Is It Time to Redefine the Four Seasons of the Year?

Astronomical seasons are seasons that are defined based on the position of the Earth in its orbit around the sun and its axial tilt. There are four astronomical seasons:

Spring

Spring begins with the vernal equinox, also known as the spring equinox, which occurs between March 19-21 in the Northern Hemisphere and around September 22 or 23 in the Southern Hemisphere.

During this time, the Earth's axial tilt is such that the sun crosses the celestial equator, moving from south to north. This marks the start of spring, characterized by longer daylight hours and generally warming temperatures.

Summer

Summer begins with the summer solstice, which happens around June 20 or 21 in the Northern Hemisphere and around December 21 or 22 in the Southern Hemisphere. During the summer solstice, the North Pole is tilted closest to the sun, resulting in the longest day and the shortest night of the year in the Northern Hemisphere.

In the Southern Hemisphere, it's the opposite, with the South Pole tilted closest to the sun. Summer is typically associated with warm to hot temperatures.

Fall (Autumn)

Fall begins with the autumnal equinox, which occurs around September 22 or 23 in the Northern Hemisphere and around March 20 or 21 in the Southern Hemisphere. Similar to the vernal equinox, the sun crosses the celestial equator during the autumnal equinox, moving from north to south.

Fall is characterized by decreasing daylight hours and cooling temperatures as the transition from summer to winter takes place.

Winter

Winter begins with the winter solstice, which takes place around December 21 or 22 in the Northern Hemisphere and around June 20 or 21 in the Southern Hemisphere. During the winter solstice, the North Pole is tilted farthest away from the sun, resulting in the shortest day and the longest night of the year in the Northern Hemisphere.

In the Southern Hemisphere, it's the opposite, with the South Pole tilted farthest away from the sun. Winter is typically associated with cold temperatures and often includes snowfall in many regions.

This system for defining seasons is based on calendar months and temperature patterns and is primarily used for the purpose of data collection and climate analysis.

Unlike astronomical seasons, which are based on the Earth's position relative to the sun, meteorological seasons divide the year into four equal three-month periods.

A Failed Attempt to Change Things Up

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. Meanwhile, 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.

Largely driven by human activity, climate change has been messing with the traditional concept of four seasons for a while now. 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 weather in the winter to warm spring temperatures happens earlier now than it did in the past, and the period of winter weather is shorter and generally milder.

Even the heatwaves are starting later in the summer months. An extended period of record-breaking heat struck the central United States in late August 2023, with the worst conditions occurring on August 23 and 24 in northern Illinois and northwest Indiana.

This marked the first time since a July 1995 heatwave that Chicago experienced consecutive days with heat indices exceeding 115 degrees Fahrenheit (46 degrees Celsius).

Longer Summers, Shorter Winters

A 2021 study published in Geophysical Research Letters examined seasons in the Northern Hemisphere from 1952 to 2011 and found that global warming has altered the length and temperatures of the four seasons.

Traditionally, a year was divided into four equal-length seasons, but this is no longer the case. In this period, summer has lengthened, while spring, autumn and winter have shortened.

Summers have started earlier and extended by 4.2 days every 10 years, resulting in a 17-day increase in summer length over the last half-century. This has led to hotter summers with more frequent and prolonged heatwaves between May and September.

In contrast, winters, springs and autumns have all become shorter. Spring has decreased by nine days, autumn by five days, and winter by three days over the same time frame. These changes are attributed to shifts in the onset and withdrawal of these seasons. Spring and summer start earlier, while autumn and winter begin later.

Temperatures have also changed, with summers becoming longer and hotter, while winters have also warmed. In northern North America, winter temperatures have risen by more than 0.4 degrees Celsius every 10 years. Shorter, warmer spring and autumn seasons have become the new norm due to these shifts in season timing and temperature patterns.

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 soon change. We'll still have winter, spring, summer and fall 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."

Even if the current rate of warming doesn't speed up, there will still be significant changes in seasons in the future. By the end of this century, spring and summer could begin a month earlier, and autumn and winter might arrive half a month later. Summers could extend to nearly half the year, with less than two months of winter by 2100.

These changes are expected to disrupt agricultural seasons and the natural rhythms of species. Early plant flowering and bird migrations could affect ecological communities.

Warmer winters may negatively impact crop yields as milder conditions lead to insufficient chilling required for bud dormancy, resulting in lower crop quality and yields. Additionally, longer summers could lead to more frequent heatwaves, severe storms and extended wildfire seasons, posing challenges for ecosystems and human societies alike.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.