Global heating incrementally boosts the intensity of extreme
rainfall at higher altitudes, putting two billion people living in or
downstream from mountains at greater risk of floods and landslides, researchers
said Wednesday.
Every degree Celsius of warming increases the density of
major downpours by 15 percent at elevations above 2,000 metres, they reported
in the journal Nature.
On top of that, each additional 1,000 metres of altitude
adds another one percent of rainfall.
A world, in other words, 3C hotter than preindustrial levels
will see the likelihood of potentially devastating deluges multiply by nearly
half.
The findings underscore the vulnerability of infrastructure
not designed to withstand extreme flooding events, the authors warned.
Earth’s surface has already warmed 1.2C, enough to amplify
record-breaking downpours that put huge swathes of Pakistan under water last
summer, and parts of California earlier this year.
On current policy trends, the planet will warm 2.8C by
century’s end, according to the UN’s IPCC climate science advisory panel.
The new study — based on data covering the last 70 years,
and climate-model projections — found two main drivers behind the upsurge in
extreme rainfall events at altitude in a warming world.
The first is simply more water: scientists have long known
that every 1C increase boosts the amount of moisture in the atmosphere by seven
percent.
Since the 1950s, heavy rainfall has become more frequent and
intense across most parts of the world, according to the World Weather
Attribution (WWA) consortium, which teases out the impact of climate change on
specific extreme weather events, including heatwaves, droughts and tropical
storms.
Extreme rainfall is more common and intense because of
human-caused climate change in Europe, most of Asia, central and eastern North
America, and parts of South America, Africa and Australia, the WWA has found.
The second factor uncovered by researchers was more
surprising.
“This is first time that anyone has looked at whether those
intense precipitation events fall as rain or snow,” lead author Mohammed
Ombadi, a researcher at Lawrence Berkeley National Laboratory in California,
told AFP.
“Unlike snowfall, rainfall triggers runoff more rapidly,
leading to a higher risk of flooding, landslide hazards and soil erosion.”
Ombadi speculated that a higher rate of snow-turned-to-rain
observed between 2,500 and 3,000 metres was due to precipitation at that
altitude occurring a just below freezing.
The mountainous regions and adjacent flood plains likely to
experience the biggest impacts from extreme rainfall events are in and around
the Himalayas and North America’s Pacific mountain ranges, according to the
study.
The findings focused only on the northern hemisphere due to
a lack of observational data from below the equator.
The regions most affected should prepare “robust climate
adaptation plans,” the authors said.
“We need to consider this increase in rainfall extremes in
the design and building of dams, highways, railroads and other infrastructure
if we want to make sure they will remain sustainable in a warmer climate,” said
Ombadi.
High-risk areas will either need to be avoided altogether,
or built up with engineering solutions that can protect the communities living
there, he added.
