As President Donald Trump prepared to pull the United States out of the global Paris climate agreement this week, scientists at NOAA reported that 2016 had recorded the second-biggest jump in atmospheric carbon dioxide on record.
Last year's increase in the atmospheric CO2 concentration was nearly double the average pace since detailed measurements started in 1979.
Once CO2 is in the atmosphere, the heat-trapping gas persists there for decades as new emissions pile in, which means that even if global emissions level off—as they have started to do—the planet is on a path toward more warming, rising sea levels and increased heat waves and droughts in the decades ahead.
Concentrations of other greenhouse gases, including methane and nitrous oxide, also increased last year, according to the National Oceanic and Atmospheric Administration's latest update to its greenhouse gas index. The heating effect of all combined greenhouses gases in the atmosphere increased by 2.5 percent in 2016, according to the index.
"The warming effect of these chemicals we're tracking has increased by 40 percent since 1990," said Steve Montzka, a NOAA scientist who co-authored the update. "Even though emissions are leveling off, CO2 is so long-lived that the concentration is still increasing."
Getting the atmospheric concentration to also level off would require reducing emissions by 80 percent, he said.
That 80 percent cut is exactly what is targeted under the Paris climate agreement, but the goal is in doubt as the Trump administration rolls back climate and energy policies meant to lower emissions in the United States, historically the world's largest sources of greenhouse gas pollution.
"All the indicators are going in the wrong direction, and warning bells are ringing so loud as to be deafening," said Kevin Trenberth, a senior scientist in the climate analysis section at the National Center for Atmospheric Research. "Without the Paris agreement, the acceleration will likely continue and we will exceed 2 degrees Celsius above pre-industrial by the 2050s or earlier."
The index was established in 1979, when NOAA expanded the global network of 80 land- and ocean-based measurement sites, including the Mauna Loa observatory in Hawaii. In the 1980s and 1990s, the CO2 level increased at about 1.5 parts per million each year. The last two years, it's been rising at nearly twice that rate—2.9 ppm—as emissions overwhelm the oceans' and forests' ability to take CO2 out of the air.
The new data also show that the powerful effect of heat-trapping and ozone-destroying chlorofluorocarbons (CFCs)—once widely used as refrigerants and propellants—continues to decrease. That decline reflects the success of the 1989 Montreal Protocol, one of the early global efforts to tackle an environmental challenge.
"That's given us a little reprieve from even more warming," according to Montzka, who said the continued use of CFCs could have had a substantial additional heating effect.
Methane, which is a much more powerful heat-trapping gas than CO2, increased in 2016 at about the same rate as the previous two years, which is double the pace set between 2007 and 2013. Scientists suspect the methane increase is mainly from decomposition of plant matter in the tropics, where global warming is speeding biological processes. Earlier spikes in methane have also been linked with warmer Arctic temperatures that release the gas by thawing permafrost.
Since 2013, the methane concentration has increased between 8.7 and 12.6 parts per billion each year, compared to an average annual increase of about 5.7 ppb between 2007 and 2013. Methane is measured in parts per billion rather than parts per million because the total amounts are much smaller.
Even though the latest figures are sobering, the fact that global carbon emissions are starting to plateau is a hopeful sign, said Pennsylvania State University climate scientist Michael Mann.
"This emphasizes a key basic truth: There is nothing Trump can do to stop the dramatic global transition away from fossil fuels toward clean and renewable energy," he said. "The world is moving on, and we will tackle this problem. At this point, it is simply a question of whether we get onboard the great economic revolution of this century, or whether we get left behind."