[From CASE Reports, Vol. 13, No. 2, 1998]


GLOBAL WARMING

Connecticut Researchers Study the Science and Economics of Climate Change

If global warming lifts the height of the oceans by 50 centimeters, as is widely expected, Connecticut could lose beaches, wetlands, and expensive homes in luxury communities. Cities, too, such as Bridgeport, could be threatened.

Wondering what to do? Dr. Gary Yohe is the man to see. The Wesleyan economist and CASE member, who researches the ways humans can adapt to changes caused by the greenhouse effect, has estimated the costs of dealing with our collapsing coastlines.

"you let the seas rise and color in what you would lose"

"In the late 1980s," he says, "we did a sample of the economic vulnerability of the United States coastline It's a lot like a coloring book-you let the seas rise and color in what you would lose." But that technique, later research showed, overestimated the cost of the damage because it ignored a critical factor: "People," points out Yohe, "are going to make adjustments."

If a piece of property is worth enough, says Yohe, its owners will not let it be swallowed by the sea. Instead they will choose to protect it, by building sea walls, for example. Miami is a prime example. "One of the sources of a huge number in the estimate of economic vulnerability was Miami, because there were a lot of really expensive properties. But the fact is, you protect Miami." Already, he says, the city has built protective walls to save hotels, and has signed a 40-year contract with the Bahamas to import sand to preserve the beaches.

In one study, Yohe estimates that the cost of protecting Miami from a sea level rise of 100 centimeters could run as high as $15.7 million. But that's less expensive than letting the city disappear beneath the waves.

Decisions to save or abandon a piece of property must depend, he says, on whether the value of the property exceeds the cost of protecting it. He estimates that protecting against a one-meter rise in sea level costs between $150 and $4,000 a linear foot, a range, he says, that provides "an upper bound for the cost of sea level rise." In terms of the national estimate, Yohe has found that when he allows for adaptation, the costs drop by about 90%.

Here in Connecticut, Yohe believes the loss of land would be slight. He regards some coastal wetlands as vulnerable, but most of the Connecticut coast, he points out, is valuable, developed property. "I'm not sure we would lose a lot in the Madison-Saybrook-Guilfords of the world-those are fairly expensive properties." The land in cities, he thinks, might be more vulnerable. "Bridgeport loses some, based on a cost-benefit analysis of whether it would be worthwhile to protect it or not-even including some historically based estimates about how property value would appreciate in the meantime."

The Economics of Climate Change

The technique of using cost-benefit analysis to make decisions such as determining which parcels of land to save is standard economic practice. However, it was not used to evaluate the potential economic effects of climate change and climate change policies until the early 1990s. The first attempt to seriously utilize this type of analysis was made at Yale, by economist William Nordhaus.

His DICE (Dynamic Integrated model of Climate and the Economy) model, along with the multi-regional version, RICE (Regional Integrated model of Climate and the Economy), track the interaction between climate and economy. DICE was, he says "the first attempt to really tie together all the different components, from the economic structure, to emissions, climate, the carbon cycle, and then back through impacts. It's an attempt to look at the entire problem, rather than looking at it piece by piece."

Nordhaus found that the effect of global warming on the US economy was likely to be slight, with only the agriculture and forestry sectors (which provide approximately 3% of US income) expected to be severely affected. Nordhaus has estimated that the overall impact of the greenhouse effect on human activity worldwide "is unlikely to be more than 2% of the total output."

This does not mean, he emphasizes, that the increase in greenhouse gases should be ignored: "While global warming may seem benign," he wrote in a recent paper, "it has major and unpredictable impacts on weather patterns, ocean currents, sea-level rise, river run-offs, storm and monsoonal tracks, desertification, and other geophysical phenomena."

