Introduction

Understanding the way carbon is cycled and recycled in nature is helpful when looking at human activities that increase the amount of carbon dioxide in the atmosphere. Included here are some of the key data which have alerted scientists to the link between increased atmospheric carbon dioxide and climate change.

The main man-made source of carbon is the burning of fossil fuels

Sections in this article

• Where the most carbon is stored

• Carbon cycle and climate change

• Where Next?

Where the most carbon is stored

Carbon is a common element that forms the building block of all organic material - plants, animals and humans.  It bonds with other elements very easily, forming a vast number of different compounds.  These carbon compounds are found throughout the earth's system, on land, in the oceans and in the atmosphere.

The list below shows the Earth's principal reservoirs of carbon (in billion tonnes).

• Marine sediments and sedimentary rocks (66,000 000 - 100,000 000)

• Deep oceans (40,000)

• Surface oceans (1,000)

• Vegetation (540-610)

• Soils and organic matter (1,600)

• Coal (3,000)

• Oil, gas (300)

• Atmosphere (750)

At a glance we can see that the amount of carbon stored in sedimentary rocks and the deep oceans is enormous and far greater than that stored in the atmosphere or in the earth's vegetation.  These large reservoirs of carbon are stable and long-term. 

In contrast, the atmospheric reservoir of carbon is less stable.  Different processes (such as photosynthesis, respiration and the absorption and release of carbon by the oceans) operate to cycle carbon between the atmosphere, biosphere and oceans.  Processes that release carbon into the atmosphere are called sources (e.g. respiration), while those that remove carbon from the atmosphere are called sinks (e.g., photosynthesis).  This exchange of carbon between the different reservoirs is simply called ‘the carbon cycle' and is most clearly illustrated with a diagram:

Carbon cycle and climate change

Most of the carbon in the atmosphere exists as carbon dioxide (CO₂) and to a lesser extent, methane (CH₄).  Although CO₂ and CH₄ are trace gases, which occur in very small concentrations, they are both important greenhouse gases.  The concentration of CO₂ and CH₄ in the atmosphere strongly affects the strength of the greenhouse effect and consequently the Earth's mean temperature.

The close correspondence between global temperature changes and carbon dioxide concentrations is clearly illustrated in polar ice core records of long-term temperature, carbon dioxide and methane variability.  The graph below shows a 420,000 year record of temperature and CO₂ concentration from the Vostok ice core in Antarctica.  It should be noted however that due to the difficulty of precisely dating the ice core records, it is still unknown whether increases in carbon dioxide concentration precede and cause temperature increases, or vice versa, or whether they increase at the same time. 

Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. 1999. Petit J.R., Jouzel J., Raynaud D., Barkov N.I., Barnola J.M., Basile I., Bender M., Chappellaz J., Davis J. Delaygue G., Delmotte M. Kotlyakov V.M., Legrand M., Lipenkov V.M., Lorius C., Pépin L., Ritz C., Saltzman E., Stievenard M. Nature 399: 429-436.
 

The amount of carbon dioxide in the atmosphere has increased by 31% since 1750 to a level which, according to the Intergovernmental Panel on Climate Change (IPCC), has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The atmospheric concentration of methane has also increased substantially (by 151% since 1750) to a level also unprecedented within the past 420,000 years.

The Mauna Loa atmospheric CO₂ measurements (a Hawaiian Island) began in 1958 and constitute the longest continuous record of atmospheric CO₂ concentrations available in the world. Since 1958, there has been a 17% increase in atmospheric CO₂ levels.  Longer records of atmospheric CO₂ have been obtained by analysing air bubbles trapped in ice cores.

The significant increase in atmospheric carbon is attributed to a range of human activities that have increased in intensity over the last two hundred years.  These activities include:

• Emission of CO₂ and CH4 from the combustion of fossil fuels (coal, oil and natural gas) as a source of energy. 
• Emission of CO₂ through deforestation
• Emission of CO₂ from land use changes (e.g., for agriculture/urbanization) 
• Emission of CO₂ from industrial processes including waste burning and the production of cement and lime.   
• Emission of CH₄ from landfills, livestock and rice cultivation.

Of these activities, about three quarters of the human (anthropogenic) emissions of carbon are attributed to the use of fossil fuels.

There is widespread consensus among the majority of scientists that this dramatic increase in CO₂ (and other GHGs) has very likely caused the observed global warming of 0.6±0.2ºC during the last century, via a strengthening of the greenhouse effect.  In their scientific report published in 2001, the Intergovernmental Panel on Climate Change concluded that ‘There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.'

Where Next?

On ClimateX.org

To find out more about the role carbon plays in climate change and what is being done to reduce its impact try reading the following three articles also by Alison Colls:

1. Lower Carbon Fuels
2. Carbon sequestration
3. Climate change

External links

Read this excellent explanation of the greenhouse effect from the University Corporation for Atmospheric Research.

Kastings, J. F. 1998 The carbon cycle, climate and longterm effects of fossil fuel burning

IPCC WGI, 2001 The Scientific Basis http://www.grida.no/climate/ipcc_tar/wg1/048.htm