The global warming issue from a geological viewpoint

The impressive achievements of our species over the past several millennia - the invention of agriculture, the building of cities, the advances of civilizations, and the development of industries and technologies - are bringing us rich rewards, but are also having inadvertent and disquieting consequences -- a rapid rise in the atmospheric concentration of carbon dioxide. This is clear evidence that we are now geologic agents capable of interfering with the processes that make this a habitable planet. We have therefore become the custodians of the Earth, the only planet known to be habitable. To make responsible decisions on behalf of future generations, it is helpful to have some familiarity with the Earth's past, especially the remarkable climate changes evident in fig.1.
Some 60 million years ago, at the time of the demise of the dinosaurs, temperatures in high latitudes were so high, in the neighborhood of 15C, that the poles were free of ice. Since that time the Earth has experienced the gradual global cooling shown in the bottom panel of fig.1. The main cause was plate tectonics with which is associated the drifting of continents, the building of mountains, and processes such as weathering that reduced the atmospheric concentration of carbon dioxide. Superimposed on the gradual cooling, whose source of energy is internal to the Earth, are the Milankovitch cycles, climate fluctuations in response to periodic variations in sunlight because of periodic variations in the tilt of the Earth's axis (at a period of 41,000 years), the precession of the axis (at a period of 23,000 years) and the eccentricity of the orbit (at a period of 100,000 years.) It is striking in the geological record that, even though the Milankovitch forcing over tens of millions of years was essentially constant, the climatic response to that forcing changed dramatically with time. The response was modest until around 3 million years ago but then started to amplify enormously as can be seen in the middle panel. The top panel is a more detailed picture of the Ice Age cycle over the past 400,000 years. Note the remarkably high correlations between fluctuations in atmospheric CO2 levels and Antarctic temperatures. The anthropogenic rise in CO2 levels over the past century is the vertical bar at the left-hand extreme.
Today is a special moment in the Earth's history for two reasons: (1) the climate is so sensitive to small perturbations that modest variations in sunlight cause recurrent Ice Ages; (2) the present is an infrequent warm, interglacial period of naturally high carbon dioxide levels in the atmosphere. The rapid increase in those levels, because of our industrial activities, is therefore at an inopportune time. The consequences, according to the empirical correlations in the top panel of fig.1, could include a return to the warm world of approximately 3 million years ago (3Ma). Recently Ravelo et al (2004) presented evidence that, at that time, equatorial Pacific sea surface temperatures were as warm in the east as in the west. Today, such a warming of the east occurs only briefly during intense El Nino episodes such as that of 1997-98. Up to 3 Ma El Nino was apparently a perennial phenomenon. Under what conditions will El Nino become a permanent phenomenon?
Studies that address this question include calculations with a variety of computer models of the atmosphere, the ocean and earth's climate. Interesting results are emerging, but how do we test the validity of these results?
Geoscientists have made enormous progress with the prediction of the daily weather, and interannual El Nino events, mainly because of the availability of data to check models and theories. The phenomena of interest do not repeat themselves - the precise weather pattern at this moment has never appeared before, and will not appear again. Data that describe a succession of many, many weather patterns are therefore needed to improve models. Fortunately the weather changes frequently. In the case of El Nino the changes are more gradual but fortunately the frequency of El Nino occurrences is sufficiently high for us, over the past thirty years, to have observed several El Nino episodes. This contributed to considerable progress. When we turn to climate changes over decades and longer, the available instrumental records are too short to provide adequate checks for models, and will continue to be so for a long time to come. Hence climate modelers have no choice but to turn to the geological record of past climates. Fortunately those records provide a wealth of information, and invaluable checks for climate models, as is evident in the figure. Explaining and predicting the climate changes in that record, the recurrent Ice Ages for example, will bolster confidence in the results from climate models enormously.