The biggest cause of global warming is all the carbon dioxide we’ve expelled into the atmosphere since the beginning of industrial times. The greenhouse gas traps heat in the atmosphere, raising temperatures on Earth. Even so, about one-quarter of the warming we’ve had so far is due to a less notorious greenhouse gas: methane, the major component of natural gas. Methane wasn’t much of a worry 20 years ago, but that’s changed since 2007, as methane emissions have accelerated, spiking in 2014 and again in 2018. Scientists still don’t know exactly what’s going on, and they face an urgent challenge to find out. Methane is a far more potent greenhouse gas than CO2, even though it only lasts about a decade in the atmosphere, whereas CO2 persists for a couple of centuries. A continued rise in the amount of methane in the air could easily cancel out any near-term progress we make in reducing CO2 emissions. Methane is the low-hanging fruit in the effort to combat planetary warming. This gas has also been in the news for political reasons, as the Donald Trump administration has moved to rescind rules requiring fossil fuel companies to monitor and fix methane leaks, despite many companies’ opposition. Fossil fuel production also contributes to methane emissions, and some studies have pointed to hydraulic fracturing, especially in the US, as the likely cause behind the recent spike in emissions. It’s certainly suspicious that fracking got underway in a big way around 2007, just as the methane surge began. However, other recent studies dispute this conclusion, and they suggest that US methane emissions have changed little over the past 15 years. More worrying are other methane sources over which humans have much less control. An international consortium of researchers has been tracking emissions with detailed measurements in the Arctic, Europe and the tropics, as well as with modeling studies. What they have found suggests that the methane surge since 2007 appears to be mostly the result of increased biogenic emissions, predominantly in the tropics. Among several factors, this activity has been accelerated by warming temperatures and increased rainfall causing the expansion of tropical wetlands. Further increases in emissions come from agricultural animals, including cows, sheep and goats. Reversing this trend in methane emissions is now probably the most urgent challenge in the fight against global warming, even more than the ongoing need to tackle CO2 emissions. An important assessment published recently offers mixed views on what can be done. The bad news is that reducing emissions from swamped wetlands and cows may be virtually impossible. But there’s good news, too. Many other sources of methane shouldn’t be too hard to tackle, especially emissions from production of coal and gas, urban gas leaks, landfills and sewage plants. New technological devices such as vehicle-mounted leak detectors and drones are making it much easier to locate the biggest sources of emissions, which can then be eliminated. Landfills can be covered. Fuel industries must also recognize the need to reduce leaks of natural gas to preserve its reputation as a green energy source. Methane emissions can also be reduced by not burning crop waste, as is common in Africa and in South and East Asia. Our success in tackling CO2 emissions has been abysmal. After 40 years of debate, emissions last year were higher than ever before. But methane is another problem that is perhaps even more urgent. Sadly, if we can’t reduce methane emissions, any tentative progress we do make on carbon dioxide will be canceled out. Bloomberg