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What are the causes of climate change?

A key process in the regulation of the climate is the greenhouse effect. For the Earth’s climate to remain stable, a balance is required between incoming solar radiation and outgoing radiation. The greenhouse effect is the process by which greenhouse gases (GHGs) – including carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O)) – and water vapour absorb and re-emit solar infrared radiation. [see reference 5] It is a natural process which contributes to keeping the Earth’s temperature at a level that is warm enough to support life.

Alterations to the concentrations of GHGs have a significant effect on the climate (see Figure 1).

Figure 1: The link between greenhouse gases and climate change


























Source: based on Stern N (2006) Stern review on the economics of climate change. Cambridge: Cambridge University Press.

There are both natural and anthropogenic factors that affect the climate:

Natural factors:
The climate is affected by a number of natural forces. Volcanic eruptions can lead to a cooling of the Earth's surface temperature as they project aerosols into the atmosphere, which in turn reflect the incoming sunlight back into space. [see reference 1] Changes in the planet’s orbit, including alterations to the eccentricity, tilt and precession (these are known as Milankovitch Cycles) can alter the seasonal and latitudinal distribution of solar radiation, which can result in changes in the climate. [see reference 1] Other natural factors that can affect the climate include differing fluctuations of energy received from the sun and changes in the radiation reflected, which can be caused by changes in cloud cover or land cover. [see reference 1]

Anthropogenic factors:
Human activities, including the burning of fossil fuels and deforestation have contributed to an increase in GHGs, intensifying the greenhouse effect and thus contributing to changes in the Earth’s climate. Analysis of ice cores has shown that over the last 650,000 years the levels of GHGs in the atmosphere have experienced natural fluctuation. [see reference 1] Atmospheric concentrations of CO₂, CH₄ and N₂O however, now far eclipse pre-industrial levels [see reference 2] (see Figure 2). CO₂ levels have risen by 36 per cent from a range of 275 to 285 ppm [a] in the pre-industrial era (AD 1000-1750) to 379 ppm in 2005. The global atmospheric concentration of methane has increased from a pre-industrial value of about 715ppb to 1774 ppb in 2005. The global atmospheric concentration of nitrous oxide has increased from a pre-industrial value of about 270 ppb to 319 ppb in 2005. The rises in CO₂ levels are mainly due to the burning of fossil fuels and land-use change, while the rises in CH₄ and N₂O are largely due to agriculture. [see reference 2]

The Earth is now very close to witnessing a global average temperature increase of 2°C (from pre-industrial levels). In order to avoid this increase, the concentration of GHGs needs to be stabilised at no more than 450 parts per million (ppm) carbon dioxide equivalent [b] (CO₂e). [see reference 6] Once a level of 450 ppm CO₂e has been reached, the 2°C increase is probable and beyond which, dangerous climate change is most likely inevitable. [see reference 6]

Figure 2: Changes in atmospheric greenhouse gases concentrations over the last 2000 years

























Increases since about 1750 are attributed to human activities in the industrial era. (Data combined and simplified from Chapters 6 and 2 of the IPCC report).
Source: Intergovernmental Panel on Climate Change (2007) Climate Change 2007: The physical science basis. Frequently Asked Questions.

While the significant majority of the scientific community directly links human activity to the recent trends in climate change, there is a small proportion that contests this. Arguments against the human impact on climate change include that the current trend is part of the climate system’s natural variation, that climate models that predict future climates are unreliable and that the methods and data used in temperature analysis are contentious. While natural factors such as volcanic eruptions and changes in the planet’s orbit can affect the Earth’s climate, the increase in levels of anthropogenic GHGs provide the only reasonable explanation for the temperature increases that have occurred over the past 50 years. [see reference 7] For a comparison of the radiative forcing [c] of the natural and anthropogenic components of climate change see the IPCC report Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change

Further sources of information:

• IPCC, 2007: Climate Change 2007: The physical science basis. Contribution of working group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Available at www.ipcc.ch
• IPCC, 2007: Climate Change 2007 FAQs: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Available at www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-faqs.pdf

[a] ppm (parts per million) and ppb (parts per billion) is the ratio of the number of greenhouse gas molecules to the total number of molecules of dry air.
[b] The CO₂ equivalent is when all the greenhouse gases are added together and expressed as an equivalent in CO₂.
[c] Radiative forcing is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. Positive forcing tends to warm the surface while negative forcing tends to cool it.