Hi All
Alexander Robinson writes that most geoengineering methods will cool
the tropics more than the poles. This might be true for stratospheric
sulphur because of its long lasting effects spreading everywhere in the
same hemisphere. But it is difficult to see how this could happen if
marine cloud brightening was done during the summer months at high
latitudes. It is unfortunate that so many modellers use steady spray
treatment through the year and often only between plus and minus 30
degrees latitude.
While there is lots of scatter in model results it does seem that marine
cloud brightening can effect precipitation in both directions depending
on when and where it is done.
Stephen
On 07/09/2015 13:05, Andrew Lockley wrote:
Poster's note : title piece is a box extract, immediately below. Main
article posted beneath, which is well worth reading for those not up
to speed with the sea level rise issue.
https://www.newscientist.com/article/mg22630253-300-latest-numbers-show-at-least-5-metres-sea-level-rise-locked-in/#bx302533B1
Can geoengineering save coastal cities?
It’s already too late to prevent massive sea level rise (see main
story). Or is it? Can geoengineering stop low-lying cities sinking
beneath the waves?
It certainly won’t be easy. “Once you kick in the melting feedbacks,
it’s very hard to shut them off,” says Alexander Robinson of the
Complutense University of Madrid. To have any chance, we have to get
the planet’s temperature back down to pre-industrial levels in the not
too distant future. “I personally see that as quite unlikely,”
Robinson says.
One key problem is that most geoengineering methods, such as pumping
sulphates into the atmosphere, rely on reflecting sunlight and would
cool the tropics more than the poles (Geophysical Research
Letters,doi.org/453 <http://doi.org/453>). Cooling the poles enough to
halt ice loss would devastate the rest of the world, slashing
rainfall, for instance.
The best solution would be to suck all the excess carbon dioxide from
the atmosphere, but the immense scale of the task and the speed
required make this seem nigh on impossible. Other suggestions, such as
building huge barriers between warming waters and glaciers, don’t look
feasible either.
Another major problem is that until cities start drowning, it is hard
to see politicians spending trillions on megaprojects. And once they
begin to drown, it will already be too late to prevent major sea level
rise.
(main article follows)
SPECIAL REPORT 10 June 2015
Latest numbers show at least 5 metres sea-level rise locked in
It’s too late to stop the seas rising at least 5 metres and only fast,
drastic action will avert a 20-metre rise, New Scientist calculates
based on recent studies
WHATEVER we do now, the seas will rise at least 5 metres. Most of
Florida and many other low-lying areas and cities around the world are
doomed to go under. If that weren’t bad enough, without drastic cuts
in global greenhouse gas emissions – more drastic than any being
discussed ahead of the critical climate meeting in Paris later this
year – a rise of over 20 metres will soon be unavoidable.
After speaking to the researchers behind a series of recent studies,
New Scientist has made the first calculations of what their findings
mean for how much sea level rise is already unavoidable, or soon will be.
Much uncertainty still surrounds the pace of future rises, with
estimates for a 5-metre rise ranging from a couple of centuries –
possibly even less – to a couple of millennia. But there is hardly any
doubt that this rise is inevitable.
We already know that we are heading for a rise of at least 1 metre by
2100. The sea will then continue to climb for many centuries as the
planet warms. The question is, just how high will it get?
No return
According to the latest report by the Intergovernmental Panel on
Climate Change (IPCC), over the next 2000 years we can expect a rise
of about 2.3 metres for each sustained 1 °C increase in the global
temperature. This means a 5-metre rise could happen only if the world
remains at least 2 °C warmer than in pre-industrial times up to the
year 4100. That doesn’t sound so bad: it suggests that if we found
some way of cooling the planet, we could avoid that calamity.
Unfortunately, the report, published in 2013, is not the whole story.
Last year, two teams reported that two massive glaciers in West
Antarctica have already passed the point of no return.
Ian Joughin of the University of Washington, Seattle, modelled the
fate of one of the glaciers. “No matter what, the glacier continued to
lose mass,” he says.
The loss of those two glaciers alone will raise sea level 1.2 metres.
If they go, Joughin says, it’s hard to see the rest of the West
Antarctic surviving.
Others agree. “I think these are very convincing studies,” says Anders
Levermann of the Potsdam Institute for Climate Impact Research in
Germany, one of the authors of the sea level chapter in the last IPCC
report. “The West Antarctic ice sheet is gone.”
The reason is that the West Antarctic ice sheet sits in a massive
basin, its base as much as 2 kilometres below sea level. At the
moment, only a little ice on the edges is exposed to the warming
waters around Antarctica. As the ice retreats, however, ever-deeper
parts of the basin will be exposed to warming waters, leading to ever
more of it being lost. The process is irreversible because once it
starts, it will continue as long as warm conditions persist. This
means a 3.3-metre rise is now unavoidable.
