My answer is the difference is the same as the difference between long-term trends and short-term fluctuations. Permafrost melts due to year-after-year temperature increases and means possibly significant change. Freakish weather is a happenstance due to particular circumstances and doesn't mean much past its own existence.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko