Cows represent a significant source of methane emissions, one of three short-lived climate pollutants being targeted by air quality regulators. Photo by Alec MacDonald.

Carbon dioxide may be a major climate change culprit, but it’s far from the only one. Unlike carbon dioxide, which grows slowly but steadily in the atmosphere and lasts a long time, the gang of bad actors known as short-lived climate pollutants hits hard and disappears fast. That might not be a concern if they were produced by a short-term source and, once gone, did not recur. However, they are common and constant, so their impact on the environment is substantial — and they’re now in the crosshairs of new policies at the regional, state, and global levels.

Short-lived climate pollutants include three categories — black carbon, methane, and hydrofluorocarbons — that share little in common, aside from their rapid and severe impacts on climate change. They have all been regulated to some degree already, but their power to heat the atmosphere has now put them under more intense scrutiny.

In September, Governor Brown signed Senate Bill 1383 (Lara), which codifies state goals for reducing short-lived climate pollutants. Using 2013 emission levels as a baseline, the state now has until 2030 to curtail methane by 40 percent, hydrofluorocarbon gases by 40 percent, and human-generated black carbon by 50 percent. The legislation also sets targets for reducing organic waste in landfills.

SB 1383 will further shape related efforts by the California Air Resources Board (CARB), which expects to adopt its Proposed Short-Lived Climate Pollutant Strategy in late 2017. In this region, the Bay Area Air Quality Management District has responsibility for implementing some of the state measures, and has been developing a short-lived climate pollutant component for its next Clean Air Plan.

Black Carbon

Also known as soot, black carbon is produced by incompletely burning fossil fuels or biomass like wood. It absorbs solar radiation, warming the atmosphere; deposited on snow and ice, it holds heat and speeds melting. In addition to its undesirable effect on the climate, black carbon also contributes to health problems such as respiratory and cardiovascular disease, cancer, and birth defects.

CARB reports that black carbon in California has already been reduced by more than 90 percent in the last 50 years, mostly through strong regulations on diesel exhaust. The next focus is residential wood fires. In the Bay Area, the Air District has been targeting this source through its Winter Spare the Air program, a ban on wood-burning fireplaces in new construction, and incentives for purchasing cleaner woodstoves. A more difficult source to target will be wildfires; reducing potential fuel requires both more efficient fire suppression and forest management.


Methane is the primary component of natural gas. It has more than 30 times as much impact on climate change as carbon dioxide over 100 years, and is more than 80 times as powerful over 20 years. The main routes for methane to reach the atmosphere are through dairy and agricultural operations, and from the decomposition of organic material in landfills. Leaks from natural gas and oil wells and pipelines are also a significant source. A source that has not been thoroughly evaluated is emissions from vegetation decomposing in wetlands and at the bottom of reservoirs.

Because nearly 60 percent of the state’s methane emissions are from agriculture, primarily from dairy farms, reducing emissions from manure will be key to reaching the 40 percent methane reduction target. In addition, cow belching would also be regulated if effective techniques are identified, such as feed changes or additives.

Dr. Ryan McCarthy, senior policy advisor at CARB, said that a new $50 million methane reduction fund includes support and incentives for dairy farmers making changes. “We are looking at a broad range of possible solutions that may be used to achieve these goals,” he explained.

Organic materials that wind up in landfills also generate methane. CARB proposes to divert that waste to food recovery programs, compost facilities, or renewable fuel production. Some landfill methane is already piped off and used as fuel, but anaerobic digestion facilities would be more efficient in harvesting the gas, which can be used as fuel in trash-management vehicles. A joint $40 million effort between CARB and CalRecycle has a target of eliminating disposal of organics in landfills by 2025. Local communities will play a large role in this, and the Bay Area is ahead of the game, as San Francisco and Alameda counties already have food waste diversion programs.

CARB is also working with the California Public Utilities Commission to develop regulations to cut methane emissions from drilling, pumping, transportation, and storage of oil and gas. This leakage is wasteful and expensive, but finding small sources across miles of pipelines and at thousands of wells will require diligence as well as new detection techniques. According to Advanced Projects Advisor Yvette DiCarlo, the Air District has provided input to the new regulations, as the agency would be responsible for implementing them at the local level through permits and enforcement.


Hydrofluorocarbons are fluorinated gases which were introduced as substitutes for ozone-depleting chemicals, but they are still harmful greenhouse gases, typically thousands of times more potent than carbon dioxide. Enforcement of current state regulations on hydrofluorocarbons is a large part of the regional strategy. “For facilities with large refrigeration systems, checking for leaks and making timely repairs is critical for reducing greenhouse gas emissions, and it can also help businesses save money on energy costs and lost refrigerant,” DiCarlo explained. Incentives may also be used in the future to encourage users to phase in alternatives such as carbon dioxide, ammonia, or other refrigerants that have significantly lower greenhouse gas impacts.

Internationally, a new agreement on hydrofluorocarbons was adopted in Kigali, Rwanda, in mid-October. It would reduce global levels by between 80-85 percent by 2047. The U.S. would stop increasing use of hydrofluorocarbons by 2018, and reduce use 10 percent by 2019, based on 2011-13 levels. “We’re excited to see [the agreement],” said McCarthy, “but it alone is not likely to meet the goals we’ve set for ourselves in California. We’ll still be pursuing our own plan to achieve an earlier reduction.”

Cutting short-lived climate pollutants now will have a quick impact while efforts to bring down carbon dioxide continue. CARB estimates that slashing these “super pollutants” will pay off within 15 years. Some climate control advocates are concerned that since reducing short-lived climate pollutants could be cheaper than the cost of reducing longer-lasting carbon sources, attention and funding may shift away from the long-term measures. However, delay in tackling any of these pollutants will have significant consequences.

Leslie Stewart covers air quality and energy for the Monitor.