Using DICE to analyze different policies for cutting the emissions of greenhouse gases, Nordhaus showed that drastic reductions could come only at substantial costs. Considering global output over the indefinite future, he found that stabilizing emissions at 1990 levels could cost $7 trillion, while reducing emissions to 80% of 1990 levels might cost $12.5 trillion. (The recent Kyoto Conference on climate change calls for the United States to reduce emissions to 7% below 1990 levels.) A more ambitious goal of preventing climate change altogether, by limiting the rise in global temperature to1.5°C over 1990 levels, could cost, over future years, an estimated $41 trillion. By comparison, the 1995 gross domestic product of the United States was about $7.3 trillion.

Most recently, Nordhaus has analyzed the ways in which countries can work together to reduce greenhouse emissions. While, theoretically, emissions reductions are most effectively achieved by a globally cooperative, economically efficient approach, Nordhaus pinpoints reasons that countries might find it difficult to work together, among them, for example, that such cooperation might allow some to benefit at the expense of others.

The Critical Role of Carbon Dioxide

The atmospheric carbon dioxide (CO2) levels which now appear so abruptly threatening have always helped regulate Earth's temperature, a fact recognized at least since 1896. And, throughout Earth's past, CO2 levels have varied widely.

Fluctuations in CO2 levels regulate the ice ages, according to Yale climatologist and CASE member Barry Saltzman. He notes that for approximately the past 800,000 years, ice ages have waxed and waned in roughly hundred thousand year cycles. Though many believe that these cycles result from changes in the Earth's orbit around the Sun, Saltzman considers such shifts, by themselves, insufficient to account for the variations. The key factor, he feels, is CO2.

According to his model, which demonstrates an interaction between CO2, ice sheets, bedrock level and ocean temperature, over the past 800,000 years CO2 has varied between approximately 180 ppm (parts per million) and 300 ppm.

To understand how it works, he says, imagine the world covered by a large amount of ice-"not just ice on the continents, but a lot of sea ice, all around in high latitudes." The CO2 level is low, which has helped cool the Earth and thus build the ice.

Even during this glacial maximum, the Earth is warmer at the equator than at the poles. Ocean currents, such as the Gulf Stream, act to redistribute the warmth, carrying it to the higher latitudes. But as they do, the cold, dense water already near the poles sinks, traveling back toward the equator in the deeper ocean levels.

A Battle of Natural Forces

"In a sense," says Saltzman, "there's a constant battle going on between the filling of the ocean with cold water from the high latitudes, and the downward diffusion of heat from the warmer low and middle latitudes." In the glacial phase, he explains, the cold water, which tends to be rich in CO2, wins out. It fills up the ocean, moving closer to the surface. "This upwelling cold, carbon-rich water will now fizz off, like a soda pop, in the tropics," he says. "When you warm up water rich in CO2, it can't hold that CO2 any more."

By filling the oceans with colder water, Saltzman feels, the ice ages sow the seeds of their own destruction. The cold water gives rise to the upwelling that increases the concentration of atmos-pheric CO2. This warms the Earth and causes the ice sheets to disintegrate. But the oceans warm too, and the growing pool of warm water at the surface traps the carbon in the colder water below. The level of CO2 in the atmosphere begins to drop. The Earth grows colder, the ice sheets begin to form, and the cycle starts all over again.

El Niño, says Saltzman, provides something of an analogy. "During the normal situation, you have this cold water upwelling off the coast of Peru and Chile, bringing up carbon, and you can actually see the increase in global CO2 content, compared to El Niño, when you get a warm barrier in the eastern Pacific, inhibiting the upward flux not only of CO2, but of nutrients as well."

If CO2 is maintained at its current level, there will be no more ice ages, according to Saltzman's model. Over the past 100 million years, CO2 levels have been decreasing, with fluctuations due to long tectonic processes. The CO2-ice age oscillations did not begin, Saltzman feels, until CO2 concentrations fell enough to allow ice and snow fields to persist through Earth's summers. That happened, Saltzman thinks, when CO2 levels dropped to about 280 ppm. But now, he says, we've changed things.

"We've shot up the CO2 level dramatically, to a point that's probably similar to what we had 5 million years ago, before the ice sheets began to grow." Because of that, he thinks, we've placed ourselves outside the cycle of ice ages. "It may be that for natural reasons, after we burn up all the fossil fuels, we'll go back to nature and the CO2 levels will come down." But, he believes, if we maintain CO2 at the level we've forced it to, massive ice sheets will never return.

