The Guardian - Fred Pearce
The Paris climate conference set the ambitious goal of finding ways to limit global warming to 1.5C, rather than the previous threshold of 2C. But what would be the difference? And how realistic is such a target?
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| Two boys walk through a dried up Chandola Lake in Ahmadabad, India, May 2016. Photograph: Ajit Solanki/AP |
How ambitious is the world? The Paris climate conference last December astounded many by pledging not just to keep warming "
well below two degrees celsius,"
but also to "pursue efforts" to limit warming to 1.5C. That raised a
hugely important question: What's the difference between a two-degree
world and a 1.5-degree world?
Given we are already at one degree above pre-industrial levels,
halting at 1.5C would look to be at least twice as hard as the
two-degree option.
So would it be worth it? And is it even remotely achievable?
In Paris, delegates called on the UN's Intergovernmental Panel on Climate Change (IPCC) to
report on the implications of a 1.5C target.
They want the job done by 2018, in time to inform renewed talks on
toughening emissions targets beyond those agreed upon in Paris.But the truth is that scientists are only now getting out of
the blocks to address what a 1.5C world would look like, because until
recently it sounded like a political and technological impossibility. As
a
commentary published online in Nature Climate Change last week warned, there is "a paucity of scientific analysis" about the consequences of pursuing a 1.5C target.
To remedy this, the paper's researchers, led by Daniel Mitchell and
others at Oxford University, called for a dedicated program of research
to help inform what they described as "arguably one of the most
momentous [decisions] to be made in the coming decade." And they are on
the case, with their own
dedicated website and a major conference planned at Oxford in the fall.
So what is at stake? There are two issues to address. First, what
would be gained by going the extra mile for 1.5? And second, what would
it take to deliver?
First, the gains. According to available research, says the Oxford
group, the biggest boost will not be measured in average temperatures.
On its own, the difference between 1.5C and 2C is marginal. But it would
have a much greater effect on the probability of extreme and
destructive weather events like floods, droughts, storms, and heatwaves.
We know extreme weather is happening more often. A
study last year
by Erich Fischer of the Institute for Atmospheric and Climate Science
in Zurich found that the risk of what was "once in a 1,000 days" hot
weather has already increased fivefold. His modelling suggests that it
will double again at 1.5C and double once more as we go from 1.5 to 2C.
The probability of even more extreme events increases even faster.
The
same will be true for droughts, says Carl-Friedrich Schleussner of the
Potsdam Institute for Climate Impact Research in Germany. Last year, he
reported that the extra half-degree
would produce dramatic increases in the likely length of dry spells
over wide areas of the globe, including the Mediterranean, Central
America, the Amazon basin, and southern Africa, with resulting declines
in river flows from a third to a half. Schleussner concluded that going
from 1.5 to 2C "marks the difference between events at the upper limit
of present-day natural variability and a new climate regime,
particularly in tropical regions."
A few studies have tried to drill down to what the difference means
for day-to-day lives. And the consequences for many will be stark. At
two degrees, parts of southwest Asia, including well-populated regions
of the Persian Gulf and Yemen,
may become literally uninhabitable without permanent air conditioning.
Some researchers predict a massive decline in the viability of food
crops critical for human survival. The extra half-degree could cut corn
yields in parts of Africa by half, says
Bruce Campbell
of the International Center for Tropical Agriculture. Schleussner found
that even in the prairies of the US, the risk of poor corn yields would
double.
Two degrees, says Johan Rockström, director of the Stockholm
Resilience Center, "contains significant risks for societies everywhere;
1.5 looks much more scientifically justifiable."
Ecosystems
would feel the difference too. Take tropical coral reefs, which already
regularly come under stress because of high ocean temperatures,
suffering "bleaching" especially during El Niño events – as happened on
the
Great Barrier Reef in Australia this year.
Most can now recover when the waters cool again, but today's
exceptional temperature may soon become the new normal. "Virtually all
tropical coral reefs are projected to be at risk of severe degradation
due to temperature-induced bleaching from 2050 onwards," as warming
slips past 1.5C , reports Schleussner.
By some estimates, curbing warming at 1.5C could be sufficient to
prevent the formation of an ice-free Arctic in summer, to save the
Amazon rainforest, and to prevent the Siberian tundra from melting and
releasing planet-warming methane from its frozen depths. It could also
save many coastal regions and islands from permanent inundation by
rising sea levels, particularly in the longer run.
In 2100, the difference in sea level rise between 1.5C and 2C would
be relatively small: 40cm versus 50cm. But centuries later, as the
impact of warmer air temperatures on the long-term stability of the
great ice sheets of Greenland and Antarctica takes hold, it would be far
greater. Michiel Schaeffer of Climate Analytics, a Berlin-based think
tank,
calculates that by 2300, two degrees would deliver sea level rise of 2.7 meters, while 1.5 degrees would limit the rise to 1.5 meters.
Historical and projected changes in global temperatures from 1850 through 2100 if greenhouse gases continue to rise unchecked through the end of the century. Photograph: Ed Hawkins/US Geological Survey
It looks like 1.5C matters a great deal. So how hard would it be to
keep warming to that level? After all, last year was one degree above
pre-industrial levels. And at various times in the past six months,
global average temperatures have sometimes gone above 1.5C.
