FIG. 25.3

How much CO₂ was in the atmosphere 1000 years ago?

BACKGROUND Direct measurements of the atmosphere show that CO₂ has increased by 25% over the past 50 years. To know whether or not such a change is unusual, we need to know CO₂ levels over a longer time interval.

OBSERVATION Scientists recognized that the snow that accumulates on glaciers to form new layers of ice is initially full of small spaces that are in contact with the air. Through time, as snow changes to ice, these spaces become sealed off from the surrounding environment and form bubbles, preserving a minute sample of air at the time of ice formation.

MEASUREMENTS Identifying chemical indicators that vary on an annual basis in successive layers of ice enabled scientists to assign ages to ice samples taken from cores drilled through the Law Dome, a large ice dome in Antarctica. The youngest ice samples overlap in age with the first part of the Keeling data, providing a check that air samples trapped in ice have CO₂ concentrations similar to those in air measured directly over the same interval.

CONCLUSIONS Before the Industrial Revolution, atmospheric CO₂ levels had varied little over 1000 years, generally falling between 270 and 280 ppm. From this, scientists have concluded that current changes in atmospheric CO₂ are unusual on the timescale of the past millennium.

SOURCE Etheridge, D. M., et al. 1996. “Natural and Anthropogenic Changes in Atmospheric CO₂ over the Last 1000 Years from Air in Antarctic Ice and Firn.” Journal of Geophysical Research 101:4115–4128.