The Geochemical Cycle

Yet, Saltzman's hundred thousand year ice age pattern reflects only part of the story. CO2 does not shift only between the ocean and the atmosphere. Through a vaster, multi-million year geochemical cycle, it also travels down to the magma beneath the Earth's crust.

Drawn from the air through chemical reactions with weathering rocks, the gas, transformed in various ways, washes to the sea, where it is used by sea creatures to build shells and bones. Later, it sinks to the ocean floor, forming sediments that are buried beneath the continents and melted by the Earth's internal heat. In time, the gas returns to the atmosphere, perhaps through an erupting volcano.

Importantly, there is a balance: an equivalence. The CO2 pulled from the air by weathering rocks eventually re-emerges. Indeed, if this were not so, all CO2 would vanish from the atmosphere in about 10,000 years. So, if there is an imbalance in the flow-if weathering removes CO2 from the atmosphere more quickly than it can be replaced, or if volcanoes (or human activity) release CO2 more quickly than it can be removed-then the concentration of CO2 in the atmosphere can drastically change.

Yale geochemist and CASE member Robert Berner, who researches the behavior of the carbon cycle over the past 600 million years, has found that the rate at which carbon dixoide is pulled from the atmosphere depends on a mingling of factors. Among the most important is the amount of available rock. The relatively low levels of atmospheric CO2 over the past 65 million years result, he says, from the uplift of mountains such as the Himalayas: high mountains provide more rock to interact with the atmosphere.

Plants too, he says, greatly influence the rate at which weathering takes place. Through studies in Hawaii, New Hamp-shire, and, most recently, Iceland, Berner has found that plants can increase the rate of weathering by about sevenfold.

In fact, he attributes an abrupt drop in atmospheric CO2 levels that occurred between 380 and 280 million years ago to the rise of vascular plants. They affect weathering in a variety of ways, Berner explains in a recent paper. As they decompose, they produce acid that attacks rocks. But even while they are still alive, they produce acids through their roots and through their associated micro-organisms. Plants also tend to increase rainfall by drawing water up through their roots and releasing it to the atmosphere, and they anchor clay, which holds the water in the soil, giving it more time to dissolve the rocks.

It's the Sun, though, that controls the overall trend of CO2 concentration, Berner believes. He finds, underlying the large fluctuations in CO2 over the past 600 million years, a gradual downward trend that he believes to be the result of an increase in solar radiation. As the Sun ages, it has gradually grown warmer: the solar constant has increased linearly by 4.6% over the past 570 million years. Berner believes that CO2 levels have changed to compensate. "Years ago, when you had more CO2, the greenhouse effect would have been greater. But you had less solar radiation. Now, we have more solar radiation, and less CO2 and less greenhouse. The two balance each other."

The mechanism is this: as the atmosphere heats up, rainfall increases, and the land itself grows warmer. This accelerates the rate of weathering, which lowers the level of CO2. In fact, Berner's research leads him to believe that CO2 levels were once nearly 18 times as high as they are now. But, because of the cooler Sun, Earth's temperature was not correspondingly high. A doubling of current CO2 levels right now, he says, would increase the temperature by about 4°C. A similar concentration 570 million years ago would result in a temperature 9° cooler than it is today.

"the ultimate long-term effect"

Despite the current increase, Berner believes that, eventually, CO2 levels will continue to drop. In about 900 million years, he predicts, the Sun will get warm enough and the CO2 low enough that the Earth will no longer support photosynthesis. No more plants could exist. "That's the ultimate long-term effect," he says.

Yet, if the long-term path of CO2 can be determined, its short-term climatic effects-over the next 20, or 50, or 100 years-remain the subject of vociferous debate. Because of human activities, atmospheric CO2 is currently increasing at an unpredented pace: over the past hundred years, we have boosted CO2 levels from a pre-industrial 280 ppm to a current 360 ppm. And while changes of such magnitude have probably occurred before, they have taken place over tens of thousands of years, not over a mere century. We have added so much CO2 that, even if we take stringent steps to reduce future emissions and even if major technological breakthroughs occur, some type of climate changes will almost certainly occur.

