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 newlyformed 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 circumstances, 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.

Earth Warmed

During the last Ice Age, glaciers covered the ocean as well as the land, killing photosynthetic algae that lived in the ocean. With algae in small numbers, they were able to remove only a fraction of CO₂ from the atmosphere. With no check on its accumulation, CO₂ increased in the atmosphere, causing the greenhouse effect. The greenhouse effect ended the Late Proterozoic Ice Age roughly 550 million years ago, inaugurating a new warm period. The lush plant growth of the Carboniferous Era (350–280 million years ago) confirms that the climate was warm and that CO₂, essential for plant growth, was abundant. Around 230 million years ago, the continents gathered into a single landmass called Pangea. Being near the equator, Pangea’s climate was tropical. Even warmer was the Cretaceous Era (135–65 million years ago). Temperatures soared 20 degrees F (11 degrees C) warmer than in the current era. Forests covered Antarctica. Ocean currents again carried warm water to the poles. The transit of warm water to the poles caused polar water to be only 40 degrees F (4 degrees C) cooler than equatorial waters. The difference today is 75 degrees F (24 degrees C). The water at the bottom of the ocean, now near freezing, was then 25 degrees F (14 degrees C) warmer. Coral reef, which requires warm water to survive, grew 1,000 mi. (1,609 km.) closer to the poles than it does today. Antarctica was warm enough to support the growth of forests.

Earth Cooled

The end of the Cretaceous suddenly reversed the climate. The consensus among scientists holds that an enormous meteor impacted Earth 65 million years ago. The meteor ejected a gigantic cloud of debris and dust and ignited widespread fires, which pumped ash into the atmosphere. The debris, dust, and ash blocked out much of the Sun’s light, chilling the climate. So severe was the reversal in climate that the dinosaurs and a large number of marine species, unable to cope with the new conditions, perished. The sudden reversal in climate has had a lasting, if erratic, effect on Earth. During the past 55 million years, temperatures have declined 20 degrees F (11 degrees C). Around 35 million years ago, the climate grew cold enough for glaciers to form on Antarctica. Yet, by 4 million years ago, the glaciers had melted and forests had returned to Antarctica. The forests were transitory: within one million years glaciers had once more spread across Antarctica and had begun to grow in the northern hemisphere.

The warmth of the Cretaceous returned briefly 130,000 years ago, when the climate was again warmer than it is today. The water from melting glaciers flowed to the oceans, raising the sea level 60 ft. (18 m.) higher than it is today. Approximately 30,000 years later, the climate cooled yet again, and glaciers once more spread across the continents, plunging Earth into its most recent Ice Age. Between 16,000 and 10,000 years ago, the glaciers in retreat, temperatures rose nearly 15 degrees F (8 degrees C). Toward the end of this era, and extending to 6,000 years ago, rain was plentiful. Africa had no deserts. Rather, Lake Chad, swollen with rain, was 10 times its current size. Salt Lake, in what is today Utah, was likewise several times larger than it is today. Variations in temperature were greater than they are today, because the tilt of Earth’s axis was extreme. If Earth did not tilt on its axis, there would be no seasons, because the northern and southern hemispheres would receive the same amount of sunlight year round. However, Earth tilts on its axis 23.5 degrees. As a consequence, the northern and southern hemispheres receive differing amounts of sunlight during the course of a year, accounting for the seasons. The greater the tilt, the greater the amount of sunlight falls in summer, and the greater the darkness in winter. About 10,000 years ago, Earth tilted on its axis 25 degrees. The northern and southern hemispheres received 7 percent more sunlight in summer than they do today, and 15 percent more sunlight in summer than in winter. By contrast, the differential in sunlight between summer and winter is only 8 percent today.

Temperatures peaked 7,000 years ago at 2–3 degrees F (1–1.5 degrees C) above current temperatures. The climate remained warm and wet another 3,000 years. There was then no desert in the American Southwest, which received enough rainfall to sustain the growth of trees. Between 6,000 and 4,000 years ago temperatures rose 5 degrees F (2.7 degrees C) , melting parts of the glaciers that remained in Antarctica and Greenland. The water from these glaciers flowed to the ocean, raising sea level 300 ft (91 m.). The end of the rainy epoch 4,000 years ago turned the climate arid. Rainfall in the American Midwest fell 25 percent, even as July temperatures peaked 4 degrees F (2 degrees C) above current July temperatures. Deserts formed in the American Southwest, Africa, Asia and Australia. Along the Atlantic coast of North America, the climate remained warm and humid.
Source: Encyclopedia of Global Warming and Climate Change (Click Here)