Forest fires, water shortages and failed harvests: the south of Europe is getting drier. Due to climate change, the boundaries of climate zones, such as the steppe climate, the polar climate and the desert climate, are shifting so quickly that the quality of life for plants, animals and people in many regions is being tested.
“India, for example, is already on the verge of quality of life,” says Marten Scheffer, professor of complex change at Wageningen University (WUR). “It is often extremely hot and conditions are becoming increasingly difficult for people to thrive in.”
In 2017, Scheffer was in Mexico for a conference with tropical biologists. “It was so incredibly hot,” he says. “I thought: this can’t be good for a person, can it?” It gave him an idea: ecological research into the so-called ‘climate niche’ of humans. A year later he published the research in a scientific journal PNAS. A niche is the place a species occupies in an ecosystem, and in which it can function optimally.
Together with colleagues, he plotted the number of people against the temperature and precipitation on earth. “To our surprise, a classic niche emerged there,” says Scheffer. “With the optimum at a temperature of about 13 degrees Celsius and extremes reaching -3 and +31 degrees Celsius.” This is the average between summer and winter temperatures and day and night temperatures. Remarkable: a 2015 economic studypublished in Nature, shows the same temperature as optimum with respect to economic productivity. The colder or warmer, the lower the economic output.
With that optimum as a starting point, Scheffer looked at shifts in climate boundaries and linked this to the ecological niche of humans. His most recent research was published last May in Nature Sustainability. The conclusion: “Shifting climate boundaries, as with animals, have consequences for where people can best live in the future.”
Significantly more difficult
The research shows that about 9 percent of people already live outside that ecological niche due to shifting climate boundaries. “That does not mean that those people die acutely, but that it is significantly more difficult for them,” explains Scheffer. At the end of this century, that percentage will be 22 to 39 percent, based on a temperature rise of 2 degrees Celsius.
Technical innovations are not included in the research. “For poorer people, innovations do not immediately solve all problems,” says Scheffer. “Simply because, for example, air conditioners are too expensive and intensive outdoor work, such as agriculture, is often necessary anyway.”
Scheffer studied all kinds of places on earth with extremely high average temperatures, where people have rarely or never lived, such as certain regions in the Sahara, Australia and the Middle East. “We have investigated where we will find that temperature in the future,” says Scheffer. “Then you see, for example, that densely populated parts of India, Brazil and North Africa will become virtually unlivable within two centuries.”
On the other hand, temperatures in areas such as Scandinavia and Canada are moving towards the optimum of the human climate niche, which will make these regions more suitable for humans to live in, Scheffer said. Just like the IPCCthe United Nations climate panel, he predicts large-scale migration: “That will have to be one of the answers to the advancement of changing climate boundaries on earth.”
Hundred year old classification
The most commonly used method to represent climate zones is the Köppen climate classification. In 1918, the Russian-German geographer, meteorologist, biologist, climatologist and botanist Wladimir Köppen invented a way to designate climate zones based on vegetation. For example, he looked at which vegetation corresponded to which climate, taking temperature and precipitation as the most important factors. Eventually he published his world map consisting of a total of 29 different climate zones.
Hylke Beck, professor of climatology at King Abdullah University of Science and Technology in Saudi Arabia, has compared existing IPCC climate scenarios to the 100-year-old Köppen climate classification and published his research in 2018 in Nature Scientific Data. “For example, we have literally visualized shifting climate boundaries up to 2100.”
The world map that Beck made shows two major differences. First of all, a big red spot that will continue to spread over the earth until 2100: the desert climate. “We see that large parts of what used to be steppe are turning into desert,” says Beck. “Dry and warm climate zones on earth are really getting bigger.” Such is the Sahara already grown 10 percent compared to 1920. The second difference is a considerably shrinking white spot, especially in the Arctic and the high mountains; the polar climate zone is shrinking considerably.
