If you live in Earth’s middle latitudes, you are used to experiencing four traditional seasons: winter, spring, summer and autumn.
The two areas around the planet, from 30 to 60 degrees north and south of the tropics, offers significantly more variety, weather-wise, than land on the equator, where there is basically either hot dry or hot rainy. For the upper latitudes, you can experience a cold winter with long dark nights, and a somewhat less-cold summer with longer daylight.
As the planet warms up, the tropics have been expanding by 0.1 to 0.2 degrees of latitude per decade. Therefore, places that once had four seasons are shifting to just two. Yet even in current regions with four seasons, climate and temperature patterns have been altered.
Astronomical definition – based upon the change in the length of days, and caused by the relative tilt of the Earth’s axis as it revolves around the Sun. In this system, winter is the period between the winter solstice — the shortest period of daylight of the year — and the vernal equinox on December 22, when day and night are roughly equal, 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 September 23, another day where day and night are equal. Then, autumn continues until the winter solstice to complete the circle.
Meteorological method – it 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 found that the meteorological definition was more closely agreed with observable weather in the continental regions of the northern hemisphere, while the astronomical definition fits better over the oceans in the southern hemisphere.
Climate change, mainly driven by human activity in combination with general warming trends, makes us question whether the concept of four seasons may ultimately become obsolete! The opinion is that it will not happen, however, the definition of the seasons may ultimately change.
A key point behind the idea of four seasons is that there are two extreme seasons – summer and winter – and two transition seasons.
A slight change in temperature is sufficient to push the spring thaw earlier, and defer the first frost until later in the autumn.
Spring is arriving earlier, winters are shorter, and the number of freezing days is decreasing. For example, an earlier spring may prompt longer growing seasons, more abundant invasive species and pests, and an earlier and longer allergy seasons. Curiously, warm weather in late winter can create a “false spring” that triggers the new growth of plants to begin too early. This will leave them vulnerable to any subsequent frosts.
Disruptions in the timing and length of these events — spring thaw or songbird migration for example – can have a variety of consequences on ecosystems and human society. This is because diverse species may respond to different environmental cues, i.e. you lose one, you may start a domino effect
Summers in Europe are coming ten days ahead of the calendar schedule. Over the past four decades, the annual switch from winter to summer weather patterns – which had occurred around April 10 at the start of the 1960s – suddenly started jumping forward. By 2010, the change was occurring on March 30. And by 2100, Europe’s “summer” could be up to 20 days sooner than in pre-industrial times, if current trends continue.
The impacts are seen with earlier budburst for grapes in France, flowering cherry trees in Switzerland, blooms of summer flowers in England, and other yearly seasonal events. For instance, birds shifting migrations and butterflies arriving early, provide further evidence.
And it is not just Europe. In the United States, springs and summers have been arriving more than a day-and-a-half earlier since 1948. In China, a study of daily temperature data found that summers have crawled forward by nearly six days between 1951 and 2000, while the winter season shrank by 11 days.
Extended T-shirt weather may sound good when shovelling snow or slogging through April muck. But even little changes to the seasonal clock can set off a torrent of negative impacts in nature.