Cooling the tropics more than the poles is also a choice for
stratospheric aerosol injection; if you want that effect you can
presumably do that, and if you’d rather cool the poles more than the
tropics you could do that instead.
*From:*[email protected]
[mailto:[email protected]] *On Behalf Of *Stephen Salter
*Sent:* Monday, September 07, 2015 12:45 PM
*To:* [email protected]
*Subject:* Re: [geo] Can geoengineering save coastal cities? | New
Scientist
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
--
You received this message because you are subscribed to the Google
Groups "geoengineering" group.
To unsubscribe from this group and stop receiving emails from it,
send an email to [email protected]
<mailto:[email protected]>.
To post to this group, send email to
[email protected]
<mailto:[email protected]>.
Visit this group at http://groups.google.com/group/geoengineering.
For more options, visit https://groups.google.com/d/optout.
--
You received this message because you are subscribed to the Google
Groups "geoengineering" group.
To unsubscribe from this group and stop receiving emails from it, send
an email to [email protected]
<mailto:[email protected]>.
To post to this group, send email to [email protected]
<mailto:[email protected]>.
Visit this group at http://groups.google.com/group/geoengineering.
For more options, visit https://groups.google.com/d/optout.
--
You received this message because you are subscribed to the Google
Groups "geoengineering" group.
To unsubscribe from this group and stop receiving emails from it, send
an email to [email protected]
<mailto:[email protected]>.
To post to this group, send email to [email protected]
<mailto:[email protected]>.
Visit this group at http://groups.google.com/group/geoengineering.
For more options, visit https://groups.google.com/d/optout.
