A Summary of Earth’s Climate History-a Geologist’s View

Thoughts from Dr. Eric T. Karlstrom, Emeritus Professor of Geography, California State University, Stanislaus

Earth scientists have learned that earth was significantly warmer than present (by perhaps about 8 to 10 °C) for about 80% of the earth’s 4.6-billion-year history.

From a historical as well as a geological perspective, warming trends are beneficial for humans, for agriculture, and for plants and animals. No “tipping points’ were reached during past geologic intervals when temperatures and CO2 concentrations were much higher than present. In fact, life flourished during these relatively warmer conditions.

For the past 2 million years (Quaternary Period), the earth has been in an ice age comprised of some 20 major glacial/interglacial cycles. Each cycle was characterized by very wide swings of temperature and precipitation.

For the past 10,000 years, the earth has experienced an unusually warm and stable (interglacial) climate known as the Holocene Epoch. The stable and favorable climate of this interglacial allowed for the development of agriculture and human civilization.

71% of the earth is covered with ocean water. 90% of the world’s ice is in Antarctica. Our instrumental climate records only extend about 100 to 150 years back. There are still not enough weather stations on the earth to determine the average temperature of the earth. The best data from satellites and the Argo (ocean robot) systems suggests the planet has been cooling slightly since 1998.

Exhaustive analyses of proxy paleoclimatic records (deep sea cores, ice cores, tree-rings, glacial deposits, soils, loess sequences, cave deposits, pollen studies, etc.) by scientists reveal that past climate changes are complex and of varying frequency and magnitude. There is much we still don’t agree on. Many scientists, including myself, believe climate change is cyclical and these cycles are of varying periodicities.

The main climate drivers include variations in solar output, ocean circulation dynamics (the ocean stores some 22 times more heat than the atmosphere and circulates that heat around the globe), and orbital variations in the earth-sun-moon system.

Carbon dioxide has a negligible effect on atmospheric temperatures. Rather, because the oceans hold about 50 times more CO2 than the atmosphere, and because the oceans and atmosphere exchange CO2, CO2 fluctuations are mainly caused by changes in ocean temperatures. And ocean temperature changes are mainly driven by the sun.

Because we have seasons, the earth is constantly warming and cooling in various locales. Weather and climate change is a constant. But is the earth as a whole warming or cooling? The answer to this question depends entirely upon the length of the climate record being analyzed. On the basis of many paleoclimatic records, earth scientists agree that the general trend over the past 3 million years has been toward cooling; the trend over the past 15,000 years has been toward warming; the trend over the past 5,000 years has been toward cooling; and there has been a warming trend since Little Ice Age maxima about 1650 AD. The earth warmed very slightly between about 1975 and 1998, and since 1998 the trend has been toward cooling. Read full post

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