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The Threat to the Planet

An Inconvenient Truth

a film directed by Davis Guggenheim

Jim Hansen is Director of the NASA Goddard Institute for Space Studies and Adjunct Professor of Earth and Environmental Sciences at Columbia University’s Earth Institute. His opinions are expressed here, he writes, “as personal views under the protection of the First Amendment of the United States Constitution.”


Animals are on the run. Plants are migrating too. The Earth’s creatures, save for one species, do not have thermostats in their living rooms that they can adjust for an optimum environment. Animals and plants are adapted to specific climate zones, and they can survive only when they are in those zones. Indeed, scientists often define climate zones by the vegetation and animal life that they support. Gardeners and bird watchers are well aware of this, and their handbooks contain maps of the zones in which a tree or flower can survive and the range of each bird species.

Those maps will have to be redrawn. Most people, mainly aware of larger day-to-day fluctuations in the weather, barely notice that climate, the average weather, is changing. In the 1980s I started to use colored dice that I hoped would help people understand global warming at an early stage. Of the six sides of the dice only two sides were red, or hot, representing the probability of having an unusually warm season during the years between 1951 and 1980. By the first decade of the twenty-first century, four sides were red. Just such an increase in the frequency of unusually warm seasons, in fact, has occurred. But most people—who have other things on their minds and can use thermostats—have taken little notice.

Animals have no choice, since their survival is at stake. Recently after appearing on television to discuss climate change, I received an e-mail from a man in northeast Arkansas: “I enjoyed your report on Sixty Minutes and commend your strength. I would like to tell you of an observation I have made. It is the armadillo. I had not seen one of these animals my entire life, until the last ten years. I drive the same forty-mile trip on the same road every day and have slowly watched these critters advance further north every year and they are not stopping. Every year they move several miles.”

Armadillos appear to be pretty tough. Their mobility suggests that they have a good chance to keep up with the movement of their climate zone, and to be one of the surviving species. Of course, as they reach the city limits of St. Louis and Chicago, they may not be welcome. And their ingenuity may be taxed as they seek ways to ford rivers and multiple-lane highways.

Problems are greater for other species, as Tim Flannery, a well-known Australian mammalogist and conservationist, makes clear in The Weather Makers. Ecosystems are based on interdependencies—between, for example, flower and pollinator, hunter and hunted, grazers and plant life—so the less mobile species have an impact on the survival of others. Of course climate fluctuated in the past, yet species adapted and flourished. But now the rate of climate change driven by human activity is reaching a level that dwarfs natural rates of change. And barriers created by human beings, such as urban sprawl and homogeneous agricultural fields, block many migration routes. If climate change is too great, natural barriers, such as coastlines, spell doom for some species.

Studies of more than one thousand species of plants, animals, and insects, including butterfly ranges charted by members of the public, found an average migration rate toward the North and South Poles of about four miles per decade in the second half of the twentieth century. That is not fast enough. During the past thirty years the lines marking the regions in which a given average temperature prevails (“isotherms”) have been moving poleward at a rate of about thirty-five miles per decade. That is the size of a county in Iowa. Each decade the range of a given species is moving one row of counties northward.

As long as the total movement of isotherms toward the poles is much smaller than the size of the habitat, or the ranges in which the animals live, the effect on species is limited. But now the movement is inexorably toward the poles and totals more than a hundred miles over the past several decades. If emissions of greenhouse gases continue to increase at the current rate—“business as usual”—then the rate of isotherm movement will double in this century to at least seventy miles per decade. If we continue on this path, a large fraction of the species on Earth, as many as 50 percent or more, may become extinct.

The species most at risk are those in polar climates and the biologically diverse slopes of alpine regions. Polar animals, in effect, will be pushed off the planet. Alpine species will be pushed toward higher altitudes, and toward smaller, rockier areas with thinner air; thus, in effect, they will also be pushed off the planet. A few such species, such as polar bears, no doubt will be “rescued” by human beings, but survival in zoos or managed animal reserves will be small consolation to bears or nature lovers.

In the Earth’s history, during periods when average global temperatures increased by as much as ten degrees Fahrenheit, there have been several “mass extinctions,” when between 50 and 90 percent of the species on Earth disappeared forever. In each case, life survived and new species developed over hundreds of thousands of years. The most recent of these mass extinctions defines the boundary, 55 million years ago, between the Paleocene and Eocene epochs. The evolutionary turmoil associated with that climate change gave rise to a host of modern mammals, from rodents to primates, which appear in fossil records for the first time in the early Eocene.

