The Arctic ice cap may be thinning, but the extent of Antarctic sea ice has been expanding for years. At least as of February, last winter was the Northern Hemisphere's coldest in decades. In May, German climate modelers reported in the journal Nature that global warming is due for a decade-long vacation. But be not not-afraid, added the modelers: The inexorable march to apocalypse resumes in 2020.
This last item is, of course, a forecast, not an empirical observation. But it raises a useful question: If even slight global cooling remains evidence of global warming, what isn't evidence of global warming? What we have here is a nonfalsifiable hypothesis, logically indistinguishable from claims for the existence of God. This doesn't mean God doesn't exist, or that global warming isn't happening. It does mean it isn't science.