The vast majority of scientists agree that there is a link between the high levels of carbon dioxide (the main greenhouse gas) in the atmosphere and global warming. In fact the IPCC, which is made up of hundreds of the world’s top climate change experts, has concluded that the rise in global temperatures observed since the mid-20th century is more than 90% likely due to the rise in greenhouse gas concentrations due to human activities, including carbon dioxide.

Human activities, such as burning fossil fuels (coal, oil and gas), agricultural practices and land clearing have increased the amount of carbon dioxide in the atmosphere by more than a third since the Industrial Revolution (250 years ago) (IPCC 2007). According to the IPCC, the amount of carbon dioxide in our atmosphere today far exceeds the natural range over the last 650,000 years (IPCC 2007).

Studies have concluded that global warming is extremely unlikely (<5% chance) to have resulted from natural causes alone (IPCC 2007). As illustrated in Figure 5, while natural factors such as changes in solar activity do influence global temperatures, only when the impact of greenhouse gases is considered can we explain recent climate changes.

Note: ‘Radiative forcing’ is a measure of the induced change to the energy balance of the atmosphere. Warming and cooling influences are indicated by positive and negative values respectively. When elements are grouped, uncertainty bands are approximated from the highest uncertainty in an individual element.
(1) Includes both the direct effect and the cloud albedo effect.
(2) Includes the cooling effect of changes to land use and the warming effect of black carbon on snow.
(3) Includes tropospheric and stratospheric ozone and stratospheric water vapour.
(4) Includes methane, nitrous oxide, HFCs, PFCs and sulphur hexafluoride.

IPCC (2007), “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 [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Garnaut, R (2008), The Garnaut Climate Change Review: Final Report, Cambridge University Press, Melbourne