If you live in Earth's middle latitudes, you're accustomed to experiencing four traditional seasons: winter, spring, summer and autumn, also known as fall. That existence in the two bands stretching across the planet from 30 to 60 degrees both north and south of the tropics offers a lot more variety, weather-wise, than on the equator, where there's basically a hot dry season and a hot rainy season. Ditto for the upper latitudes, whose residents get a cold winter with long dark nights and a slightly less-cold summer with longer daylight.
To be technical about it, there actually are two different ways of defining the seasons. There's the astronomical definition, which is based upon change in the length of days, which is caused by the relative tilt of the Earth's axis as it revolves around the Sun. In that system, winter is the period between the winter solstice — the shortest period of daylight of the year — and the vernal equinox on Dec. 22, when day and night are roughly equal, which occurs around March 21. Spring then lasts from the vernal equinox to the summer solstice on June 22, which is the longest daylight period. Summer runs from then until the autumnal equinox on Sept. 23, another day when light and darkness are equal. Then, autumn continues until the winter solstice.
The meteorological method, on the other hand, has been around since at least the mid-1900s and is a lot simpler. The year is divided by four, so that winter is December-January-February, spring is March-April-May, summer is June-July-August, and autumn is September-October-November. A 1983 study conducted by climate researcher Kevin Trenberth found that the meteorological definition more closely agreed with observable weather in the continental regions of the northern hemisphere, while the astronomical definition only fit reality better over the oceans in the southern hemisphere.
Nevertheless, the astronomical definition of the seasons continues to be the one generally used in the United States. "After my article, there were a few places that tried to change, but it petered out," says Trenberth, a distinguished scientist at the National Center for Atmospheric Research and one of the co-recipients of the 2007 Nobel Peace Prize, via email.
But climate change largely driven by human activity has been messing with the traditional concept of four seasons. Scientists have discovered that as the planet warms up, the tropics have been expanding by 0.1 to 0.2 degrees of latitude every decade, so that places that once had four seasons are shifting to having just two. But even in regions with four seasons, weather and temperature patterns have been altered. Across the United States, the shift from cold winter weather to warm spring temperatures happens earlier now than it did in the past, and the period of winter weather is shorter and generally milder. In the first few months of 2017, some eastern U.S. cities even were beset by startling summerlike temperatures in February, with many reaching or surpassing their all-time temperature records.
Climate change-induced seasonal creep, coupled with the overall warming trend, might make you wonder whether the concept of four seasons may eventually become obsolete. Trenberth doesn't see that happening, but the definition of the seasons may change. We'll still have winter, spring, summer and autumn in the middle latitudes — but the timing and duration will be different.
"A key point though is the idea of four seasons, and the way I think of it is the two extreme seasons, summer and winter, and two transition seasons," says Trenberth. "One could redefine the latter to be shorter. In some sense, the summer is becoming longer. In some of our analyses we use N-D-J-F-M, and M-J-J-A-S with October and April as transition months! Spring has warmed a bit more than autumn in the U.S."
He adds, "The summer is not just temperature, but also the character of the weather — more convective, thunderstorms, etc., versus winter's more extra-tropical storms, cold fronts, etc. There is one analysis that suggests summers are now 13 days longer and winters are 20 days shorter than they used to be."