But, troublingly, we do not understand yet what those changes will be. A gradual, mild warming may even temporarily benefit some portions of the world, perhaps by increasing agricultural yields. But scientists cannot rule out the possibility of abrupt climatic changes capable of toppling the infrastructures that support our lives. Nor can they eliminate the chance that even a gradual warming will, if unchecked, eventually produce an unfamiliar and inhospitable world.

Uncertainties pervade all aspects of our knowledge, from the interrelationships of climatic systems, to the effects of mitigation policies. By studying the far-reaching effects of CO2 level changes in the past, and by evaluating and developing climate-change related economic policies, the diverse research being done in Connecticut provides information critical to the task of dealing with the forces mankind has already set in motion.-- Karen Miller, science writer

 How Do They Know?

How is it that geologists can speak of events that took place millions of years before humans evolved? How can they know when ice ages came and went, or when mountains rose?

In fact, traces of these events linger subtly, often as minute changes in the ratios of types of atoms. Differences in oxygen atoms help track the ice ages. Comparisons of forms of carbon date recent events, while studying the products of the radioactive decay of uranium helps pinpoint the age of the Earth.

In a technique developed at Yale, measuring different forms of the element osmium preserved in ocean sediments can track tectonic events such as the uplifting of mountains.

Osmium, like all atoms, exists in a variety of forms (isotopes). The method used at Yale focuses on two of these isotopes, which are distributed unevenly throughout the planet. The isotope 187 Os predominates on the continents, but in the Earth's mantle, and in the oceanic crust, the amount of the isotopes 187 Os and 186 Os are about the same. As continents erode, the 187 Os they contain is deposited in the ocean, washed down by rivers, or carried in dust blown by the winds. And the ratio of 187 Os to 186 Os-- that is, the amount of 187 Os compared to the amount of 186 Os-increases. By itself, the continental crust averages a 187 Os/186 Os ratio of between 10 and 15. In the Earth's mantle, the ratio is close to 1. Right now, the ratio of 187 Os to 186 Os in the oceans is about 8. But that number changes over time. It depends on the mix, on how much osmium comes from the mantle, and how much from the land. It depends on how fast the land is eroding, and on how much land there is to erode.

And there's a complication: some mountains are formed of oceanic crust. The Himalayas were created like this: ocean floor crumpled upward as two continental plates collided. When they first formed, the Himalayas possessed the low osmium ratio typical of the oceanic crust. As a result, their rise (and subsequent weathering) temporarily increased the amount of 186 Os found in the oceans, lowering the 187 Os/186 Os ratio from about 4 to 3.

Yale geochemistry professor and CASE member Karl Turekian, who has developed techniques for interpreting osmium, tracks changes in isotopic ratios by analyzing clay samples taken from Long Lines 44-Giant Piston Core 3 (GPC3), a 24-meter long stack of ocean sediments that goes back 70 million years.

Over that time, the osmium ratio has varied from a low of about 2 to the current high of about 8. One abrupt drop, about 65 million years ago, records the impact of the meteorite widely believed to have caused the extinction of the dinosaurs: meteorites have an osmium ratio of about 1, close to that of the Earth's mantle. So when the crashing meteorite vaporized and dissolved, it temporarily lowered the 187 Os/186 Os ratio of the oceans.

Another drop, about 33 million years ago, records the closing of the Tethys sea, which once separated India and Asia; another, at 17 million years, marks the beginning of the uplift of the Himalayas. A steeper slope in the ratio, beginning about 7 million years ago and continuing today, reflects the swifter pace at which the Himalayas are rising.

All these tectonic changes are reflected in Earth's changing climate. The tall mountains bring new wind patterns, forcing drought and flood. Moving continents alter the paths of ocean currents. The faster rising ranges increase the rate of weathering, which draws more carbon dioxide from the air, which changes the climate of the Earth.-- K.M.


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