Warmer nights turn vegetation into dry fuel (Image: David McNew/Getty Images)
Read more: “The night: The nocturnal journey of body and mind“
NIGHTS are not just dark; they are also cool. In colder climes, this fall in temperature is seldom welcome. But in hot regions, the night comes as a sweet relief. Many animals – including people – take it easy during the sweltering day and kick into action only when the sun goes down.
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Now nights are getting hotter. That’s not surprising on a warming planet, but what is surprising is that our nights are warming up much more than our days. The consequences will be far-reaching. There may be some upsides, such as fewer nights when you need to don warm pyjamas, but there are plenty of downsides, too.
Between 1950 and 2010, the number of warm days – defined in relation to the 1961 to 1990 norm – increased worldwide by about 50 per cent. But over the same period, the number of warm nights increased by 70 per cent. Some places have seen close to a doubling in the number of warm nights and a halving of the number of cold ones.
The trend of warming nights is a long-predicted effect of rising greenhouse gases. If the warming was due to the sun getting hotter, there would be extra heat arriving during the day but nothing in particular to stop heat loss after dark, so days would warm faster than nights. Greenhouse gases, by contrast, trap heat 24/7, which warms nights and days.
As the planet warms, all kinds of feedbacks kick in, which also play a role. One factor could be an increase in clouds, says Lisa Victoria Alexander of the University of New South Wales in Sydney, Australia, who studies day-night warming. “During the day that would cool, and at night it would warm. Of course, the story isn’t quite that simple.”
While many questions remain about the details, nights are expected to continue to warm dramatically, and . Nocturnal temperatures are rising especially fast in urban areas, where concrete radiates solar heat after sundown, buildings block cooling winds and ever more waste heat is released.
On the face of it, warmer nights might seem like a good thing. It has been claimed, for instance, that . But having can also allow more pests to survive winters, including the pine beetle now chomping its way through North America’s forests. In the mountains of Costa Rica, warmer nights and cooler days due to increased cloudiness may also have aided the frog-killing chytrid fungus, by providing the conditions it thrives in.
“Fewer overnight frosts have allowed pine beetles to chomp their way through forests”
Warmer nights also affect the activity of all sorts of other organisms. “What we worry about a lot is mosquitoes being more active at night,” says Andrew Dobson, an infectious disease ecologist at Princeton University. “Being warmer for longer is likely to speed up the life cycle of pathogens like malaria, which means it can infect more people.”
Plants, too, are being forced to be more active, which means they burn more energy overnight. This “heavy breathing” can boost photosynthesis the following day. In some cases, plants grow faster; in others, the boost is not enough to compensate for the increased energy loss at night and it stunts their growth.
According to a , this is already affecting the “g°ů±đ±đ˛Ô˛Ô±đ˛ő˛ő” of the northern hemisphere. Warmer nights have led to reduced growth in wet temperate regions like Japan and the northwestern US, but boosted growth in drier regions like the grasslands of China. This effect is also , by favouring some and hurting others.
Rising nocturnal temperatures may be stunting growth in tropical forests, . That’s bad news, as is the effect on one of the world’s staple foods: rice. A study in the Philippines concluded that a rise in night-time temperatures of about 1 °C since 1979 had led to .
The risk of wildfires could also increase. During cool nights, dew settles on vegetation, making it harder to burn. “Warmer nights will add to dry fuels,” says Mike Flannigan, a meteorologist and wildfire expert at the University of Alberta in Edmonton, Canada. “It’ll be like a tinderbox.” That probably won’t matter when a fire is already raging but it could mean the difference between a damp nothing and an inferno when a lit cigarette is dropped.
“Warmer nights turn vegetation into dry fuel, turning a forest into a tinderbox”
It is difficult to disentangle this effect from the increase in wildfire risk caused by global warming as a whole. Indeed, many of the consequences of warmer nights are subtle, especially in temperate regions. Most of us will not notice that nights are not quite as cold as they were.
There is one important exception: heatwaves. Many of the records being set around the world are for the highest minimum rather than the highest maximum. During the 2010 heatwave in the US, for instance, than at any time since records began in 1895. The European heatwave of 2003, which killed more than 21,000 people, also .
Hot nights mean that our bodies don’t get a chance to cool off, greatly increasing heat stress. And they are hard to avoid in homes without air conditioning. “During the day you can go somewhere to escape the heat,” says Alexander. “At night you’re stuck at home and there’s a point where your body can’t cope any more.”
So while warmer nights may be good for California’s wine-makers, they are also making heatwaves deadlier, feeding forest fires and helping pests. “It’s not just an inconvenience,” says Kim Knowlton of the Natural Resources Defense Council in New York City. “It kills people.”
This article appeared in print under the headline “The heat of the night”
