Levels of methane in our atmosphere are rising and scientists haven’t yet been able to pinpoint the cause. And while scientists love a good mystery this particular one is pretty starting considering that methane is a potent greenhouse gas. As the LA Times reported, twenty years ago the level of methane in the atmosphere stopped increasing, giving humanity a bit of a break when it came to slowing climate change. But the concentration started rising again in 2007 — and it’s been picking up the pace over the last four years, according to new research.

Phys also explained that methane is produced when dead stuff breaks down without much oxygen around. In nature, it seeps out of waterlogged wetlands, peat bogs and sediments as well as from forest fires. In addition:

• Human activities churn out about half of all methane emissions.
• Leaks from fossil fuel operations are a big source, as is agriculture—particularly raising cattle, which produce methane in their guts.
• Even the heaps of waste that rot in landfills produce the gas.
• The atmosphere contains far less methane than carbon dioxide. But methane is so good at trapping heat that one ton of the gas causes 32 times as much warming as one ton of CO₂ over the course of a century.

Why This Matters: Euan Nisbet, an Earth scientist at Royal Holloway, University of London, and lead author of a recent study reporting that the growth of atmospheric methane is accelerating doesn’t yet know why there’s such an uptick in methane. Some molecules of methane weigh more than others, because some atoms of carbon and hydrogen are heavier than others. And Nisbet’s research has found that lately, the average weight of methane in the atmosphere has been getting lighter which seems to implicate biological sources such as wetlands and livestock. This could be for a variety of reasons such as the fact that climate change could be causing permafrost in the tundra to melt and release methane, but more research needs to be done to reach a conclusion.