■ FIGURE 8.27 This figure shows the broad climate trends of the past 150,000 years, with significant details for the Holocene. Climatologists have been remarkably successful in dating recent climate change.

What is some of the evidence for past climates that can be studied directly?

The evidence for past climate can be studied directly.

The studies of climate change are made possible by two major developments. They are radioactive decay studies and the ocean floor sediments studies. In the first method, the radioactive decay rate of elements can be used to measure the time of death of fossilized organisms. These types of radiometric dating techniques include radiocarbon dating. The time of glacial retreat can also be calculated from the radiocarbon dating of organic materials in glacial deposits. The sediments that are deposited on the floors of ocean can provide detailed evidence about the change in climate. The continuous and slow deposition of deep sea sediments preserves the climate history during the past several million years.

The rapid rate of change in climate over a short span of time can be studied only with the aid of a more detailed record of climate than the deep sea sediments can provide. It is due to the fact that only small centimetres of mud accumulate on the deep floors of ocean in a thousand years. The sudden change in climate over a period of few hundred years is not clearly recorded in the sediments of seafloor. In this situation, studying the glacial ice cores of an area is a better method since it records the annual accumulation of snowfall in ice layers. It can provide short term evidence of change in climate. To analyse the glacial ice cores, oxygen isotope analysis is used along with other techniques and it can provide a detailed record of climate variation during the past 250,000 years.

In oxygen isotope analysis, the ratio of O16 to O18 is used to gather information about the past global temperatures and thereby past climate