Geoengineering the Sahara
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John Nissen
Sep 21, 2009, 12:37:28 PM9/21/09
to Geoengineering
http://www.popsci.com/environment/article/2009-09/scientists-concoct-2-trillion-year-plan-geoengineer-sahara-desert
Scientists Concoct a $2-Trillion-Per-Year Plan To Geoengineer The Sahara Desert
Now that scientists agree that humans have profoundly changed the Earth's climate, many have begun asking if we can use our globe-altering power to simply change it back. Geoengineering, essentially terraforming on Earth, has been floated as a cure for global warming a number of times over the past year, but now some scientists have published a plan to transform a part of the Sahara desert into a lush forest, and in the process, absorb enough carbon to offset the world's current fossil fuel use. The catch: it will cost $2 trillion a year, and possibly destroy the Amazon jungle while unleashing giant swarms of locusts across Africa.
Writing in next month's issue of the journal Climatic Change, Leonard Ornstein, a cell biologist at the Mount Sinai School of Medicine, and David Rind and Igor Aleinov, researchers at NASA's Goddard Institute for Space Studies, lay out a plan to pump desalinated seawater from the coast to the desert. The pipes, buried underground to avoid loss to evaporation, would irrigate fields of Eucalyptus grandis. As the trees take root, they will replenish the soil and cause more rainfall, allowing for even more growth. The researchers estimate that as the forest grows, it will fix 8 million tons of atmospheric carbon, equivalent to the total emissions of the planet today.
Of course, there might be a few side effects. For one, sand from the Sahara is carried into the air, across the Atlantic, and deposited in South America. The rich dust that falls from the sky, and the rain storms caused by that dust picking up moisture during it's transoceanic journey both fertilize the Amazon rain forest. No desert, no dust. No dust, no rain forest. During that journey, the dust also feeds a variety of sea life.
Plus, the rain could cause massive swarms of locusts. Currently, wet years in the Sahara trigger serious population spikes of the destructive insects. With a permanent forest and heavy rain every year, Exodus-level clouds of locusts could spread across the entire continent.
And did I mention this would cost $2 trillion a year?
There is no doubt that global warming is a clear and present danger, and geoengineering may be part of the solution to that problem. But I think this is one case where I have to agree with the inevitable comments and say this cure sounds worse than the disease.
Oliver Morton
Sep 22, 2009, 5:29:23 AM9/22/09
to geoengineering
I blogged this last week
http://heliophage.wordpress.com/2009/09/15/terraforming-the-sahara/
Text
An interesting paper in Climatic Change: Irrigated afforestation of
the Sahara and Australian Outback to end global warming by Leonard
Ornstein, Igor Aleinov and David Rind Doi: 10.1007/s10584-009-9626-y.
(Mason Inman has a nice write up with some background and comment over
at ScienceNow; [update] and corresponding author Len Ornstetin
chronicles the idea’s rocky research road on his own site). The
central idea is that with enough irrigation you can turn big deserts
into big forests: forests big enough to suck up a large part of total
carbon dioxide emissions for decades or even centuries. I think that
you can take this notion as a serious plan, a thought experiment, a
jeu d’esprit, a warning or a jumping off point, depending on
predisposition. Aspects of all that in what follows.
Here are the basic numbers: The Sahara is about a billion hectares in
area, on which you could fit a trillion eucalyptus trees. Those trees,
if working flat out, could each put on twenty kilos of biomass a year.
If roughly half that biomass is carbon, that would mean a net annual
sink on the order of ten billion tonnes of carbon. That’s about the
amount that humans currently emit.
