Excerpt from The Whale and the Supercomputer

by Charles Wohlforth
From Chapter 6

Why carbon dioxide affects climate

The NOAA-CMDL Barrow lab was the best equipped in the Arctic for measuring climate change indicators. It was the tip of a long, air-sniffing antenna reaching from the globe’s inhabited zones to this perimeter of the human habitat. Numbers flowed to scores of investigators with individual projects all over the world, and to thousands of researchers trying to grasp the climate as a system, mathematically, and predict its future. This was the longest continuous record of atmospheric carbon dioxide in the Arctic and the second longest on earth after the record started in 1958 at Mona Loa, Hawaii, by Charles Keeling himself, who first discovered the rapid increase caused by human activities (a project funded in part by the International Geophysical Year). Manager Dan Endres kept handy a plot of his lab’s entire twenty-eight-year record of atmospheric carbon dioxide and temperature to show visitors. He said, “That’s the most famous data to come out of the Arctic anywhere at any time.” It was an extraordinary graph, almost too clear and unequivocal to be true. Each year, it showed atmospheric carbon dioxide rising and falling with the season, but with each winter peak higher, in an inexorable stair-step up the page. Winter temperatures trended upward along the same path, one degree C per decade.

The relationship between temperature and atmospheric carbon dioxide has been understood since the nineteenth century and is conceptually simple. The sun’s energy, which powers the weather and virtually all life, arrives at our planet in short wavelengths, such as visible light, that pass through the atmosphere relatively easily. But when that energy reflects off the earth and bounces back toward space, much of it has become radiated heat, or long-wavelength infrared energy. Certain gases in the atmosphere that are transparent to sort wavelengths--including water vapor, carbon dioxide and methane--instead absorb long wavelengths. The sun’s energy bouncing up from the earth heats those gasses instead of escaping to the stratosphere or out to space. This warms the earth and the lower atmosphere, called the troposphere, and cools the stratosphere. The phenomenon is called the greenhouse effect because glass in a greenhouse works essentially the same way: it lets energy come in as light but won’t let it go out as heat. The greenhouse effect is powerful, the second most important factor after the sun in determining the earth’s temperature; without greenhouse gases, the planet would average -18 degrees C, too cold for most life (the current average is around 14 degrees C). Carbon dioxide heats up so readily under infrared energy that instruments built to detect minute quantities of it, including the one at Dan Endres’ lab, do so by exposing air samples to infrared and measuring the change in pressure.

Besides being a greenhouse gas, carbon dioxide is necessary for life. Plants capture energy from the sun by combining the carbon atom in CO₂ with water’s one oxygen and two hydrogen atoms (H₂O) to produce sugar (CH₂O) and an extra O₂ oxygen molecule that returns to the atmosphere. That’s the process of photosynthesis in its simplest form. Animals make use of the binding energy in the sugar molecules by reversing photosynthesis, combining CH₂O with O₂ from the atmosphere and emitting CO₂ and H₂O, the process of respiration by which we live, which is also the basic chemistry of fire and decomposition. The earth’s plants and animals constantly exchange the same carbon, oxygen and hydrogen atoms back and forth; we spend only the binding energy of the sugar molecules originally captured by photosynthesis. That’s called the carbon cycle.

Air bubbles trapped under Antarctic ice showed that for the last 420,000 years the carbon cycle was in a rough range of balance, with carbon, oxygen and hydrogen exchanged between plants and animals in generally equal quantities. The carbon in the atmosphere seesawed regularly from 180 to 280 parts per million. As it did, temperatures rose and fell through glacial periods and intervening warm periods in fairly close correlation. In the last fifty years, however, our human activities raised the carbon dioxide in the atmosphere well above any they attained during those hundreds of millennia.