The expansion of the dry and warm climate zones, as is the case in Spain, is a lot more complicated
Hilke Beck professor of climatology
In his research, Beck used the most extreme SSP5-8.5 scenario, which assumes a temperature increase of 4.4 degrees Celsius compared to pre-industrial periods. But in a study to be published soon, Beck looked at how climate boundaries will shift across all climate scenarios. In the middle of the roadscenario, in which the temperature will rise by 2.7 degrees Celsius until 2100, the area of a tropical, dry, temperate, cold and polar climate will change by +9%, +3%, -3%, -2% and -33% in the period 2071 to 2099 compared to 1991 to 2020. A side note: ecosystems do not necessarily migrate as fast as the climate is now changing. Therefore, the actual changes in the landscape are likely to be slower.
“The drastic shrinkage of the polar climate zone is simply due to rising temperatures,” says Beck. “But the expansion of the dry and warm climate zones, as is the case in Spain, is a lot more complicated.”
Atmospheric expansion
For an explanation, scientists look to the atmosphere. “The driving force behind this shift comes from changes in air circulations, known as atmospheric cells,” says Michael Byrne, a climatologist at the University of St Andrews in Scotland and director of the Climate Dynamics Lab. “Those cells comprise a global system of winds that transport heat from the tropics to the poles.”
In both the northern and southern hemispheres there are three of these so-called cells – the polar cells, the ferrel cells and the hadley cells – in which air circulates from the ground to an altitude of 10 to 15 kilometers.
Until now, climate change mainly causes changes in the hadley cell, around the equator. Byrne: “In the Hadley cell, warm, moist air rises around the equator to the troposphere, about six miles above the Earth’s surface. We call this region the Intertropical Convergence Zone (ITCZ). There is a lot of precipitation here, but once it has finished raining, the dry warm air travels towards the poles and descends again around the thirtieth degree of latitude. It’s a lot drier here.”
Existing ecosystems become unbalanced as a result
Hilke Beck professor of climatology
Looking at the earth, you see deserts and steppes at 30 degrees north and south latitude (the Sahara and South Africa), while tropical green forests can be found around the equator. “The hadley cell therefore determines the distribution between wet and dry in this region,” says Byrne. “But we see that distribution changing.”
First of all, the ITCZ, the area around the equator where a lot of rain falls, gets smaller and smaller as the temperature of the atmosphere rises. Byrne researched these so-called deep tropics squeezeand published about this in 2018 in the scientific journal Current Climate Change Reports. “We see that the shrinkage of the ITCZ almost goes hand in hand with climate change, and that is most likely related to rising ocean temperatures.” This narrowing of the ITCZ causes more extreme rainfall around the equator, but in an increasingly smaller area.
“On the other hand, there is a widening of the drier parts in the hadley cell,” says Byrne. “As higher parts of the atmosphere – the so-called troposphere – warm up, the boundary with the even higher stratosphere – the tropopause – will be higher.” For example, climate change creates more space in the atmosphere for moist rising air around the equator, pushing the dry parts of the cell outwards, as it were. This is how the Hadley cell has been expanding since 1980 with an average of 0.1 to 0.5 degrees of latitude per decade. “We see that areas that are not yet in the hadley cell in particular are slowly being swallowed up,” says Byrne. “As a result, southern Europe, the southern United States and India are getting drier and drier.” Regions at the same latitudes in the southern hemisphere are also seeing increasingly drier conditions due to expansion of the hadley cell, as in Chili and Australia.
Poleward shift
Due to climate change, according to Michael Byrne, the hadley cell will continue to grow in size, causing global boundaries of a dry and warm climate to move further and further towards the pole. Hylke Beck explains the consequences: “Ecosystems depend for their health on the presence of certain temperatures and precipitation patterns,” he says. “Due to a new, much drier climate, different vegetation is created and alternative animal species are emerging, in short, existing ecosystems are getting out of balance.”
The recent drought report (from June) of the European Joint Research Center makes no difference. Since October 1, the Mediterranean regions have had 28 percent less rain than average. The temperature is 2.5 to 4 degrees higher than the average of the past thirty years. Already in May the temperature rose to – sometimes far – above 40 degrees Celsius. The result: failed harvests, fallow fields and water reservoirs that are only 25 percent full in Andalusia, for example.
‘Un sol de justicia’, they sometimes say in Spain: the sun of justice. That expression stems from medieval torture practices in which criminals had to stand in the blazing sun for hours as punishment. It is doubtful whether they still find the sun in Spain so fair.
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