If natural gas begins to flow through the Mountain Valley Pipeline a year from now, as its developers expect, the operation will produce about 730,000 metric tons of greenhouse gases per year. Airborne emissions of carbon dioxide from three compressor stations along the 303-mile pipeline, along with methane expected to leak from the buried steel pipe, have long been a concern of opponents who say that delivering huge amounts of fossil fuel to markets will only worsen a climate change problem that is rapidly overheating the earth. On July 12, Mountain Valley announced a plan: The company will spend at least $150 million over the next 10 years on carbon offsets, which will be used to construct a massive methane abatement system at a coal mine in far Southwest Virginia.