The
Observer
Climate
change
Many parts of the Earth’s climate system have been
destabilised by warming, from ice sheets and ocean currents to the Amazon
rainforest – and scientists believe that if one collapses others could follow
Michael
Marshall
Sat 19 Sep
2020 17.00 BST
The warning
signs are flashing red. The California wildfires were surely made worse by the
impacts of global heating. A study published in July warned that the Arctic is
undergoing “an abrupt climate change event” that will probably lead to dramatic
changes. As if to underline the point, on 14 September it was reported that a
huge ice shelf in northeast Greenland had torn itself apart, worn away by warm
waters lapping in from beneath.
That same
day, a study of satellite data revealed growing cracks and crevasses in the ice
shelves protecting two of Antarctica’s largest glaciers – indicating that those
shelves could also break apart, leaving the glaciers exposed and liable to
melt, contributing to sea-level rise. The ice losses are already following our
worst-case scenarios.
These
developments show that the harmful impacts of global heating are mounting, and
should be a prompt to urgent action to cut greenhouse gas emissions. But the
case for emissions cuts is actually even stronger. That is because scientists
are increasingly concerned that the global climate might lurch from its current
state into something wholly new – which humans have no experience dealing with.
Many parts of the Earth system are unstable. Once one falls, it could trigger a
cascade like falling dominoes.
Tipping points
We have
known for years that many parts of the climate have so-called tipping points.
That means a gentle push, like a slow and steady warming, can cause them to
change in a big way that is wholly disproportionate to the trigger. If we hit
one of these tipping points, we may not have any practical way to stop the
unfolding consequences.
The Greenland ice sheet is one example of a tipping
point. It contains enough ice to raise global sea levels by seven metres, if it
were all to melt. And it is prone to runaway melting.
This is
because the top surface of the ice sheet is gradually getting lower as more of
the ice melts, says Ricarda Winkelmann of the Potsdam Institute for Climate
Impact Research in Germany. The result is familiar to anyone who has walked in
mountains. “If we climb down the mountain, the temperature around us warms up,”
she says. As the ice sheet gets lower, the temperatures at the surface get
higher, leading to even more melting. “That’s one of these self-reinforcing or
accelerating feedbacks.”
We don’t
know exactly how much warming would cause Greenland to pass its tipping point
and begin melting unstoppably. One study estimated that it would take just 1.6C
of warming – and we have already warmed the planet 1.1C since the late 19th
century.
The
collapse would take centuries, which is some comfort, but such collapses are
difficult to turn off. Perhaps we could swiftly cool the planet to below the
1.6C threshold, but that would not suffice, as Greenland would be melting uncontrollably.
Instead, says Winkelmann, we would have to cool things down much more – it’s
not clear by how much. Tipping points that behave like this are sometimes
described as “irreversible”, which is confusing; in reality they can be
reversed, but it takes a much bigger push than the one that set them off in the
first place.
In 2008,
researchers led by Timothy Lenton, now at the University of Exeter, catalogued
the climate’s main “tipping elements”. As well as the Greenland ice sheet, the
Antarctic ice sheet is also prone to unstoppable collapse – as is the Amazon
rainforest, which could die back and be replaced with grasslands.
A
particularly important tipping element is the vast ocean current known as the
Atlantic meridional overturning circulation (AMOC), which carries warm
equatorial water north to the Arctic, and cool Arctic water south to the
equator. The AMOC has collapsed in the past and many scientists fear it is
close to collapsing again – an event that was depicted (in ridiculously
exaggerated and accelerated form) in the 2004 film The Day After Tomorrow. If
the AMOC collapses, it will transform weather patterns around the globe –
leading to cooler climates in Europe, or at least less warming, and changing
where and when monsoon rains fall in the tropics. For the UK, this could mean
the end of most arable farming, according to a paper Lenton and others
published in January.
Tumbling dominoes
In 2009, a
second study took the idea further. What if the tipping elements are
interconnected? That would mean that setting off one might set off another – or
even unleash a cascade of dramatic changes, spreading around the globe and
reshaping the world we live in.
For
instance, the melting of the Greenland ice sheet is releasing huge volumes of
cold, fresh water into the north Atlantic. This weakens the AMOC – so it is
distinctly possible that if Greenland passes its tipping point, the resulting
melt will push the AMOC past its own threshold.
“It’s the
same exact principles that we know happen at smaller scales,” says Katharine
Suding of the University of Colorado, Boulder, who has studied similar shifts
in ecosystems. The key point is that processes exist that can amplify a small
initial change. This can be true on the scale of a single meadow or the whole
planet.
