AS A GROUP, CLIMATE SCIENTISTS are reluctant to claim even the soundest projections as incontrovertible facts. The climate system is by definition exceedingly complex, and theories must always be open to revision as new observations become available. When I was a graduate student we learned that the warming that defined the end of an ice age occurred very gradually, over the course of many thousands of years. This view of sluggish climate changes was shattered about ten years ago when scientists discovered that the main warming events that ended the last ice age took place within less than a decade. In Greenland, air temperatures warmed by about fifteen degrees centigrade within the time it takes to complete a college degree.
Still, most climate scientists today agree that Earth’s climate is warming and changing as a result of human activity, and that the projected changes in coming decades will affect nearly all parts of the globe. This combination of exceptional risk and uncertainty has led to a lack of clear consensus among policy makers on how to address the global warming crisis. National-level planning and preparation for current and future climate change remain mired in dysfunction and polarized along a scientific/political divide. There are those who are convinced that there is a big problem and those who would make the case that there is no problem at all. A path of least resistance has led to a cul-de-sac of inaction.
The divide between science and policy is marked by the fundamentally different motivations, accountability, and time frames that characterize the players. Scientists are generally motivated by intellectual curiosity and achievement; they are accountable mainly to their profession; and they have open time frames in which to produce results.
Politicians, on the other hand, are generally motivated by the need for conflict resolution and political viability. They are accountable to their constituents and have very restricted time frames in which to produce results. Global warming is of far larger scope than any single administration can address, and effective policy demands a decades-long, committed, political response.
To complicate matters, political solutions to global warming call for critical decisions based on imperfect observations and seemingly nebulous risks, in contrast to problems like world hunger or AIDS, for which the evidence and consequences are all too apparent. Consequently, few American politicians have had the courage to take on the issue. Given the short-term economic costs and the perceived lifestyle changes required, addressing the problem of global warming is a platform perfectly ill-suited to election or re-election.
WITH THE START of the Industrial Revolution, humankind began a vast global climate experiment of which we are only now realizing the effects. Combustion of fossil fuels, burning, and land-use changes have led to a nearly 34 percent increase in atmospheric carbon dioxide (CO2), reaching the highest levels in four hundred thousand years, and a nearly 150 percent increase in methane, both greenhouse gases that warm the planet.
Over the last 150 years the Earth’s surface temperatures have risen by about 0.8 degrees centigrade, with the majority of the rise taking place in just the last fifty years. A true though seemingly improbable statistic is that all of the ten warmest years on record have occurred since 1990.
The warming we’ve experienced so far may not seem like much, but it is just the beginning of a steadily upward trend that is expected to reach between 1.4°C and 5.8°C by the end of this century when CO2 levels will have more than doubled. The Earth hasn’t seen such high CO2 levels for over 25 million years, and it was then a very different place: London had groves of palm trees, and crocodiles nested in the swamps of a warm and humid Canadian arctic.
Current projections indicate that if nothing is done to slow greenhouse gas emissions, rising global temperatures will produce a wide range of climate impacts. Large-scale changes in rainfall patterns and growing seasons are predicted, as are more intense and frequent storms, typhoons, and hurricanes. Higher frequencies of extreme weather events may also occur, with record temperatures and high heat-index days, like the August 2003 heat wave across Europe that claimed thousands of lives. Sea levels are predicted to continue to rise (perhaps by as much as one meter by 2100), an especially pernicious risk since roughly half of the world’s population lives within sixty miles of an ocean. In addition to causing the loss of productive land in highly populated coastal regions such as Bangladesh, rising seas would force the relocation of tens or hundreds of millions of people and lead to a refugee crisis of unprecedented scope.
Perhaps one of the greatest challenges to society, however, and one often overlooked, is the likelihood of drought events more severe than any we have experienced. The continental interiors, home to the breadbaskets of North America and Eurasia, are projected to become markedly drier in future decades, leading to a greater frequency of protracted regional drought. How a modern, urbanized society of today might respond to a period of pervasive, extended drought is yet to be seen, but climate history may offer some lessons in at least understanding the effects of this aspect of our climatically uncertain future.
FOR ALL ITS DEVASTATION, the Dust Bowl of the 1930s — one of the most well-documented agricultural, economic, and social disasters in the history of the United States — was the result of a merely “above average” drought. But in a rare display of unanimity the scientific community has determined that this is not the type of drought we need to prepare for today. Cardinal among the climate risks presented, under numerous scenarios, by global warming, is the higher probability of “megadroughts.” A megadrought is a very different beast in that it persists for many decades, not just a few years, and affects broad sectors of whole continents. Though we have no modern instrumental records of past megadroughts, there is very good scientific evidence that vast regions of North America witnessed several such periods during the last millennium, with devastating cultural consequences.
Paleoclimatology, the branch of science to which I belong, involves the reconstruction of ancient climatic conditions using evidence such as tree rings, lake and ocean sediment records, and ice cores. Our particular corner of science affords us a different view of climate change than that available to scientists who, for example, examine historical (recorded) climate changes. Such records extend back only a century or two. But by comparing paleoclimate evidence of climate changes to archeological records of cultural changes, we have been able to learn a great deal about the broader social impacts of exceptionally large changes in climate.
