Greenhouse Gases and the Carbon Cycle

Contents:

See also the paper by J. Kasting, 1998 on the web.

1. Greenhouse Gases in the Atmosphere.

Since 1990 the international discussion of scientific as well as societal and economic aspects of the possible influence of human activities on global climate has been guided by the Intergovernmental Panel on Climate Change (IPCC), which is publishing a continuing series of reports that represent an unprecedented degree of international consensus, although there remain a few non-convinced scientists. This consensus includes the conclusion that the mean surface temperature of the Earth will probably rise by 1-2° C (2-4° F) over the next fifty to one hundred years, if we continue to burn fossil fuels at increasing rates. It also includes the opinion that the effects of what we have already burned are not at present unequivocally apparent in global temperature records.

The mean surface temperature of the Earth has systematically increased over the last hundred years, and most pronouncedly over the last few decades, and it is widely suspected that greenhouse gases introduced in the atmosphere by human activities have caused this long-term rise in temperatures at least in part. But the average increase is less than 1° C (about 0.6° C), and it still might just be within the range of natural variation in climate. After all, the warm period of the Mediaveal Climate Optimum (peaking at about 1000 AD) was followed by the very cold Little Ice Age, and warming after this naturally occurring cold period started only about 150 years ago. The important question is whether we want to start to take measures against emission of more greenhouse gases now, before we can be sure that it is necessary to take these measures, or whether we want to wait until we are certain, by which time it will be too late for any effective response.

Not only CO2, but several other greenhouse gases have been increasing in concentration in the atmosphere as a result of human activities. The most important of these are methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs). Recent studies indicate that about 60% of the combined warming effect of the greenhouse gases is caused by CO2, 20% by CH4, 12% by CFCs, and 6% by N2O. Each of these gases causes greenhouse warming, but the long-term effects of all is different.

Methane has an atmospheric lifetime of only about twelve years, and most of the anthopogenic CH4 added in 1999 will have left the atmosphere by 2011. The most important anthropogenic sources of methane are bacterial fermentation in rice paddies and in the intestines of cattle; these are related to food production, and can be expected to go up linked to the total population on Earth. If the population doubles in he next 50 years, we expect the concentration of CH4 to double, but not much more than that. This would add only a few tenths of a degree to the mean temperature of the Earth.

In contrast to CH4, nitrous oxide and chlorofluorocarbons remain in the atmosphere for a century or more. The production of N2O, however, is only indirectly dependent on human activities. Its principal source is the bacterial removal of nitrogen from soils, which is expected to increase as human use of nitrogen-containing fertilizer increases. This use is expected to be linked to human population growth, and the amount in the air should increase only slowly.

The most abundant of the man-made CFCs, freon-11 and freon-12, are by international agreement (Montreal Protocol) being phased out of production because of their effects on depletion of stratospheric ozone. The concentration of freon-11, for insatnce, peaked in 1994 and is in a slow decline that should continue for the next century or so. The freon-12 concentration is expected to level off within the next few years. In terms of climatic effects, the main threat from CFCs comes from other long-lived compounds that may be used to replace the ones that have been phased out, and that could also act as greenhouse gases. These gases are as yet present only in very low concetrations and we are not yet certain of their effect.

CO2 will remain in the atmosphere from tens to thousands of years: it is estimated that about 65 % of carbon dioxide generated by humans since the start of the Industrial Revolution is still in the air we breathe today. Most of the anthropogenically added CO2 results from either fossil fuel burning or deforestation (see below). Expected future increases in CO2 are not simply linked to food production and simple population increase, but they are linked to the number of people as well as the energy use per person. We expect not only the world population to grow, but also an increase in energy use as nations of the world try to improve standards of living of their citizens. China's recently improved standard of living, for instance, is fueled by extensive coal burning: coal is a fuel of which has large deposits. We thus expect future CO2 increases to extend into the future and increase much faster than population growth.

 

2. The Carbon Cycle.

The carbon on Earth is continually exchanged and recycled among the biosphere, the soil (lithosphere), the atmosphere and the hydrosphere (see earlier lectures). In some of these temporary storage places (called reservoirs) carbon is securely held (e.g., limestone), but in others it readily combines with oxygen in the air to form CO2. In order to predict how atmospheric CO2 levels and climate may change in the future, we need to understand where carbon is stored and how it moves about; to understand this we use box models (Figure 1).

Table 1.

