By Jason Mark for Earth Island Journal
“Methane is 21 times more heat-trapping that carbon dioxide.” If you’re a frequent reader of environmental websites, no doubt you’ve seen some version of that sentence many times. The “twenty-times” figure is the most common way of explaining how methane (or CH4, or uncombusted natural gas) reacts in the atmosphere.
Just one problem: It’s not entirely accurate — at least not in the time-scale we should be using to think about how to tackle greenhouse gas emissions.
Actually, any CH4 released today is at least 56 times more heat-trapping than a molecule of C02 also released today. And because of the way it reacts in the atmosphere, the number is probably even higher, according to research conducted by Drew Shindell, a scientist at NASA’s Goddard Space Center.
So why is the 21 times figure the one that gets bandied about? Because methane breaks down much faster than carbon dioxide.
While CO2 remains in the atmosphere for at least a century (and probably much, much longer, according to Stanford’s Ken Caldeira), CH4 lasts only about a dozen years. When the Intergovernmental Panel on Climate Change had to come up with a way for comparing different greenhouse gases, it decided to use a century baseline to calculate a molecule’s “global warming potential.”
See this graph on global warming potential values for some key GHGs by the GHG Management Institute (you’ll have to scroll down a bit to get to it)
While these various comparisons are well understood by climatologists and climate change policy wonks, I’m guessing they’re not that well known among the general public.
Why does it matter? Because we don’t have a century to get serious about the impacts of runaway greenhouse gas emissions.
Read the rest of this article at http://www.earthisland.org/journal/index.php/elist/eListRead/methanes_contribution_to_global_warming_is_worse_than_you_thought/
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