Maria Gilardin: TUC Radio, San Francisco. Time of Useful Consciousness. Dangers of climate change: global warming and the Gulf Stream. This is program four in a three hour series on dangerous climate change. The science presented here comes from a symposium held in Exeter, England in February of 2005. For the first time in the history of scientific meetings, the issue of dangerous climate change and how to avoid it was raised. The conference made headlines in the European media, but was not reported in any detail by the US media. The US administration and the so called climate skeptics either deny that human activities play a role in climate changes that we are already experiencing, or claim that it is too early to act. In this series you already heard reports about accelerated melting in the Antarctic and on Greenland, and the appeal from scientists involved to take the warming of the polar region seriously. Today's program deals with the Gulf Stream. Warm ocean currents from the tropic flow along the North American coast towards Europe and make that area 5-10 degrees Celsius warmer than it should be at its latitude. The movie "The Day After Tomorrow" showed a scenario of a sudden collapse of the Gulf Stream. In that movie, within days, Europe and parts of the US east coast found themselves in ice-age conditions. The kernel of truth in that story is that the Gulf Stream did collapse before. However, it took several hundred years to set up that collapse and it happened not in a few days, but within ten years. Temperatures over Greenland and Europe dropped by 10 degrees Celsius, or 18 Fahrenheit. We know that because of temperature and air composition data that can be extracted from ice-cores. So, the Gulf Stream has collapsed before and you will hear in a moment how scientists reconstruct that event and why they believe it can happen again. The Gulf Stream is also called the thermohaline circulation; thermo for heat and haline for salt. The underwater movement of these huge amounts of water from the tropics north depends on three factors: the heat that drives the water north, and the cooling and salt that add weight to the water and make it sink. In this thermohaline circulation, the water moves north driven by heat, sinks down as it cools and becomes heavy, and returns back south at a much lower ocean level.
Michael Schlesinger, one of the scientists at the Exeter conference who gave a report about the Gulf Stream, showed that the circulation of the Gulf Stream could be slowed or even stopped by the addition of fresh water, since fresh water is less heavy than salt water it is less capable of sinking. The more fresh water is added to the North Atlantic by increased rainfall because of global warming, the greater the risk to the functioning of the Gulf Stream, and an addition of fresh water from a different source, scientists believe, has stopped the Gulf Stream before. Michael Schlesinger is professor of atmospheric sciences at the University of Illinois at Urbana-Champaign. He directs the University's climate research group, and is an expert in the modeling, simulation, and analysis of climate and climate change. I called professor Schlesinger at his home in Urbana in early June 2005. You will hear the family parakeets chirping in the background, and I began by asking him about the statement he made at the beginning of his presentation at the conference at the Hadley Centre in Exeter in February of 2005.
MG: You said: absent a climate policy the collapse of the Gulf Stream over the next 200 years is more than two chances in three. I saw that that statement of yours created quite a stir in the European media.
Michael Schlesinger: Yes, in fact, when we obtained that result I was shocked. This is usually called a low probability high consequence event, this shutdown of the Gulf Stream. The bottom line was this, that over the next 200 years there's roughly a 70% likelihood of the Gulf Stream collapsing. The two talks following mine were also on this Gulf Stream shutting down or collapsing. The second one after mine, by Peter Challenor from the Southampton Oceanography centre in the UK. He obtained a similar result using very different techniques from the ones that we used. The first calculations of this likelihood, first quantitative calculations, came out with a rather alarming result.
MG: You do use different computer models apparently. Do you all use the same data?
