Tim Radford / Climate
News Network
NOV 12, 2019 NEWS
In an unprecedented step, more
than 11,000 scientists from 153 nations have united to warn the world that,
without deep and lasting change, the climate emergency promises humankind unavoidable
“untold suffering.”
And as if to underline that
message, a US research group has predicted that – on the basis of experiments
so far – global heating could reduce
rice yields by 40% by the end of the century, and at the same time
intensify levels of arsenic in the cereal that provides the staple food for
almost half the planet.
And in the same few days a
second US group has forecast that changes to the world’s vegetation in an
atmosphere increasingly rich in carbon dioxide could mean that – even though
rainfall might increase – there
could be less fresh water on tap for many of the peoples of Europe,
Asia and North America.
Warnings of climate hazard
that could threaten political stability and precipitate mass starvation are not
new: individuals, research groups, academies and intergovernmental agencies
have been making the same point, and with increasing urgency, for more than two
decades.
New analysis
The only argument has been
about in what form, how badly, and just when the emergency will take its
greatest toll.
But the 11,000 signatories to
the statement in the journal BioScience report that
their conclusions are based on the
new analysis of 40 years of data covering energy use, surface
temperature, population growth, land clearance, deforestation, polar ice melt,
fertility rates, gross domestic product and carbon emissions.
The scientists list six steps
that the world’s nations could take to avert the coming catastrophe: abandon
fossil fuel use, reduce atmospheric pollution, restore natural ecosystems,
shift from animal-based to plant diets, contain economic growth and the pursuit
of affluence, and stabilise the human population.
Their warning appeared
on the
40th anniversary of the first world climate congress, in Geneva in 1979.
Surprising rice impact
“Despite 40 years of major
global negotiations, we have continued to conduct business as usual and have
failed to address this crisis,” said William
Ripple of Oregon State University, one of the leaders of the coalition.
“Climate change has arrived and is accelerating faster than many scientists
expected.”
Both the warning of catastrophic
climate change and the steps to avoid it are familiar. But researchers at
Stanford University in the US say they really did not expect the impact of
world temperature rise on the rice crop – the staple for two billion people
now, and perhaps 5 bn by 2100 – to be so severe.
Other groups have already
warned that changes in seasonal temperature and rainfall could reduce
both the
yields of wheat, fruit
and vegetables, and the
nutritional values of rice and other staples.
The Stanford group report in
the journal Nature
Communications that they looked more closely at what climate
change could do to rice crops. Most soils contain some arsenic. Rice is grown
in flooded paddy fields that tend to loosen the poison from the soil particles.
But higher temperatures combined with more intense rainfall show that, in
experiments, rice plants absorb more arsenic, which in turn inhibits nutrient
absorption and reduces plant development. Not only did the grains contain twice
the level of arsenic, the yield fell by two-fifths.
“By the time we get to 2100,
we’re estimated to have approximately 10bn people, so that would mean we have 5
billion people dependent on rice, and 2bn who would not have access to the
calories they would normally need,” said Scott Fendorf, an earth
system scientist at Stanford.
“I didn’t expect the magnitude
of impact on rice yield we observed. What I missed was how much the soil
biogeochemistry would respond to increased temperature, how that would amplify
plant-available arsenic and then – coupled with temperature stress – how that
would really impact the plant.”
And while the rice croplands
expect heavier rains, great tracts of the northern hemisphere could see vegetation
changes that could have paradoxical consequences. In a wetter, warmer world
plants could grow more vigorously. The stomata on the leaves through which
plants breathe are more likely to close in a world of higher levels of
atmospheric carbon dioxide, meaning less water loss through foliage.
And while this should mean
more run-off and a moister tropical world, a team at Dartmouth College in the
US report in the journal Nature Geoscience that
in the mid-latitudes plant response to climate change could actually make the
land drier instead of wetter.
Water consumption rises
“Approximately 60% of the
global water flux from the land to the atmosphere goes through plants, called
transpiration. Plants are like the atmosphere’s straw, dominating how water
flows from the land to the atmosphere. So vegetation is a massive determinant
of what water is left on land for people,” said Justin Mankin, a
geographer at Dartmouth.
“The question we’re asking
here is, how do the combined effects of carbon dioxide and warming change the
size of that straw?”
The calculations are complex.
First, as temperatures soar, so will evaporation: more humidity means more rain
– in some places. As atmospheric carbon dioxide levels soar, driven by fossil
fuel combustion, plants need less water to photosynthesise, so the land gets
more water. As the planet warms, growing seasons become extended and warmer, so
plants grow for a longer period and consume more water, and will grow more
vigorously because of the fertility effect of higher carbon dioxide
concentrations.
The calculations suggest that
forests, grasslands and other ecosystems will consume more water for longer
periods, thus drying the soil and reducing ground water, and the run-off to the
rivers, in parts of Europe, Asia and the US.
Avoiding the worst
And that in turn would mean
lower levels of water available for human consumption, agriculture, hydropower
and industry.
Both studies are indicators of
possible hazard, to be confirmed or challenged by other scientific groups. But
both exemplify the complexity of the challenge presented by temperature rises
of at least the 2°C set
by 195 nations in Paris in 2015 as the limit by the century’s end; or the
3°C that
seems increasingly likely as those same nations fail to take the
drastic action prescribed.
The world has already warmed
by almost 1°C above the long-term average for most of human history. So both
papers shore up the reasoning of the 11,000 signatories to the latest warning
of planetary disaster. But that same warning contains some steps humankind
could take to avert the worst.
“While things are bad, all is
not hopeless,” said Thomas
Newsome, of the University of Sydney, Australia, and one of the
signatories. “We can take steps to address the climate emergency.”
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