Why is carbon dioxide important?

The consensus view of scientists reflected in the 2001 report of the Intergovernmental Panel on Climate Change (IPCC) is that the recent rapid rise in global temperature is caused by rising human emissions of greenhouse gases. Of these gases, carbon dioxide is by far the most important contributor (>50%) to global warming. Nitrogen and oxygen make up the greater part of the gaseous composition of the atmosphere, but because of their molecular structure they do not absorb or reflect thermal (infrared) energy and thus do not contribute to the greenhouse effect. Since the work of John Tyndall in the mid 19th century it has been known that while nitrogen and oxygen, the gases which comprise 99% of the atmosphere, are transparent to visible and infra-red radiation, gases like carbon dioxide, water vapour and methane are not. These gases trap heat in the atmosphere leading to the green house effect (http://www.tyndall.ac.uk/general/history/john_tyndall.shtml). Water vapour is the most important contributor to the natural greenhouse effect which ensures that the Earth has a temperature that is conducive to life; without this natural process the Earth would be 30°C colder than at present. A number of other gases that absorb thermal (infrared) radiation also contribute to the greenhouse effect; these include methane (CH₄), nitrogen oxide (N₂O) and ozone (O₃) as well as man made gases such as the chlorofluorocarbons (CFCs). The effects of changing levels of carbon dioxide in the atmosphere on the Earth’s energy balance were calculated in 1895 by Svante Arrhenius who was concerned about the effects of human activity, particularly the burning of coal, the principal fossil fuel used at that time (http://earthobservatory.nasa.gov/Library/Giants/Arrhenius/arrhenius_2.html). Uncertainty as to whether natural processes have removed the human input of CO₂ have been resolved by a time series of measurements of atmospheric concentrations (http://cdiac.esd.ornl.gov/trends/co2/sio-mlo.htm) and results from ice cores (http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/icecore.html).

Carbon dioxide trends through time

Measurements of CO₂ have been made at the Mauna Loa Observatory in Hawaii since 1958 and show

Mauna Loa Observatory, Hawaii, USA

an accelerating increase to a record high concentration of 380 ppm in May 2004 (a 100 ppm increase above the pre-industrial concentration). A pronounced seasonal cycle is evident in the record that reflects the annual growth of plants in the oceans and on land in the northern hemisphere. As plants grow and photosynthesise they take up CO₂ and as their growth decreases in the autumn, winter levels of CO₂ in the atmosphere increase.

Monthly mean measurement of carbon dioxide at Mauna Loa

Similar measurements are now being made at a wide range of sites around the world, including Antarctica. As one might expect at some sites the seasonal cycle is less pronounced, but the general trend through time remains the same.

Locations of greenhouse gas observing stations

It is difficult to conclude from these short time series what causes the rise in CO₂. Fortunately, bubbles of atmospheric gas are trapped in snow when it falls. In cold regions of the world such as Antarctica and Greenland, annual layers of snow are formed that turn into ice and can be measured and dated. Deep drilling through the ice sheet at Law Dome, in Antarctica for example, has revealed the evolution of CO₂ in the atmosphere over the last 1000 years. Until the beginning of the industrial revolution levels remained relatively constant at about 208 ppm.

           

Position of ice cores in Antarctica                 Levels of CO₂ measured in ice with extension in red
                                                                from atmospheric measurements at Mauna Loa.

A 3623 metre core taken at Vostok, Antarctica sampled ice as old as 420,000 years. This core showed that CO₂ levels had oscillated between approximately 200 and 280 ppm over this period reflecting different glacial and interglacial periods of the last ice age. The data clearly show that the present levels in the atmosphere are much higher than at any time over the last 420,000 years. Estimates obtained from geological and modelling studies indicate that the present levels in the atmosphere were last experienced on Earth more than 40 million years ago.

New results from the Dome C core in Antarctica (EPICA community members, 2004) extends the climate record back to 740,000 years ago. The new results show that prior to 430,000 years as recorded in the Vostok core interglacial periods were less warm, but lasted longer compared to more recent glacial/interglacial cycles.

What is the cause of the recent rapid increase in CO₂ levels?

On 7 June 2005 scientists from the National Academies of 68 nations, plus Brazil China and India, strongly endorsed the view reported in the third report of the Intergovernmental Panel on Climate Change (IPCC) in 2001 that the recent rapid rise in global temperature is caused by rising anthropogenic emissions of greenhouse gases with CO₂ being the main culprit.

Changes in temperature are preceded by concentrations of CO₂

There has been an inexorable rise in emissions by mankind to the atmosphere, mainly from the use of fossil fuels (oil, gas and coal) and cement production. The industrial revolution (around 1751) has contributed to increasing atmospheric concentrations of CO₂. Emissions in 2002 of 6975 million metric tons of carbon were the highest ever recorded. Changes in land use and increasing exploitation of forests for wood, especially in the tropics, further add to atmospheric concentrations each year.

Role of the oceans in the carbon cycle

The carbon cycle is crucial to climate because it governs the amount of important greenhouse gases, such as carbon dioxide and methane in the atmosphere with the oceans playing a key role as the major reservoir of carbon. Methane adds to the carbon dioxide trend as it is mostly converted to CO₂ in the atmosphere over an approximately ten year period.

Carbon continuously cycles between different components of the climate system.

A schematic representation of the carbon cycle

The carbon cycle in the diagram above (from Sarmiento and Gruber, 2002) shows the size of the different ‘reservoirs’ and the fluxes between them for both the natural carbon cycle and the human (anthropogenic) contribution. It is clear from these diagrams that the oceans have a dominant role in the carbon cycle and that the deep ocean is the main long-term depository for carbon. Without the oceans the rate of increase in atmospheric concentrations of CO₂ would be much greater. The increasing trend in the atmosphere is less than half that of anthropogenic emissions as a considerable proportion is taken up by the oceans and land. The take-up of carbon dioxide by seawater is dependent on what is known as the partial pressure of carbon dioxide (pCO₂); transfer requires that the pressure of the gas in the sea is lower than the adjacent air, otherwise outgassing to the atmosphere takes place. In the sea carbon dioxide reacts with water to produce bicarbonate and carbonate ions. These products, together with the small amount of CO₂ that remains are known as Dissolved Inorganic Carbon (DIC). Recent measurements of DIC in the World’s oceans have shown that the oceans have taken up ~50% and the land ~25% of fossil fuel emissions since the beginning of industrialisation around 1800. Transfer of carbon to the deep ocean takes place via two processes known as the solubility and biological pumps.

Atmospheric concentrations a century from now

In the 2001 IPPC report projections calculated by a range of Global Climate Change models forecast a continuing rise in carbon dioxide by 2100 of between 540 and 970 ppm compared to 360 ppm in 2000.

The same models predicted that globally-averaged surface temperatures would rise between 1.4 and 5.8°C over the same period. More recent modelling from the Hadley Centre includes feedback mechanisms from the oceans and land and has predicted much greater concentrations of 980 ppm within 100 years time, assuming that emissions continue to grow at their present rate. Global surface temperature would increase to 5.5°C (8°C on land) if carbon dioxide reached this level. A number of studies have indicated that future climate change will reduce the ability of the Earth's system (both ocean and land reservoirs) to absorb human carbon emissions. If this should happen the trend in atmospheric carbon dioxide levels and global temperature would increase even more rapidly. The prognosis for the future, without a radical change in living habits and major technological developments is not bright, given that global energy demand over the next 25 years is expected to increase by 60%.