MS: We have a suit of climate models, ranging from very simple to the most complex and demanding of computer time. It's the most comprehensive models, so called atmosphere/ocean general circulation models that we used to simulate what happens to the Gulf Stream when you add fresh water to the North Atlantic ocean. We also have evidence from so called proxy data, of course thermometers go back only about 150 years before that we don't have any instrumental temperature measurements and we have to use so called proxies like tree rings that are sensitive to temperature and precipitation to try and determine what the temperature was. One of the pieces of information that we have is a core in the ice down to bedrock in the middle of Greenland. This goes back perhaps 20,000 years, and you can use the information from the ice-core to reconstruct, as it's called, what the temperature was in central Greenland as a function of time in the past. That record shows that the temperatures were at the ice-age temperatures until about 15,000 years ago, and then as the orbital configuration of the earth changed the earth was coming out of the ice-age about 15,000 years ago, and then there was an abrupt, of course abrupt is a relative term, return back almost to ice-age conditions, so called younger dryas period of about 13,000 years ago when the temperatures in Greenland and in the United Kingdom dropped 10 degrees Celsius or roughly 18 degrees Fahrenheit. The reason this occurred is believed is because as we were coming out of the ice-age the ice-sheet on North America, and on Europe, were melting back, and during that melt-back, at least in North America, most of the time the water went down the Mississippi and into the gulf of Mexico down there. As the ice-sheet got far enough northward that melt-water start went out instead into the St Lawrence and flowed on top of the North Atlantic, and that was the addition of fresh water, and that shut down or slowed down the Gulf Stream sufficiently that the heat that was transported northward from the southern hemisphere into the high latitudes of the northern hemisphere seized. That's what resulted in this cooling over Greenland, cooling over the United Kingdom and western Europe. We have information both from the so called paleoclimate record, which is these proxy indicators, that this Gulf Stream either slowed down or shut down more than once in the passed. Our models can reproduce that if you put enough fresh water on the North Atlantic. The concern, strangely enough perhaps, is that future global warming due to the increase in greenhouse gases that we're putting into the earth's atmosphere will result in a slow down or a shut down of the Gulf Stream in the future, and that would lead to a relative cooling in the North Atlantic and western Europe, relative to the case where there wasn't this slowdown or shutdown. People have made use of that in ways that are probably inappropriate, for example the movie "The day after tomorrow" makes it that we're going into an ice-age, whereas in fact all models' simulation show that the slowdown or shutdown of this Gulf Stream due to global warming, and I can explain how that works, results in a smaller warming over North Atlantic and western Europe, but not a net cooling. That film depiction is really an exaggeration of what we understand is likely to occur. The reason why global warming results in this relative local cooling over the North Atlantic is that when the Earth's atmosphere warms, there is more precipitation in the latitudes where this Gulf Stream is sinking, around 60 degrees north, and there's also more evaporation, but the difference between the precipitation and the evaporation increases so the fresh water is put on to the North Atlantic not from the melting of the ice-sheet, which don't exist anywhere anymore except in Greenland, but in these model simulations because there is more rainfall over the North Atlantic. This addition of fresh water results in the water there being less dense, and it either seizes to sink at all or it sinks more slowly, and that sinking motion and the connecting branches of water moving from the south to the north at the surface then sinking and returning back to the southern hemisphere, that transports a huge amount of heat and that gets reduced or completely shut down, and that's what results in this local cooling, but not a net cooling. The Gulf Stream transports an absolutely fantastic amount of heat from the southern hemisphere into the northern hemisphere, ultimately deposited around the North Atlantic and western Europe. It's as if the southern of the tropical Atlantic were a tremendous solar collector on the earth not in space. The Gulf Stream transports poleward a petawatt of heat. What's a petawatt? A petawatt is a million billion watts. That's a number that's so large it's really hard to put it in perspective, but all of humanity now uses ten terawatts, which means that the Gulf Stream is transporting enough heat poleward to run a hundred earths. Or put it another way; if we could extract 1% of the heat transported poleward by the Gulf Stream we could run the entire planet, so that's absolutely a staggering amount of heat that is being transported due to this solar collector in the tropics transporting the heat towards the north pole.
MG: Let's go back to that really interesting statement about the abruptness of a climate change. I guess the hopes of everybody looking at this is that whatever is happening will happen slowly enough so we can do something about it. But you sorted out the Gulf Stream as one of the events that would actually create an abrupt change, and I even asked that of Stephen as well. I brought up "The Day After Tomorrow" and he said: "Oh, it definitely would not happen in a few days, it would take at least ten years", which of course in geologic time is not a whole lot longer than three days or four. And it would take a few hundred years to set up, but we may already be 100 years into the setting up of it, possibly.
