Natural Longterm Climate Change

Many controls operate to change the Earth's climate on different timescales. some cause the global temperature to rise and fall within a time interval between warming and cooling influences of billions to hundreds of millions of years; others operate on time frames of millions to tens of millions of years. These slowly operating forces include the sluggish evolution of the composition of the planet's atmosphere from an early greenhouse atmosphere when the 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 temperatures on Earth's surface were not as high as they could have been, given the early greenhouse conditions. Changes in solar luminosity have been significant in Earth history, and will be significant again in the future.

Plate tectonics exhibits different types of controls and with different timescales of influence on changing the atmospheric composition and climate. one type of influence of plate tectonics is on a planetary scale—plate tectonics goes through intervals of time in which seafloor spreading and volcanism is very active and periods when it is less active. During the active times the volcanism releases a lot of carbon dioxide and other greenhouse gases into the atmosphere, causing global warming. During inactive times global cooling can result. These changes operate on timescales 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 carbonate ion (Co32-) is dissolved by rainwater, and the free calcium ion (Ca2+) then combines with atmospheric Co2 to form new layers of limestone in the ocean, while drawing down Co2 from the atmosphere and causing global cooling.

scientists have shown that the interaction between these different long-term drivers of global climate is largely responsible for the long-term fluctuations in global climate on the billions to tens of millions of years timescales. Many aspects of these changes are not yet understood by geologists and paleoclima-tologists, but the mechanisms described above seem fairly well understood and represent the most likely explanation for the causes of the changes.

Continue reading here: Role of the Atmosphere in Climate Change

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