To create such a forest in a century, you would have to plant as many
hectares of trees every year as are currently lost to deforestation
worldwide. And, even harder, you’d have to provide them with what they
need to order to grow. You need a great many things to turn a desert
into a forest — soil nutrients, microbiota, possibly pioneer plants, a
compelling reason for doing the work, and so on — but the biggest
hurdle, pretty obviously, is water. Eucalyptus, the authors say, needs
about a metre of rainfall a year. For a billion hectares, that’s 10
trillion tonnes of water. The authors assume, reasonably for all that
I know, that if you have smart irrigation getting the water to just
where it is needed you can get away with half that amount. Even so,
even the vast aquifers beneath the Sahara don’t contain the amount of
water required, so it will have to come from desalination plants on
the and be pumped it up to where it is needed (the average elevation
of the Sahara is about 450m). The size of this undertaking — more than
50 new Niles, flowing in reverse — may explain why the authors feel
they need to use that fine old-school term “terraforming” for their
undertaking. The power requirement, if I’m reading their figures
right (4.04kWh/m^3 fresh water delivered), is a bit to the north of
2.2 terawatts, about 40% of it for desalination by reverse osmosis and
about 60% for pumping.
The world’s electricity generators currently provide about 18,000 TWh
of energy, which averages out at 2TW of constant supply. So in energy
terms the desalination and pumping needed for the Sahara forest would
use a bit more electricity than the world currently generates for
every other purpose. This unavoidably sounds nutty. But that is at
least in part because of the nuttiness of the situation, rather than
its proposed solution — the nutty situation in which we burn fossil
carbon at tens or hundreds of thousands of times the rate at which it
is sequestered over geological time. If humanity insists on putting so
much carbon dioxide into the air every year that it would take a brand
new forest the size of the Sahara to suck it all up, then that’s where
the madness starts. That creating such a forest would have to be a
large undertaking — large in terms of the whole world economy — is
just a consequence of the initial folly.
And in practice the investment would be smaller. A nice thing about
forests is that they can go some way to creating their own weather,
and the authors have looked at this effect with some climate modelling
work. If a forest with irrigation dampened soil is imposed on the
Sahara, rain begins to fall, in some places as much as a metre of it
every year. This rainfall doesn’t obviate the need for irrigation,
because it is strongly seasonal — basically an extension of the West
African monsoon of April to November. But it might significantly
reduce the irrigation requirements. Maybe you could get away with just
a terawatt…
The Sahel, to the south of the Sahara, also gets damper in those
enhanced and extended monsoon rains, which is definitely a plus, I’d
guess, and the African Easterly Jet, a feature which is driven in
large part by the temperature contrast between the desert and
surrounding land, seems to more or less vanish. Since a large number
of Atlantic hurricanes get their starts as kinks in the AEJ, that
might be a pretty significant change, too. Beyond that, the rest of
the world seems pretty much unaffected. In particular, the authors say
that their models show no additional warming that might be laid at the
door of the change of albedo which comes with replacing light desert
with darker trees. (I think this fits with the 2007 Bala et al paper
in PNAS, which suggested that warming associated with afforestation
would be due to changes in boreal, rather than tropical, forest
cover).
There is, however, a fly in the ointment. The Bodélé depression in
Northern Chad is only a small part of the Sahara, but it is the
world’s greatest source of mineral dust, with the winds drawing some
700,000 tonnes a day off the surface. According to Koren et al in
ERL, 2007 40 million tonnes of dust a year travels from the Bodélé to
the Amazon rain forest, half the total annual mineral inputs into the
forest basin (the dust fertilises the mid Atlantic, too, and it may
play a role in abating hurricanes too — Jim Giles wrote a lovely piece
on this for Nature some time back). There’s a real chance that this
dust is crucial to maintaining the soil fertility of the forest, and
even if the Bodélé itself were left unirrigated and unforested, the
increase in precipitation all round it, and the wetter atmosphere
downwind of it, would probably shut it down as a dust producer. If
growing a forest in the Sahara hurts the one we already have in the
Amazon it obviously becomes a less attractive proposition (though if
we are going to lose the Amazon forest anyway, things might look
different…). That said, if you are pumping trillions of tonnes of
water across continental scales, then paying to air dump a few tens of
millions of tonnes of fine-particle mineral fertiliser upwind of where
you want it is hardly going to break the bank.
