“Global warming skeptics sometimes say rising temperatures are just another naturally occurring shift in Earth’s climate, like the Medieval Warm Period of the years 800 to 1200 or the Little Ice Age, a period of cooling that spanned from roughly 1300 to 1850. But a pair of studies published Wednesday provides stark evidence that the rise in global temperatures over the past 150 years has been far more rapid and widespread than any warming period in the past 2,000 years — a finding that undercuts claims that today’s global warming isn’t necessarily the result of human activity.”