Committee on Climate Change

Human-caused emissions of greenhouse gases (GHGs) have now driven atmospheric concentrations well outside the natural cycle of the last million years:

• Global CO₂ emissions rose six-fold over the 20th Century (see Figure 1.2 below); this is largely a result of fossil fuel burning, with additional contributions from deforestation and cement production.
• The concentration of CO₂ in the atmosphere is now nearly 390 parts per million by volume (ppm) and rising, compared to around 280ppm in pre-industrial times (see Figure 1.2 below). This increase is clearly traceable to emissions from human activity (as shown for instance by the changing balance of carbon isotopes in the atmosphere).

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• The magnitude and rate of CO₂ increase in recent decades are far greater than anything seen in the ice core record (see Figure 1.3 below).

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Similar trends in other long-lived GHGs are taking place:

• Methane (CH₄) concentrations have increased to nearly 1,800 parts per billion (ppb) from pre-industrial levels of around 700ppb, as a result of emissions from agriculture, waste and fossil fuel use.
• Emissions of nitrous oxide (N₂O), primarily from intensive agriculture, have led to concentrations exceeding 320ppb, up from a pre-industrial level of around 270ppb.
• A range of halocarbons (artificial compounds such as CFCs and HFCs which did not exist in pre-industrial times) are now present in the atmosphere.

Fossil fuel combustion gives rise to additional short-lived gases, particles and aerosols which cause both warming and cooling. Amongst these:

• Sulphate aerosol concentrations have increased. These cause local cooling and have partially offset the global warming trend, however estimates suggest emissions flattened during 1980-2000 following steps taken to improve air quality.
• Ground level ozone (O₃) concentrations have increased, causing further local warming.

The influence of different climate change drivers can be measured by a quantitiy known as radiative forcing. There is high confidence that current overall forcing from human activity is strongly positive (leading to warming), and is most likely an order of magnitude larger than the forcing from changes in the Sun (see Figure 1.4 below).

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Atmospheric GHG concentrations have clearly increased due to human activity since the Industrial Revolution, and it is close to certain that this activity has led to an overall warming influence on climate.