changes in the types of rocks exposed to constant rates of weathering (e.g., Harris, 1995). The relative contributions of these factors will have to be sorted out before we can determine how to translate the Sr isotopic data into a quantitative estimate of global weathering rates. Other possible proxies for weathering include Os, Ca, and Mg isotopes, although these elements are still in early stages of study.

Summary

The geological record teaches us that Earth’s climate has always been changing, but remarkably the surface temperature has remained within a range suitable for life for the past 3.5 billion to 4 billion years. The primary factors responsible for this relatively benign climate are believed to be volcanic emissions of carbon dioxide to the atmosphere, removal of CO2 by weathering of surface rocks, and more subtle effects, such as the positions of the drifting continents, the patterns of ocean currents, the orientation of Earth’s rotational axis and orbit around the Sun, and the luminosity of the Sun. Other chemical and biological effects are also likely to be important, such as the oxidation state of the atmosphere and the concentrations of other greenhouse gases. Interspersed in this vast and mostly life-supporting history are a few periods when Earth was considerably warmer than it is at present, and completely ice free, and a few times when Earth might have been extremely cold and completely ice covered.

At present the greenhouse gas content of the atmosphere is increasing rapidly. The greenhouse gas content of the atmosphere is the most important determinant of climate on geologically short timescales, and models can be used to predict how climate will change over the next decades and centuries. Over longer geological time periods, natural geological processes control the greenhouse gas content of the atmosphere, and other geological and astronomical factors are influential. We have a good qualitative understanding of the factors that contribute to Earth’s natural climate states, but we still lack a comprehensive model that can account for the climate changes of the past or predict climate changes into the distant future. Better models for both the volcanic and weathering components of the climate cycle, more quantitative descriptions of erosion and its relation to weathering, and the incorporation of inputs from the biosphere and other factors will likely lead to a more accurate understanding of Earth’s climate and climate history.

QUESTION 8:
HOW HAS LIFE SHAPED EARTH—AND HOW HAS EARTH SHAPED LIFE?

It is not surprising that many Earth scientists have viewed the geological evolution of Earth as a fundamentally inorganic process—dominated by titanic mechanisms such as mantle convection and plate tectonics. After all, virtually all of Earth’s organic mass exists as a veneer of frail and short-lived creatures within a few vertical miles of the outermost surface, a seemingly insignificant afterthought to this massive planetary body of rock. And yet this multitude of organisms—most of them microscopic packages composed primarily of carbon, hydrogen, nitrogen, and oxygen—determines major features of the atmosphere, oceans, and continents. Biologically influenced processes like erosion and weathering, for example, continually shape and reshape Earth’s surface. And as we have seen in Questions 4 and 5, the erosion and weathering influenced by life forms affect not only the topography and composition of continents but also the chemical composition of subducted crust and therefore the mechanism of plate tectonics and the composition of the mantle.

Life scientists, in the same spirit, have regarded the evolution of life as a fundamentally biological issue, dependent primarily on time, chance, and competition to trend toward increasing diversity and complexity. We now know that Earth itself is not the mere substrate or background for life’s activities as once supposed but rather an active partner in evolution. Geological processes and astronomical events have strongly and repeatedly influenced the story of life on Earth and often determine the kinds of life that can survive and flourish.

The interconnectedness of life and the environment has been a subject of continuing research and debate. An extreme view is that life controls Earth’s surface environment and does so in ways that are most beneficial to the continuation of life (Lovelock, 1979). But evidence in the geological record, especially of mass extinctions, suggests that life cannot always maintain



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