Committee on Climate Change

The primary source of Earth’s energy is the Sun. Some sunlight falling on Earth is reflected away, but most is absorbed. This warms the Earth’s surface, causing it to radiate heat out into space. The average temperature of the Earth is determined by the balance between absorbed solar energy and outgoing heat.

But without an atmosphere, this balance cannot explain the temperatures we observe – the average temperature we experience at the surface is about 33°C warmer than would be expected for a planet of Earth’s reflectivity and distance from the Sun.

Scientists have known since the early 1800's that the extra warmth is due to the atmosphere. It traps some of the outgoing heat, creating a natural ‘greenhouse effect’. By 1900 scientists had identified the specific gases in the atmosphere responsible (water vapour, carbon dioxide and methane and other trace chemicals) and had already calculated that changes in GHG levels would lead to changes in global surface temperature.

Several factors can lead to global climate change:

• Changes in incoming solar radiation.
• Changes in the atmosphere (including greenhouse gases, but also other atmospheric constituents such as particles and clouds).
• Changes in the reflectivity of Earth’s surface (e.g. clearing of forest, development of urban areas and melting of ice & snow).

Earth’s average temperature does show some natural variability, even when not forced by these factors. Complex interactions between the atmosphere, land surface and oceans cause the daily fluctuations we know as weather, but also give rise to variability over years or decades.

The fact that the Earth’s surface is warmer than it would otherwise be without atmospheric GHG's is as close to certain as any scientific finding, based on fundamental laws of physics.

The fact that other potential drivers of global climate change also exist is well understood.

A key challenge in climate science is to identify the extent to which these different GHG and non-GHG drivers are responsible for observed changes.