Climate change is
altering terrestrial water availability
by Michael Keller, ETH
Zurich
Credit: CC0 Public
Domain
June 30, 2020
The amount and
location of available terrestrial water is changing worldwide. An international
research team led by ETH Zurich has now proved for the first time that
human-induced climate change ...
phys.org
|
The amount and location
of available terrestrial water is changing worldwide. An international research
team led by ETH Zurich has now proved for the first time that human-induced
climate change is responsible for the changes observed in available terrestrial
water.
Water is the lifeblood
of ecosystems and one of the most important natural resources for human beings.
But available terrestrial water—that is, the amount of water left from
precipitation after evaporation—is not just distributed unevenly across the
planet, it is also changing over time. Observations show that the available
volume of water has been falling in some regions of the world for a few
decades. One example is southern Europe, where aridity is increasing. But in
other areas water supplies are trending upwards.
The causes of this
change in water availability pose an urgent question—and not only for those
countries suffering from acute water shortages. Is anthropogenic climate change
to blame, or is it simply random fluctuations in the climate system? To date,
there has been no definitive answer at a global level.
World water dynamics
over 112 years
It is scientifically
indisputable that increased atmospheric concentrations of CO2 influence the
complex global water cycle in various ways. But until now it has been
impossible to prove a direct effect of global warming on available terrestrial
water resources over recent decades. The historical observation series,
sometimes too brief and qualitatively inadequate, did not enable exclusion of
natural climate variability as the cause of the changes observed.
Now, an international
research team led by Sonia Seneviratne, ETH Professor of Land-Climate Dynamics,
has proved this. As the scientists report in the current edition of Nature
Geoscience, they reconstructed worldwide water availability in the driest month
of years between 1902 and 2014 using climate models and new observations-based
data.
In order to determine
how water availability changed over time, the researchers compared the reconstructed
water resources of the years 1985 to 2014 with those of the first half of the
20th century. In this way they mapped out a global pattern of changes in
available water over the past three decades. In this pattern, the researchers
found the fingerprint of climate change.
Climate signal detected
through simulation
"We were able to
show that this global pattern of observed changes is consistent with the
effects of human-induced climate change and highly unlikely to be the product
of natural fluctuations," says Ryan Padrón, a postdoc in Seneviratne's
group and lead author of the study.
It is not possible to
prove the effect of climate change directly in an observation series. To verify
its role, the team used what is known as the attribution method. This involves
a comparison of observational series with simulations of climate models
calculated both with and without human-made CO2 emissions. "If only the
model simulations with human influence agree with the pattern of observed
changes, as is the case here, we can conclude that a response to climate change
is verifiable in the observations," explains Padrón.
Drier dry seasons in
extratropical areas
Seneviratne adds: "Our
study is the first to establish at a global level the connection between the
water availability during dry seasons, which is so important to society and
ecosystems, and human-induced climate change. The results also show a tendency
towards greater aridity in the northern mid-latitudes—which include, for
example, Switzerland—where conditions have become drier in summer months."
In general, the
reconstructed water availability data point to more intense dry seasons in
extratropical latitudes. Affected regions include Europe, western North
America, northern Asia, southern South America, Australia, and East Africa. The
researchers note that the increased intensity of dry seasons is generally
caused by greater evaporation (due to higher temperatures and radiation) rather
than reduced precipitation.
But there are also
regions in which the volume of available water has increased in dry seasons,
including the interior of China, south-east Asia and the Sahel region.
Explore further
How can man-made climate
change be proven?
More information:
Observed changes in dry-season water availability attributed to human-induced
climate change. Nature Geoscience (2020), DOI: 10.1038/s41561-020-0594-1
Journal information:
Nature Geoscience
Provided by ETH Zurich
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