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The human hand in climate change
by Kerry Emanuel

Two strands of environmental philosophy run through the course of human history. The first holds that the natural state of the universe is one of infinite stability, with an unchanging earth anchoring the predictable revolutions of the sun, moon, and stars. Every scientific revolution that challenged this notion, from Copernicus’ heliocentricity to Hubble’s expanding universe, from Wegener’s continental drift to Heisenberg’s uncertainty and Lorenz’s macroscopic chaos, met with fierce resistance from religious, political, and even scientific hegemonies.

The second strand also sees the natural state of the universe as a stable one but holds that it has become destabilized through human actions. The great floods are usually portrayed in religious traditions as attempts by a god or gods to cleanse the earth of human corruption. Deviations from cosmic predictability, such as meteors and comets, were more often viewed as omens than as natural phenomena. In Greek mythology, the scorching heat of Africa and the burnt skin of its inhabitants were attributed to Phaeton, an offspring of the sun god Helios, who, having lost a wager to his son, was obliged to allow him to drive the sun chariot across the sky. In this primal environmental catastrophe, Phaeton lost control and fried the earth, killing himself in the process.

These two fundamental ideas have permeated many cultures through much of history. They strongly influence views of climate change to the present day.

The myth of natural stability

In 1837, Louis Agassiz provoked public outcry and scholarly ridicule when he proposed that many puzzles of the geologic record, such as peculiar scratch marks on rocks, and boulders far removed from their bedrock sources, could be explained by the advance and retreat of huge sheets of ice. This event marked the beginning of a remarkable endeavor, today known as paleoclimatology, which uses physical and chemical evidence from the geological record to deduce changes in the earth’s climate over time. This undertaking has produced among the most profound yet least celebrated scientific advances of our era. We now have exquisitely detailed knowledge of how climate has varied over the last few million years and, with progressively less detail and more uncertainty, how it has changed going back in time to the age of the oldest rocks on our 4.5-billion-year-old planet.

For those who take comfort in stability, there is little consolation in this record. Within the past three million years or so, our climate has swung between mild states, similar to today’s and lasting from ten to 20 thousand years, and periods of 100,000 years or so in which giant ice sheets, in some places several miles thick, covered northern continents. Even more unsettling than the existence of these cycles is the suddenness with which the climate can apparently change, especially as it recovers from glacial eras.

Over longer intervals of time, the climate has changed even more radically. During the early part of the Eocene era, around 50 million years ago, the earth was free of ice, and giant trees grew on islands near the North Pole, where the annual mean temperature was about 60°F, far warmer than today’s mean of about 30. There is also some evidence that the earth was almost entirely covered with ice at various times around 500 million years ago; in between, the planet was exceptionally hot.

What explains these changes? For climate scientists, the ice cores in Greenland and Antarctica provide the most intriguing clues. As the ice formed, it trapped bubbles of atmosphere, whose chemical composition—including, for example, its carbon dioxide and methane content—can now be analyzed. Moreover, it turns out that the ratio of the masses of two isotopes of oxygen locked up in the molecules of ice is a good indicator of the air temperature when the ice was formed. And to figure out when the ice was formed, one can count the layers that mark the seasonal cycle of snowfall and melting.

Relying on such analyses of ice cores and sediment cores from the deep ocean, climate scientists have learned something remarkable: the ice-age cycles of the past three million years are probably caused by periodic oscillations of the earth’s orbit that affect primarily the orientation of the earth’s axis. These oscillations do not much affect the amount of sunlight that reaches the earth, but they do change the distribution of sunlight with latitude. This distribution matters because land and water absorb and reflect sunlight differently, and the distributions of land and water—continents and oceans—are quite different in the northern and southern hemispheres. Ice ages occur when, as a result of orbital variations, the arctic regions intercept relatively little summer sunlight so that ice and snow do not melt as much.

The timing of the ice ages, then, is the combined result of the earth’s orbit and its basic geology. But this combination does not explain either the slow pace of the earth’s descent into the cold phases of the cycle or the abrupt recovery to interglacial warmth evident in the ice-core records. More disturbing is the evidence that these large climate swings—from glacial to interglacial and back—are caused by relatively small changes in the distribution of sunlight with latitude. Thus, on the time scale of ice ages, climate seems exquisitely sensitive to small perturbations in the distribution of sunlight.

And yet for all this sensitivity, the earth never suffered either of the climate catastrophes of fire or ice. In the fire scenario, the most effective greenhouse gas—water vapor—accumulates in the atmosphere as the earth warms. The warmer the atmosphere, the more water vapor can accumulate; as more water vapor accumulates, more heat gets trapped, and the warming spirals upward. This uncontrolled feedback is called the runaway greenhouse effect, and it continues until the oceans have all evaporated, by which time the planet is unbearably hot. One has only to look as far as Venus to see the end result. Any oceans that may have existed on that planet evaporated eons ago, yielding a super greenhouse inferno and an average surface temperature of around 900°F.

Death by ice can result from another runaway feedback. As snow and ice accumulate progressively equatorward, they reflect an increasing amount of sunlight back to space, further cooling the planet until it freezes into a “snowball earth.” It used to be supposed that once the planet reached such a frozen state, with almost all sunlight reflected back to space, it could never recover; more recently it has been theorized that without liquid oceans to absorb the carbon dioxide continuously emitted by volcanoes, that gas would accumulate in the atmosphere until its greenhouse effect was finally strong enough to start melting the ice.

It would not take much change in the amount of sunlight reaching the earth to cause one of these catastrophes. And solar physics informs us that the sun was about 25 percent dimmer early in the earth’s history, which should have led to an ice-covered planet, a circumstance not supported by geological evidence.

So what saved the earth from fire and ice?

Life itself may be part of the answer to the riddle of the faint young sun. Our atmosphere is thought to have originated in gases emitted from volcanoes, but the composition of volcanic gases bears little resemblance to air as we know it today. It is thought that the early atmosphere consisted mostly of water vapor, carbon dioxide, sulfur dioxide, chlorine, and nitrogen. There is little evidence that there was much oxygen—until the advent of life. The first life forms helped produce oxygen through photosynthesis and transformed the atmosphere into something like today’s, consisting mostly of nitrogen and oxygen with trace amounts of water vapor, carbon dioxide, methane, and other gases. Carbon-dioxide content probably decreased slowly with time owing to chemical weathering, possibly aided by biological processes. As the composition changed, the net greenhouse effect weakened, compensating for the slow but inexorable brightening of the sun.

Thus early life dramatically changed the planet. We humans are only the most recent species to do so.

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