Summers in Earth’s midlatitudes are starting earlier, ending later and becoming more intense.
New research is showing that they now last about 30 days longer than in the 1960s.
Summer length has grown roughly six days per decade since 1990.
Summers in the earth’s midlatitudes, which are the subtropical and temperate zones, are arriving earlier and more abruptly, staying longer and are hotter than previous decades, scientists have assessed.
These rapid changes may impact the physiological ability of humans to adapt and increase energy demands to cool bodies.
The study conducted by the University of British Columbia, Canada published in the Environmental Research Letters said that between 1990 and 2023, the average summer in the geographic regions between the tropics and polar circles lengthened to an average of about six days per decade.
The increase in days is longer than the 4.8 days per decade noticed by previous research investigations until the 2012. “This increase means that on average across the midlatitudes, summer-like conditions in the mid-2020s last roughly 30 days longer than in the 1960s, starting earlier and ending later each year. Summer length has increased for all surfaces: land (excluding coasts), coasts, and oceans, with land experiencing longer summers up until the most recent decade, when ocean summer lengths become longest. Ocean summer length can increase faster than land under similar warming due to a reduced seasonal range in temperature,” the study said.
During the same time period (1990s), the cumulative summer heat stress or accumulated heat grew at 44 degrees Celsius per decade for the northern hemisphere to over three times faster at 14°C per decade between 1961 and 1990.
“This increase means that on average across the midlatitudes, summer-like conditions in the mid-2020s last roughly 30 days longer than in the 1960s, starting earlier and ending later each year,” it said.
The study said that the midlatitude mean summer length is influenced by earlier onset of the seasons and late withdrawal in majority part of the regions except for the coasts in the northern hemisphere where withdrawal is shifting faster than the onset.
“Across land, coasts and oceans in the midlatitudes, the length of time where summer conditions are experienced each year has expanded over 1961-2023. Focusing on 1990-2023, the average rate of growth of summer length across all surface types is roughly six days/decade, yielding a ∼20 d longer summer in 2023 than in 1990. For land, the average rate of growth since 1990 has increased by 100 per cent from that during the baseline period in this study, 1961-1990,” it noted.
In some cities, summer is lengthening by more than one day per year. For instance, Sydney has seen a summer length trend of 14.8 days per decade which means they now last for about 130 days compared to 80 days in the 1990. While Minneapolis shows a trend of 9.3 days per decade, Toronto summers are expanding by eight days per decade.
The increase in accumulated summer heat may challenge the ability of humans in the midlatitudes to physiologically adapt and will likely increase the energy expended for daytime and nighttime cooling, the study said.
Lengthening of the portion of the year with summer-like conditions, the authors of the report noted, has altered the timing of the midlatitude growing season, drought severity, heatwave frequency and severity, energy demand for cooling and economic growth and productivity. In addition, the speed and timing of the transition from spring into summer and from summer into autumn provide important cues to organisms, it added.
The study introduced a concept of accumulated heat, a metric used to describe cumulative summer heat stress by measuring the total intensity and duration of temperatures that exceed a specific local threshold.
It noted that as expected from an upward shift in seasonal temperature profiles, the rate of growth of accumulated heat in summer shows a non-linear increase. “Over northern hemisphere
land, the rate of increase in accumulated heat since 1990 is 300 per cent higher than during the baseline period of 1961–1990,” it said.
This increase in accumulated summer heat with the increasingly sharp transition from spring to summer, and from summer to fall, that this study documents for the first time, leaves less time to adjust for systems that are dependent on these transitions, the authors said.
“More abrupt transitions may lead to more rapid snowmelt events, increasing springtime flooding of saturated or still-frozen ground, more rapid melting of lakes in high latitudes and thus more rapid changes in transportation pathways, as well as deadlier early season heat-waves as people have not yet had a chance to adjust to warmer temperatures,” they added.