*[Enwl-eng] Arctic Thawing May Set Off A Cascade of Tipping Points

[ENWLine]

*Arctic thaw may be first in cascade of tipping points*

* 27 February 2013 by Michael Marshall
* Magazine issue 2906. Subscribe and save

ONE climate domino has fallen, and it may start toppling others. A
recent study outlined an interconnected web of climate tipping points,
some of which make the next ones more likely. Now, an analysis of data
from the last 23 years suggests we passed the first of these tipping
points in 2007, when Arctic sea ice flipped into a new, less stable
state. That may speed the world towards the next tipping point -- the
thaw of a vast expanse of Siberian permafrost.

When it comes to the consequences of climate change, few are more
dramatic than tipping points -- a small push unleashes a big change,
which may be unstoppable. According to Tim Lenton of the University of
Exeter, UK, and Valerie Livina of the UK's National Physical Laboratory,
Earth saw its first tipping point in 2007 when the Arctic sea ice hit a
record low. The pair analysed data on ice cover going back to 1979, and
found that every year since then, the extent of sea ice in the summer
has been hovering around a new, shrunken state (The Cryosphere,
doi.org/kkq). "This wasn't a one-off, it was a permanent change," Lenton
says. He notes that since 2007, the ice has consistently taken longer to
recover from small changes, suggesting it has entered a new, less stable
state.

The claim is controversial. Anders Levermann of the Potsdam Institute
for Climate Impact Research in Germany argues that the ice loss cannot
be called a tipping point because it could still be reversed. Peter
Ditlevsen of the University of Copenhagen in Denmark says it is clear
from the ice cover data that 2007 marked a dramatic turning point for
Arctic sea ice.

A little further south, in the expanse of Siberian permafrost known as
Yedoma, another tipping point could be looming. Ecologists predict that
once the region begins to thaw, microbes will start breaking down the
carbon-rich soil, producing heat and releasing greenhouse gases, which
will accelerate the thaw.

Anton Vaks of the University of Oxford and colleagues used stalagmites
in caves beneath Russia and China to reconstruct the 500,000-year
history of the Siberian permafrost. Stalagmites cannot form when the
soil is frozen, Vaks explains. "They only grow when water flows into
caves." He found that those in the northernmost cave -- nearest to
modern-day continuous Siberian permafrost -- only grew once, during a
particularly warm period 400,000 years ago when global temperatures were
1.5 °C warmer than pre-industrial temperatures. That suggests the
permafrost is likely to become vulnerable when we hit 1.5 °C of global
warming (Science, doi.org/kkt).

Global temperatures have already risen by 0.8 °C. Even if humanity
stopped all emissions tomorrow, temperatures would rise another 0.3 °C,
suggesting the permafrost tipping point is likely to be reached.

These two tipping points -- the Arctic sea ice and permafrost -- are the
first two in a network of points outlined recently by Lenton and
Levermann (see diagram). The pair argue that periods of rapid ice loss
in the Arctic change regional weather patterns, to warm Asia more
quickly and speed up the thaw.

"No climate model has ever induced the tipping of one by tipping
another," cautions Levermann. But he says that could be a quirk of the
models, which climate modellers build to study aspects of climate that
don't involve tipping points. "We observe more violent changes in the
past than our models are capable of simulating," agrees Ditlevsen. "That
points to the idea of dominoes."

In Lenton and Levermann's cascade, a critical point appears to be the
shutdown of the Atlantic thermohaline circulation. This vast current
pumps water around all the Earth's oceans, and interacts with many of
the other areas susceptible to tipping points, including the Greenland
and Antarctic ice. The good news is that this tipping point could act as
a safety valve, slowing the progress of the others.

There are huge arguments over whether it will ever be hit. "All the
models show the overturning circulation declining with global warming,"
says Levermann. But that doesn't mean it will collapse. Ditlevsen and
Jan Sedlácek of ETH Zurich in Switzerland believe internal mechanisms
would reboot the circulation in the event of a collapse -- in part
because so far, there is no evidence of a complete collapse in the past.

Perhaps the worst news of all is that there may be no warning of
impending flips. Lenton has developed tipping point forecasts that look
for warning signs. Historical records and chaos theory applied to
ecosystem models suggest that as a system nears a threshold, it will
struggle to bounce back from small disturbances. So if a system is
approaching a tipping point, its response to extreme events should
become more sluggish. The trouble is that the one tipping point we have
already passed, according to Lenton -- melting of the Arctic sea ice --
gave us no such warning signs. Ditlevsen is not surprised. He found that
there was no warning before similar events during the last ice age
either (Geophysical Research Letters, doi.org/fd7vkj). Both researchers
say the behaviour of other unstable systems, like the Amazon rainforest,
glaciers and monsoons, may be more predictable.

So what's next? According to the temperatures on Lenton and Levermann's
cascade, the collapse of Greenland's ice sheet would become inevitable
shortly after the Yedoma permafrost thaws. This would raise sea levels 7
metres, over many centuries. A 2012 study (Nature Climate Change,
doi.org/kkw) suggested it would only take 1.6 °C, because as soon as the
south-eastern ice sheet starts losing surface mass, the entire sheet
destabilises.

This article appeared in print under the headline "Domino effect tips
climate over edge"

Trashing Siberia

A mere 1.5 °C of warming may be enough to start the widespread thaw of
Siberia's vast permafrost (see main story). We are already committed to
1.1 °C. What are the consequences?

The greatest concern, says Tim Lenton of the University of Exeter in the
UK, is the regional landscape. Buildings and infrastructure are often
built on hard permafrost, and will start to subside. "Ice roads won't
exist any more," he says.

The increasingly soggy permafrost will also threaten the pipelines that
transport Russian gas to Europe. "The maintenance and upkeep of that
infrastructure is going to cost a lot more," says Ted Schuur of the
University of Florida in Gainesville.

A study published last year predicted that the Russian cities of Nadym,
Yakutsk and Salekhard would be worst affected. The amount of weight the
ground underneath could support could fall 20 per cent or more (Arctic,
Antarctic and Alpine Research, doi.org/kkz). The ecosystem will also
change as plants move north.

The thawing permafrost will also release methane and carbon dioxide,
which will warm the planet still further. Though Lenton says the effect
will be small compared to humanity's emissions from burning fossil fuels.
Issue 2906 of New Scientist magazine

* From issue 2906 of New Scientist magazine, page 6-7.

*** NOTICE: In accordance with Title 17 U.S.C. Section 107, this
material is distributed, without profit, for research and educational
purposes only. ***




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