Committee on Climate Change

The climate of the Earth has indeed varied greatly over very long timescales. Past changes have been caused by various drivers, but it is clear throughout that CO₂ and other greenhouse gases (GHGs) have played an important role either as an initial trigger of change or as an amplifying feedback.

50 million years ago, geological records suggest that the Earth was around 6-7°C warmer. CO₂ concentrations were also high, and there were no major ice sheets (such as those currently over Greenland and Antarctica). Since then there has been a slow, long-term decline in both CO₂ concentration and global temperature.

Over the last million years or so Earth’s climate has been cooler than today on average, characterised by a natural cycle between ice ages and warmer interglacial periods:

• Records from ice cores spanning 800,000 years show a series of ice ages and interglacials (see figure). Shifts between the two have taken about 5,000 years and are estimated to have led to an eventual 4-7°C of global temperature change. Atmospheric CO₂ is directly measurable over this time from air bubbles trapped in the ice cores.
• There is strong evidence that these climate shifts were triggered by regular cycles in Earth’s orbit which altered the distribution of sunlight over the Earth.
• But while orbital cycles acted as the initial driver, other factors amplified the change. For example, reduced sunlight over the Northern Hemisphere caused snow and ice to advance, reflecting more sunlight away from the Earth’s surface. CO₂ concentrations also decreased, amplifying the cooling effect further.
• This record shows directly that altered CO₂ concentrations influence global temperature, even if acting as a feedback rather than being the initial driver. It also shows that feedback processes in general are important for past and future climate change. 

Click on the image to enlarge chart

In the last 10,000 years (since the end of the last ice age) we have lived in a relatively warm period with stable CO₂ concentrations. Some regional changes have occurred in this time, affecting local societies and ecosystems. For instance, persistent droughts have occurred in Africa and North America, and both El Niño and the Asian monsoon have undergone changes in frequency and intensity. These have not however been part of a coherent global change.

There is a high degree of confidence that temperature changes over the last million years have been amplified by closely-linked changes in atmospheric CO₂ concentration.