Re: Geo-engineering
[ Follow Ups ] [ Post Followup ] [ Class Discussion Groups ] Posted by James Risbey on September 16, 1997 at 18:10:33:
In Reply to: Geo-engineering posted by Neil Strachan on September 12, 1997 at 17:08:02:
: Given the worst case scenario (or one of them), geo-engineering would be required to give us some breathing space to make radical changes in our emissions and/or adapt to the new environment.
: During the class we talked about the possible shutdown of the Atlantic ocean current system (and being Scottish I an rather keen to ensure the continuing warming effect of the Gulf stream!) - it was then postulated that the blocking of the Mediterranean at the Rock of Gibralter would prevent this.
: On this general theme I have 3 questions.
: 1. How would blocking the Med, ensure the North Atlantic currents kept functioning?
: 2. How long would geo-engineering measures (eg seeding the atmosphere with inhibiting aerosols give us?
: 3. Could we use these scenarios to 'scare' people into acting now or would it merely lull them into a false sense of security?Dear Neil,
The role of the Mediterranean Sea (Med) on climate is not well known
and so the answer to your question involves a certain amount of
speculation. The speculation has been stirred chiefly by an article
in the July 8 issue of EOS by R. Johnson titled "Climate control
requires a dam at the Strait of Gibraltar."
Johnson's theory involves not so much a shut down of the Gulf Stream,
as a displacement of it. The theory entails quite a house of cards
built on top on one another, but I'll try to outline it briefly here.
Don't blink or you could end up in another ice age:
The salinity and hydrologic balance of the Med is determined by the
amount of fresh water lost through evaporation and by the amount of
freshwater inflow from the Nile river. Prior to past ice ages the
insolation configuration was conducive to weak African
monsoons leading to drought in the Nile (low outflow). This
creates a more salty Med, which then increases its outflow at the
Straits of Gibraltar as a result of the increase in salinity.
This dense water then sinks and mixes with cold water as it moves
northwards. Then it hits the shallow banks off Ireland and upwells
off Ireland and Scotland and flows into the Nordic seas. This flow of
cold water in the Nordic seas then chills Europe. The cold upwelling
off Scotland also has the effect of displacing the warm Gulf Stream
currents westward into the Labrador Sea. The warmer Labrador Sea
increases storminess and the advection of moisture into Canada,
resulting in the growth of the Canadian ice sheets. The ice sheets
then kicked in with their own positive ice-albedo feedback and the
next ice age was initiated.
That is the basic mechanism. According to Johnson, we are in the
process of triggering this mechanism off. By diverting water out of
the Nile for irrigation we are radically reducing the Nile freshwater
outflow into the Med (some of the diverted water makes its way back to
the Nile, but much of it is lost to evaporation and the subsurface).
Greenhouse induced warming is also increasing the evaporation of fresh
water from the Med. So we are in effect recreating the conditions
that led to an increase in salinity of the Med prior to the ice ages.
Thus, we will be increasing outflow from the Med and trigerring cold
upwelling, a displaced gulf stream, and growth of ice sheets.
Johnson's solution is thus to build a dam across the Med so that we
can reduce the volume of water that would otherwise outflow, upwelling
off Scotland would diminish, warm surface water now being increasingly
diverted to Labrador would enter the Nordic seas, Canada would remain
dry, and Europe's climate would remain mild and stable. So, time to
start the Gibraltar dam fund!
Your question 2 was on how long geoengineering measures such as
seeding the atmosphere with aerosols would give us?
Here there are multiple tradeoffs at stake and there is no definitive
time answer to your question. It depends how much we want to trade
off some things versus other things. For instance we can continue to
seed the atmosphere with aerosols (or put sails in space) to increase
the earth's albedo to keep pace with the increase in greenhouse
forcing. We will just have to keep putting more and more aerosols up
as the greenhouse forcing increases with time. This will get more and
more expensive (unless the delivery cost of putting them into the
stratosphere comes down with time) and the side effects of spreading
aerosols through the stratosphere will also presumably increase the
more we put up there (aerosols will fall out of the stratosphere into
the troposphere and so will pollute the tropsosphere. On the plus
side, we'll have great sunsets!). At some point we might decide that
the cure (aerosols in the stratosphere and attendent costs and side
effects) is worse than the disease (greenhouse warming). But that is
a multi-valued trade-off and it is not easy to say just when the
tradeoff point is reached.I've blabbed enough. Does anyone else want to respond to
Neil's third question?