Introduction to Climate

Past climate

Climate, like other parts of the Earth system, is not static but changes over time on both human and geologic time scales. Latitude, for example, has a very direct effect on climate, so as the continents shift over geologic time, the climates on them also shift. Furthermore, the conditions on Earth as a whole have varied through time, altering what kinds of climates are possible. 

Ancient climates are reconstructed through many methods. Written records, tree rings, and glacial ice cores go back hundreds of thousands of years, whereas fossils and rocks that indicate different climates go back as far as hundreds of millions of years. These clues, coupled with modeling and a knowledge of physics and chemistry, help climatologists put together an increasingly detailed history of the Earth's climate. Unfortunately, we do not have as clear an understanding of climate for the earliest part of Earth history as we do for the later parts, because the oldest rocks are much more difficult to find. However, we can still say something about the climate of the ancient Earth, in large part due to our knowledge of atmospheric chemistry.

Hadean to Archean (4.6 to 2.5 billion years ago)

Not long after the Earth first formed more than 4.5 billion years ago, its atmosphere was composed mostly of hydrogen and helium. Volcanic activity and collisions with meteorites and comets added water vapor, carbon dioxide (CO₂), and nitrogen (N₂) to the atmosphere. As the Earth cooled enough for liquid water to form, water vapor formed clouds. Rain poured forth in volumes the planet will never experience again. These torrential rains were constant for millions of years. The rainwater absorbed salt and other minerals as it flowed to the lowest areas on the surface of the planet, forming Earth's oceans and seas.

During the early formation of the Earth and its atmosphere, the sun produced significantly less energy than it does today. Thus, one might expect that once the oceans formed, they would continue to cool and eventually freeze. Yet temperatures stabilized and remained high enough for liquid water to exist. Temperatures may have remained above freezing because there was a greater concentration of potent greenhouse gases in the atmosphere and less land surface to reflect light than there is today.

Oxygen

Earth's early atmosphere had little oxygen (O₂). Cyanobacteria—a group of bacteria sometimes also "blue-green algae" for their unusual color—appeared perhaps as early as 3.5 billion years ago. These bacteria lived in the ocean and made their own food through photosynthesis. Photosynthesis is a process that coverts carbon dioxide and water to sugar and oxygen in the presence of sunlight. The sunlight serves as an energy source for the reaction. The sugar produced by photosynthesis is a food source, whereas the oxygen is released as a waste product.

Cyanobacteria sometimes produce layered structures called stromatolites that can be found in the fossil record. The oldest stromatolites are from Australia and are about 3.5 billion years old.

The cyanobacteria released oxygen into the early oceans. The oxygen did not immediately start to build up in the atmosphere, however, because it was quickly absorbed by reactions with the abundant iron and organic matter present on Earth. Evidence for the absorption of oxygen on the early Earth occurs in some geological deposits like banded iron formations. These ancient rocks have iron-rich layers that formed when dissolved iron particles reacted with oxygen in the ocean to form iron-oxide minerals like hematite (Fe₂O₃) and magnetite (Fe₃O₄).

After hundreds of millions of years, much of the iron and organic matter had combined with oxygen, so much less oxygen could be absorbed by these sinks. Thus, free oxygen could finally build up in the atmosphere. With this addition, the modern atmosphere was complete, although the relative amounts of the gases composing it would, and still continue to, shift. 

Proterozoic climate (2.5 billion to 541 million years ago)

Paleoproterozoic to Mesoproterozoic climate (2.5 to 1 billion years ago)

There is evidence suggesting that the entire surface of the planet has been covered in ice several times, a hypothesis called Snowball Earth. Glacial deposits discovered near Lake Huron and elsewhere show that starting about 2.4 billion years ago the entire surface of the Earth may have been covered in ice for as long as 300 million years. In North America, this event is known as the Huronian glaciation. At that time the continental plates covered less than half as much of the Earth's surface as they do today and were unified as the continent Arctica.

It is possible that oxygen produced by microorganisms during this time reacted with methane in the atmosphere. Methane is a potent greenhouse gas. If its concentration in the atmosphere was lowered, it may have tipped the Earth toward a series of cooling feedbacks that caused ice to spread from pole to pole.

An ice-covered planet might remain that way because the ice will reflect almost all of the sun's energy back into space, preventing warming. (In other words, little of the sun's heat would be absorbed by the Earth.) The Earth did not remain cold because of plate tectonics. Volcanoes continued to erupt, dumping carbon dioxide (CO₂) and methane into the atmosphere. Although these greenhouse gases are usually removed from the atmosphere by organisms (for example, during photosynthesis) and the weathering of rocks, the widespread ice impeded these processes.

