The level of carbon dioxide in the atmosphere is not just rising, it's accelerating, and another potent greenhouse gas, methane showed a big spike last year, according to the latest annual greenhouse gas index released by the National Oceanic and Atmospheric Administration.

CO2 emissions totaled between 35 and 40 billion tons in 2015, according to several agencies. Some of that is absorbed by forests and oceans, but those natural systems are being overwhelmed by the sheer volume of new CO2. As a result, the inventory shows, the average global concentration increased to 399 parts per million in 2015, a record jump of almost 3 ppm from the year before.

Methane levels jumped 11 parts per billion from 2014 to 2015, nearly double the rate they were increasing from 2007 to 2013. Methane, and other greenhouse gases like nitrous oxide and tropospheric ozone, are measured in parts per billion because the concentrations are lower.

"This inventory shows the rate of releases are increasing. It's going completely in the wrong direction, with no sign that the planet as a whole has the problem under control," said Kevin Trenberth, a senior scientist in the climate analysis section at the National Center for Atmospheric Research who wasn't involved in compiling the inventory.

The greenhouse gas inventory shows an accelerated rise in concentrations in recent years. Credit: NOAA

The index, now in its 10th year, measures how much of the sun's warmth is trapped in the atmosphere by gases like CO2, methane and nitrous oxide. The data is compiled from a global network of measuring stations, including the famed observatory atop Mauna Loa, known for having the longest continuous record of atmospheric CO2 concentrations. Mauna Loa's CO2 levels for the northern hemisphere are currently about 4 ppm higher than this time last year. Scientists there predict it may not dip below 400 ppm again.

NOAA's index shows that CO2 concentration has risen by an average of 1.76 parts per million since it was established in 1979, and that increase is accelerating. In the 1980s and 1990s, it rose about 1.5 ppm per year. Over the last five years, the rate of increase has been about 2.5 ppm, said Ed Dlugokencky, a senior scientist with NOAA's Earth Systems Research Laboratory who helped compile the inventory.

That means since 1990, global atmospheric CO2 has resulted in a 50 percent increase in its direct warming influence on climate, Dlugokencky said.

"This isn't a model. These are precise and accurate measurements, and they tell us about how humans are changing the balance of heat in the Earth system," said Jim Butler, director of NOAA's Global Monitoring Division, in a statement. "We're dialing up Earth's thermostat in a way that will lock more heat into the ocean and atmosphere for thousands of years."

Since humans started burning fossil fuels at the beginning of the industrial age—releasing gaseous carbon that had been locked up in solid form for millions of years—the concentration of carbon dioxide in the atmosphere has been rising. At first, it crept up from 278 parts per million, where it had stayed for at least 20,000 years, and then began accelerating.

In the 1950s, when scientists first figured out a way to accurately measure greenhouse gases in the atmosphere, the concentrations had already increased to the point that their heat-trapping effect was becoming apparent. Models have since shown this trend will kill forests and coral reefs, melt ice sheets and glaciers, turn fertile farm lands into deserts and swamp densely populated coastal areas with rising sea levels by the end of the century.

The number perhaps most potentially troubling from the current inventory is methane, which traps heat 25 times more effectively than CO2. It accounts for about 10.6 percent of U.S. greenhouse gas emissions, according to the EPA, which has only recently begun cracking down on methane emissions from the oil and gas industry.

Dlugokencky said it's not clear if last year's spike in methane is the beginning of a new trend or a one-time aberration, because methane concentrations vary widely from year to year. He said annual changes can be linked in part to emissions from tropical wetlands.

"It can change with weather. When we're in an El Niño, the tropics are drier, which means less methane. It's not absolute," he said. Concentrations of nitrous oxide, another potent greenhouse gas, are also building up at a faster rate in recent years, he added. The warming impact of gases other than CO2 are equal to an additional 85 ppm of carbon dioxide. In other words, the atmosphere is warming as if it contained 21 percent more carbon dioxide than it does today.

An Eye Toward Meeting Paris' Goals

All the measurements add up to bad news for global efforts to keep global warming below 2 degrees Celsius by cutting emissions of greenhouse gases.

"They're increasing in spite of things like the Kyoto Protocol just at a time when we need to bend this curve back down," Trenberth said.

The warming effects of the greenhouse gases will be felt for centuries even if greenhouse gas emissions were to be cut to zero immediately.

"A fraction of them are going to remain in the atmosphere for millennia," Dlugokencky said. "Once we have a reduction in emissions, there are a number of different processes that take up CO2."

For now, the oceans are still taking up a lot of heat, which will continue to warm the planet for centuries even if the blanket of greenhouse gases gradually starts to thin, he added.

Trenberth said it's important to view the inventory's data in the context of the 2015 Paris climate agreement.

"There are two key aspects of this that are often confused by the public—the greenhouse gas emissions and the concentrations. We measure...the concentrations quite well, but how they connect to the emissions is a tougher problem," he said.

"Under the Paris agreement, all the countries are supposed to report what their emissions are. The problem is under-reporting of various kinds," he said, highlighting methane from fracking as particularly problematic.

"We know that methane escapes from wells and pipelines, but it's probably greatly under-reported how much is going into the atmosphere. And how good are China's numbers on emissions?" he said.

To meaningfully tackle global warming means tracking the emissions and rising concentrations of greenhouse gases to their sources. The best hope of doing that is via the satellites of NASA's OCO-2 orbiting carbon observatory, Trenberth said. Readings from sensitive instruments, combined with computer models, will help pinpoint where the heat-trapping pollution originates, and also identify which parts of Earth are helping remove carbon from the atmosphere.