Dreams of Earth and Sky (11 page)

Bohr’s understanding of quantum mechanics was based on a philosophical principle, which he called complementarity. Two descriptions of nature are said to be complementary when they are both true but cannot both be seen in the same experiment. In quantum mechanics, the wave picture and the particle picture of an electron or a light quantum are complementary. You see the wave picture when you do an experiment with electrons or light reflected from a diffraction grating and observe the diffracted waves. You see the particle picture when you detect the electrons or light quanta in an electronic counter and count them one at a time. Complementarity in quantum mechanics is an established fact. But in 1932 Bohr proposed to extend the idea of complementarity to biology, suggesting that the description of a living creature as an organism and the description of it as a collection of molecules are also complementary. In this context, complementarity would mean that any attempt to observe and localize precisely every molecule in a living creature would result in the death of the organism. The holistic view of a creature as a living organism and the reductionist view of it as a collection of molecules would be both correct but mutually exclusive. Bohr believed strongly in this application of complementarity to the understanding of life. Delbrück believed in it too when he decided to become a biologist.

It is one of the ironies of history that Delbrück chose to study the phage, which may be the only organism simple enough to be described without invoking complementarity. The life of the phage is
pure replication without metabolism. Replication is a chemical process that was completely explained by the double-helix structure of the DNA molecule discovered by Francis Crick and James Watson in 1953. When Crick and Watson discovered the double helix, they loudly claimed to have discovered the basic secret of life. The discovery came as a disappointment to Delbrück. It seemed to make complementarity unnecessary. Delbrück said it was as if the behavior of the hydrogen atom had been completely explained without requiring quantum mechanics. He recognized the importance of the discovery, but sadly concluded that it proved Bohr wrong. Life was, after all, simply and cheaply explained by looking in detail at a molecular model. Deep ideas of complementarity had no place in biology.

Segrè agrees with this judgment. He says dogmatically, “Bohr’s conjecture was provocative, as it was meant to be, but in the end it turned out to be wrong. DNA and RNA are the answer to life, not complementarity.” In the middle years of the twentieth century, this was the verdict of the majority of scientists. The historic debate over complementarity between Bohr and Einstein was over. Bohr had won in physics. Einstein had won in biology.

Now, fifty years later, Segrè’s opinion is widely held by physicists, less widely by biologists. I disagree with it profoundly. In my opinion, the double helix is much too simple to be the secret of life. If DNA had been the secret of life, we should have been able to cure cancer long ago. The double helix explains replication but it does not explain metabolism. Delbrück chose to study the phage because it embodies replication without metabolism, and Crick and Watson chose to study DNA for the same reason. Replication is clean while metabolism is messy. By excluding messiness, they excluded the essence of life. The genomes of human and other creatures have now been completely mapped and the processes of replication have been thoroughly explored, but the mysteries of metabolism still remain mysteries.

The phage is still the only living creature whose behavior is simple enough to be completely understood and predicted. To understand other kinds of creatures, from fruit flies to humans, we need also a deep understanding of metabolism. The understanding of metabolism will perhaps be the theme of the next revolution in biology. I have already discussed a seminal paper by the biologist Carl Woese with the title “A New Biology for a New Century,” pointing the way toward the next revolution.
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Woese’s new biology is based on the idea that a living creature is a dynamic pattern of organization in the stream of chemical materials and energy that passes through it. Patterns of organization are constantly forming and reforming themselves. If we try to observe and localize every molecule as it passes through an organism, we are likely to destroy the patterns that constitute metabolic life. In Woese’s picture of life, complementarity plays a central role, just as Bohr said it should.

At the same time, while Woese and others are debating the future of biology, the great debate over the future of physics continues. It is still a debate over the same questions that caused the disagreement between Bohr and Einstein. Does the quantum theory of the 1920s, together with the standard model of particles and interactions that grew out of it, give us a solid foundation for understanding nature? Or do we need another revolution to reach a deeper understanding?

Theoretical physicists are now divided into two main factions. Those who look forward to another revolution mostly believe that it will grow out of a grand mathematical scheme known as string theory. Those who are content with the outcome of the old revolution are mostly studying more mundane subjects such as high-temperature superconductors and quantum computers. String theory may be considered to be the counterattack of those who lost the debate over
complementarity in physics in Copenhagen in 1932. It is the revenge of the heirs of Einstein against the heirs of Bohr. The new discipline of systems biology, describing living creatures as emergent dynamic organizations rather than as collections of molecules, is the counterattack of those who lost the debate over complementarity in biology in 1953. It is the revenge of the heirs of Bohr against the heirs of Einstein.

I BEGIN THIS
review with a prologue, describing the measurements that transformed global warming from a vague theoretical speculation into a precise observational science.

There is a famous graph showing the fraction of carbon dioxide in the atmosphere as it varies month by month and year by year (see below). It gives us our firmest and most accurate evidence of the effects
of human activities on our global environment. The graph is generally known as the Keeling graph because it summarizes the lifework of Charles David Keeling, a professor at the Scripps Institution of Oceanography in La Jolla, California. Keeling measured the carbon dioxide abundance in the atmosphere for forty-seven years, from 1958 until his death in 2005. He designed and built the instruments that made accurate measurements possible. He began making his measurements near the summit of the dormant volcano Mauna Loa on the big island of Hawaii.