The carbon reservoirs that are most relevant to the global warming question with the total amount of carbon, in gigatons. A gigaton is a billion (109) metric tons, 1012 kg, or about 2200 billion pounds (after J. Kasting, 1998).

 

The World's Carbon Reservoirs

RESERVOIR

SIZE (GtC)

Atmosphere

750

Forests

610

Soils

1580

Surface Ocean

1020

Deep Ocean

38,100

FOSSIL FUELS

Coal

4,000

Oil

500

Natural Gas

500

Total Fossil Fuels

5000

The atmosphere contained about 750 gigatons of carbon (Gt C) as CO2 in 1994, or an average atmospheric concentration of CO2 of 358 parts per million (ppm) by volume. During the past decade, the average concentration of CO2 has increased by about 1.5 ppm per year, reaching about 365 ppm at the beginning of 1999, which is about 765 Gt C. Note the sizes of the other carbon reservoirs as compared to the atmosphere. All vegetation on land together has less total carbon that the atmosphere! Also note that all fossil fuels together hold much more carbon than the atmosphere, and that the deep ocean is a really huge reservoir. That there is so much more carbon stored in fossil fuels than in the air is important, for it shows that burning these fuels can bring about some very large changes in atmospheric CO2, especially if it occurs on a time scale that is faster than that of the natural removal processes.

 

Impacts of burning all remaining fossil fuel

A look at this table (and at figure 1) shows that if we were to burn all the world's fossil fuel reserves in a short period of time, atmospheric CO2 would rise by about a factor of eight. The air around us would then hold almost ten times more CO2 than in pre-industrial times, when for millennia the concentration held relatively steady at 280 ppm.

Climate model calculations predict that each doubling of atmospheric CO2 should produce an increase of 1.5 to 5° C (about 3 to 9° F) in the mean surface temperature of the Earth, so three of them could drive the temperature 4.5 to 15° C higher than what it is today.This compares to a climate during the reign of the dinosaurs.

 

3. Human perturbations to the carbon cycle

There is no question, today, that the global carbon cycle is out of balance. For some time we have been perturbing it in a variety of ways, the most telling of which is the burning of fossil fuels. The worldwide consumption of coal, oil (and its derivatives), and natural gas now releases CO2 at a rate of about 5.5 Gt C/yr (Table 2).

Human Perturbations to the Global Carbon Budget

CO2 sources

Flux (Gt C/yr)

Fossil fuel combustion and cement production

5.5 ± 0.5

Tropical deforestation

1.6 ± 1.0

Total anthropogenic emissions

7.1 ± 1.1

CO2sinks

Storage in the atmosphere

3.3 ± 0.2

Uptake by the ocean

2.0 ± 0.8

Northern hemisphere forest regrowth

0.5 ± 0.5

Other terrestrial sinks (CO2 fertilization, nitrogen fertilization, climatic effects)

1.3 ± 1.5
Source: Climate Change 1995, published by the IPCC

 

Carbon dioxide is released by deforestation of the tropical regions, mainly for agriculture, at a rate roughly estimated to be about 1.6 Gt C/yr. Of the 7.1 gigatons of carbon released each year by fossil fuel burning and deforestation, about half is remaining in the atmosphere. The rest is removed by a combination of natural processes; we are not certain exactly what goes where, but we think that most is absorbed in the surface water of the oceans (about 2.0 Gt C/yr) It is difficult to obtain enough measurements to define a precise global number for this process.

The remaining 1.8 Gt C/yr of anthropogenic CO2 is being removed by other prcoesses, most probably increased carbon storage in forests and soils. This piece of the global carbon budget is the least understood, and was referred to as the 'missing sink' for CO2. We are now fairly confident that this missing carbon is stored in the terrestrial biosphere, and we have at least a qualitative understanding of the processes involved. About a third of it is probably absorbed by re-growth of northern hemisphere forests that were cleared in the 1800s, as in parts of New England and the Midwest today. Other factors that are thought to be contributing to CO2 uptake by the terrestrial biosphere include the increased fertilization of plants by CO2 and by anthropogenic nitrogen oxides.

Figure 1: The global carbon cycle. All numbers in Gigatons carbon. Note the numbers and reservoirs chosen are somewhat different than in Table 1. The arrows with numbers give the flux of carbon in Gigatons per year (how much moves from one reservoir into the other per year. Note the large size of many of the fluxes.