Fossil fuels and greenhouse gases

Let us now start to look at one potential cause of the observed warming, namely the level of greenhouse gases in the atmosphere. We discussed in chapter 1 that the presence of such gases maintains the surface temperature at a level higher that it otherwise would be: the current average surface temperature is about 15°C, whereas the radiative equilibrium temperature, which would hold if Earth had no atmosphere, is about -18°C. Thus, a rather obvious possible cause of the observed increase in temperatures is an increase in the concentration of greenhouse gases in the atmosphere, and such an increase has indeed occurred.

The main greenhouse gas in the atmosphere is water vapor; however, we do not control its concentration. Rather, as we discussed in chapter 1, there is balance between evaporation (mainly from the oceans, but also from lakes and moist land areas) and precipitation, and the overall level of water vapor is largely determined by the temperature of the atmosphere. The second most important greenhouse gas is carbon dioxide (C02), which is an unavoidable product of the combustion of fossil fuels. It has been directly measured at Mauna Loa Observatory in Hawaii for about 50 years, as shown in figure 7.4. The measurements were first taken by C. D. Keeling, and the curve, now one of the most famous in science, is known as the Keeling curve.5 There are two features about the plot worth noting. The most obvious is the steady increase in CO2 level over the past few decades, from a level of about 310 ppm (parts per million) to about 390 ppm in 2010. The second is the presence of an annual cycle. There is more land and more vegetation in the Northern Hemisphere, and land takes up CO2 and releases oxygen during the boreal (Northern Hemisphere) summer, so that there is a minimum of CO2 at the end of summer (in October), and a maximum in May.

There are two main ways that we know that the increase in CO2 is caused by the burning of fossil fuels. The first, and most obvious, is that we know how much fossil fuels (mostly petroleum and coal, followed by natural gas) have been extracted from Earth and burned, and from this amount we can calculate approximately how much CO2 has been added to the atmosphere. Human carbon emissions from fossil fuel combustion (coal, petroleum, and gas) currently contribute almost 9 gigatons of carbon a year to the atmosphere. In fact, more fossil

Figure 7.4. The levels of C02 measured at Mauna Loa Observatory in Hawaii from 1958 to 2010 in parts per million by volume.

fuel has been burned than can be accounted for by the increase in the atmosphere, and most of the remainder is taken up by the ocean. The second way is via the isotopic composition: the burning of fossil fuels leaves a characteristic fingerprint in the atmosphere. There are three naturally occurring isotopes of carbon, namely, carbon 12, carbon 13, and carbon 14. Carbon 12 is by far the most abundant. Now, carbon 14 is produced in the atmosphere by cosmic rays; it is then incorporated into C02 and taken up by plants during photosynthesis. However, carbon 14 is radioactive and decays with a half-life of about 5,700 years. Thus, since the fossil fuels are remnants of plant matter millions of years old, they contain virtually no carbon 14, as it has all decayed away. When fossil fuels are burned, they therefore put CO2 into the atmosphere that has a lower abundance of carbon 14 than the CO2 already in the atmosphere, and it is indeed found that the ratio of carbon 14 in the atmosphere, relative to the other isotopes, is decreasing at about the right rate to be explained by fossil fuel burning.

Although a continuous record of CO2 levels from direct measurements goes back only about 50 years, we have a good record of carbon dioxide going much further back, mainly from measurements of CO2 trapped in bubbles in ice cores in Greenland and Antarctica. There are also a number of somewhat isolated measurements at various periods in the past; for example, a series of measurements were made near Paris from 1876 to 1910. The ice cores reveal that CO2 levels were about 200 ppm at the last glacial maximum some 20,000 years ago, rising over the course of deglaciation to about 260-270 ppm 10,000 years ago, then slowly rising again to about 280 ppm in 1750, just before the Industrial Revolution.6 The rate of increase since then, to 390 ppm today, is thus far faster than anything else in the past 10,000 years. The level of methane, another greenhouse gas, has also been increasing over the past few decades, although it has leveled off in the past decade.

0 0

Post a comment