Earth's Climate History

Climate is not a static set of weather conditions, constant over eons; rather it varies, sometimes in dramatic ways, over time. The hot climate of the newly formed Earth gave way to glaciers in a little more than a billion years, an immense time by human reckoning, but not nearly so long by geological standards. Earth's climate has alternated many times between hot and cold periods of varying magnitudes. Radiation from the sun, the ocean currents, rainfall, wind, continental drift, the concentration of greenhouse gases in the atmosphere, volcanic activity, radioactivity in the Earth's core, the eccentricity of Earth's orbit around the sun, the tilt of Earth's axis, and photosynthesis all affect climate. Climate has myriad causes. Disentangling these causes is not easy, but it is necessary to understanding why climate changes over time. The current climate is warming. The culprit, carbon dioxide, has been increasing in the atmosphere, driving up temperature, and prompting speculation over Earth's future climate.

The interlocking scientific theories of the 19th century—Darwinism and uniformitarianism—implied that climate changed little, and then only gradually. Uniformitarianism holds that the climactic conditions prevailing today are very similar to the conditions that prevailed centuries and even eons ago. Charles Darwin matched his theory of evolution to the dictates of uniformitarianism. Darwin posited that species evolve gradually in response to slow and small changes in the environment. Climate might change, but neither abruptly, nor by a large magnitude. In contrast, the advocates of catastrophism asserted that Earth has been racked by sudden changes. Modern geology has retained the kernel of catastrophism. The climate has changed quickly by geological standards, and by large swings in temperature and precipitation.