading the dice, however, the likelihood of 2016’s temperatures increased to as high as 27 percent.