If you live in the central or eastern United States and have been outside lately, you can attest to the downright summery weather we’ve been experiencing. In fact, this March weather is not just unusual; it is unprecedented. In Detroit, there has not been a comparable spring heat wave since 1886 , and that warm spell occurred a full month later (April 16-24). In Chicago, last week’s high temperatures in the low 80’s  are similar to Chicago’s average high in August (82°).
Daily record highs have been falling in droves across the region, with some remarkable occurrences . One weather station in Michigan hit 85°F, breaking the previous daily record high by an unheard of 32°, which is also 48° above average. Two stations recorded low temperatures that beat the previous record high, something that experienced weatherman Jeff Masters had never seen before . This record warmth is not confined to the United States. Several Canadian cities surpassed both their all-time March and April records this week, an amazing feat considering the vast differences between March and April during a normal spring.
This historic heat wave was also associated with the most humid air ever observed this early in the year in the Midwest. An increase in high-humidity heat waves  is undesirable because the humidity increases the heat index and prevents nighttime cooling, both of which increase the health impacts of heat events.
Unfortunately, global warming is loading the dice in favor of events like this. As Weather Channel Meteorologist Stu Ostro  said, “This remarkable warmth is associated with a bulging ridge of high pressure aloft that is exceptionally strong and long-lasting for March. While natural factors are contributing to this warm spell, given the nature of it and its context with other extreme weather events and patterns in recent years there is a high probability that global warming is having an influence upon its extremity.” In other words, we might well be having an unusually warm spring in any event, but global warming probably pushed it into unprecedented territory.
Interestingly, the jet stream configuration that is generating heat in the east has also caused very un-summerlike weather in the western United States—on the other side of the high-pressure dome. While the Midwest was baking under summer temperatures, Arizona and New Mexico were experiencing an unusually late snowstorm , and in Eugene, Oregon, the largest snowstorm ever recorded this late in the season  blanketed the city with fresh snow. Extreme meandering of the jet stream, responsible for both late snow and early summer, has been connected recently to global warming through decreases in the arctic sea ice cover.
Although some people may be enjoying the early arrival of summer, others are concerned about the impacts. Warm winters  are associated with an increase in pests that are able to survive locally or penetrate farther north. These pests can damage crops or spread diseases (e.g., more ticks carrying Lyme disease). And allergists have recorded the highest pollen counts ever in the South and Midwest. One clinic in Nashville recorded 11,000 grains of pollen per cubic meter, far above the “high” threshold of 1,500. And the moist conditions that warm air brings can cause heavy rains that inundate soils and delay spring planting. The more often such conditions prevail from year to year, the more often such problems will cost money for farmers, make people sick from allergies and climate-sensitive diseases, and put pressure on existing infrastructure.
We are fortunate that this heat wave did not occur during the peak of the summer. At 4-5 standard deviations warmer than normal  we could be talking about temperatures in the 110s in Michigan instead of the 80s, with all of the associated power shortages, heat stroke, livestock deaths, etc. Even so, this historic bout of warm weather offers important lessons about the consequences of climate change. This event is outside the historical experience of the United States in more than 100 years of records. We know that the risk of extreme weather events is increasing with global warming, and this most recent episode represents further evidence that history is not a good guide for assessing the level of risk we face in the future.
Dan Huber is a Science & Policy Fellow at C2ES.