The history of the Maya culture in Central America provides possibly the best example of how an extended drought can impact a highly developed, technologically advanced urban society. The Maya had thrived for nearly two thousand years and their cultural achievements were comparable in many ways to those of any modern G-8 nation. They were accomplished astronomers, mathematicians, and urban planners. They built large, well-engineered cities and had established trade networks. Their society was stratified, closely governed, and populous, with an estimated eight to fifteen million people in cities and rural villages across Central America and the YucatÃ¡n Peninsula.
This thriving civilization collapsed at the peak of its cultural and scientific development, between 750 and 950 a.d., and the decline coincided precisely with a 150-year drought that gripped the region. A paleoclimate record consisting of annually laminated sediments revealed that the three regional waves of societal collapse (occurring around 810, 860, and 910 a.d.) corresponded with three decade-long extreme droughts that hit the region during this already dry period. Many archeologists recognize the importance of social conditions in contributing to the collapse, but the extended drought appears to have been a primary factor in gradually reducing the carrying capacity of the land, which already suffered from overpopulation and overexploitation of resources.
THE SCIENTIFIC COMMUNITY is now very confident that much of the warming over the past century is attributable to human activities. Global temperatures have changed for many natural reasons, of course, but when scientists try to account for the full record of global temperature changes over the past several hundred years, it is impossible to obtain a close match to the observed temperature curve without including the effects of very recent increases in greenhouse gas concentrations.
With global warming comes greater probabilities of “climate surprises” — unexpected climate changes such as megadroughts, which would seriously challenge our ability to adapt. Wally Broecker, a National Medal of Science awardee for his paleoclimate research and my colleague at Columbia University, likens the climate system to an angry beast that we’re poking with a stick — provoking something we know to be extremely sensitive and immeasurably powerful.
Of perhaps still greater concern than megadrought, though even more difficult to predict, is the possibility that continued global warming may lead to changes in ocean circulation — the giant flywheel of global climate, and also its Achilles heel. Modern ocean observations provide some very early indications that the initial phases of circulation change are already underway. A team of oceanographers at the Woods Hole Oceanographic Institution have observed that all Nordic seas have freshened tremendously over the last forty years, and that the salinity of the deep ocean is now lowering as well. These changes may be signaling a shift or disruption in ocean circulation, and if that happens, we’re in for all sorts of surprises. Atmospheric circulation is often likened to a nimble mouse scurrying through the forest — it responds quickly to imposed changes. Ocean circulation, on the other hand, is more like a giant lumbering elephant — reluctant to alter its course but suddenly clearing the forest when it does.
The kinds of climate change that could result from a shift in ocean circulation would be as devastating to contemporary culture as the decimating drought was to the Maya. The scenario that current climate models show goes like this: Today, as northward-flowing warm, salty North Atlantic Drift water is stripped of its heat by the atmosphere each winter, it brings surface warmth to the North Atlantic, and is partly responsible for the mild climate of northwestern Europe. In the Nordic seas chilled surface waters sink and massive volumes of cold, salty deep water flow southward toward the Antarctic, where they resurface, forming a giant ocean conveyor. In some climate model simulations of our future, after a gradual, decades-long warming and freshening trend in the North Atlantic due to increased Arctic warming, ice melting, and river runoff, this ocean conveyor slows down abruptly — within several decades. The stability of the ocean conveyor has been likened to a light switch that flips suddenly from the “on” to “off” position given a steady application of finger pressure. The resulting changes in ocean temperatures would likely lead to equally abrupt, very large, and lasting changes in land temperature and rainfall patterns globally. A shift in the ocean conveyor, once initiated, is essentially irreversible over a time period of many decades to centuries, and would permanently alter the climatic norms for some of the most densely populated and highly developed regions of the world, multiplying the risks — and magnifying the effects — of major climatic disruptions from global warming.
While most climate models indicate the conveyor is responsive to global warming, few models project a complete conveyor shutdown within the next century. However, we do know that this scenario has happened several times before, and scientists have a very good idea of how large and how far-reaching the impacts can be. When the Earth was thawing out of the last ice age about 14,500 years ago due to gradual, periodic changes in the Earth’s orbit, the melting of large ice sheets on the continents funneled large volumes of fresh water into the North Atlantic. Analyses of ice cores, deep-sea sediment cores, and other geologic evidence have shown that the surge of river runoff did in fact form a freshwater lid over the North Atlantic, which subsequently shut down formation of deep water currents for about a millennium between 12,800 and 11,700 years ago.
Known as the Younger Dryas event, this period marked a sudden reversal in the warming trend, and regional temperatures plummeted. Greenland and northern European temperatures dropped by as much as ten to fifteen degrees centigrade within a decade. Arctic conditions were established in the United Kingdom and Europe; icebergs reached as far south as Portugal; Africa and parts of Asia became much drier. The event has been detected in paleoclimate records from nearly all regions of the globe. It took nearly a thousand years for ocean salinities to gradually increase until the conveyor switched on again. An equally rapid warming, completed in less than a decade, marked the end of the Younger Dryas event and the beginning of the current warm period, the Holocene, bringing with it the birth of agriculture and the subsequent rise of complex urban societies.
