World View of Global Warming

This graph shows the measured record of temperature readings back through the mid 19th century, when thermometers were first in widespread use. 

To estimate temperatures before that, the scientists used temperature reconstructions based on information from indirect "proxy" sources, such as ice cores, tree rings, and sediment cores.  These estimates are less certain, which is indicated by the yellow shading of possible temperature ranges.  Temperature changes are shown as deviations from the average temperature from 1961 to 1990. Thus it was cooler and getting colder for almost 2000 years, and then in mid 20th Century, it began to get much warmer.

There is a tight correlation between the amount of CO2 and methane in the atmosphere and the average world air temperature.  More carbon dioxide = higher temperature.  Scientists investigating ancient rocks and ocean sediment can see the relationship going back millions of years.  None of the correlations is as tight as that extracted from the deep ice core drilled into the Antarctic ice cap at the Russian Vostok base. 

Measuring CO2 trapped in the ice and using oxygen isotopes as an indicator of temperatures, J. R. Petit and his co-workers were able to draw this double chart of 420,000 years.  None of the CO2 levels they found is as high as we have today: "Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years," they wrote in Nature.

Other studies suggest that today's 370 parts per million of CO2 is the greatest in 20 million years -- and it is still going up.

Estimates of more recent past temperatures are based largely on tree rings --  the mark of yearly growth of a tree that is extremely sensitive to local conditions and which over the years etches a chart of climate.   Dr. Malcom Hughes of the University of Arizona dendrochronology lab is among a small group of scientists who have cored trees and made cross sections of dead stumps all over the world.

One tree that has been extremely important in reading deep into history is the bristlecone pine of the White Mountains, California, where Dr. Hughes and associates worked in 2002 to extract ring data from thousands of years old wood from pines that died many hundreds of years ago.   The dry climate at nearly 12,000 feet preserves these old stumps and their record of climate going back 10,000 years.

Other living things that react to environmental changes also can be read for clues to ancient temperature and growth conditions.  Among the most commonly used are coral cores, which are similar to tree rings, and studied here by   Kevin Helmle of the  National Coral Reef Institute at Nova Southeast University, Dania Beach, Florida.

Each year sediment falls to the bottom of oceans and lakes, and this record can be extracted by inserting hollow tubes into the mud (See Antarctic Section, also).

Sediment cores from both oceans surrounding North America have been studied by scientists like Lloyd Keigwin at Woods Hole Oceanographic Institute to make a picture of geologically recent climate events like the Little Ice Age.

The oldest climate records come from glaciers and ice caps, for which cores extend back almost half a million years (in Antarctica).