Something the authors don’t look into is that the higher the CO2 level
in the atmosphere gets, the easier this all becomes. Higher carbon
dioxide levels make plants more water efficient, all other things
being equal. All other things are not, necessarily, equal — higher CO2
also makes things hotter, which plants don’t much care for. In a world
with some solar radiation management, though (such as aerosols in the
stratosphere) all things might indeed be kept equal, or at least
temperature might be. Martin Claussen has been working for some time
on the idea that the Sahara is a “tipping element” in the climate
regime, one that can be pushed from a dry state to a wetter one
relatively easily. In a more carbon rich but not-too-hot world the
circumstances might be right for it to tip the other way, and it might
take rather less than a 50-Nile terraforming project to nudge it over.
In the final analysis, I don’t think I take this paper very seriously
as a practical proposition. Doubling global electricity generation for
a single project seems far fetched. For such a thing to be put
anywhere near the top of one’s list of African infrastructure
investments would require that a great many other large and important
development initiatives (provision of power, water, roads, cold
chains, vastly improved agronomical advice, etc to the vast majority
of the population, for starters) would already have had to have been
put in place. But it’s kind of nice to imagine a world in which we
were wealthy and together enough to have actually taken the pressing
need for those changes to heart, and were thus in a position to
consider greening a great desert too.
And regardless of practicalities I think there’s real value in taking
the analysis further. A big idea like this throws off many fascinating
questions that force you to look at the earth, and what we know about
it, in new ways (or old ways but with a new twist):
What polycultures would you build the new forest with? (all-eucalyptus-
all-the-time is fine for first calculations, but doesn’t sound like
anyone’s idea of a proper landscape. Baobabs? Laurels? And what fauna
might be good, or bad?)
What genetic engineering — reduced flammability, higher albedo
leaves, more refractory soil carbon, who knows what else — might help?
How much bioenergy with carbon capture could be built into the scheme,
perhaps initially to power some of the inland the pumping stations?
Can biochar help? (and a million other soil-creation questions)
What are the best silvicultural ways to make the new woodlands pay, as
that is something people by and large like their environments to do,
and can there be room for some agriculture too?
How could local people best be convinced this was a good idea? And
what are the property title reforms that would be prerequisite?
If the AEJ stops, do hurricanes stop too? Or does some other mechanism
initiate them, maybe somewhere else? And does the dust really have an
effect?
When the Sahara was wetter and less dusty in the past, did the Amazon
actually suffer from lack of nutrients? (I think there is actually
some research already out there on that — but can’t offhand think
where)
How can the transformation be made stunningly beautiful?
What regions and landforms do you want to keep as monuments/heritage
sites/national or world parks? There would undoubtedly be a real
aesthetic/biodiversity loss in the removal of the desert, not to
mention risks to some utterly wonderful buildings.
How to stop the Fremen becoming soft and decadent now that Arrakis has
become a land of milk and honey?
and so on.
In particular, it would be nice to see some analysis of halfway
houses; where in the Sahel and points north might merely huge, as
opposed to planet-sized, afforestation be attempted, and what would be
the costs and benefits? It is possible to transform land on very large
scales, if not quite this large: 40m hectares of the Brazilian cerrado
have been brought into agricultural production over the past fifty
years. Can afforestation/silvicultural interventions on such scales
ever make sense? And where else might be suitable for such things?
And on the topic of where else: My apologies to any Australian readers
for not going into the paper’s analysis of foresting the Outback in
addition, or as an alternative, to the Sahara. Basically the arguments
are largely the same but the costs and effects are a bit smaller.
There’s also a risk of interfering with El Nino that would definitely
merit further attention. If anyone wants to blog more on that aspect
of the subject send me a link and I’ll post it up here.
