Dennis Dorney, of Dunedin asks :-

The amount of carbon dioxide (CO2) in the atmosphere has been deduced for the past 650,000 years, varying cyclically with ice ages. Our present CO2 levels are already above those that in the past have tripped an ice age but there is no talk of a coming ice age. Why?

Martin Manning, of the School of Geography Environment and Earth Studies at Victoria University of Wellington, responded.

The amount of CO2 in the atmosphere has varied in ways that are closely linked to the past ice ages, but the ice ages occurred when there was less CO2 and the warm periods when there was more. This is a field of science that has evolved for more than 100 years as it became realised that the Earth’s temperature has changed significantly in the past.

In the 1890s, Svante Arrhenius, a future Nobel prize winner, calculated how the Earth’s temperature would change if there was less or more CO2 in the atmosphere and what he discovered has been confirmed by much more detailed analyses in the last 40 years. If the amount of CO2 were reduced to half of what it is, then we expect a cooling of between 2 and 4.5°C, and if CO2 is doubled then there will be a warming in that range.

There has to be a driver for major atmospheric changes like this. In the past this is related to continuing cycles in the Earth’s orbit around the Sun that repeat over tens and hundreds of thousands of years. These lead to changes in incoming energy that cause small shifts in the climate. But that can then trigger transitions in the extent of ice sheets and vegetation on land as well as in the large amounts of carbon dissolved in the ocean which all combine to cause much more rapid changes in atmospheric CO2 and in global temperature.

This close linkage between past temperature and atmospheric CO2 is shown in the figure that summarises a number of ice core records from Antarctica which show past changes in each of these important features of our planet over the last 250,000 years.