Climate - Carbon dioxide

From Worldtraveller

The temperature of the Earth's atmosphere is determined by the balance between incoming and outgoing radiation. The ground absorbs sunlight, which warms it up, and it emits infrared radiation. The atmosphere is transparent to visible light, but absorbs strongly at many infrared wavelengths. Because radiation from the ground is absorbed, the atmosphere is warmer than it would otherwise be. All else being equal, increasing the amount of infrared absorption will make the atmosphere warmer; decreasing the absorption will make it cooler. If the atmosphere was totally transparent to infrared radiation, its temperature would be about 33° lower than it is.

So the questions to be answered are a) is carbon dioxide an important absorber of infrared radiation? b) is its concentration increasing? c) If so, is the increase due to human activity? d) and if so, is the increase enough to cause significant warming?

Contents 1 Is carbon dioxide a significant greenhouse gas? 2 Is the concentration of carbon dioxide increasing? 3 Is this rise due to human activities? 4 Is the increase enough to cause significant warming?

Is carbon dioxide a significant greenhouse gas?

Water vapour is the main part of the atmosphere which absorbs infrared radiation. If all the water vapour in the atmosphere were removed, between 66% and 85% of the infrared radiation that is currently absorbed would escape, and the atmosphere would therefore cool dramatically, by 22-28°C.

Carbon dioxide has a lesser effect than water vapour but is the next most important greenhouse gas. If all the CO₂ in the atmosphere was removed, between 9 and 26% of the infrared radiation currently absorbed would escape. At a very rough estimate, the temperature would drop by between 3 and 9°C.

The reason there is a range of possible values for the effect of removing all the water vapour or carbon dioxide is that the lower number comes from seeing how much radiation is absorbed when you remove the constituent, and the higher number comes from how much radiation is absorbed when the only thing you have is the constituent. The reason for the difference between the values is that the wavelengths at which the molecules of water vapour and CO₂ absorb radiation sometimes overlap.

It is known beyond any reasonable doubt that CO₂ is a significant greenhouse gas. The basic physics has been known for more than a century, since the pioneering work of Swedish Nobel prize-winning chemist Svante Arrhenius. Later work has refined the numbers and we now have a very good knowledge of the effect of CO₂ on the temperature of the atmosphere. Some important papers on the subject are these:

• Svante Arrhenius, 1897: On the influence of carbonic acid in the air upon the temperature of the earth.
• Ramanathan and Coakley, 1978: Climate modeling through radiative-convective models
• Dickinson & Cicerone, 1986: Future global warming from atmospheric trace gases

Is the concentration of carbon dioxide increasing?

Without any doubt at all, the concentration of carbon dioxide in the atmosphere has risen significantly. Its concentration can be measured in the present day atmosphere accurately and easily, and since such measurements began in 1958, the concentration has risen quite steadily by about 1-2 parts per million per year. Measuring the CO₂ content of the atmosphere before 1958 is not too hard either: you can take ice cores from Antarctica. When snow falls and is compressed into ice by subsequent snowfall, it traps small bubbles of atmosphere. This has been happening in Antarctica for hundreds of thousands of years. Careful measurements of ice cores show that over the last several hundred thousand years, the concentration of CO₂ in the atmosphere has pretty much always been between 180 and 280 parts per million. The concentration began to rise above 280ppm in about 1850, and had reached 315ppm by the time direct measurements started in 1958. Today it's about 385ppm, and continues to rise.

Useful references here are:

Is this rise due to human activities?

How can we tell whether all the new CO₂ that has appeared in the atmosphere since 1850 has come from? There are a couple of ways. One is to estimate how much CO₂ has been emitted over the last 150 years and compare it to what has come out of and gone into the oceans and the biosphere. It's thought that about 500 billion tonnes of carbon dioxide has been emitted over the last 150 years - if it was all still in the atmosphere the concentration of CO₂ would be about 500ppm. So humankind has emitted more than enough CO₂ to raise the levels of CO2 to those we observe. No observation has ever found a decreasing amount of CO₂ in the oceans or in the biosphere, so those two sources cannot account for the rise.

A more sophisticated approach requires a detailed look at the composition of the CO₂. Carbon atoms come in three different types; ¹²C contains six protons and six neutrons, ¹³C contains six protons and seven neutrons, and ¹⁴C has six protons and eight neutrons.

The fraction of carbon in each of the three forms can tell you where it came from: carbon in fossil fuels has a slightly different composition to carbon in the pre-industrial oceans and atmosphere, with less of the heavy ¹³C. Ice core records show that atmospheric CO₂ had a ¹³C/¹²C ratio that hardly varied at all until 1850 or thereabouts, when it began to drop. Observations of the CO₂ dissolved in the oceans find the same - the isotopic ratio varied very little until about 1850 and then began to drop.

The total change in isotopic ratios is exactly what you'd expect if you mixed in 500 billion tonnes of CO₂ from the burning of fossil fuels into the pre-industrial atmosphere. So, we can say without doubt that essentially all of the increase in the concentration of atmospheric CO₂ since 1850 is attributable to human activities.

Some papers on this are:

• Stuiver, M., Burk, R. L. and Quay, P. D. 1984. 13C/12C ratios and the transfer of biospheric carbon to the atmosphere. J. Geophys. Res. 89, 1731–1748.
• Francey, R.J., Allison, C.E., Etheridge, D.M., Trudinger, C.M., Enting, I.G., Leuenberger, M., Langenfelds, R.L., Michel, E., Steele, L.P., 1999. A 1000-year high precision record of d13Cin atmospheric CO2. Tellus 51B, 170–193.
• Quay, P.D., B. Tilbrook, C.S. Wong. Oceanic uptake of fossil fuel CO2: carbon-13 evidence. Science 256 (1992), 74-79

Is the increase enough to cause significant warming?

We know, then, that CO₂ is an important greenhouse gas, that there is more of it about than there used to be, and that the increase is due to human activities. As a crude estimation of the effect the increased CO₂ will have on temperature, we can start by saying that the pre-industrial CO₂ had a warming effect of 3-9°C, and that there is about 38% more CO₂ in the atmosphere today than there was in 1850. So, the extra CO₂ should warm the earth by 1-3°C. This very simple calculation gives an overestimate, but given that the global average temperature typically varies by only about 0.2°C between years, a rise of 1°C is certainly large enough to be noticeable.

Climate / Temperatures / Carbon dioxide / The Sun / Models / Consequences