Taking Control Of Co2

Convinced that humans had taken control of the atmospheric methane trend by 5,000 years ago, I began to wonder if we might have had a significant effect on carbon dioxide millennia ago. Because CO2 is a more abundant greenhouse gas than methane, and its effects on climate are generally larger, this question was potentially more important than the methane story, but for a while I resisted pursuing it. One reason for the delay was that CO2 changes are more difficult to interpret than those of methane. While natural variations in methane are controlled mainly by growth and shrinkage of wetlands at the 22,000-year cycle of orbital change (chapter 5), CO2 variations occur at all three orbital cycles and are harder to disentangle.

Scientists are also less certain what causes natural CO2 cycles. The problem is that carbon exists in almost every part of the climate system: in the air as CO2; in vegetation as grass and trees; in soils as organic carbon; and in the ocean mainly in dissolved chemical form but also in the tissue of organisms. Each of these reservoirs of carbon interacts with the others in different ways and at different rates. Three of the reservoirs exchange carbon relatively rapidly (within days to years): the air, the vegetation, and the upper sunlit layer of the ocean where plankton live. For example, vegetation takes some CO2 out of the air each spring in the process of photosynthesis but gives it back each autumn when the leaves fall or the grass dies. The amount of CO2 exchanged between the uppermost ocean layers and the overlying atmosphere depends on factors like ocean temperature and wind strength. The deep ocean holds by far the largest amount of carbon, but it is somewhat isolated from the processes at the surface and interacts with the other reservoirs more slowly. The carbon system is complicated.

One thing we know for certain is the longer history of CO2 changes. The same ice cores at Vostok Station in Antarctica that yielded the long record of methane also hold a history of CO2 changes (fig. 9.1). This record has four major cycles, each marked by a slow and erratic drift to lower CO2 concentrations, and then rapid shifts to higher values every 100,000 years. Smaller cycles at 22,000 and 41,000 years are also present but less obvious.

The concentration of CO2 ranges from peaks of 280 to 300 parts per million during warm interglacial climates to lows of just under 200 parts per million during major glaciations. These changes are equivalent to transfers of some 200 billion

9.1. CO2 concentrations in the atmosphere vary naturally at a cycle of 100,000 years, with peak values occurring a few thousand years before the ice sheets reach minimum (interglacial) size.

Thousands of Years Ago

9.1. CO2 concentrations in the atmosphere vary naturally at a cycle of 100,000 years, with peak values occurring a few thousand years before the ice sheets reach minimum (interglacial) size.

tons of CO2. It may seem strange to think of a gas in the atmosphere as weighing billons of tons, but the atmosphere does have "mass" (as in the everyday term "air mass" used by weather forecasters). Consider the weight of a gallon of gasoline you carry in a metal can. Every gallon you burn while driving your car doesn't simply vanish into space; it releases several pounds of carbon to Earth's atmosphere. So if you drive 20,000 miles a year and your car gets 20 miles per gallon, you are personally adding several thousand pounds of gaseous carbon (mostly CO2) to the atmosphere each year.

Climate scientists are still wrestling with the problem of accounting for the 200 billion tons of CO2 that was removed from the atmosphere during major glaciations. Where did it all go? One place it did not go was into vegetation. Huge Northern Hemisphere ice sheets covered regions that had been occupied by forests during warm interglacial climates, so the amount of carbon there in glacial times was much smaller. Also, the glacial world in most places south of the ice sheets was drier and dustier than today, with less vegetation to stabilize the soils from strong winds (chapter 4). As a whole, the continents had less vegetation than they did during interglacial times, at least 500 billion tons less in terms of carbon. So now the challenge is even larger: scientists are faced with explaining what happened to more than 700 billion tons of carbon in glacial times.

The only place left for the carbon to have gone is the ocean, specifically the deep ocean, the largest carbon reservoir of all except for the carbon trapped in sediments and rocks (including coal and oil). How the carbon was transferred to the deep ocean is currently a major area of research. Some of it could have been sent down by plankton that lived in the surface layers and settled to the sea floor after they died, taking carbon-rich soft tissue with them. Another way to deliver more carbon to the deep ocean is for cold water sinking in polar regions to carry it down in dissolved chemical form. In any case, the experts on this subject know that CO2 changes occurred at all three orbital cycles, and they know that the carbon taken from the atmosphere during glaciations went into the deep ocean, but they don't know for certain which processes were most important.

So, with my expertise lying in other aspects of the climate system, I was reluctant at first to look for possible human impacts on the CO2 trend in recent millennia. And yet, once again, I noticed a trend that just didn't seem right. If you look at the CO2 changes in figure 9.1 and compare the last four times when the CO2 values were highest, initially they all appear similar. But if you look more closely, you will see that they are not.

The first three interglaciations followed the same basic sequence. Peak CO2 values were reached late in the interval of ice melting and several thousand years before the times of minimum ice volume (marked by asterisks). All of these CO2 peaks were coincident with times of maximum summer solar radiation and also with the largest methane peaks. The fact that these CO2 peaks occur just before the ice sheets melted makes sense: the high CO2 values were helping summer radiation and high methane concentrations melt the ice sheets. Following these early peaks, the CO2 concentrations began steady decreases that lasted for about 15,000 years. Not until some 100,000 years later did the CO2 values return to the levels of the previous peaks.

How does this basic pattern compare with the most recent interval of ice melting? A high-resolution CO2 record spanning the last 11,000 years (fig. 9.2) shows a trend that looks similar, but only in part: the CO2 concentration rose rapidly to a peak of almost 270 parts per million nearly 10,500 years ago. This CO2 maximum is the same age as the most recent peaks in summer solar radiation and methane, and it precedes the final melting of the ice (again marked by an asterisk) by 5,000 years. As a result, this CO2 maximum 10,500 years ago must be the direct equivalent of those CO2 peaks that occurred during the earlier interglacial intervals. After this peak was reached, the CO2 concentration began to decrease, just as it had on all three of the earlier interglaciations.

But then, 8,000 years ago, something different happened. Instead of continuing to decrease as expected, the CO2 concentration began a slow rise that continued until the Industrial Revolution. This behavior may sound familiar. Here again is the same pattern that occurred with methane: an anomalous recent increase instead of the decrease expected from earlier (natural) trends.

By the start of the Industrial Revolution, CO2 values had risen from a low of about 260 parts per million to between 280 and 285 parts per million. But, as

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Minimum Ice-

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