Latitude and longitude play a significant role in determining where people live. About half of the world’s population resides north of 27 degrees latitude. Equatorial regions experience hot, dry, and rainy seasons. However, the areas between 30 and 60 degrees latitude north and south witness more climatic variation, experiencing distinct seasonal changes.
But are transitional seasons like spring and fall shortening due to global climate change? There are two ways to define the four seasons: astronomical and meteorological. This article will explore these two methods and discuss whether we should redefine our understanding of the seasons.
Astronomical Seasons: Based on Earth’s Orbit
Astronomical seasons are defined by Earth’s position in its orbit around the sun and the tilt of its axis. The year is divided into four astronomical seasons: 
Spring: The Start of Spring
Spring begins at the vernal equinox. This occurs on March 19-21 in the Northern Hemisphere and September 22 or 23 in the Southern Hemisphere. Earth’s tilt causes the sun to cross the celestial equator, moving from south to north. This marks the start of spring, with increasing daylight and rising temperatures.
Summer: The Longest Day of the Year
Summer begins at the summer solstice, occurring on June 20 or 21 in the Northern Hemisphere and December 21 or 22 in the Southern Hemisphere. On this day, the North Pole is closest to the sun, resulting in the longest day and shortest night of the year in the Northern Hemisphere. In the Southern Hemisphere, the opposite occurs. Summer is typically associated with warm to hot temperatures.
Fall: The Transition to Winter
Fall begins at the autumnal equinox, which occurs on 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, moving from north to south. Fall is marked by decreasing daylight, cooling temperatures, and the transition into winter.
Winter: The Coldest Season
Winter begins at the winter solstice, occurring on December 21 or 22 in the Northern Hemisphere and June 20 or 21 in the Southern Hemisphere. On this day, the North Pole is farthest from the sun, resulting in the shortest day and longest night of the year in the Northern Hemisphere. In the Southern Hemisphere, the situation is reversed. Winter is characterized by cold temperatures and snow.
Meteorological Seasons: Defined by Calendar and Temperature
Meteorological seasons are based on calendar months and temperature patterns, primarily used for data collection and climate analysis. Unlike astronomical seasons, meteorological seasons divide the year into four equal three-month periods:
Spring: End of Winter
Spring is defined as March, April, and May in the Northern Hemisphere and September, October, and November in the Southern Hemisphere. This period generally marks the end of winter and a rise in temperatures.
Summer: The Hottest Months
Summer occurs in June, July, and August in the Northern Hemisphere and December, January, and February in the Southern Hemisphere. These months typically have the highest temperatures of the year.
Fall: Cooling Down
Fall spans September, October, and November in the Northern Hemisphere and March, April, and May in the Southern Hemisphere. This season is characterized by decreasing temperatures and the shedding of leaves from deciduous trees.
Winter: The Coldest Months
Winter is defined as December, January, and February in the Northern Hemisphere and June, July, and August in the Southern Hemisphere. These months generally have the lowest temperatures.
Failed Attempts to Change Season Definitions
In 1983, climate researcher Kevin Trenberth suggested that meteorological seasons better align with weather patterns in continental areas of the Northern Hemisphere. In contrast, astronomical seasons were more suitable for oceanic regions in the Southern Hemisphere. Despite this, the U.S. continues to use astronomical definitions of the seasons. Trenberth noted, “Although there were attempts to make a change after my research, it never really took hold.”
Climate Change Impacts on the Seasons
Human activities have caused climate change, which is disrupting the traditional concept of four seasons. Scientists have observed that as the world warms, tropical areas are expanding outward by 0.1 to 0.2 degrees of latitude each decade. This is causing some areas that once experienced four seasons to transition to having only two.
Even in regions with well-defined seasons, weather and temperature patterns are shifting. In the U.S., warming from winter to spring is occurring earlier, leading to shorter and milder winters. Heat waves are also emerging later in the summer. For example, in late August 2023, the central U.S. saw record-breaking heat, with northern Illinois and northwest Indiana experiencing extreme heat on August 23rd and 24th. This marked the first time since July 1995 that Chicago had consecutive days with heat indexes exceeding 115°F (46°C).
A 2021 study published in Geophysical Research Letters showed that global warming has altered the length and temperatures of Northern Hemisphere seasons between 1952 and 2011. The study found that summers have become longer, while spring, fall, and winter have shortened. Summers are extending by an average of 4.2 days per decade, adding up to 17 extra days over the last 50 years. This results in more frequent and prolonged heat waves between May and September. Conversely, winter, spring, and fall have shortened by 9, 5, and 3 days, respectively. Additionally, the starts and ends of seasons are shifting, with spring and summer beginning earlier, and fall and winter starting later. Temperatures are rising, with longer and hotter summers and warmer winters. In northern North America, winter temperatures have risen by more than 0.4°C each decade.
Will Seasons Become Obsolete?
With the changing seasons and overall warming trends driven by climate change, one might wonder if the concept of four seasons will become obsolete. Trenberth doesn’t believe this will happen, but the definitions may soon change. Mid-latitude regions will still have winter, spring, summer, and fall, but their start times and durations will be different.
He explains that the key factor is the idea of four seasons, especially two extremes—summer and winter—and two transitional periods. These transitional seasons could be redefined and shortened. “In some of our analyses, we use November through March and May through September, leaving October and April as transition months,” said Trenberth. In the U.S., spring is now warmer than fall.
Trenberth emphasizes that summer involves more than just temperature. It also includes weather characteristics like increased convection, instability, and thunderstorms. Winter, on the other hand, is about mid-latitude storms and cold fronts. Analyses have shown that summer has gained 13 extra days, while winter has lost 20 days compared to the past.
What Will Future Seasons Look Like?
Even if the current pace of warming doesn’t accelerate, future seasons will look dramatically different. By the end of the century, spring and summer could start a month earlier, while fall and winter could begin half a month later. Summers may last almost half the year, and winters may last less than two months by 2100.
While the traditional four seasons in mid-latitude regions may not disappear, their lengths and intensities are changing. These changes are expected to disrupt agricultural seasons and the natural rhythms of species. For example, plants may flower earlier, and bird migrations could shift, disrupting ecological communities. Warmer winters could negatively impact crop yields because mild conditions may fail to provide enough chilling for bud dormancy, affecting crop quality and productivity. Longer summers may also lead to more frequent heat waves, severe storms, and longer wildfire seasons, presenting challenges for ecosystems and human societies.