Most researchers agree that, short of some global economic meltdown,
even decade-long averaged temperatures are destined to go above 1.5C of
warming by mid-century. So delivering the target by the end of the
century will require drawing down temperatures by using technologies and
energy systems that can
extract carbon dioxide from the atmosphere on a large scale.
For
some, this would be nonsensical geoengineering. Kevin Anderson, a
climate scientist at the University of Manchester in the UK,
writing in Nature
after the Paris conference, declared "the world has just gambled its
future on the appearance, in a puff of smoke, of a carbon-sucking fairy
godmother."
But it could be done. The calculations are inexact. Nobody, even now,
knows quite how sensitive global temperatures are to rising
concentrations of greenhouse gases in the atmosphere. But here is the
task, as outlined by Joeri Rogelj, of the Austria-based International
Institute for Applied Systems Analysis (IIASA), in
an article in Nature Climate Change in March.
The planet's primary thermostat is the concentration of CO2 in the
atmosphere. Pre-industrial levels were 280 parts per million. We just
hit 400 ppm with warming at one degree and some more in the pipeline,
due to time lags. The IPCC, in its most recent report, estimated that to
stop at 1.5C will mean holding concentrations to around 430 ppm.
Because much of our CO2 emissions stay in the atmosphere for
centuries, that means bringing annual emissions to zero. Impossible?
Maybe, but the good news is that
greenhouse gas emissions actually fell in 2015 despite rising global economic activity,
thanks to the growing use of renewable energy. If we could build on
that and bring emissions to zero by 2050, then we might limit emissions
from here on out to 800bn tons.
If we could somehow find ways to extract 500bn tons from the
atmosphere, Rogelj concluded, we would likely be able to have our wish
of CO2 concentrations of 430 ppm and warming capped at 1.5C. The fairy
godmother would have delivered.
But how? While there are chemical processes for removing CO2 out of
the air, they remain very expensive. More likely are biological methods —
using plants to soak up CO2 and then preventing that CO2 from getting
back into the atmosphere when the plants die or are burned.
The trick that puts a glint in the eye of some technologists and
climate scientists is known by the acronym BECCS, which stands for
"biomass energy, carbon capture, and storage." The idea is to convert
the world's power stations to burning biomass, such as trees or marine
algae. The industrialized production of this biomass on such a scale
would accelerate the natural drawdown of CO2 by plants during
photosynthesis. If the CO2 created by burning the biomass could then be
captured from the stacks and
buried in geological strata
— the prototype technology known as carbon capture and storage — then
the net effect would be a permanent extraction of CO2 from the
atmosphere.
It would be the reverse of the current fossil-fuel energy system. And
the more energy generated, the more CO2 would be drawn out of the air.
There are huge questions about such a strategy. Wouldn't such a vast
new industry have its own absurdly high-energy requirements, putting us
back at square one?
Is there the land available to cultivate all that biomass? Would we
end up chopping down forests to make room for growing the biomass,
creating a
massive new source of emissions? While there are back-of-the-envelope calculations,
nobody has yet satisfactorily answered these questions.
Other geo-engineering options that have been proposed include
fertilizing the oceans so that more algae can grow, sucking up CO2 as
they do, or a terrestrial equivalent – burying charred biomass known as
biochar into soils, where it could provide a kind of deep fertilizer
that would turn soils into carbon-suckers over many centuries. But says
IIASA's Florian Kraxner, "Of all the ways of achieving negative
emissions, BECCS seems to be the most promising."
Is
this all scientific pie in the sky? Some analysts argue that, whatever
was said in Paris, there is little chance of hitting even two degrees,
let alone anything tougher. David Victor, of the University of
California at San Diego, for instance,
wrote in Yale Environment 360
at the conclusion of the Paris agreement that "the world has dithered
for too long and must now brace for the consequences. Even a realistic
crash program to cut emissions will blow through 2C; 1.5C is
ridiculous."
Others say that even trying to paint a picture of what a 1.5C world
would look like is a fool's errand. Mike Hulme of King's College London
in England
wrote recently that it could result in bad science,
because predictions about future local climate come with such wide
error bars. He wondered whether, even at the request of the Paris
conference, science should be "corralled into servicing a tightly
determined political agenda."
But the Oxford team is not having such defeatism. "It is our job as
scientists, first and foremost, to inform. Whether or not the
information we provide makes a difference is ultimately up to others,"
they say in their new paper. Moreover, they point out, "if additional
research is not undertaken as a matter of urgency, there is a danger…
that the 2018 special report will present all the negative economic
constraints of achieving 1.5C" without reporting on the potential
positive impacts of reduced extreme weather activity that such a
scenario could bring.
Ultimately, this is a highly political issue about who should be in
charge of setting targets: those most vulnerable nations, who led the
call in Paris for a 1.5C target, or those less vulnerable nations in the
rich world, who were ready to stick with two degrees? Them or us?
As Petra Tschakert of Penn State University put it in a
paper last year,
"danger, risk, and harm would be utterly unacceptable in a 2C warmer
world, largely for 'them' – the mollusks, and coral reefs, and the poor
and marginalized populations, not only in poor countries – even if this
danger has not quite hit home yet for 'us'."
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