On Sep 21, 5:37 pm, John Nissen <j...@cloudworld.co.uk> wrote:
> http://www.popsci.com/environment/article/2009-09/scientists-concoct-2-trillion-year-plan-geoengineer-sahara-desertScientists Concoct a $2-Trillion-Per-Year Plan To Geoengineer The Sahara Desert
>
http://heliophage.wordpress.com/2009/09/15/terraforming-the-sahara/
Text
An interesting paper in Climatic Change: Irrigated afforestation of
the Sahara and Australian Outback to end global warming by Leonard
Ornstein, Igor Aleinov and David Rind Doi: 10.1007/s10584-009-9626-y.
(Mason Inman has a nice write up with some background and comment over
at ScienceNow; [update] and corresponding author Len Ornstetin
chronicles the idea’s rocky research road on his own site). The
central idea is that with enough irrigation you can turn big deserts
into big forests: forests big enough to suck up a large part of total
carbon dioxide emissions for decades or even centuries. I think that
you can take this notion as a serious plan, a thought experiment, a
jeu d’esprit, a warning or a jumping off point, depending on
predisposition. Aspects of all that in what follows.
Here are the basic numbers: The Sahara is about a billion hectares in
area, on which you could fit a trillion eucalyptus trees. Those trees,
if working flat out, could each put on twenty kilos of biomass a year.
If roughly half that biomass is carbon, that would mean a net annual
sink on the order of ten billion tonnes of carbon. That’s about the
amount that humans currently emit.
To create such a forest in a century, you would have to plant as many
hectares of trees every year as are currently lost to deforestation
worldwide. And, even harder, you’d have to provide them with what they
need to order to grow. You need a great many things to turn a desert
into a forest — soil nutrients, microbiota, possibly pioneer plants, a
compelling reason for doing the work, and so on — but the biggest
hurdle, pretty obviously, is water. Eucalyptus, the authors say, needs
about a metre of rainfall a year. For a billion hectares, that’s 10
trillion tonnes of water. The authors assume, reasonably for all that
I know, that if you have smart irrigation getting the water to just
where it is needed you can get away with half that amount. Even so,
even the vast aquifers beneath the Sahara don’t contain the amount of
water required, so it will have to come from desalination plants on
the and be pumped it up to where it is needed (the average elevation
of the Sahara is about 450m). The size of this undertaking — more than
50 new Niles, flowing in reverse — may explain why the authors feel
they need to use that fine old-school term “terraforming” for their
undertaking. The power requirement, if I’m reading their figures
right (4.04kWh/m^3 fresh water delivered), is a bit to the north of
2.2 terawatts, about 40% of it for desalination by reverse osmosis and
about 60% for pumping.
The world’s electricity generators currently provide about 18,000 TWh
of energy, which averages out at 2TW of constant supply. So in energy
terms the desalination and pumping needed for the Sahara forest would
use a bit more electricity than the world currently generates for
every other purpose. This unavoidably sounds nutty. But that is at
least in part because of the nuttiness of the situation, rather than
its proposed solution — the nutty situation in which we burn fossil
carbon at tens or hundreds of thousands of times the rate at which it
is sequestered over geological time. If humanity insists on putting so
much carbon dioxide into the air every year that it would take a brand
new forest the size of the Sahara to suck it all up, then that’s where
the madness starts. That creating such a forest would have to be a
large undertaking — large in terms of the whole world economy — is
just a consequence of the initial folly.
