The cumulative heat in summers is rising, meaning there is less relief from warm temperatures once summer begins. (Unsplash/Evgeniy Beloshytskiy)Do you have the sense that summers feel different than when you were younger? That they start earlier, arrive quickly and remain intense until the fall? If you live in the mid-latitudes of either the Northern or Southern Hemispheres, chances are you answered yes.For many, the idea of a longer and warmer summer conjures up images of spending more time at the beach, playing sports or enjoying family picnics, but there are concerning downsides. Summer is also the season of wildfires, droughts and heatwaves, like the June 2021 heat dome in the Pacific Northwest of North America. Recent research shows that devastating heat dome event was amplified by its proximity to the summer solstice, which is the calendar start of summer, and by ground already dried out from earlier spring warmth. Earlier starts to the warmest season are making preconditions worse in areas prone to wildfire, extending fire seasons.My colleagues and I at the University of British Columbia recently published research into how summer conditions are lasting longer and transitions into and out of summer are becoming more abrupt. The cumulative heat in summers is rising, meaning there is less relief from warm temperatures once the season begins. Human-driven climate change is impacting the warmest season of the year. These changes challenge our expectations of the natural seasonal cycle being gradual and predictable. Read more: Extreme heat is breaking global records: Why this isn’t ‘just summer,’ and what climate change has to do with it Longer summersTo allow for a flexible definition of summer length, we defined typical summer weather based on daily average temperatures during the warmest 25 per cent of days from 1961 to 1990. This gave us a threshold daily temperature to define when summer began and ended in a given year and location.We found that the number of days with typical summer weather has been growing 1.5 times faster over the past 30 years than in prior reports. On average, in mid-latitude regions, summers have lengthened by around six days every decade since 1990. These rates are similar across land, ocean and coastal areas.We examined 10 cities using local weather station data, including Paris, St. Petersburg and Tokyo. A few of the cities stood out: in my hometown of Minneapolis, Minn., summertime has been lengthening by almost one additional day every year since 1990. Toronto summers are gaining an average of eight days every decade, with summer conditions lasting four weeks longer now than in 1990. Sydney, Australia, added 1.5 additional days in each of those years. Sydney summers now last over one-third of the year. Impacts of accumulating heatThe buildup of heat during summer is also rising quickly; it’s three times faster over land since 1990. As heat builds up in more areas, cooling demand can be expected to increase. This change is not confined to continental interiors but is also happening in coastal areas. Perceived as favourable with their maritime climates, these areas face growing populations and often higher climate risk. The percentage of Canadians with air conditioning varies by province and by income, but we know those who are most vulnerable struggle to stay cool.Incentives for heat pumps will help those who can afford to make the switch, and have the benefit of also replacing natural gas heating in the winter, but regardless of cooling method, electricity needs will rise.Longer summers and earlier starts will undoubtedly also affect agriculture, perhaps encouraging earlier planting. However, a complication is that the hours of daylight are not shifting. The impact of seasonal changes on farming practices and food supply is an active area of study.We also found that spring and autumn seasons are shrinking because the transitions from spring to summer and from summer to fall are becoming more abrupt. For areas that rely on mountain snow for fresh water throughout the year, this snow will melt earlier and more quickly, potentially leading to flooding. Additionally, those batteries of fresh water are running out and drought seasons are lasting longer. Read more: Warming winters are reshaping Canada’s snowpack Adapting to longer summerMany other aspects of society are linked to the timing of the summer season, like the start and end of the school year, or outdoor sports. How should we adjust if it’s simply too warm for strenuous outdoor activity, whether it’s recreational or work-related?If these trends continue, we can expect further impacts on the planting season, the timing and pace of snowmelt and the connection with water supply, the length of the fire season, and especially on the energy demand for cooling. Governments and experts have a lot of work to do to mitigate and adapt to the consequential changes humans have brought about through our dependence on fossil fuels. These changes to summer are noticeable because they are already disrupting lives. While some places will still occasionally have cooler years and significantly warmer years, the data tells us the trends for summers are headed in one direction.Ted Scott does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.