Natural Longterm Climate Change

Atmospheric Evolution, Plate Tectonics, Supercontinents, and Solar Evolution

There are many controls that operate to change Earth's climate on different time scales. Some cause the global temperature to rise and fall with a time interval between warming and cooling influences of billions to hundreds of millions of years; others operate on millions to tens of millions of years time frames. These very slowly operating forces include the very slow evolution of the composition of the planet's atmosphere from an early greenhouse atmosphere when Earth had recently formed to its present day composition. During the earliest history of the solar system, the Sun was about 30 percent less luminous, so the temperature on Earth's surface was not as high as it would have been had those atmospheric conditions been extant when the atmosphere had its present composition. Changes in solar luminosity have been significant in Earth history and will be significant again in the future.

Plate tectonics exhibits controls of different types and with different time scales of influence on changing the atmospheric composition and climate. One type of influence of plate tectonics is that on a planetary scale, plate tectonics goes through intervals of time in which sea-floor spreading and volcanism is very active, and periods where it is less active. During the active times, the volcanism releases a lot of carbon dioxide and other greenhouse gases to the atmosphere, causing global warming. During inactive times, global cooling can result. These changes operate on time scales of tens to hundreds of millions of years. Periods of very active seafloor spreading are often associated with periods of breakup of large continental landmasses known as supercontinents, and thus breakup of continents is often associated with global warming. Periods of less active seafloor spreading are often associated with continental amalgamations, formation of supercontinents, and global cooling.

When continents collide, this process uplifts large sections of carbonate rocks from passive margins and exposes them to atmospheric weathering. When the calcium carbonate (CaCO3) in these rocks is broken down by chemical weathering, the CO3 ion is dissolved by rainwater, and the free Ca ion then combines with atmospheric CO2 to form new layers of limestone in the ocean, drawing CO2 down out of the atmosphere and causing global cooling.

The interaction between these different long-term drivers of global climate is thought to be largely responsible for the long-term fluctuations in global climate on the billions to hundreds to tens of millions of years time scales. Geologists and paleoclimatologists do not yet understand many aspects of these changes, but the mechanisms seem

Plot showing how the average temperature on the surface of Earth has changed with time over the past several hundreds of millions of years. These represent slow, long-term changes in global temperature (modified from C.R. Scotese, PALEOMAP)

fairly well understood, and represent the most likely explanation for the causes of the changes.

In this chapter, the structure and evolution of the atmosphere is discussed, and then the long-term plate tectonic influences on climate are discussed.

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