If human beings follow a business-as-usual course, continuing to exploit fossil fuel resources without reducing carbon emissions or capturing and sequestering them before they warm the atmosphere, the eventual effects on climate and life may be comparable to those at the time of mass extinctions. Life will survive, but it will do so on a transformed planet. For all foreseeable human generations, it will be a far more desolate world than the one in which civilization developed and flourished during the past several thousand years.


The greatest threat of climate change for human beings, I believe, lies in the potential destabilization of the massive ice sheets in Greenland and Antarctica. As with the extinction of species, the disintegration of ice sheets is irreversible for practical purposes. Our children, grandchildren, and many more generations will bear the consequences of choices that we make in the next few years.

The level of the sea throughout the globe is a reflection primarily of changes in the volume of ice sheets and thus of changes of global temperature. When the planet cools, ice sheets grow on continents and the sea level falls. Conversely, when the Earth warms, ice melts and the sea level rises. In Field Notes from a Catastrophe, Elizabeth Kolbert reports on the work of researchers trying to understand the acceleration of melting, and in his new book and film An Inconvenient Truth, Al Gore graphically illustrates possible effects of a rising sea level on Florida and other locations.

Ice sheets waxed and waned as the Earth cooled and warmed over the past 500,000 years. During the coldest ice ages, the Earth’s average temperature was about ten degrees Fahrenheit colder than today. So much water was locked in the largest ice sheet, more than a mile thick and covering most of Canada and northern parts of the United States, that the sea level was 400 feet lower than today. The warmest interglacial periods were about two degrees Fahrenheit warmer than today and the sea level was as much as sixteen feet higher.

Future rise in the sea level will depend, dramatically, on the increase in greenhouse gases, which will largely determine the amount of global warming. As described in the books under review, sunlight enters the atmosphere and warms the Earth, and then is sent back into space as heat radiation. Greenhouse gases trap this heat in the atmosphere and thereby warm the Earth’s surface as we are warmed when blankets are piled on our bed. Carbon dioxide (CO2), produced mainly by burning fossil fuels (coal, oil, and gas), is the most important greenhouse gas made by human beings. Methane (CH4), which is “natural gas” that escapes to the atmosphere from coal mines, oil wells, rice paddies, landfills, and animal feedlots, is also an important greenhouse gas. Other significant warming agents are ground-level ozone and black soot, which arise mainly from incomplete combustion of fossil fuels and biofuels.

In order to arrive at an effective policy we can project two different scenarios concerning climate change. In the business-as-usual scenario, annual emissions of CO2 continue to increase at the current rate for at least fifty years, as do non-CO2 warming agents including methane, ozone, and black soot. In the alternative scenario, CO2 emissions level off this decade, slowly decline for a few decades, and by mid-century decrease rapidly, aided by new technologies.

The business-as-usual scenario yields an increase of about five degrees Fahrenheit of global warming during this century, while the alternative scenario yields an increase of less than two degrees Fahrenheit during the same period. Warming can be predicted accurately based on knowledge of how Earth responded to similar levels of greenhouse gases in the past. (By drilling into glaciers to analyze air bubbles trapped under layers of snow, scientists can measure the levels of each gas in the atmosphere hundreds of thousands of years ago. By comparing the concentrations of different isotopes of oxygen in these air bubbles, they can measure the average temperature of past centuries.) Climate models by themselves yield similar answers. However, the evidence from the Earth’s history provides a more precise and sensitive measure, and we know that the real world accurately included the effects of all feedback processes, such as changes of clouds and water vapor, that have an effect on temperature.

How much will sea level rise with five degrees of global warming? Here too, our best information comes from the Earth’s history. The last time that the Earth was five degrees warmer was three million years ago, when sea level was about eighty feet higher.

Eighty feet! In that case, the United States would lose most East Coast cities: Boston, New York, Philadelphia, Washington, and Miami; indeed, practically the entire state of Florida would be under water. Fifty million people in the US live below that sea level. Other places would fare worse. China would have 250 million displaced persons. Bangladesh would produce 120 million refugees, practically the entire nation. India would lose the land of 150 million people.

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