Climate change will not only increase the world's average temperature but also intensify extreme heat waves that are harming people and wildlife even now, scientists warn. Climate change is often talked about in terms of averages -- like the goal set by the Paris Agreement to limit the Earth's temperature increase to 2 degrees Celsius, according to the review of over 140 scientific studies published in the journal Physiology.
Summertime is quickly becoming a deadly season for life on Earth," said Professor Jonathon Stillman from San Francisco State University in the US.
Heat waves have already produced striking images of mass mortality in animals, from the bleached skeletons of corals across swaths of the Great Barrier Reef to the deaths of horses during Australian summers, researchers said.
Heat stroke from such extreme events is also a present danger for people, especially the elderly, albeit in a less obvious form.
"Human mortality is different in that a lot of it is not visible in that way. It's happening in homes or in doctors' offices, but it's striking all the same," Stillman said.
For instance, a 2003 heat wave in Europe killed more than 70,000 people across the continent, researchers said.
To get a comprehensive view of the effects of future heat waves on humans and wildlife, Stillman gathered information from over 140 scientific studies on the topic.
As carbon dioxide and other greenhouse gases in the atmosphere trap heat and raise the Earth's average temperature, the heat waves we are accustomed to will worsen and become more frequent.
In some more extreme projected scenarios, temperatures rivalling or exceeding those seen in Europe in 2003 could last as much as four times as long by the end of the 21st century, the researchers found. "This suggests that (during some years), all summer long we will have heat that is more intense than what happened in 2003," Stillman said.
There are ways to cope with heat waves, but they won't be available for everyone, or for every species, the researchers said.
A lack of available infrastructure may make migrating to cooler climates difficult for many vulnerable human communities and cause large-scale conflict, they said.
Due to humanity's sprawling ecological footprint, many animals won't have a clear path to cooler climes unless natural space is specifically set aside for that purpose.
Heat waves can also have more subtle effects on the bodies of animals, such as prompting increases in the amount of specialised proteins that protect other molecules from the warping effects of heat.
"If populations of wildlife are experiencing more near-lethal temperatures, you won't see mortality but you might see shifts in their physiology that show they're getting close to mortality," Stillman said.
By studying responses like these, scientists could potentially get an early warning signal before heat waves start to produce more dire consequences.