Emissions agreements under negotiation in Paris at the climate talks this week depend on the rates at which greenhouse gases have accumulated in Earth’s atmosphere and how fast they are expected to build for decades to come. Yet just as the climate negotiations are wrapping up—and during the weeks leading up to them—a more complex, and in some ways puzzling, picture of greenhouse gas trends has been emerging.
The rapidly increasing emissions rate of the No. 1 greenhouse gas, carbon dioxide (CO2), has suddenly slowed, mainly because the fast-climbing annual growth of coal burning in China abruptly leveled out, according to a commentary published Monday in Nature Climate Change. However, global atmospheric concentrations of the world’s No. 2 greenhouse gas, methane, have risen steeply in recent years, according to a briefing 1 month ago by the World Meteorological Organization (WMO) in Geneva, Switzerland. That surge follows a previous and yet unexplained 7-year pause in the rise of global methane concentrations.
CO2 Emissions to Decline?
In the new commentary on CO2 emissions, scientists reported this week that the world is on target to see an unprecedented 0.6% reduction in the annual rate of emissions of carbon dioxide in 2015. The projected decline contrasts sharply with the previous decade’s annual average rise of 2.4%.
“Our findings are encouraging because they show that global emissions do not necessarily need to grow at the incredible rates that we have seen in the past,” climate scientist Corinne Le Quéré, director of the Tyndall Centre for Climate Change Research at the U.K.’s University of East Anglia and a coauthor of the report, told Eos. On the other hand, she noted the wide margin of uncertainty surrounding the projection, ranging from −1.6% to +0.5%.
Already last year, the growth rate of global CO2 emissions had slowed markedly to just a 0.6% increase over 2013, the researchers reported. Most notably, they pointed out, the new, lower-emissions trend began during a time of global economic growth, unlike previous slowdowns, which coincided with periods of economic crisis.
Although the halt of China’s previous expansion of coal use accounted primarily for the unexpected turnabout, below-average global demand for oil and natural gas and a boost in the use of renewable energy sources also played important roles, the team found. In China, most new energy production in 2013 and 2014—58%—came from nuclear power or hydropower and other renewable sources, according to the study.
Despite the CO2 slowdown, atmospheric concentrations of the gas hit a record 397.7 parts per million in 2014, the WMO reported in its 9 November briefing. This spring, the time of year when the CO2 concentration seasonally rises, the global average also crossed the symbolically significant 400 parts per million threshold, the agency reported. “Next year it is likely that this threshold will be reached…also on an annual average,” said Michel Jarraud, WMO’s secretary-general, in a press release. “We will soon be living with globally averaged CO2 levels above 400 parts per million as a permanent reality.”
Near-Term Methane Threat
The recognition of the recent deceleration of CO2 emissions might influence what sorts of deals are struck this week at the Paris meeting known as the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change, or COP21. However, even the most stringent restrictions on emissions of the predominant greenhouse gas would leave the world vulnerable to a more imminent threat, said climate scientist Robert Howarth of Cornell University in Ithaca, N.Y., who is a delegate to COP21. “Reducing carbon dioxide emissions is essential for the long term, yet will not appreciably slow global warming over the critical next few decades,” he told Eos. “The only way to slow global warming in the next 15 to 35 years—and avert warming to the dangerous levels of 1.5 to 2 degrees—is to reduce methane emissions.”
Whereas the global atmospheric concentration of CO2 has steadily risen for decades—reaching 143% of its preindustrial level in 2014, according to the 9 November WMO briefing—methane concentrations have fluctuated widely. Since 2007, Earth’s global atmospheric methane reading has skyrocketed, reaching 254% of its preindustrial value in 2014, the WMO reported. However, between 1999 and 2006, the gas’s accumulation in the atmosphere took a long and still mysterious pause.
“The scientific community has not resolved the problem of methane growth rate,” Oksana Tarasova, chief of WMO’s Atmospheric Environment Research Division, told Eos.
“It’s another one of those lingering mysteries where we really need some better observations to sort out,” added climate modeler Lesley Ott at NASA’s Goddard Space Flight Center in Greenbelt, Md. Some ideas researchers are exploring include changes in how methane may be removed from the atmosphere chemically, changes to the tundra in the high latitudes, and the increase in fracking. Still, “the prevailing theory is that we are seeing impacts to changes in wetlands, which are a major source of methane into the atmosphere,” she noted.
Indeed, some researchers who will present new methane research at the 2015 American Geophysical Union Fall Meeting in San Francisco next week contend that the pause was the result of natural reductions in methane emissions masking the overall effect of increases from human-caused emissions. “It’s a climate response,” said Earth scientist Euan Nisbet of the University of London, U.K., who, along with colleagues in the United States and France, will be presenting a poster about the global methane budget.
The culprits, they venture, are the pattern of eastern Pacific Ocean warming known as El Niño, which spurs climate changes worldwide, and its counterpart, La Niña. “The reason behind this is increased tropical precipitation during the recent, persistent La Niña phase,” Earth scientist Ed Dlugokencky of the National Oceanic and Atmospheric Administration in Boulder told Eos.
He contrasted the relatively wet conditions that prevailed on and off in tropical regions from 2007 to 2013 with drier El Niño conditions that dominated during the earlier time span when methane emission rates stood still. Under El Niño, Dlugokencky suggested, “the decreasing wetland emissions may have masked increases in manmade methane emissions, making the overall concentration look like it was reaching steady state.”
—Christina Reed, Freelance Writer
Citation: Reed, C. (2015), Greenhouse gas patterns offer promise, puzzlement, Eos, 96, doi:10.1029/2015EO041351. Published on 10 December 2015.