As late as the second half of the last century, April often brought alternating warm and cold spells, May tended to be a period of changeable weather, and genuinely summery conditions usually arrived only in June. In recent decades, however, many people have begun to feel as though spring is disappearing.
According to a new study, this is not merely a feeling, but a keen awareness of real changes in the world. After a few warm days in spring, summer weather arrives quickly; the first tropical temperatures are appearing earlier and earlier, and the heat often persists well into September. Research published in the journal Environmental Research Letters suggests that this is not merely a subjective impression. According to an international team of scientists, summer in many temperate regions around the world is not only getting longer, but is also starting earlier than before, and so-called accumulated heat, or heat stress, is also increasing. Climate change is therefore not only affecting average temperatures, but is altering the very rhythm of the seasons.
The study’s authors analysed long-term temperature trends over land and oceans, focusing on how periods with typically summer-like conditions are changing. They did not, of course, use a calendar or astronomical definition of summer. Instead, they defined summer from a meteorological perspective, based on temperature characteristics — as the period when temperatures match what was historically typical for the summer months in a given area between 1961 and 1990. This method therefore takes local climatological conditions into account.
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The results showed that, since the 1990s, the length of summer, as defined here, has been increasing at a rate of approximately five to seven days per decade. Over the last thirty years, the length of summer has thus increased by two to three weeks in a number of regions. In the preceding period, between 1961 and 1990, the rate of increase was somewhat slower, at around four days per decade. This means, however, that since the 1960s, summer has lengthened by roughly 30 days.
At the same time, the transitional periods between spring and summer, and between summer and autumn, are becoming shorter. It is precisely this change in the rate at which the seasons transition that ranks among the study’s most interesting findings. The atmosphere is warming to such an extent that ‘summer’ temperature thresholds are being exceeded with increasing frequency. As a result, spring lasts for a shorter period in many areas and the onset of summer conditions tends to be more sudden.
This is consistent with the experience of recent years in Central Europe as well. Whereas it used to be quite common to have prolonged periods of mild weather, we are now increasingly seeing situations where temperatures jump from (sometimes early) spring levels straight to summer or even tropical levels within a matter of days. The same applies to the transition from summer to autumn.
The study also examined what is known as ‘accumulated heat’ – this metric calculates the cumulative total for the period during which the average daily air temperature exceeds a given summer threshold. This indicator describes not only the number of warm days, but also the total amount of thermal energy that accumulates over the course of the season. And it is here that the changes are even more pronounced.
Over the land masses of the Northern Hemisphere, the rate of increase in accumulated heat since 1990 has more than tripled compared with the trend for the period 1961 to 1990. However, this non-linear increase in accumulated heat is to be expected – a shift in the seasonal temperature profile towards higher temperatures leads both to an increase in intensity (the number of degrees above the threshold) for all summer days and to an increase in the number of days included, resulting in a non-linear increase in accumulated heat even with linear warming.
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It is therefore true that summer is not only longer, but also tends to be more intense. Hot spells last longer, night-time temperatures remain higher, and both the atmosphere and the soil heat up more during the summer season.
This has a number of consequences for both nature and society. Above all, it places a significant burden on health, particularly for the elderly, young children and those with chronic illnesses. The health risk is not only linked to high daytime temperatures, but much more frequently to the fact that the body struggles to recover during warm nights, particularly when people in homes without air conditioning are unable to cool their bedrooms effectively. This is especially true at the start of the summer season, when the human body has not yet adapted to high temperatures.
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Longer and hotter summers are also having a significant impact on the landscape. Higher temperatures increase water evaporation from the soil and vegetation, which often exacerbates drought and thus increases the need for irrigation of agricultural crops. In high mountain areas, the prolonged melting of glaciers and snow increases the risk of late-spring flooding in these regions. Longer summers also extend the forest fire season. This is clearly evident, for example, in the Mediterranean, in the western part of North America, and in some parts of Australia, where fires now often start earlier in the spring and end later in the autumn.
It is worth noting that significant changes are taking place not only over land but also over the oceans. Although the oceans are warming more slowly than the continents, even a relatively small rise in temperature can significantly extend the period of summer conditions. In some coastal areas, therefore, the length of summer is increasing very rapidly.
According to the authors of the study, this is not merely further evidence that climate change simply means a gradual rise in average temperatures. A change in the global average temperature of a few tenths of a degree is not something people feel directly. However, people are much more likely to notice that hot weather is arriving as early as May or even April and lasting into the second half of September. This highlights another impact of climate change – the gradual transformation of the very nature of the seasons, to which both society and natural systems have been accustomed for many decades.
An article written by Michal Žák, Tomáš Karlík (CT), initially published on 23 May 2026 at 07:00 (CEST)