Field Notes From a Catastrophe: Man, Nature, and Climate Change (11 page)

For many years, the events described in “The Curse of Akkad” were thought, like the details of Sargon’s birth, to be purely fictional.

In 1978, after scanning a set of maps at Yale’s Sterling Memorial Library, a university archaeologist named Harvey Weiss spotted a promising-looking mound at the confluence of two dry riverbeds in the Khabur plains, near the Iraqi border. He approached the Syrian government for permission to excavate the mound, and, somewhat to his surprise, it was almost immediately granted. Soon, he had uncovered a lost city, which in ancient times was known as Shekhna and today is called Tell Leilan.

Over the next ten years, Weiss, working with a team of students and local laborers, proceeded to uncover an acropolis, a crowded residential neighborhood reached by a paved road, and a large block of grain-storage rooms. He found that the residents of Tell Leilan had raised barley and several varieties of wheat, that they had used carts to transport their crops, and that in their writing they had imitated the style of their more sophisticated neighbors to the south. Like most cities in the region at the time, Tell Leilan had a rigidly organized, state-run economy: people received rations—so many liters of barley and so many of oil—based on how old they were and what kind of work they performed. From the time of the Akkadian empire, thousands of similar potsherds were discovered, indicating that residents had received their rations in mass-produced, one-liter vessels. After examining these and other artifacts, Weiss constructed a time line of the city’s history, from its origins as a small farming village (around 5000 B.C.), to its growth into an independent city of some thirty thousand people (2600 B.C.), and on to its reorganization under imperial rule (2300 B.C.).

Wherever Weiss and his team dug, they also encountered a layer of dirt that contained no signs of human habitation. This layer, which was more than three feet deep, corresponded to the years 2200 to 1900 B.C., and it indicated that, around the time of Akkad’s fall, Tell Leilan had been completely abandoned. In 1991, Weiss sent soil samples from Tell Leilan to a lab for analysis. The results showed that, around the year 2200 B.C., even the city’s earthworms had died out. Eventually, Weiss came to believe that the lifeless soil of Tell Leilan and the end of the Akkadian empire were products of the same phenomenon—a drought so prolonged and so severe that it represented, in his words, an example of “climate change.”

Weiss first published his theory, in the journal
Science
, in August 1993. Since then, the list of cultures whose demise has been linked to climate change has continued to grow. They include the Classic Mayan civilization, which collapsed at the height of its development, around A.D. 800; the Tiwanaku civilization, which thrived near Lake Titicaca, in the Andes, for more than a millennium, then disintegrated around A.D. 1100; and the Old Kingdom of Egypt, which collapsed around the same time as the Akkadian empire. (In an account eerily reminiscent of “The Curse of Akkad,” the Egyptian sage Ipuwer described the anguish of the period: “Lo, the desert claims the land. Towns are ravaged … Food is lacking … Ladies suffer like maidservants. Lo, those who were entombed are cast on high grounds.”) In each of these cases, what began as a provocative hypothesis has, as new information has emerged, come to seem more and more compelling. For example, the notion that Mayan civilization had been undermined by climate change was first proposed in the late 1980s, at which point there was little climatological evidence to support it. Then, in the mid-1990s, American scientists studying sediment cores from Lake Chichancanab, in north-central Yucatán, reported that precipitation patterns in the region had indeed shifted during the ninth and tenth centuries, and that this shift had led to periods of prolonged drought. More recently, a group of researchers examining ocean-sediment cores collected off the coast of Venezuela produced an even more detailed record of rainfall in the area. They found that the region experienced a series of severe, “multiyear drought events” beginning around A.D. 750. The collapse of the Classic Mayan civilization, which has been described as “a demographic disaster as profound as any other in human history,” is thought to have cost several million lives.

The climate shifts that affected past cultures predate industrialization by hundreds—or, in some cases, thousands—of years. They reflect the climate system’s innate variability and could not have been foreseen by the societies that experienced them. Caught by surprise, the Akkadians made sense of their suffering as divine retribution. The climate shifts predicted for the coming century, by contrast, are attributable to forces whose causes we know and whose magnitude we will determine.

