Oil seeping from the seafloor may have contributed to climate change long before the internal combustion engine did. The petroleum deposits are rich in the powerful greenhouse gas methane, which, according to a new study, may have played a major role in two previous episodes of global warming. Bedrock below the ocean bottom keeps a lid on oil reservoirs, but it's not an impermeable cap. Small cracks allow petroleum and methane to bubble to the surface. Once there, the petroleum oxidizes and turns to tar, which sinks. Meanwhile, the methane drifts into the atmosphere, where it makes up about 15%...