He chose this place for his observatory because the ambient air is far from any continent and is uncontaminated by local human activities or vegetation. The measurements have continued after Keeling’s death, and show an unbroken record of rising carbon dioxide abundance extending over fifty years. The graph has two obvious and conspicuous features. First, a steady increase of carbon dioxide with time, beginning at 315 parts per million in 1958 and reaching 385 parts per million in 2008. Second, a regular wiggle showing a yearly cycle of growth and decline of carbon dioxide levels. The maximum happens each year in the Northern Hemisphere spring, the minimum in the Northern Hemisphere fall. The difference between maximum and minimum each year is about six parts per million.

Keeling was a meticulous observer. The accuracy of his measurements has never been challenged, and many other observers have confirmed his results. In the 1970s he extended his observations from Mauna Loa, at latitude 20 north, to eight other stations at various latitudes, from the South Pole at latitude 90 south to Point Barrow on the Arctic coast of Alaska at latitude 71 north. At every latitude there is the same steady growth of carbon dioxide levels, but the size of the annual wiggle varies strongly with latitude. The wiggle is largest at Point Barrow where the difference between maximum and minimum is about fifteen parts per million. At Kerguelen, a Pacific island at
latitude 29 south, the wiggle vanishes. At the South Pole the difference between maximum and minimum is about two parts per million, with the maximum in the Southern Hemisphere spring.

The only plausible explanation of the annual wiggle and its variation with latitude is that it is due to the seasonal growth and decay of annual vegetation, especially deciduous forests, in temperate latitudes north and south. The asymmetry of the wiggle between north and south is caused by the fact that the Northern Hemisphere has most of the land area and most of the deciduous forests. The wiggle is giving us a direct measurement of the quantity of carbon that is absorbed from the atmosphere each summer north and south by growing vegetation, and returned each winter to the atmosphere by dying and decaying vegetation.

The quantity is large, as we see directly from the Point Barrow measurements. The wiggle at Point Barrow shows that the net growth of vegetation in the Northern Hemisphere summer absorbs about 4 percent of the total carbon dioxide in the high-latitude atmosphere each year. The total absorption must be larger than the net growth, because the vegetation continues to respire during the summer, and the net growth is equal to total absorption minus respiration. The tropical forests at low latitudes are also absorbing and respiring a large quantity of carbon dioxide, which does not vary much with the season and does not contribute much to the annual wiggle.

When we put together the evidence from the wiggles and the distribution of vegetation over the earth, it turns out that about 8 percent of the carbon dioxide in the atmosphere is absorbed by vegetation and returned to the atmosphere every year. This means that the average lifetime of a molecule of carbon dioxide in the atmosphere, before it is captured by vegetation and afterward released, is about twelve years. This fact, that the exchange of carbon between atmosphere and vegetation is rapid, is of fundamental importance to the
long-range future of global warming, as will become clear in what follows. Neither of the books under review mentions it.

William Nordhaus is a professional economist, and his book
A Question of Balance: Weighing the Options on Global Warming Policies
describes the global-warming problem as an economist sees it.
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He is not concerned with the science of global warming or with the detailed estimation of the damage that it may do. He assumes that the science and the damage are specified, and he compares the effectiveness of various policies for the allocation of economic resources in response. His conclusions are largely independent of scientific details. He calculates aggregated expenditures and costs and gains. Everything is calculated by running a single computer model that he calls DICE, an acronym for Dynamic Integrated model of Climate and the Economy.

Each run of DICE takes as input a particular policy for allocating expenditures year by year. The allocated resources are spent on subsidizing costly technologies—for example, deep underground sequestration of carbon dioxide produced in power stations—that reduce emissions of carbon dioxide, or placing a tax on activities that produce carbon emissions. The climate model part of DICE calculates the effect of the reduced emissions in reducing damage. The output of DICE then tells us the resulting gains and losses of the world economy year by year. Each run begins at 2005 and ends either at 2105 or 2205, giving a picture of the effects of a particular policy over the next one or two hundred years.

The practical unit of economic resources is a trillion inflation-adjusted dollars. An inflation-adjusted dollar means a sum of money,
at any future time, with the same purchasing power as a real dollar in 2005. In the following discussion, the word “dollar” will always mean an inflation-adjusted dollar, with a purchasing power that does not vary with time. The difference in outcome between one policy and another is typically several trillion dollars, comparable with the cost of the war in Iraq. This is a game played for high stakes.

Nordhaus’s book is not for the casual reader. It is full of graphs and tables of numbers, with an occasional equation to show how the numbers are related. The graphs and tables show how the world economy reacts to the various policy options. To understand these graphs and tables, readers should be familiar with financial statements and compound interest, but they do not need to be experts in economic theory. Anyone who knows enough mathematics to balance a checkbook or complete an income tax return should be able to understand the numbers.

For the benefit of those who are mathematically illiterate or uninterested in numerical details, Nordhaus has put a nonmathematical chapter at the beginning with the title “Summary for the Concerned Citizen.” This first chapter contains an admirably clear summary of his results and their practical consequences, digested so as to be read by busy politicians and ordinary people who may vote the politicians into office. He believes that the most important concern of any policy that aims to address climate change should be how to set the most efficient “carbon price,” which he defines as “the market price or penalty that would be paid by those who use fossil fuels and thereby generate CO
₂
emissions.” He writes: