In yet another signal to global governments that greater ambition is needed to combat the climate crisis, an annual United Nations report released Monday revealed that levels of long-lived greenhouse gases in the atmosphere reached record highs last year.

"We face a stark choice: set in motion the radical transformations we need now, or face the consequences of a planet radically altered by climate change."

—Inger Andersen, UNEP

The latest World Meteorological Organization (WMO) Greenhouse Gas Bulletin (pdf) provided figures for globally averaged concentrations of three key climate-heating gases in 2018:

Carbon dioxide (CO 2 ), which reached 407.8 parts per million;

), which reached 407.8 parts per million; Methane (CH 4 ), which reached 1869 parts per billion; and

), which reached 1869 parts per billion; and Nitrous oxide (N 2 O), which reached 331.1 parts per billion.

"These values represent, respectively, 147%, 259%, and 123% of pre-industrial (before 1750) levels," the bulletin noted. In terms of contributions to warming the climate, "carbon dioxide is the single most important anthropogenic GHG in the atmosphere" among all long-lived greenhouse gases, the primary focus of the report.

Both methane and nitrious oxide are emitted by natural sources, but about 60 percent of CH 4 emitted into the atmosphere comes from human activities such as biomass burning, cattle farming, fossil fuel exploitation, landfills, and rice agriculture, according to the bulletin. About 40 percent of N 2 O comes from human sources, including fertilizer use and various industrial processes.

From 2017 to 2018, concentrations of all three gases surged by higher amounts than the yearly increases documented over the past decade.

Another year. Another record. CO2 concentrations reached 407.8 parts per million in 2018. #ClimateAction #COP25 pic.twitter.com/lbDuGNQPPE — WMO | OMM (@WMO) November 25, 2019

"Since 1990, there has been a 43% increase in total radiative forcing—the warming effect on the climate—by long-lived greenhouse gases," the WMO said in a statement announcing the new bulletin.

The U.N. agency warned that "this continuing long-term trend means that future generations will be confronted with increasingly severe impacts of climate change, including rising temperatures, more extreme weather, water stress, sea level rise, and disruption to marine and land ecosystems."

The bulletin was released a week before the next U.N. Climate Change Conference, COP 25, is scheduled to start. Representatives from across the globe will meet at the conference in Madrid to discuss governments' commitments to tackling the human-caused climate emergency, including obligations under the 2015 Paris accord.

Greenhouse gas concentrations in the atmosphere hit new record high in 2018, leading to rising temperatures

extreme weather

sea level rise & more. Get details in new @WMO report, ahead of next week's UN Climate Conference #COP25: https://t.co/XEcoZHNujw #ClimateAction pic.twitter.com/Ukfcp0e7Y8 — United Nations (@UN) November 25, 2019

WMO Secretary-General Petteri Taalas, in the agency's statement, tied the bulletin's findings to the necessity of bolder climate action on a global scale.

"There is no sign of a slowdown, let alone a decline, in greenhouse gases concentration in the atmosphere despite all the commitments under the Paris agreement on climate change," Taalas said. "We need to translate the commitments into action and increase the level of ambition for the sake of the future welfare of... mankind."

"It is worth recalling that the last time the Earth experienced a comparable concentration of CO 2 was 3-5 million years ago," he added. "Back then, the temperature was 2-3°C warmer, sea level was 10-20 meters higher than now."

Greenhouse gas concentrations in atmosphere reach yet another high, warned @WMO Petteri Taalas as he presented latest science @UNGeneva today. The future welfare of mankind is at stake, he said. More: https://t.co/5s5z5KZshH#ClimateActionNow pic.twitter.com/K8xy5xytdW — UN Geneva (@UNGeneva) November 25, 2019

Inger Andersen, executive director of the U.N . Environment Program—which last week released a report detailing how projections for fossil fuel production by 2030 are "dangerously out of step" with global climate goals—connected the bulletin to UNEP's next annual Emission Gap Report, set to be published Tuesday.

"The findings of WMO's Greenhouse Gas Bulletin and UNEP's Emissions Gap Report point us in a clear direction—in this critical period, the world must deliver concrete, stepped-up action on emissions," Andersen said. "We face a stark choice: set in motion the radical transformations we need now, or face the consequences of a planet radically altered by climate change."

U.N. Secretary-General António Guterres shared the WMO's bulletin on Twitter Monday and wrote, "another record that shows we are not doing enough to address the climate emergency."

The level of greenhouse gases in the atmosphere has reached another record high, according to @WMO. Another record that shows we are not doing enough to address the climate emergency. Strong & decisive #ClimateAction cannot wait. https://t.co/2g0Andj1Am pic.twitter.com/GgI2xHzsxh — António Guterres (@antonioguterres) November 25, 2019

The bulletin elicted renewed demands from activists for both governments and the private sector to ramp up efforts to drive down greenhouse gases, particularly by phasing out fossil fuels.

"The record rise in greenhouse gas concentrations is a cruel reminder that for all the real progress in clean technology, we have yet to even stop global emissions increases," Nick Mabey, chief executive of global think tank E3G, told The Guardian. "The climate system cannot be negotiated with. Until we stop new investment in fossil fuels and massively scale up green power the risks from catastrophic climate change will continue to rise."