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 not one, but, rather, 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 held 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.
EARTH'S EARLY CLIMATE
Earth is roughly 4.5 billion years old. Its early climate was the hottest in the planet's long history. Temperatures were hot enough to liquefy rock, a circumstance that accounts for the absence of rock from the early geological record. The mass of radioactive elements in Earth's core was the maximum at the origin of Earth. The radioactive elements generated heat and pressure as they decayed, pushing molten rock toward Earth's surface. Volcanoes were active, bringing molten rock to the surface, where it liberated its heat. Volcanoes spewed carbon dioxide (CO2) into the atmosphere, causing the greenhouse effect. Sunlight passed through the atmosphere and struck Earth, which absorbed some sunlight as heat and radiated the rest into space as infrared radiation. Rather than disappearing into space, the infrared radiation was absorbed by CO2, heating the atmosphere. The amount of CO2 in the atmosphere of the newly-formed Earth was 1,000 times higher than it is today, more than making up for the fact that the young Sun burned with only 70 percent the luminosity of its mature, current phase. At its origin, Earth received only as much sunlight as Mars receives today. The young Earth was too hot for water to liquefy. Instead, the atmosphere held all of Earth's water as vapor. Like CO2, water vapor traps heat in the atmosphere, intensifying the greenhouse effect.
As the mass of radioactive elements in Earth's core diminished, the climate cooled, and the first rock formed roughly 3.8 billion years ago. The cooling of the atmosphere liquefied water vapor, which fell to Earth as rain. The deluge was greater than any rainfall since, filling Earth to a depth of 2 mi. (3.2 km.) and forming the primordial ocean. The origin of life around 3.5 billion years ago enhanced the cooling of the climate, for among the first life were single celled photosynthetic algae. Like modern plants, these algae consumed CO2 and exuded oxygen. The reduction of CO2 in the atmosphere weakened the greenhouse effect. Rainfall also diminished the amount of CO2 in the atmosphere. CO2 dissolves in rainwater to form carbonic acid, a process that removes carbon dioxide from the atmosphere. With the reduction in CO2, temperatures dropped below freezing, causing the planet's first ice age roughly 3 million years ago.
The retreat of the glaciers one million years later inaugurated a long period of warm climate. The sun, burning steadily brighter, bathed Earth in its heat. Warm inland seas covered Earth, moderating the climate. Ocean currents circled the globe, spreading warm water from the equator to the poles. The warm climate persisted until 800 million years ago, when a series of ice ages and interglacials alternated the climate between cold and warm cycles.
The multiple changes in climate may have an astronomical cause, in addition to terrestrial ones. Earth's orbit around the Sun is not constant, but changes its geometry. The orbit traces an ellipse that puts Earth far from the Sun at its greatest distance and near the Sun at its shortest distance. Because Earth's distance from the Sun varies, the amount of heat that Earth receives from the Sun fluctuates. Under these circum stances, Earth's climate alternates between hot and cold extremes, accounting for the advance and retreat of glaciers. At other times, Earth's orbit is nearly circular. Earth, at roughly a constant distance from the Sun, receives nearly the same amount of heat from it. Under these conditions, Earth's climate is uniformly warm, as it was during the Cretaceous Era.
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