Ice core evidence
Some of the best evidence for the link between carbon dioxide, human activity and temperature has come from glaciers. Ice cores are drilled and carefully stored at well below 0 ˚C. Teams of researchers have produced reliable and consistent results from sites in Greenland and central Antarctica. The growing interest and concern about climate change has enabled researchers to carry out expensive experiments in some of the most hostile environments in the world.
As a glacier forms, tiny bubbles of air are trapped and become isolated in the ice. Each bubble is a tiny sample of air from the past; Scientists can measure how old they are from the depth they come from in the ice. In the Northern Hemisphere samples go back about 125,000 years. Antarctic records for the last 650,000 years have been recovered.
The trapped air contains a record of the carbon dioxide and methane levels at the time. The balance of isotopes of oxygen and hydrogen can be used to reconstruct the temperature. By comparing these readings from a large number of ice cores and with evidence from other methods like tree rings, we can have a high degree of confidence in these results.
Ice core data shows a complex picture, but there are some very obvious features.
- The climate has been very variable over the last 650,000 years.
- Greenhouse gas concentrations change in step with temperature, though in a complicated way.
- Greenhouse gas concentrations in the atmosphere are now at a higher level than they have ever been in the last 650,000 years.
- Temperature levels are relatively high at present.
Unless we reduce greenhouse gas emissions, their levels in the atmosphere will continue to rise, leading to further increases in global temperature.
We know a lot about why climate change happened in the past and can try to predict what is likely to happen in the future. The most important lesson from the ice core record is that the levels of carbon dioxide in the atmosphere are now higher than they have been for more than half a million years and that this will have a profound effect on global climate.