Should we reconsider how we define the four seasons of the year?

For those residing in the Earth's middle latitudes, experiencing four distinct seasons—winter, spring, summer, and fall—is a common occurrence.

Living between 30 and 60 degrees north and south of the tropics provides a far more diverse climate compared to the equator, where the year is divided into a hot dry season and a hot rainy season. Similarly, regions in the higher latitudes face cold winters with extended dark hours and less harsh summers with prolonged daylight.

Though it may seem surprising to some, there are actually two main ways to define the seasons: astronomical and meteorological. In this article, we’ll explore these methods and examine whether it's time to rethink our approach to the seasons as we know them.

Astronomical Seasons

Astronomical seasons are defined by Earth's position in its orbit around the sun and its axial tilt. There are four such seasons:

Spring

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

At this point, the Earth's axial tilt aligns in such a way that the sun crosses the celestial equator, moving from south to north. This signals the start of spring, bringing longer days and typically warmer temperatures.

Summer

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

In the Southern Hemisphere, the opposite happens, with the South Pole tilted closest to the sun. Summer is generally associated with warm to hot weather.

Fall (Autumn)

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

Fall is marked by a reduction in daylight and a drop in temperatures as the shift from summer to winter begins.

Winter

Winter starts with the winter solstice, which occurs around December 21 or 22 in the Northern Hemisphere and June 20 or 21 in the Southern Hemisphere. On this day, the North Pole is tilted furthest away from the sun, leading to the shortest day and longest night in the Northern Hemisphere.

In the Southern Hemisphere, the situation is reversed, with the South Pole tilted farthest away from the sun. Winter is generally associated with cold weather and is often accompanied by snowfall in various regions.

Meteorological Seasons

This system for categorizing the seasons relies on calendar months and temperature trends, primarily for purposes such as data collection and climate studies.

Unlike astronomical seasons, which are determined by the Earth's alignment with the sun, meteorological seasons divide the year into four equal three-month periods.

An Unsuccessful Effort to Redefine the Seasons

A 1983 study by climate scientist Kevin Trenberth revealed that the meteorological approach better aligned with observable weather patterns in the Northern Hemisphere's continental regions. In contrast, the astronomical model was more accurate over the oceans in the Southern Hemisphere.

Despite the changes, the astronomical definition of the seasons remains the most widely used in the United States. "After my article, some places attempted to make a change, but it eventually fizzled out," says Trenberth, a renowned scientist at the National Center for Atmospheric Research and co-recipient of the 2007 Nobel Peace Prize, via email.

Climate Change's Impact on the Seasons

Primarily driven by human activities, climate change has been disrupting the traditional idea of four seasons for some time. Researchers have found that as the Earth warms, the tropics have expanded by 0.1 to 0.2 degrees of latitude each decade, causing regions once experiencing four seasons to shift toward a two-season pattern.

Even in areas that still experience four seasons, weather and temperature trends have been altered. In the United States, the transition from winter’s chill to warmer spring temperatures occurs earlier than before, while the winter period has become shorter and generally milder.

Heatwaves are also beginning later in the summer months. A prolonged period of record-breaking heat hit the central United States in late August 2023, with the most extreme conditions occurring on August 23 and 24 in northern Illinois and northwest Indiana.

This marked the first time since the 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 analyzed seasonal changes in the Northern Hemisphere from 1952 to 2011 and concluded that global warming has impacted the duration and temperatures of the four seasons.

Historically, the year was divided into four equal-length seasons, but this is no longer true. Nowadays, summer has become longer, while spring, autumn, and winter have become shorter.

Summers have begun earlier and are extending by 4.2 days every 10 years, resulting in a 17-day increase in summer length over the past 50 years. This shift has led to hotter summers, with more frequent and longer heatwaves occurring between May and September.

In contrast, winters, springs, and autumns have all become shorter. Over the same period, spring has shortened by nine days, autumn by five days, and winter by three days. These changes are a result of shifts in when these seasons begin and end. Spring and summer are starting earlier, while autumn and winter are beginning later.

Temperatures have also changed, with summers growing longer and hotter, and winters also warming up. In northern North America, winter temperatures have increased by more than 0.4 degrees Celsius every decade. These shorter, warmer spring and autumn seasons have become the new normal due to shifts in the timing of seasons and temperature patterns.

Will the Seasons Become Obsolete?

With climate change driving seasonal shifts and the ongoing warming trend, one might wonder if the concept of four seasons will eventually fade away. Trenberth doesn't foresee this happening, but the way we define the seasons may change. While we will still have winter, spring, summer, and fall in the middle latitudes, the timing and length of each season 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, 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, "Summer is not only defined by temperature but also by the nature of the weather — more convective, with thunderstorms, etc., compared to winter's extra-tropical storms, cold fronts, and the like. One analysis suggests that summers are now 13 days longer, while winters have shortened by 20 days."

What Does the Future Hold for the Seasons?

Even if the current rate of warming remains steady, major changes in seasons will still occur in the future. By the end of this century, spring and summer may start a month earlier, while autumn and winter could arrive about half a month later. Summers could stretch to nearly half of the year, leaving less than two months for winter by 2100.

These shifts are anticipated to disturb agricultural cycles and the natural rhythms of various species. Earlier plant blooming and altered bird migration patterns could have significant effects on ecosystems.

Milder winters may harm crop production as insufficient cold temperatures fail to meet the chilling requirements for bud dormancy, leading to reduced crop quality and yields. Additionally, longer summers could bring about more frequent heatwaves, extreme storms, and extended wildfire seasons, all of which present challenges for both ecosystems and human societies.