We had good reason for doing that. We outgrew the amount of energy captured by the plants living around us. Much more energy was available from ancient photosynthesis, in the form of coal and petroleum. It allowed societies to stop killing whales for light, stop deforesting land for heat, and stop using human beings and animals as machines for work. Fossil fuels saved the environment. And they made life immeasurably easier and more fulfilling for people freed from the limits of the energy available in their immediate surroundings. Don’t try to tell an Eskimo elder that life was better before fuel oil heaters. Burning blubber and seal oil didn’t work as well; the Iñupiat suffered in cold in frame houses, and in their more energy-efficient sod houses they developed chronic lung disease from childhood because of the smoke they breathed. Besides, it’s doubtful today’s population on the North Slope could be sustained that way, just as the balance of the world’s population in cold and temperate regions has grown too large to heat with wood. Transportation using fossil fuels also improved life vastly. Not many people remain alive in western society who remember making the switch from using animals for transportation, but plenty of elders in Barrow were around in the 1960s when snowmachines took over from dog teams. Suddenly, instead of taking a day to get to hunting camp, you could do it in an hour or two and use that day for something else. No longer did you have to fish and hunt to feed dogs, or care for them all year round. The gifts of leisure time and greater freedom, the time to learn, think and create, to accomplish more and go more places, all came in large part when we were released from the limits of the energy available from contemporary photosynthesis. The fact that many people wasted these gifts doesn’t diminish their intrinsic value, as people who lived the other way can testify.

Once we started burning fossil fuels, however, the carbon cycle was bound to become imbalanced. Current levels of human energy use bumped up the respiration side of the equation by 8 percent. Fossil fuels multiplied the energy one person could use by many thousands. A human’s daily diet of 2,000 calories equaled 8000 BTUs of energy, enough to continuously operate a 100-watt lightbulb. A gallon of gasoline contains 125,000 BTUs of energy, more than the human body uses in two weeks. Burning one six-pound gallon of gasoline puts twenty pounds of carbon dioxide into the air, of which five pounds is carbon and the rest is oxygen (we’ll just talk about the carbon, for simplicity). Wood is half carbon, so to recycle the carbon in one gallon of gasoline a tree has to grow by ten pounds. To recycle the carbon released to fly coast to coast round trip on a passenger jet takes about 1,750 pounds of wood growth for each passenger. The total energy use of the average American loads five tons of carbon into the atmosphere annually. A growing forest (not a mature forest) can use about a ton of carbon per acre per year, so to balance the energy use of each American would require about five acres of young, healthy forest. That’s far more forest than we have. More fundamentally, however, forests don’t only grow, they also burn, age and decay, releasing carbon back to the atmosphere; only if all the forests were cut down regularly and buried deep underground would they really negate carbon taken from underground and put into the atmosphere.

In 1850, atmospheric CO₂ was 288 parts per million; by 1958 it was 315 parts per million; as I wrote this, it was over 370 and rising at more than 3 parts per million per year. As a rule of thumb, an increase of 3 ppm in the atmosphere resulted each year from the 6 billion metric tons of fossil fuels burned on earth. But rules of thumb can be misleading; the true picture was much more complicated. The biosphere was absorbing more carbon than it did before fossil fuel use. Images from space showed a greener world; more carbon dioxide helped plants grow. Forests that were cut down for fuel and other uses were growing back: from 1950 to 1992, the amount of carbon stored in the forests of the eastern United States rose by 80 percent as formerly logged and farmed lands regrew. But when those forests reached maturity, they wouldn’t soak up as much carbon anymore. Moreover, more forest fires, caused in part by warmer weather, were cycling carbon back out of the forests at a faster rate. The oceans were the most important cushion for our carbon emissions. Currently, ocean water dissolved about 2 billion metric tons of carbon a year from human activities. Phytoplankton floating on the surface used immense quantities of carbon for photosynthesis, too, some of which sank down deep into the ocean and out of the contemporary carbon cycle. But ocean chemistry limited the amount of carbon the water could dissolve. And the oceans’ ability to dissolve CO₂ and to use it biologically declined as the water warmed. At a certain point, that drop could be severe.

Forests in temperate and tropical regions were like checking accounts for carbon: photosynthesis made deposits but fires and decay made withdrawals. Because these forests grew fast and covered large areas, the carbon on deposit at any one time was large, but over the long term it cycled through with debits and credits in rough equality. Deep ocean sediments that formed carbonate rocks such as limestone were more like permanent investments: once carbon entered, it was out of circulation until geological forces uplifted and eroded the rock. Arctic tundra and northern forests, called boreal forests, were somewhere in between--they were like savings accounts. Plants and trees there grew slowly and didn’t capture much carbon in a year, but much of that organic material fell to ground that was cold and damp on top and frozen a little deeper, so the material would not decay, building up a positive bank balance over time. As long as it stayed cold, that carbon was mostly out of circulation. Over the eight millennia since the last glacial period this account had grown large. The first meter of soil under the Arctic and the boreal forest were thought to contain 450 billion metric tons of carbon, more than humans have ever released and comparable to about two-thirds of the carbon in the atmosphere.