And in practice the investment would be smaller. A nice thing about
forests is that they can go some way to creating their own weather,
and the authors have looked at this effect with some climate modelling
work. If a forest with irrigation dampened soil is imposed on the
Sahara, rain begins to fall, in some places as much as a metre of it
every year. This rainfall doesn’t obviate the need for irrigation,
because it is strongly seasonal — basically an extension of the West
African monsoon of April to November. But it might significantly
reduce the irrigation requirements. Maybe you could get away with just
a terawatt…
The Sahel, to the south of the Sahara, also gets damper in those
enhanced and extended monsoon rains, which is definitely a plus, I’d
guess, and the African Easterly Jet, a feature which is driven in
large part by the temperature contrast between the desert and
surrounding land, seems to more or less vanish. Since a large number
of Atlantic hurricanes get their starts as kinks in the AEJ, that
might be a pretty significant change, too. Beyond that, the rest of
the world seems pretty much unaffected. In particular, the authors say
that their models show no additional warming that might be laid at the
door of the change of albedo which comes with replacing light desert
with darker trees. (I think this fits with the 2007 Bala et al paper
in PNAS, which suggested that warming associated with afforestation
would be due to changes in boreal, rather than tropical, forest
cover).
There is, however, a fly in the ointment. The Bodélé depression in
Northern Chad is only a small part of the Sahara, but it is the
world’s greatest source of mineral dust, with the winds drawing some
700,000 tonnes a day off the surface. According to Koren et al in
ERL, 2007 40 million tonnes of dust a year travels from the Bodélé to
the Amazon rain forest, half the total annual mineral inputs into the
forest basin (the dust fertilises the mid Atlantic, too, and it may
play a role in abating hurricanes too — Jim Giles wrote a lovely piece
on this for Nature some time back). There’s a real chance that this
dust is crucial to maintaining the soil fertility of the forest, and
even if the Bodélé itself were left unirrigated and unforested, the
increase in precipitation all round it, and the wetter atmosphere
downwind of it, would probably shut it down as a dust producer. If
growing a forest in the Sahara hurts the one we already have in the
Amazon it obviously becomes a less attractive proposition (though if
we are going to lose the Amazon forest anyway, things might look
different…). That said, if you are pumping trillions of tonnes of
water across continental scales, then paying to air dump a few tens of
millions of tonnes of fine-particle mineral fertiliser upwind of where
you want it is hardly going to break the bank.
Something the authors don’t look into is that the higher the CO2 level
in the atmosphere gets, the easier this all becomes. Higher carbon
dioxide levels make plants more water efficient, all other things
being equal. All other things are not, necessarily, equal — higher CO2
also makes things hotter, which plants don’t much care for. In a world
with some solar radiation management, though (such as aerosols in the
stratosphere) all things might indeed be kept equal, or at least
temperature might be. Martin Claussen has been working for some time
on the idea that the Sahara is a “tipping element” in the climate
regime, one that can be pushed from a dry state to a wetter one
relatively easily. In a more carbon rich but not-too-hot world the
circumstances might be right for it to tip the other way, and it might
take rather less than a 50-Nile terraforming project to nudge it over.
In the final analysis, I don’t think I take this paper very seriously
as a practical proposition. Doubling global electricity generation for
a single project seems far fetched. For such a thing to be put
anywhere near the top of one’s list of African infrastructure
investments would require that a great many other large and important
development initiatives (provision of power, water, roads, cold
chains, vastly improved agronomical advice, etc to the vast majority
of the population, for starters) would already have had to have been
put in place. But it’s kind of nice to imagine a world in which we
were wealthy and together enough to have actually taken the pressing
need for those changes to heart, and were thus in a position to
consider greening a great desert too.
And regardless of practicalities I think there’s real value in taking
the analysis further. A big idea like this throws off many fascinating
questions that force you to look at the earth, and what we know about
it, in new ways (or old ways but with a new twist):
What polycultures would you build the new forest with? (all-eucalyptus-
all-the-time is fine for first calculations, but doesn’t sound like
anyone’s idea of a proper landscape. Baobabs? Laurels? And what fauna
might be good, or bad?)
What genetic engineering — reduced flammability, higher albedo
leaves, more refractory soil carbon, who knows what else — might help?
How much bioenergy with carbon capture could be built into the scheme,
perhaps initially to power some of the inland the pumping stations?
