"One problem: Natural variability in Earth’s climate makes some years warmer than others, says Richard Seager of Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. “The dominant features,” he notes, “are El Ni±os, during which the global mean temperature goes up, and [countervailing] La Ni±a events where the global mean temperature goes down.” But other factors — from volcanic eruptions and local cloudiness to human-caused changes like urban development and crop irrigation — can also affect winds, rainfall and sunlight.
This variability tends to smooth out on the global scale, as local effects blend together. “But as you move to smaller spatial and temporal scales,” Seager says, “the amount of random variability in climate gets bigger and bigger relative to any global-warming signal.”
He says that’s why attribution studies have largely focused on weather events that tend to be regionally broad and long-lived."