However,
the tipping point cascade is very difficult to simulate. In many cases the
feedbacks go both ways – and sometimes one tipping point can make it less
likely that another will be triggered, not more. For example, the AMOC brings
warm water from equator up into the north Atlantic, contributing to the melting
of Greenland. So if the AMOC were to collapse, that northward flow of warm
water would cease – and Greenland’s ice would be less likely to start
collapsing. Depending whether Greenland or the AMOC hit its tipping point
first, the resulting cascade would be very different.
What’s
more, dozens of such linkages are now known, and some of them span huge
distances. “Melting the ice sheet on one pole raises sea level,” says Lenton,
and the rise is greatest at the opposite pole. “Say you’re melting Greenland
and you raise the sea level under the ice shelves of Antarctica,” he says. That
would send ever more warm water lapping around Antarctica. “You’re going to
weaken those ice shelves.”
“Even if
the distance is quite far, a larger domino might still be able to cause the
next one to tip over,” says Winkelmann.
In 2018,
Juan Rocha of the Stockholm Resilience Centre in Sweden and his colleagues
mapped out all the known links between tipping points. However, Rocha says the
strengths of the interconnections are still largely unknown. This, combined
with the sheer number of them, and the interactions between the climate and the
biosphere, means predicting the Earth’s overall response to our greenhouse gas
emissions is very tricky.
Into the hothouse
The most
worrying possibility is that setting off one tipping point could unleash
several of the others, pushing Earth’s climate into a new state that it has not
experienced for millions of years.
Since
before humans existed, Earth has had an “icehouse” climate, meaning there is
permanent ice at both poles. But millions of years ago, the climate was in a
“hothouse” state: there was no permanent polar ice, and the planet was many
degrees warmer.
If it has
happened before, could it happen again? In 2018, researchers including Lenton
and Winkelmann explored the question in a much-discussed study. “The Earth
System may be approaching a planetary threshold that could lock in a continuing
rapid pathway toward much hotter conditions – Hothouse Earth,” they wrote. The
danger threshold might be only decades away at current rates of warming.
Lenton says
the jury is still out on whether this global threshold exists, let alone how
close it is, but that it is not something that should be dismissed out of hand.
“For me,
the strongest evidence base at the moment is for the idea that we could be committing
to a ‘wethouse’, rather than a hothouse,” says Lenton. “We could see a cascade
of ice sheet collapses.” This would lead to “a world that has no substantive
ice in the northern hemisphere and a lot less over Antarctica, and the sea
level is 10 to 20 metres higher”. Such a rise would be enough to swamp many
coastal megacities, unless they were protected. The destruction of both the
polar ice sheets would be mediated by the weakening or collapse of the AMOC,
which would also weaken the Indian monsoon and disrupt the west African one.
Winkelmann’s
team studied a similar scenario in a study published online in April, which has
not yet been peer-reviewed. They simulated the interactions between the
Greenland and west Antarctic ice sheets, the AMOC, the Amazon rainforest and
another major weather system called the El Niño southern oscillation. They
found that the two ice sheets were the most likely to trigger cascades, and the
AMOC then transmitted their effects around the globe.
What to do?
Everyone
who studies tipping point cascades agrees on two key points. The first is that
it is crucial not to become disheartened by the magnitude of the risks; it is
still possible to avoid knocking over the dominoes. Second, we should not wait
for precise knowledge of exactly where the tipping points lie – which has
proved difficult to determine, and might not come until it’s too late.
Rocha
compares it to smoking. “Smoking causes cancer,” he says, “but it’s very
difficult for a doctor to nail down how many cigarettes you need to smoke to
get cancer.” Some people are more susceptible than others, based on a range of
factors from genetics to the level of air pollution where they live. But this
does not mean it is a good idea to play chicken with your lungs by continuing
to smoke. “Don’t smoke long-term, because you might be committing to something
you don’t want to,” says Rocha. The same logic applies to the climate dominoes.
“If it happens, it’s going to be really costly and hard to recover, therefore
we should not disturb those thresholds.”
“I think a
precautionary principle probably is the best step forward for us, especially
when we’re dealing with a system that we know has a lot of feedbacks and
interconnections,” agrees Suding.
“These are
huge risks we’re playing with, in their potential impacts,” says Lenton. “This
is yet another compulsion to get ourselves weaned off fossil fuels as fast as
possible and on to clean energy, and sort out some other sources of greenhouse
gases like diets and land use,” says Lenton. He emphasises that the tipping
points for the two great ice sheets may well lie between 1C and 2C of warming.
“We actually
do need the Paris climate accord,” says Winkelmann. The 2016 agreement
committed most countries to limit warming to 1.5 to 2C, although the US
president, Donald Trump, has since chosen to pull the US out of it. Winkelmann
argues that 1.5C is the right target, because it takes into account the
existence of the tipping points and gives the best chance of avoiding them.
“For some of these tipping elements,” she says, “we’re already in that danger
zone.”
Cutting
greenhouse gas emissions is not a surprising or original solution. But it is
our best chance to stop the warning signs flashing red.
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