Can biochar help? (and a million other soil-creation questions)
What are the best silvicultural ways to make the new woodlands pay, as
that is something people by and large like their environments to do,
and can there be room for some agriculture too?
How could local people best be convinced this was a good idea? And
what are the property title reforms that would be prerequisite?
If the AEJ stops, do hurricanes stop too? Or does some other mechanism
initiate them, maybe somewhere else? And does the dust really have an
effect?
When the Sahara was wetter and less dusty in the past, did the Amazon
actually suffer from lack of nutrients? (I think there is actually
some research already out there on that — but can’t offhand think
where)
How can the transformation be made stunningly beautiful?
What regions and landforms do you want to keep as monuments/heritage
sites/national or world parks? There would undoubtedly be a real
aesthetic/biodiversity loss in the removal of the desert, not to
mention risks to some utterly wonderful buildings.
How to stop the Fremen becoming soft and decadent now that Arrakis has
become a land of milk and honey?
and so on.
In particular, it would be nice to see some analysis of halfway
houses; where in the Sahel and points north might merely huge, as
opposed to planet-sized, afforestation be attempted, and what would be
the costs and benefits? It is possible to transform land on very large
scales, if not quite this large: 40m hectares of the Brazilian cerrado
have been brought into agricultural production over the past fifty
years. Can afforestation/silvicultural interventions on such scales
ever make sense? And where else might be suitable for such things?
And on the topic of where else: My apologies to any Australian readers
for not going into the paper’s analysis of foresting the Outback in
addition, or as an alternative, to the Sahara. Basically the arguments
are largely the same but the costs and effects are a bit smaller.
There’s also a risk of interfering with El Nino that would definitely
merit further attention. If anyone wants to blog more on that aspect
of the subject send me a link and I’ll post it up here.
On Sep 21, 5:37 pm, John Nissen <j...@cloudworld.co.uk> wrote:
> http://www.popsci.com/environment/article/2009-09/scientists-concoct-2-trillion-year-plan-geoengineer-sahara-desertScientists Concoct a $2-Trillion-Per-Year Plan To Geoengineer The Sahara Desert
>
Stephen Salter
Sep 22, 2009, 6:34:27 AM9/22/09
to oemo...@googlemail.com, geoengineering, leno...@pipeline.com, Climate Intervention
Hi All
The Sahara ploy may not be that hard.
A very interesting paper at the techie Copenhagen meeting was from Kerry
Cook. She has a climate model that lets you plant trees. If she plants
them in Africa from the present forests up to 17.9 deg north they
quickly die back. But if she grows them slightly further north they
spread all the way up to the Mediterranean, just as they used to be 5500
years when the Sahara was packed out with hippos.
This means that you do not have to provide water for the whole area or
go on paying the $2 Trillion every year. Some papers are attached.
Stephen
Emeritus Professor of Engineering Design
School of Engineering and Electronics
University of Edinburgh
Mayfield Road
Edinburgh EH9 3JL
Scotland
tel +44 131 650 5704
fax +44 131 650 5702
Mobile 07795 203 195
S.Sa...@ed.ac.uk
http://www.see.ed.ac.uk/~shs
--
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.
The Sahara ploy may not be that hard.
A very interesting paper at the techie Copenhagen meeting was from Kerry
Cook. She has a climate model that lets you plant trees. If she plants
them in Africa from the present forests up to 17.9 deg north they
quickly die back. But if she grows them slightly further north they
spread all the way up to the Mediterranean, just as they used to be 5500
years when the Sahara was packed out with hippos.
This means that you do not have to provide water for the whole area or
go on paying the $2 Trillion every year. Some papers are attached.
Stephen
Emeritus Professor of Engineering Design
School of Engineering and Electronics
University of Edinburgh
Mayfield Road
Edinburgh EH9 3JL
Scotland
tel +44 131 650 5704
fax +44 131 650 5702
Mobile 07795 203 195
S.Sa...@ed.ac.uk
http://www.see.ed.ac.uk/~shs
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.
