Ars Technica: The rise of atmospheric carbon dioxide has been unrelenting over the past century. But that isnt the only greenhouse gas humans are adding to the atmosphere, and other have different stories to tell. Methane levels, for example, actually flattened out in the late 1990s, holding pretty steady until continuing an upward climb in 2006. Why, you ask? Well, you arent the only one. Methane is a little harder to get a handle on than CO2, partly because human emissions from things like livestock, rice...