After millions of years, the concentrations of methane and CO₂ increased to the point that greenhouse warming began to melt the ice sheets. Once the melting started, more of the sun's energy was absorbed by the Earth's surface, and warming feedbacks began. Because the oceans had been covered with ice, nutrients from volcanic gases and chemical changes in submerged rocks had accumulated in ocean waters. Once the oceans were again exposed to light, the light and accumulated nutrients drove an explosion in the population of cyanobacteria. The cyanobacteria produced more and more oxygen through photosynthesis. This oxygen combined with carbon from freshly thawed sources, making carbon dioxide, which further enhanced warming.

For the next 1.5 billion years, the continental crust that was to become North America drifted around the surface of the Earth. A new supercontinent called Rodinia formed about 1.1 billion years ago.

Neoproterozoic climate (1 billion to 541 million years ago)

Perhaps beginning late in the Tonian Period (about 1 billion to about 720 million years ago) and continuing throughout the Cryogenian period (about 720 to 635 million years ago), the Earth entered a 200-million-year ice age during which there were several intervals of glaciation. Two extensive periods of glaciation occurred during this time, called the Sturtian glaciation (717 to 660 million years ago) and the Marinoan glaciation (640 to 635 million years ago). Although the part of Rodinia that would eventually become North America was located near the equator, it was glaciated. The fact that North America was at such a low latitude yet had glaciers is strong evidence that the Earth really did freeze over completely. 

At the end of the Cryogenian, about 635 million years ago, the prolonged ice age came to an end, and the following Ediacaran period saw the flourishing of complex multicellular life forms.

The Phanerozoic: Ancient Life and Climate

Paleozoic climate (541 million years ago to 252 million years ago)

Cambrian to Ordovician (541 to 444 million years ago)

With the start of the Paleozoic era, climates across the world were warm, and North America (at this time called Laurentia) was located in the low, warmer latitudes of the Southern Hemisphere. As the Cambrian progressed, Laurentia moved northward. Cambrian fossils of archaeocyathids, trilobites, brachiopods, and mollusks reveal that most of the continent was probably covered by warm, shallow seas that persisted into the Ordovician.

In the late Ordovician (about 460 to 430 million years ago), the Earth fell into another brief but intense ice age. Glaciers covered most Gondwana (the continent made up of present-day South America, Antarctica, Africa, Australia, and India), which was located over the South Pole. This led to global cooling. Although sea level dropped during this event, North America's climate relatively warm because the continent was near the equator.

Silurian to Permian (444 to 252 million years ago)

From 430 to 300 million years ago (from the mid-Silurian to nearly the end of the Carboniferous), ancient North America moved north across the equator, and the cycle of warming and cooling was repeated yet again. Glaciation in the Southern Hemisphere occurred during the late Devonian, when the supercontinent Gondwana was located over the South Pole.

During the Devonian, shallow seas were especially productive. In fact, plankton productivity became so high that it depleted all the oxygen from the seafloor and sediments. The lack of oxygen allowed organic matter to accumulate instead of decay, leading to the deposition of black, carbon-rich shale. Although all geologic periods have experience such deposits, some Devonian-age marine rocks that are currently especially important sources for natural gas and petroleum.

In the late Devonian and early Carboniferous (Mississippian), complex terrestrial ecosystems developed, including the world's earliest forests. Plants took in carbon dioxide during photosynthesis to fuel their growth, and they used this carbon to build tissues. As the plants died, thick, organic peat deposits accumulated that locked away carbon; over time, these peats became that coal deposits.

As plants lowered the amount of carbon dioxide in the atmosphere, their growth contributed to global cooling. By the early Carboniferous (Mississippi), ice capped the South Pole and began to expand northward. Although the Earth's temperature fell during this time and glaciers growing in the southern hemisphere caused sea levels to drop, North America still remained relatively warm because of its low-latitude position.

By the late Carboniferous (Pennsylvanian), North America—which was part of the continent of Laurussia—had collided with Gondwana. This collision was part of the formation of Pangaea, a supercontinent made up of nearly all the landmass on Earth. Global temperatures again declined, an glaciation continued near the South Pole. In unglaciated regions, extensive coal swamps grew. This ice age lasted well into the Permian, ending about 260 million years ago. The end of the Permian was a time of dry, warming conditions and lower sea levels.

