Carbon dioxide is the gas we most associate with global warming, but methane gas also plays an important role. For reasons that are not well understood, methane gas stopped increasing in the atmosphere in the 1990s. But now it appears to be once again on the rise. Scientists are trying to understand why — and what to do about it.Methane gas comes from all sorts of sources including wetlands, rice paddies, cow tummies, coal mines, garbage dumps and even termites.Molecule for molecule, methane is much more effective than carbon dioxide at trapping heat in the atmosphere. And that’s just part of the trouble.

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