IN LEARNING HOW and why past shifts in climate have occurred and what their effects have been, paleoclimatology has made remarkable progress in defining just how large and abrupt natural climate swings can be and what these may mean for our global-warming future. In his statement to a U.S. Senate committee in 2003, Richard Alley, a leading paleoclimatologist from Pennsylvania State University, presented a consensus view of past and present climate change that was recently summarized in the National Academy of Sciences report Abrupt Climate Change: Inevitable Surprises. The main points were that climate is very sensitive to even weak forcing; that climate changes, when they occur, tend to happen very quickly (on timescales of years to decades); and that the present and projected greenhouse-gas climate forcing greatly exceeds the relatively weak forcing that caused, for example, the Dust Bowl, or even past megadroughts.
“Many current policies and practices are likely to be inadequate in a world of rapid and unforeseen climatic changes,” said Alley. “Identifying ways to improve these policies will be beneficial even if abrupt climate change turns out to fit a best-case, rather than a worst-case, scenario. Societies will have no regrets about the new policies, because they will be good policies regardless of the magnitude of environmental change.”
The “no-regrets” and “good policy” political solutions are still distant dreams, however, as the U.S. government has been reluctant to face the political dimension of the problem. In 2001, the Bush administration abandoned the Kyoto Protocol, the first legally binding international treaty for reducing greenhouse gas emissions. The administration deemed it unfair (developing nations such as China and India were exempted in the short term) and viewed it as too costly to the U.S. economy. Yet the cost of doing nothing may be far greater. In the state of Florida alone, the projected cost of a one-meter rise in sea level has been estimated to exceed $300 billion (in 1991 dollars), not including future development losses for the nearly fourteen thousand square miles of inundated real estate.
A recent European Union economic assessment concluded that Kyoto compliance will cost the EU only about 0.1 percent of its gross domestic product by 2010. And with less than 5 percent of the world’s population here in this country responsible for nearly 25 percent of global fuel consumption and emissions, there can be little question that the U.S. has a huge historical carbon debt. The current gas-guzzling culture only deepens the international community’s growing disappointment with the lack of American leadership on the issue.
Even in the U.S. there is a growing recognition that the magnitude of the changes that could ensue from global warming would very likely challenge political stability worldwide. A report commissioned in 2002 by the Pentagon entitled Imagining the Unthinkable explored a scenario in which continued global warming leads to the eventual shutdown of the conveyor after 2010. The resulting changes in climate bring about protracted megadroughts in Europe, China, and North America. Food, water, and energy shortages create border conflicts that are exacerbated by mass migrations. Citing historical precedent, the authors note, “Every time there is a choice between starving or raiding, humans raid.” Widespread famine and political chaos eventually lead to a “world of warring states.” Many scientists dismissed the report as overly alarmist. “Exaggerated scenarios serve only to intensify the existing polarization over global warming,” said Wally Broecker. “What is needed is not more words but rather a means to shut down CO2 emissions to the atmosphere.”
IT MAY BE TOO LATE to reverse changes that have already begun, but there is still much that can be done to minimize future increases of greenhouse gases and to mitigate and prepare for the effects of large climate change. Policy makers must begin serious discussion on immediate implementation of political solutions to reduce emissions and increase adaptive capacity. While some of the coping strategies learned from previous shorter droughts will no doubt prove useful, the sheer duration and intensity of a megadrought pose an entirely new set of technological hurdles. One of the world’s leading freshwater resource experts, Dr. Peter Gleick of the Pacific Institute, recommends that “water managers should begin a systematic reexamination of engineering design, operating rules, contingency plans, and water allocation policies under a wider range of climatic conditions and extremes than has been traditionally used. For example, the standard engineering practice of designing for the worst case in the historical observational record may no longer be adequate.”
Presciently, some industries are already incorporating future global warming into their business plans. Perhaps most revealing are the concerns of the reinsurance industry — those who insure the insurers — which has been paying out an escalating number of claims in recent years as a result of vastly increased storm-related property damage. Between 1989 and 1994, insurers paid out more than $67 billion in storm damage claims — $20 billion more than was paid out during the previous five years. Natural disaster claims in 2003 were up 36 percent from 2002, and claims from the August hurricanes of 2004 exceeded $20 billion in Florida alone. Facing potentially crippling future claims, the industry has been lobbying on Capitol Hill to jump-start discussions on climate-change mitigation. In a richly significant statement, H. R. Kaufmann, general manager of Swiss Re, the world’s second-largest reinsurer, dryly announced, “In light of the magnitude of these losses, it would be prudent for the property/casualty industry to act as if that theory [global warming] is correct. Failure to act would leave the industry and its policyholders vulnerable to truly disastrous consequences.”