Dave Roberts coined the term. Yglesias provides a definition:
Well of course much like a deficit hawk or a national security hawk or an inflation hawk, a climate hawk is tough-minded and awesome and entitled to worshipful media coverage. We’re very serious people who want to confront the major challenges of our time. Are we environmentalists? Perhaps. But many of us aren’t really “nature-lovers,” we just think it would be unfortunate if low-lying areas were flooded, while vast new regions of the earth are stricken with drought. We recognize that the particulate pollution from burning coal and the geopolitical consequences of oil dependence are both dire enough to make a compelling case for energy reform even apart from the greenhouse gas issue.
(Hat tip: Cohn)