Alvia Gaskill
Sep 22, 2009, 8:47:56 AM9/22/09
to S.Sa...@ed.ac.uk, oemo...@googlemail.com, geoengineering, leno...@pipeline.com, Climate Intervention
Nor might it have to take as long as several centuries to get things going.
From reading both of the papers, it is clear that the surface albedo has an
important role in the strengthening and weakening of the monsoon which in
turn supplies the rainfall for the plants. Revegetation of the Sahel has
been a work in progress for decades, with the fluctuations in climate
usually drowning out any man-made efforts. And as the other paper pointed
out, the water requirements for a man-made greening of the entire Sahara are
unacceptably large. I might add that there have been numerous
proposals/patents over the years to vegetate the Sahara, the one that
started this discussion just the most recent. The authors or someone else
sent it to Holdren as it appeared in the OSTP/FOIA dump I reported on a
while back.
I proposed whitening parts of the Sahara to increase the surface albedo.
But it is the relatively high natural albedo of 0.3-0.4 that is the end
product of the natural desertification observed since the humid period ended
rather abruptly 5500 years ago. Someone should look into seeing what the
inpact of applying a BLACK cover to the region just outside the Sahel would
be on the monsoon. Black plastic lasts much longer under solar radiation
than white due to the fact that the carbon black that gives it the black
color absorbs most of the UV that breaks down the polymer. I've seen pieces
of black plastic in Durham that have been outside for more than a decade and
are still intact!
Since the black plastic surface would absorb much more solar radiation than
the existing natural surface or even green vegetation, it would give a much
more enhanced effect. Of course, the black plastic can't evaporate any
water, but it could be applied concurrently with tree planting, the land
around the trees covered with the plastic. I would predict that a much
smaller area would have to be covered to get things going in terms of the
feedback effects. We can't make the sun stronger, but we can make the land
warmer. The additional GHG forcing from the plastic vs. the carbon
sequestered by the vegetation would have to be factored in. I would think
it would be much less once a sufficient area is revegetated. While some
water would be required for the tree or shrub planting, irrigation via
desalination or other means would not be necessary as the monsoon would
supply the water. The area in question stretches about 2400 miles from the
Atlantic Ocean to the Red Sea. A 10-mile North/South band of black surface
would cost around $15 billion to install and could be completed in a few
years.
The concerns expressed about impacts on dust/nutrient flows would need to be
addressed, but it appears that iron and phosphorous are in excess now for
the Amazon and also for the Atlantic, so a decrease could be tolerated.
Since it would likely take more than a century to complete the revegetation,
there would be ample time to determine any impacts. Only a few tropical
waves are affected by dust storms, so the effect on hurricane development
would be minimal. The alteration of the surface and strengthening of the
monsoon might also offset some of the weakening predicted from use of
stratospheric aerosols.
From reading both of the papers, it is clear that the surface albedo has an
important role in the strengthening and weakening of the monsoon which in
turn supplies the rainfall for the plants. Revegetation of the Sahel has
been a work in progress for decades, with the fluctuations in climate
usually drowning out any man-made efforts. And as the other paper pointed
out, the water requirements for a man-made greening of the entire Sahara are
unacceptably large. I might add that there have been numerous
proposals/patents over the years to vegetate the Sahara, the one that
started this discussion just the most recent. The authors or someone else
sent it to Holdren as it appeared in the OSTP/FOIA dump I reported on a
while back.
I proposed whitening parts of the Sahara to increase the surface albedo.
But it is the relatively high natural albedo of 0.3-0.4 that is the end
product of the natural desertification observed since the humid period ended
rather abruptly 5500 years ago. Someone should look into seeing what the
inpact of applying a BLACK cover to the region just outside the Sahel would
be on the monsoon. Black plastic lasts much longer under solar radiation
than white due to the fact that the carbon black that gives it the black
color absorbs most of the UV that breaks down the polymer. I've seen pieces
of black plastic in Durham that have been outside for more than a decade and
are still intact!