MS: That's right. The calculations we did was it's a two out of three likelihood of a collapse of the Gulf Stream. That collapse happens not in this century but in the next. Of course, if you're an economist and you use a discount rate, what happens in the century after this has no bearing on anything whatsoever. If you're not an economist you may look at it in a slightly different way. So, what may be abrupt to one pair of eyes may not be abrupt to another pair of eyes. But you raise an interesting point about earth. This planet is unique in two very important ways. First of all it has an ocean, and no other planet has an ocean in our solar system. And the earth has partial cloudiness. Most other planets in our solar system either has no clouds or they complete cloud cover. Earth is basically the only one with partial cloud cover. But getting back to the ocean; if you have an all-land planet then the heat that's added to the surface by the greenhouse effect cannot go below the surface of the land very far, because there is no way of transporting that heat except by conduction, which is pretty inefficient. So, in an all-land planet pretty much the temperature change you see at any instant in time will be the temperature change that you would see if you stopped increasing the concentration of greenhouse gases, that is the so called equilibrium temperature. You have a planet that's 75% water, which can transport heat vertically downward by its motion, say by the thermohaline circulation or the Gulf Stream as well as convection that goes on in high latitudes in both polar regions, then the heat that would have stayed at the surface had you not any ocean can be taken down into the depth of the ocean, thereby reducing the warming that you see at any given time. So, on such a planet, if you wait until the uncertainty that global warming is occurring is reduced, and that it's due to human factors, increasing greenhouse gases. If you stopped putting greenhouse gases in the earth's atmosphere today, and held the concentration constant, then we would have, in the future, a warming about as big as we have had already in the past, which is a little over a degree Fahrenheit averaged over the entire planet. So, this inertia of the system, or the hiding of the warming in the deep ocean, can fool you to thinking that the problem is less severe than it would otherwise be in an all-land covered planet. So, if you wait, if you hesitate to take actions, then you foreclose options. We showed this in a science paper last October, where we talked about hedging against an uncertain climate future. We looked at the effect of taking action now by imposing a really small tax on carbon, that turned out to be ten dollars a ton of carbon, which translates into an almost laughable five cents a gallon of gasoline, and that would increase at the inflation rate into the future. Although it starts small, after a hundred or so years of inflationary increases it can be substantial. We looked at suppose we impose this tax as a hedge against the uncertainty of the future climate compared to waiting 30 years and then taking action. We found that if you delay by 30 years until presumably, but not necessarily, the uncertainty is diminished, then you foreclose certain options that you hold open by taking action now. It turns out that taking action now in the form of this low cost carbon tax was less expensive than doing nothing for 30 years and then adding the tax on at that point. This was the first step that we made in looking at the effect of uncertainty on decision making, because the current administration in the US has used the argument that there is this large uncertainty and because there is we shouldn't do anything, or very little, until the uncertainty is reduced. In fact, there was an article by Andrew Revkin in the New York Times last Tuesday documenting that there was somebody in the White House who for all climate documents that came out of the government was adding words that enhanced the apparent uncertainty to bolster this argument, that in the face of this deep uncertainty, as I and my colleagues call it, you shouldn't do anything. But this paper we published in Science came to the opposite conclusion. That because there was this deep uncertainty it is better to take a low cost hedging strategy now by this low cost carbon tax than doing nothing for thirty 30 years until presumable, but as I said not necessarily, the uncertainty is diminished. We likened this to, we use a metaphor of insurance. Most people of a certain age that have children buy a life insurance. Nobody expect that their life is gonna terminate this week or next month or in the near future, but there is uncertainty about how long anyone of us will live, and this carbon tax is very much along the same lines.
MG: I looked at the Cooler Heads Coalition statement about the possibility of the alteration of the Gulf Stream, and they say it won't happen as long as winds blow and the earth turns, and the only way to stop the Gulf Stream is to stop the earth.