And that’s not all (see chart). Even in the unlikely event we manage
to limit warming to 2 °C, we’re in for a 0.8-metre rise as the oceans
warm and expand. Mountain glaciers around the world will contribute
0.4 metres. Adding those figures to the 3.3 metres, we get 4.5 metres
in total, or 5 metres rounded up. That’s conservative, given that it
doesn’t count any melting from East Antarctica or Greenland.
Latest numbers show at least 5 metres sea-level rise locked in
Most of the ice in East Antarctica is more stable than that in West
Antarctica as it rests on land above sea level. There are two large
basins, the Aurora and the Wilkes, whose floors are below sea level,
but these are shallower than the West Antarctic one. We had thought
only massive warming would destabilise the ice here.
Trough threat
However, Totten, the main glacier that drains the Aurora basin, is
thinning, says Jamin Greenbaum of the University of Texas at Austin.
His team reported in March that radar sounding has revealed a trough
under the ice that could let warm water enter the basin and trigger
enough melting to eventually raise sea level by 5.1 metres (Nature
Geoscience,doi.org/27w <http://doi.org/27w>). “The mind-blowing thing
is that there is as much ice in one glacier in East Antarctica as in
all of West Antarctica,” says Greenbaum.
The situation is similar in the Wilkes basin. It’s not losing ice yet,
but once a small amount on the margins is lost it will continue
disintegrating until enough ice has melted to raise sea level 3.5
metres, Levermann’s team reported last year (Nature Climate
Change,doi.org/snz <http://doi.org/snz>).
What will it take to kick-start the loss of all this ice? Not much.
During the Pliocene period around 4 million years ago, for instance,
when the planet was 2 or 3 °C warmer at times, sea level was over 20
metres higher than now. Researchers suspect that much of this came
from the Aurora and Wilkes basins.
Support for this idea comes from an improved ice sheet model that, for
the first time, includes dynamic processes such as cliff collapse
resulting from ice sheets being undercut by warming waters. In
January, a team including Richard Alley of Pennsylvania State
University reported that Pliocene conditions will lead, so the model
indicates, to ice loss not only in Aurora and Wilkes but also in
several smaller East Antarctic basins. Together, they hold enough ice
to add at least 15 metres to global sea level (Earth and Planetary
Science Letters,doi.org/42m <http://doi.org/42m>).
We are currently on course for a world even warmer than the Pliocene,
which means we could soon trigger the loss of the Wilkes and Aurora
ice – if we haven’t already.
Latest numbers show at least 5 metres sea-level rise locked in
This break-up will be traumatic (Image: NASA)
Then there’s Greenland. The ice here mostly rests on land above sea
level, so should take thousands of years to melt. You might think,
then, that there is plenty of time left to save it. Not so, says
Alexander Robinson of the Complutense University of Madrid, Spain.
He says his team’s studies show that we are already nearing the point
of no return for Greenland (Nature Climate Change,doi.org/kkw
<http://doi.org/kkw>). “Within the next 50 years, we could be
committing ourselves to continuous sea level rise from Greenland over
the next thousands of years,” he says. “That’s a very profound thing
to think about.”
The reason is that as warming continues, various positive feedbacks
will kick in. As the surface of the ice sheet lowers, for instance, it
experiences higher temperatures. In theory, the melting could still be
stopped if temperatures fall, but because carbon dioxide persists in
the atmosphere for many centuries, says Robinson, it is hard to see
how that could happen (see “Can geoengineering save coastal cities?“).
The loss of Greenland’s ice would add at least 6 metres to global sea
level. And in this business-as-usual scenario, ocean warming would
contribute 1.6 metres or more. Adding all this up leads to the
frightening conclusion that we don’t have much time left before we’re
on a one-way street to a world with seas 20 metres higher. “It’s kind
of scary,” says Robinson.
It will take thousands of years for the seas to rise to this extent,
but much of the rise could happen early on – within the first few
centuries – although no one can say for sure. Joughin thinks the IPCC
estimate of up to 1.2 metres by 2100 could still be in the right ball
park. “It’s likely to be on the high end [of the IPCC estimate] but
not far outside.”
Yet in the improved ice model that Alley’s team ran, Antarctica alone
added 5 metres to sea level in the first two centuries. That model was
run with warm Pliocene-like conditions from the start, not where we
are at now.
It might not take too long to reach a similar point, though. We’re in
danger of soaring past Pliocene levels of warmth as early as the
middle of the century if we don’t slash emissions soon. In the study,
the West Antarctic ice sheet collapsed in mere decades in response to
this kind of warmth.
What’s more, the model might still leave out some melting processes,
Alley says. “It is possible that this rather short timescale is not
the worst possible case.”
This article appeared in print under the headline “Five metres and
counting”
By Michael Le PageMagazine issue 3025 published 13 June 2015
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