Since the black plastic surface would absorb much more solar radiation than
the existing natural surface or even green vegetation, it would give a much
more enhanced effect. Of course, the black plastic can't evaporate any
water, but it could be applied concurrently with tree planting, the land
around the trees covered with the plastic. I would predict that a much
smaller area would have to be covered to get things going in terms of the
feedback effects. We can't make the sun stronger, but we can make the land
warmer. The additional GHG forcing from the plastic vs. the carbon
sequestered by the vegetation would have to be factored in. I would think
it would be much less once a sufficient area is revegetated. While some
water would be required for the tree or shrub planting, irrigation via
desalination or other means would not be necessary as the monsoon would
supply the water. The area in question stretches about 2400 miles from the
Atlantic Ocean to the Red Sea. A 10-mile North/South band of black surface
would cost around $15 billion to install and could be completed in a few
years.
The concerns expressed about impacts on dust/nutrient flows would need to be
addressed, but it appears that iron and phosphorous are in excess now for
the Amazon and also for the Atlantic, so a decrease could be tolerated.
Since it would likely take more than a century to complete the revegetation,
there would be ample time to determine any impacts. Only a few tropical
waves are affected by dust storms, so the effect on hurricane development
would be minimal. The alteration of the surface and strengthening of the
monsoon might also offset some of the weakening predicted from use of
stratospheric aerosols.
Andrew Lockley
Sep 28, 2009, 2:20:09 AM9/28/09
to agas...@nc.rr.com, geoengineering
As an alternative to black plastic sheeting in Alvia's idea, you could
use biochar. This has several advantages, in that it will attenuate
flooding, hold water for dry periods, act as a substrate on which soil
can begin to form in currently desertified areas. It also locks up
carbon in the soil.
I agree that the desalination/irrigation idea isn't sensible.
However, the practical limitation to the irrigation process is
desalination, not water supply or the cost of moving the water. It
would seem sensible to look at pumping seawater into the Sahara, and
letting evaporation do the desalination for you. If you pumped water
onto absorbent surfaces (think large bath towels) in the hottest,
windiest, driest part of the desert, then evaporation rates would be
enormous. The resulting moist air should be able to make rain
whenever it rises or cools significantly. You'd probably have to
include a return flow which took away saltier water and returned it to
the sea, to prevent the build up of large salt deposits.
Steps would need to be taken to prevent contamination of aquifers with
saltwater. However, there are lots of plants that seem to be able to
tolerate high salt levels, so maybe these could be grown on
salt-contaminated areas.
A
2009/9/22 Alvia Gaskill <agas...@nc.rr.com>:
use biochar. This has several advantages, in that it will attenuate
flooding, hold water for dry periods, act as a substrate on which soil
can begin to form in currently desertified areas. It also locks up
carbon in the soil.
I agree that the desalination/irrigation idea isn't sensible.
However, the practical limitation to the irrigation process is
desalination, not water supply or the cost of moving the water. It
would seem sensible to look at pumping seawater into the Sahara, and
letting evaporation do the desalination for you. If you pumped water
onto absorbent surfaces (think large bath towels) in the hottest,
windiest, driest part of the desert, then evaporation rates would be
enormous. The resulting moist air should be able to make rain
whenever it rises or cools significantly. You'd probably have to
include a return flow which took away saltier water and returned it to
the sea, to prevent the build up of large salt deposits.
Steps would need to be taken to prevent contamination of aquifers with
saltwater. However, there are lots of plants that seem to be able to
tolerate high salt levels, so maybe these could be grown on
salt-contaminated areas.
A
2009/9/22 Alvia Gaskill <agas...@nc.rr.com>:
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