Storms of My Grandchildren (22 page)

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 about 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 one hundred miles over the past several decades. If greenhouse gases continue to increase at business-as-usual rates, then the rate of isotherm movement will double in this century to at least seventy miles per decade.

Species at the most immediate 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.

Earth’s history provides an invaluable perspective about what is possible. Fossils in the geologic record reveal that there have been five mass extinctions during the past five hundred million years—geologically brief periods in which about half or more of the species on Earth disappeared forever. In each case, life survived and new species developed over hundreds of thousands and millions of years. All these mass extinctions were associated with large and relatively rapid changes of atmospheric composition and climate. In the most extreme extinction, the “end-Permian” event, dividing the Permian and Triassic periods 251 million years ago, nearly all life on Earth—more than 90 percent of terrestrial and marine species—was exterminated.

None of the extinction events is understood in full. Research is active, as increasingly powerful methods of “reading the rocks” are being developed. Yet enough is now known to provide an invaluable perspective for what is already being called the sixth mass extinction, the human-caused destruction of species. Knowledge of past extinction events can inform us about potential paths for the future and perhaps help guide our actions, as our single powerful species threatens all others, and our own.

We do not know how many animal, plant, insect, and microbe species exist today. Nor do we know the rate we are driving species to extinction. About two million species—half of them being insects, including butterflies—have been cataloged, but more are discovered every day. The order of magnitude for the total is perhaps ten million. Some biologists estimate that when all the microbes, fungi, and parasites are counted, there may be one hundred million species.

Bird species are documented better than most. Everybody has heard of the dodo, the passenger pigeon, the ivory-billed woodpecker—all are gone—and the whooping crane, which, so far, we have just barely “saved.” We are still losing one or two bird species per year. In total about 1 percent of bird species have disappeared over the past several centuries. If the loss of birds is representative of other species, several thousand species are becoming extinct each year.

The current extinction rate is at least one hundred times greater than the average natural rate. So the concern that humans may have initiated the sixth mass extinction is easy to understand. However, the outcome is still very much up in the air, and human-made climate change is likely to be the determining factor. I will argue that if we continue on a business-as-usual path, with a global warming of several degrees Celsius, then we will drive a large fraction of species, conceivably all species, to extinction. On the other hand, just as in the case of ice sheet stability, if we bring atmospheric composition under control in the near future, it is still possible to keep human-caused extinctions to a moderate level.

It’s important to describe the specific extinction events throughout Earth’s history, but first we should look at how information is gleaned from fossil records. Fossils are remains, impressions, or traces of life preserved in rock or sedimentary layers. All or portions of deceased organisms, from microscopic to dinosaur-size, and commonly deposited on the floor of the ocean, lakes, bogs, and the alluvia of streams and rivers, are preserved in sediments and rocks that were formed after these deposits were buried under sufficient pressure. A variety of methods is now available to date these deposits, which makes it possible to catalog the history of species at many locations. Extinctions are defined by the times beyond which fossils of a given species are not found anywhere in the world.

Causes of the end-Permian extinction, when life nearly died, are still debated, but some facts are reasonably clear. The extinction event took place during a time of massive volcanic eruptions in Siberia that spread basalt lava over an area the size of Europe in a layer as much as two miles thick. The lava outflow occurred over a period of about a million years. By itself the lava outflow probably could not be responsible for the extinctions—there have been a few larger lava outflows in Earth’s history without such extreme loss of life. One factor may have been noxious gases from the eruptions, including acid rain produced by the volcanic sulfur dioxide emissions, which placed a stress on late Permian life-forms.

However, the biggest stress on life may have been the strong global warming that occurred at that time, about 6 degrees Celsius at low latitudes and probably more at high latitudes. This warming was a puzzle to scientists for some time, because the size and slow pace of the basalt eruption should not have produced enough carbon dioxide to yield such a great increase in temperature. It was only with the help of carbon isotope studies of end-Permian sediments that a likely explanation for the large magnitude of this global warming emerged.

Carbon isotopes are extremely important to climate studies. The properties of an element depend mainly on the number of protons in its nucleus. Some elements exist in more than one form (different isotopes), depending on the number of neutrons in the nucleus. The number of neutrons has only small effects on the element’s properties, but the small effects turn out to be very useful for climate studies. The most common form of carbon, carbon-12, has six protons and six neutrons. About 99 percent of the carbon atoms in carbon dioxide is carbon-12, and about 1 percent is carbon-13, with seven neutrons in the nucleus.

Plants prefer carbon-12, the light carbon. In other words, as plants grow by taking carbon dioxide out of the air, they take in more of the carbon dioxide that has carbon-12 than would be expected from its proportion in the air. Thus sedimentary deposits derived from biological material, such as coal, have an unusually large proportion of light carbon.

One of the characteristics of rocks formed during end-Permian time was an even greater proportion of light carbon. That meant the atmosphere at the time had an excessive amount of light carbon. How could that be? One possibility was that a huge amount of coal “burned” during that period—it was exposed to the surface and oxidized. But it would have required burning almost all the coal on the planet to reach such light carbon levels, and how could the coal have been unearthed? It did not seem plausible.

