Summers are lasting longer and the rate at which they are lengthening is also increasing. This conclusion results from a recent study for the northern and southern hemisphere, excluding the tropics and polar regions, for the period 1961–2023. 

In this study, daily mean air temperatures 2 metres above the surface were analysed. This was done for two periods, 1961–1990 and 1990–2023, whereby the latter period was compared with the former. The beginning and end of the summer were defined as the day when daily temperature intersected a threshold value. This threshold is the so-called 75th percentile of daily mean temperature for the 1961–1990 period. So, the summer in this period is the quarter of the year with the highest temperatures.

Over land, summer length increased by roughly 3 days every decade over the 1961–1990 period. This increase accelerated to above 6 days every decade over the 1990–2023 period.

Worldwide, excluding the tropics and polar regions, summers last on average about 30 days longer now than in the 1960s. They start earlier and end later every year.

The transition to and from summer is also becoming increasingly abrupt: temperatures change more rapidly at the beginning and end of summer than decades ago.

Source: Scott et al., 2026. Environmental Research Letters 21: 074009.