Mesozoic climate (252 to 66 million years ago)

Triassic to Jurassic (252 to 145 million years ago)

As the Triassic period began, North America was still part of Pangaea, and was shifting northwards. The Earth remained warm and ice-free at the poles through much of the Mesozoic era, although global temperatures began to dip again slightly around 150 million years ago. After reaching its greatest size during the Triassic period, Pangaea began to rift apart, and the climate gradually shifted, becoming wetter and warmer.

Cretaceous (145 to 66 million years ago)

The Earth warmed near the beginning of the Cretaceous, and sea level rose. Throughout the Cretaceous, sea level was as much as 100 meters (330 feet) higher than it is today. High sea levels were caused by the displacement of water due to continental rifting and rapid sea-floor spreading. Shallow seaways spread over many of the continents. By the start of the Late Cretaceous, North America was divided in two by an inland sea known as the Western Interior Seaway. Global temperatures during the Cretaceous were very warm, as much as 10°C (18°F) above present. There was likely little or no glacial ice anywhere on Earth. Temperatures were highest in lower latitudes.

At the very end of the Cretaceous, the Gulf Coast of the United States experienced an enormous disruption when an asteroid or comet collided with Earth in what is now the northern Yucatán Peninsula in Mexico. The impact vaporized both water and rock, blocking out sunlight for weeks to years, which led to a collapse of photosynthesis and food webs on land and in the oceans. After this event, the climate may have cooled briefly, but it soon rebounded to a warmer state.

Cenozoic climate (66 million years ago to present)

Paleogene (66 to 23 million years ago)

As the Cenozoic era began, global temperatures remained high, and rose even further into the early Eocene. At the boundary between the Paleocene and Eocene epochs (around 56 million years ago), temperatures spiked upward in what geologists call the Paleocene-Eocene Thermal Maximum (PETM). During this event, which lasted perhaps only around 10,000 years, the atmosphere and ocean warmed by as much as 8°C (14°F) in as little as 4000 years. Deep oceans became acidic, with low levels of dissolved oxygen. The causes of the PETM remain unclear, but may have involved the sudden release of methane from sediments on the seafloor. The resulting greenhouse effect persisted for 100,000 years.

In the late Eocene, the Earth began cooling, and global temperatures fell sharply at the boundary between the Eocene and Oligocene epochs around 35 million years ago. Global cooling was caused in part by the separation of the southern tip of South America from the Antarctic Peninsula. This allowed for the formation of the Antarctic Circumpolar Current, which insulated Antarctica from warm ocean water coming from lower latitudes and led to the formation of the continent's glaciers.

Neogene (23 to 2.6 million years ago)

Climates warmed slightly during the Miocene, reaching a small peak at the mid-Miocene Climatic Optimum. Afterwards, global temperatures began falling. Global cooling was possibly driven by the continuing collision of the India subcontinent with Asia, which was forming the Himalayas. The rise of the Himalayas had a significant impact on global climate, as newly exposed rock reacted with atmospheric CO₂ during the process of weathering, removing the greenhouse gas from the atmosphere. With less carbon dioxide in the atmosphere, temperatures cooled worldwide. This cooling has continued more-or-less to the present day.

Around 3.5 million years ago, glacial ice began to form over the Arctic Ocean and on the northern parts of North America and Eurasia. A major factor contributing to the formation of glaciers in the northern hemisphere was a geological change that occurred in Central America. The Isthmus of Panama, formed by undersea volcanoes, rose out of the ocean.

The new, dry-land isthmus blocked the warm ocean currents that had been flowing east-to-west from the Atlantic to the Pacific for more than 100 million years. The currents where diverted into the Gulf of Mexico and, ultimately, into the western Atlantic Gulf Stream. As a result, the strengthened Gulf Stream carried more warm, moist air with into the northern Atlantic, which caused increased snowfall at high latitudes, which further accelerated cooling. These changes in ocean circulation also affected nutrient supplies in the coastal ocean throughout the Caribbean and Gulf of Mexico, which may have caused an increase in the extinction of marine animals during the late Pliocene.

Quaternary (2.6 million years ago to present)

Eventually, a sheet of sea ice formed over the Arctic, and ice sheets spread over northern Asia, Europe, and North America, signaling the start of the most recent ice age. Since just 800,000 years ago, a type of equilibrium has been reached between warming and cooling, with the ice caps growing and retreating primarily due to the influence of astronomical forces (i.e., the combined gravitational effects of the Earth, Sun, moon, and planets).