The Goddard Institute for Space Studies, or GISS, is situated just south of Columbia University’s main campus, at the corner of Broadway and West 112th Street. The institute is not well marked, but most New Yorkers would probably recognize the building: its ground floor is home to Tom’s Restaurant, the coffee shop made famous by
Seinfeld
.

GISS, an outpost of NASA, started out, forty-five years ago, as a planetary-research center; today, its major function is making climate forecasts. GISS employs about a hundred and fifty people, many of whom spend their days working on calculations that may—or may not—end up being incorporated in the institute’s climate model. Some work on algorithms that describe the behavior of the atmosphere, some on the behavior of the oceans, some on vegetation, some on clouds, and some on making sure that all these algorithms, when they are combined, produce results that seem consistent with the real world. (Once, when some refinements were made to the model, rain nearly stopped falling over the world’s rainforests.) The latest version of the GISS model, called ModelE, consists of 125,000 lines of computer code.

GISS’s director, James Hansen, occupies a spacious, almost comically cluttered office on the institute’s seventh floor. (I must have expressed some uneasiness the first time I visited him, because the following day I received an e-mail assuring me that the office was “a lot better organized than it used to be.”) Hansen, who is sixty-three, is a spare man with a lean face and a fringe of brown hair. Although he has probably done as much to publicize the dangers of global warming as any other scientist, in person he is reticent almost to the point of shyness. When I asked him how he had come to play such a prominent role, he just shrugged. “Circumstances,” he said.

Hansen first became interested in climate change in the mid-1970s. Under the direction of James Van Allen (for whom the Van Allen radiation belts are named), he had written his doctoral dissertation on the climate of Venus. In it, he had proposed that the planet, which has an average surface temperature of 876 degrees Fahrenheit, was kept warm by a smoggy haze; soon afterward, a space probe showed that Venus was actually insulated by an atmosphere that consists of 96 percent carbon dioxide. When solid data began to show what was happening to greenhouse gas levels on Earth, Hansen became, in his words, “captivated.” He decided that a planet whose atmosphere could change in the course of a human lifetime was more interesting than one that was going to continue, for all intents and purposes, to broil away forever. A group of scientists at NASA had put together a computer program to try to improve weather forecasting using satellite data. Hansen and a team of half a dozen other researchers set out to modify it, in order to make longer-range forecasts about what would happen to global temperatures as greenhouse gases continued to accumulate. The project, which resulted in the first version of the GISS climate model, took nearly seven years to complete.

At that time, there was little empirical evidence to support the notion that the earth was warming. Instrumental temperature records go back, in a consistent fashion, only to the mid-nineteenth century. They show that average global temperatures rose through the first half of the twentieth century, then dipped in the 1950s and ’60s. Nevertheless, by the early 1980s Hansen had gained enough confidence in his model to begin to make a series of increasingly audacious predictions. In 1981, he forecast that “carbon dioxide warming should emerge from the noise of natural climate variability” around the year 2000. During the exceptionally hot summer of 1988, he appeared before a Senate subcommittee and announced that he was “99 percent” sure that “global warming is affecting our planet now.” And in the summer of 1990 he offered to bet a roomful of fellow scientists a hundred dollars that either that year or one of the following two years would be the warmest on record. To qualify, the year would have to set a record not only for land temperatures but also for sea-surface temperatures and for temperatures in the lower atmosphere. Hansen won the bet in six months.

Climate models divide the world into a series of boxes. Credit:
Global Warming: The Complete Briefing,
Cambridge University Press.

Like all climate models, GISS’s divides the world into a series of boxes. Thirty-three hundred and twelve boxes cover the earth’s surface, and this pattern is repeated twenty times moving up through the atmosphere, so that the whole arrangement might be thought of as a set of enormous checkerboards stacked on top of one another. Each box represents an area of four degrees latitude by five degrees longitude. (The height of the box varies depending on altitude.) In the real world, of course, such a large area would have an incalculable number of features; in the world of the model, features such as lakes and forests and, indeed, whole mountain ranges are reduced to a limited set of properties, which are then expressed as numerical approximations. Time in this grid-world moves ahead for the most part in discrete, half-hour intervals, meaning that a new set of calculations is performed for each box for every thirty minutes that is supposed to have elapsed in actuality. Depending on what part of the globe a box represents, these calculations may involve dozens of different algorithms, so that a model run that is supposed to simulate climate conditions over the next hundred years involves more than a quadrillion separate operations. A single run of the GISS model, done on a supercomputer, usually takes about a month.