The likely source of the light carbon came to light in the past decade or so, as scientists began to focus on methane ice, also called methane clathrates or methane hydrates. This is essentially “frozen” methane, with each methane molecule enclosed in a “cage,” or crystal, of water ice. Large amounts of methane ice are found today in arctic tundra (frozen ground) and, especially, beneath sediments on the seafloor of the Arctic Ocean. The methane was produced by bacterial degradation of organic matter in a low-oxygen environment—in other words, the rotting and decay of buried plant and animal remains—which yields an even a higher concentration of carbon-12 than coal does. This is the only carbon repository on Earth with enough light carbon to plausibly explain the end-Permian data.

The mystery of all the light carbon in the air during the end-Permian extinction finally was solved, but the details remain unclear. The large Siberian lava flows may have caused the melting and release of methane ice from the Siberian tundra and the ocean floor; or global warming due to carbon dioxide from the Siberian basalt eruptions caused melting of the methane ice, which then amplified the global warming, leading to the great warming of 6 or more degrees Celsius.

There are still many questions and theories about exactly what happened during the end-Permian extinction and why it was so devastating. Some scientists believe that an asteroid collided with Earth at that time, perhaps helping to initiate the Siberian basalt eruptions or methane hydrate release or both. But no convincing evidence of an asteroid collision has been provided. Most geologists agree that methane hydrates played a role, probably an important role. There is simply no other known source for such a large amount of light carbon. It also seems likely that the large global warming was an important factor in this great crisis for life on the planet.

What is certain is the magnitude of the devastation. It took about 50 million years for life to again develop the diversity that it had prior to the event.

Other extinctions, albeit less devastating ones, took place more recently and can be studied in more detail. The famous end-Cretaceous extinction, which wiped out about half the species on the planet, including the dinosaurs, occurred 65 million years ago when an asteroid struck Earth, producing a crater about one hundred miles wide on the Yucatán peninsula in Mexico. The extinctions are believed to have been caused, at least in part, by a massive injection of gas and dust into the atmosphere. Aerosols in the stratosphere would have blocked sunlight for a few years, reducing photosynthesis and causing a temporary global cooling.

A slightly more recent extinction event, about 55 million years ago, deserves greater scrutiny, because it is the most relevant to ongoing human actions. The Paleocene-Eocene thermal maximum (PETM) is classified as a minor extinction event—almost half the deep ocean foraminifera (microscopic shelled animals) species disappeared, but there was little extinction of land plants and animals. The range of some flora expanded poleward by hundreds and even thousands of miles. And the diversity, dispersal, and body sizes of terrestrial mammals changed rapidly.

Global warming of about 5 to 9 degrees Celsius occurred in the PETM, almost as great as in the end-Permian and comparable to the warming that may occur in the next century or so if business-as-usual greenhouse gas emissions continue. But the fact that most terrestrial species survived the PETM does not mean we shouldn’t be concerned about the effect of future global warming, for two major reasons. First, the PETM warming occurred over millennia, not in a century. That means the power in the human punch is an order of magnitude greater. Climatic zones are moving poleward ten times faster now than in the PETM. Second, humans are simultaneously causing other stresses on animals and plants, by overharvesting, deforesting, and simply taking over large parts of the planet.

We need to dig deeper, to understand the PETM better, before drawing conclusions. But the PETM cannot be reliably interpreted in isolation. It needs to be looked at in the broader context of Earth’s climate history, which has much to teach us.

That’s easier said than done. The paleoclimate literature is voluminous and arcane. My own minor contribution, an empirical evaluation of climate sensitivity by comparing the last ice age and the Holocene, only scratched the surface of paleoclimate data. But in early June 2007 I received phone calls from the media that gave me an added push to dig a little deeper into paleoclimate.

The calls requested my reaction to a statement made by NASA administrator Michael Griffin on National Public Radio. This was Griffin’s response to a question about global warming:

I am not sure that it is fair to say that it is a problem we must wrestle with. To assume that it is a problem is to assume that the state of the Earth’s climate today is the optimal climate, the best climate that we could have or ever have had, and that we need to take steps to make sure that it doesn’t change. First of all, I don’t think it’s within the power of human beings to assure that the climate does not change, as millions of years of history have shown. And second of all, I guess I would ask which human beings—where and when—are to be accorded the privilege of deciding that this particular climate that we have right here today, right now, is the best climate for all other human beings. I think that’s a rather arrogant position for people to take.

Upon reflection, I realized that many well-educated people might draw conclusions similar to Griffin’s. It is not easy to appreciate the implications of paleoclimate time scales—Griffin obviously did not. But his ignorance underlined a broader problem. Paleoclimate data actually reveal the opposite of what Griffin concluded. So why have we been unable to make that clear, especially the staggering implications for global energy policies?