Countries won't be able to limit global warming to 1.5 degrees Celsius above pre-industrial levels, considered by some scientists and policymakers to be the "safe" limit of climate change, without immediate and rapid reductions in a wide range of greenhouse gases, not just carbon dioxide, according to a new United Nations report.

The report, released Oct. 8 by the Intergovernmental Panel on Climate Change, sums up the research into how 1.5°C of warming will affect the world and how global warming can most effectively be stopped.

The planet has already warmed about 1°C since the start of the industrial era, and that's likely to reach 1.5°C between 2030 and 2052 if emissions continue at their current rate, the IPCC says. It describes how recent warming has been accompanied by a trend toward more intense and frequent climate, temperature and weather extremes, and how those risks will rise with the temperature.

The warming can be stopped, the IPCC writes in its summary for policymakers. Doing so will require countries to reduce global net emissions of carbon dioxide to zero by around 2050 and to also significantly reduce short-lived climate pollutants, including methane, black carbon and hydrofluorocarbons, or HFCs.

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That emphasis on reducing short-lived climate pollutants, which are many times more potent than CO2 but don't last as long in the atmosphere, is stronger than what has been written into past international agreements.

That's partly because, with the clock running out before the world busts through its carbon budget, curtailing short-lived pollutants can buy valuable time.

In analyzing the least disruptive pathways for keeping global warming under 1.5°C, the IPCC found that all involve deep reductions in both methane and black carbon emissions of at least 35 percent by 2050.

Where to Cut Short-Lived Climate Pollutants?

The report's summary for policymakers points to three industries in particular for reducing short-lived climate pollutants: energy, agriculture and waste.

"One of the lowest-hanging fruits by far would be reducing methane from oil and gas operations," said Tiy Chung, a spokesman for the Climate & Clean Air Coalition, an advocacy group focused on reducing short-lived climate pollutant emissions. "The process to finding and fixing those leaks is relatively easy, and then the saved gas helps pay for that work."

A study earlier this year in the journal Science estimated that in the U.S., methane equivalent to 2.3 percent of all the natural gas produced in the nation leaks into the atmosphere during the production, processing and transportation of oil and gas every year. The Obama administration set rules aimed at reducing these emissions, but the Trump administration has been rolling back the regulations.

Agriculture is another leading source of methane, particularly from livestock and their manure and from rice fields. Landfills, like oil and gas fields, contribute methane as organic material decomposes.

Groups that are encouraging reducing short-lived climate pollutants emphasize that doing so reduces health hazards at the same time. For example, black carbon, also known as soot, can damage the lungs and cause heart problems, particularly for people who live or work around sources of it, such as diesel engines or wood- or coal-burning cookstoves.

"If you reduce things like black carbon emissions from the tailpipes of vehicles, for example, you are providing these important air quality improvement benefits which is going to help local populations as well," said Katherine Ross, an associate with the World Resources Institute's Climate Program.

The damage from short-lived climate pollutants is already showing up in the Arctic, where the new IPCC report says temperatures have warmed two to three times more than the global average and where warming can trigger feedback loops, including thawing permafrost that releases even more methane into the atmosphere.

Capturing a Missed Opportunity

The recognition of the need to reduce emissions of methane and other short-lived climate pollutants addresses what some advocates for action on climate change see as a key shortcoming of initial pledges made under the Paris Agreement.

Of the 195 nations that signed the Paris Agreement, eight included specific targets and policies for reducing short-lived climate pollutants in their national commitments, though many others describe measures targeting sources of methane, black carbon or HFCs.

"There was this real missed opportunity," Ross said. "There was this general lack of detail and specificity with regard to how countries planned to take action to reduce these highly potent pollutants."

To limit warming to 1.5°C, countries will have to act quickly.

"It's really hard to get to 1.5 under any conceivable set of policies," Drew Shindell, a professor of climate science at Duke University and a coordinating lead author on the IPCC report said. "So, you really need to do pretty much everything you can think of, which means SLCPs (short-lived climate pollutants) are a key part."

Read more on the overall conclusions of the IPCC's 1.5 degrees report.