The winter of 2011-2012 was, across much of the Northern Hemisphere, conspicuously absent. In Chicago, where winter typically announces itself with conviction in November and doesn't release its grip until April, residents found themselves wearing light jackets in January and watching crocuses emerge weeks ahead of schedule. Similar reports came from the East Coast, from Europe, and from Russia.

The meteorological explanation involved a persistent pattern in the atmospheric circulation over the North Atlantic — the kind of blocking high pressure that routes cold Arctic air away from populated areas — combined with La Niña conditions in the Pacific that suppressed moisture and cold air delivery across North America.

For residents accustomed to shoveling and salt, the mild winter was mostly delightful. Heating bills were lower. Fewer people slipped on ice. Outdoor café dining extended well past its normal season.

The environmental and ecological consequences were more complicated. Fruit trees bloomed early, exposing blossoms to the late frosts that eventually arrived. Invasive species that normally die back in cold winters survived in higher numbers. Ski resorts, dependent on natural snowfall, struggled.

Scientists were careful about attribution — one mild winter does not confirm a trend, and natural variability in weather patterns can produce anomalous seasons in either direction. But the pattern fit projections about how climate change would affect winter severity in temperate regions, and the conversation it prompted about what "normal" weather might mean in a warming world was one that would grow louder in years to come.

That winter, for many people, was the first time climate change felt like something personal rather than abstract.