Very broadly speaking, there are two types of equations that go into a climate model. The first group expresses fundamental physical principles, like the conservation of energy and the law of gravity. The second group describes—the term of art is “parameterize”—patterns and interactions that have been observed in nature but may be only partly understood, or processes that occur on a small scale and have to be averaged out over huge spaces. Here, for example, is a tiny piece of ModelE, written in the computer language FORTRAN, which deals with the formation of clouds:

C**** COMPUTE THE AUTOCONVERSION RATE OF CLOUD WATER TO PRECIPITATION

RHO=1.E5*PL(L)/(RGAS*TL(L))

TEM=RHO*WMX(L)/(WCONST*FCLD+1.E–20)

IF(LHX.EQ.LHS) TEM=RHO*WMX(L)/(WMUI*FCLD+1.E=20)

TEM=TEM*TEM

IF(TEM.GT.10.) TEM=10.

CM1=CM0

IF(BANDF) CM1=CM0*CBF

IF(LHX.EQ.LHS) CM1=CM0

CM=CM1*(1.–1./EXP(TEM*TEM))+1.*100.*(PREBAR(L+ 1) +

* PRECNVL(L+1)*BYDTsrc)

IF(CM.GT.BYDTsrc)CM=BYDTsrc

PREP(L)=WMX(L)*CM

END IF

C**** FORM CLOUDS ONLY IF RH GT RH00

219 IF(RH1(L).LT.RH00(L)) GO TO 220

C**** COMPUTE THE CONVERGENCE OF AVAILABLE LATENT HEAT

SQ(L)=LHX*QSATL(L)*DQSATDT(TL(L), LHX)*BYSHA

TEM=–LHX*DPDT(L)/PL(L)

QCONV=LHX*AQ(L)-RH(L)*SQ(L)*SHA*PLK(L)*ATH(L)

* –TEM*QSATL(L)*RH(L)

IF(QCONV.LE.0.0.AND.WMX(L).LE.0) GO TO 220

C**** COMPUTE EVAPORATION OF RAIN WATER, ER

RHN=RHF(L)

IF(RHF(L).GT.RH(L)) RHN=RH(L)

All climate models treat the laws of physics in the same way, but, since they parameterize phenomena like cloud formation differently, they come up with different results. Also, because the real-world forces influencing the climate are so numerous, different models tend, like medical students, to specialize. GISS’s model specializes in the behavior of the atmosphere; other models in the behavior of the oceans; and still others in the behavior of land surfaces and ice sheets.

One rainy November afternoon, I attended a meeting at GISS that brought together members of the institute’s modeling team. When I arrived, about twenty men and five women were sitting in battered chairs in a conference room across from Hansen’s office. At that particular moment, the institute was performing a series of runs for the U.N. Intergovernmental Panel on Climate Change. The runs were overdue, and apparently the IPCC was getting impatient. Hansen flashed a series of charts on a screen on the wall summarizing some of the results obtained so far.

The obvious difficulty in verifying any particular climate model or climate-model run is the prospective nature of the results. For this reason, models are often run into the past, to see how well they reproduce trends that have already been observed. Hansen told the group that he was pleased with how ModelE had reproduced the aftermath of the eruption of Mount Pinatubo, in the Philippines, which took place in June 1991. Volcanic eruptions release huge quantities of sulfur dioxide—Pinatubo produced some twenty million tons of the gas—which, once in the stratosphere, condenses into tiny sulfate droplets. These droplets, or aerosols, tend to cool the earth by reflecting sunlight back into space. Man-made aerosols, produced by burning coal, oil, and biomass, also reflect sunlight and are a countervailing force to greenhouse warming, albeit one with serious health consequences of its own. (The impact of man-made aerosols is difficult to quantify; without it, however, the earth almost certainly would have warmed even faster than it has.) The cooling effect of aerosols lasts only as long as the droplets remain suspended in the atmosphere. In 1992, following the Pinatubo eruption, global temperatures, which had been rising sharply, fell by half a degree. Then they began to climb again. ModelE had succeeded in simulating this effect to within nine hundredths of a degree. “That’s a pretty nice test,” Hansen observed laconically.