MS: Well, like all statements, there's some truth in this. There are two kinds of circulation in the North Atlantic. One is this thermohaline, which is due to the temperature and salinity, which determines the density of the water and the relative density of the surface water compared to the density below determines whether it will sink or not. Just like air in a balloon; if the air inside is less dense then the balloon will rise to the level where it's the same density as the atmosphere and it won't rise any further than that. There is wind-driven circulation, the so called gyre circulation in the middle latitudes of both hemispheres. The winds blow from the west to the east, and in the tropics they blow from the east to the west. This difference in the wind causes a gyre circulation in the North Atlantic, which moves in a clockwise direction in the northern hemisphere. In middle latitudes from the west to the east, and in the tropics from the east to the west. A part of the Gulf Stream is driven by that. So, when we in fact put on enough fresh water to shut down the thermohaline circulation, let me call it that now as opposed to the Gulf Stream, there is a residual, small residual transport from the south to the north which is due to this wind-driven gyre circulation. But it is very very small, and it doesn't go very far into the middle latitudes, let alone the high latitudes of the northern hemisphere. So the statement they make, as long as the earth is rotating and there are winds, that wind-driven circulation will exist. It doesn't contribute hugely to the heat transport. Most of the heat transport is due to this density-driven, temperature and salinity driven, transport in the North Atlantic. So the statement, on the face of it, part of it's right, but most of it's wrong.
MG: I should go back just a moment to an interview I did for my program prior to this. I talked to Jay Zwally, who's a Greenland researcher. He is observing accelerated melting on Greenland. Just looking at the store of water that's bound up on the Greenland ice sheet, that equals to seven meters sea level. So if all that were to come down, or a lot of it were to come down, that would have an influence on the Gulf Stream.
MS: That would have a fantastic influence on everything. In fact, all of these model simulations of the future; none of them include the melt of the Greenland ice sheet. In fact, my group recently submitted a proposal wherein we would have many different climate models simulate this slowdown or shutdown of the Gulf Stream in the past, and compare there results not only with each other, but with these ***[24:24] climate reconstructions to see how well the models do. But none of those model simulations of the future include Greenland, and we propose that we, since we have an ice-sheet model, would put in the ice-sheet in Greenland and Antarctica, and try to determine exactly what would happen in the warmer world. All the simulations that I talked about before simulate this increase in precipitation minus the evaporation which is the net addition of fresh water. That fresh water basically comes from the tropics where it evaporates from the ocean and is transported polewards by the motion of the atmosphere and then rains out over the middle latitudes. We can see that that addition by itself is enough to slowdown or perhaps even shutdown the Gulf Stream. So what we're talking about now is the additional fresh water added by Greenland. Recent observations over the past four or five years, in fact there's one paper, I believe, in press in Science by Ruth Curry has actually made measurements of the change in salinity of the top part of the North Atlantic, and they can see a significant freshening of the North Atlantic, that's the addition of fresh water. It's not clear where that fresh water is coming from. It may very well be from Greenland, and certainly if we melt any, let alone all, of the Greenland ice sheet then that's gonna be such a dramatic change that we probably don't wanna go there.
MG: That was an interview with Michael Schlesinger, professor of atmospheric sciences at the University of Illinois at Urbana-Champaign. He was trained at the University of California, Los Angeles, and holds degrees in engineering and meteorology. Professor Schlesinger directs the University of Illinois climate research group within the department of atmospheric sciences. He's an expert in the modeling, simulation, and analysis of climate, and climate change, with interest in understanding past, present, and possible future climates. This was part four in a three hour series on the dangers of climate change. The next segment will be on changes in the rising acidity of ocean water caused by the absorption of CO2 . These changes will most likely affect the entire marine food chain, and reduce the oceans ability to store heat. You can find out more about this series on the dangers of climate change on TUC Radio's website www.tucradio.org. Look under newest programs. You can get information on how to order a copy of the complete one hour program on Greenland and the Gulf Stream by calling us. TUC Radio is free to all radio stations. Your tape or CD order is the only support we receive and helps us stay on the air. Call us toll free anytime at 877-TUCTAPE for information on how to order. You can get your tape or CD by mail or credit card, by phone or on the web. Our toll free phone number 877-TUCTAPE translates into 18778828273, my name is Maria Gilardin. Thank you for listening, give us a call.
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