Climate Through Human History

Reconstructing Past Climates

Were the climates of Earth always as they are today?

There is much evidence in historic geological records that says no!

The figure on the left shows that in the northern hemisphere, the mean annual temperature rose somewhat from the late nineteenth century until 1938, then fell until around 1965, and has been rising again since then.

Not only is the earth's climate always changing, but a mere 18,000 years ago the earth was in the grip of a cold spell, with alpine glaciers extending down river valleys and huge ice sheets covering vast areas of North America and Europe.

Presently, glaciers cover only about 10% of the earth's land surface. Most of the ice is in the Greenland and Antarctic ice sheets. If global temperatures were to rise enough so that all of this ice melted, the level of the ocean would rise about 65 m (213 ft.).

Why do we need to understand the climates of the past?

• useful to understand present-day climate and how it might change in the future;
• test hypothesis about the causes of climate change.

How do we know what climate was like in the past?

Instrumental weather records are available for only the last 200 or so years.

Qualitative weather observations and indirect indicators, such as historical and archaeological data, allow reconstructions of climate in some parts of the world back to medieval times.

A wide range of scientific techniques can be applied to different kinds of environmental evidence to provide a picture of climatic variation. A few of these are:

• Plant and animal remains
• Glacier fluctuations
• Lake levels
• Dendroclimatology
• Oxygen isotope data
• Ice cores

These are collectively refered as environmental indicators