Changes of Climatic Regimes in Earth’s History
Scientific investigations have revealed that 3.8 billion years ago, the Earth, and indeed the solar system was a completely different place from what it is today. The sun shined with less luminescence – as much as 30 percent weaker – which means the Earth should have been really cold. So cold, in fact, that liquid water could not have existed. Whereas this is an established scientific fact, the geologic record shows that water was, indeed, present and provided the foundation for the proverbial “primordial soup” that gave rise to life.
Many theories have been proposed to explain these facts, one of which suggests that the Earth’s reflectivity at that time was lower because of smaller continents, allowing more sunlight to be absorbed. But one of the leading theories examines the atmosphere of the Archaean period, specifically the presence of greenhouse gases like carbon dioxide and methane that might have warmed the atmosphere to temperatures at or above today’s.
The same greenhouse gases that, in abundance, are getting us into trouble today, may have been fundamental to the Earth’s life-creating conditions. As geochemist James Kasting of Penn State University points out, methane and carbon dioxide could have been abundant in the earth’s atmosphere during the first several hundred million years of its history because of degassing during the planet’s formation.
The concentrations of these greenhouse gases may have declined over time – methane converting to carbon dioxide, and carbon dioxide converting into carbonate rocks. But the storage of carbon in rocks would have slowed with dropping temperatures. Meanwhile, near continuous volcanic activity could have then provided the carbon dioxide boost to the atmosphere to spike temperatures back up again. The feedback loop, called the carbonate-silicate cycle, goes on even to this day.
Using fossils and many geologic clues, geologists have reconstructed Earth's climate going back hundreds of millions of years. Over long time scales (tens- to hundreds- of millions of years), Earth's climate can be broadly characterized as "greenhouse" or "icehouse" climates. During greenhouse times, there is little, if any, permanent ice on either pole. Warm temperate climates are found at high latitudes. During icehouse conditions, global climate is cool enough to support large ice sheets at one or both poles.
Earth's climate has transitioned between these two categories only a few times in the past 540 million years. The most recent transition occurred during the Cenozoic Era. We live on an "icehouse" Earth, one that is quite cold compared to other periods in Earth history.
Precambrian Climates (prior to 542 million years ago)
The Precambrian accounts for more than 80% of Earth's history and is the "Age of Early Life." Climate varied widely during different periods of the Precambrian. Shallow seas—like today's Caribbean—covered some of the early continents. During warm climates, mats, or mounds of algae called grew in those shallow seas. Shark Bay, Australia is famous for its modern stromatolites. Billion-year-old stromatolites are found in (Montana). During the later Precambrian, Earth's climate flip-flopped between very warm periods and very cold periods. The cold periods were so cold that extensive glaciers were found near the equator – a true "snowball" Earth!
Paleozoic Era Climates (542 to 251 million years ago)
Global climate was relatively cool during many periods of the "Age of Fishes." However, North America was located near the equator and experienced generally warm climates for much of the Paleozoic Era. Shallow seas advanced and retreated over vast areas of North America, depositing immense amounts of limestone and other marine sediments. These sediments contain a fantastic fossil record of evolving sea life from ubiquitous trilobites to vertebrates. Vertebrates first evolved in the oceans. Land plants and animals first appeared about 350 million years ago. Visitors can walk through the limestone layers and among Paleozoic fossils in many NPS cave parks including (Kentucky), (South Dakota), and (South Dakota). Many other fossil parks preserve fossils from the Paleozoic Era.
Mesozoic Era Climates (251 to 65.5 million years ago)
Global climates much warmer than today existed during most of the Mesozoic Era – the "Age of Reptiles". Carbon dioxide was likely many times higher than today contributing to a "greenhouse" planet. During the early part of the Mesozoic, all of Earth's continents were assembled into a supercontinent called Pangaea. Pangaea began to break apart during the Mesozoic and the continents began moving towards their present locations. The last of North America's shallow inland seas covered the continent's interior during the Cretaceous Period of the Mesozoic Era. Lush forests, including newly-evolved flowering plants (angiosperms) blanketed much of North America. (Utah, Colorado) and (Arizona) were established to tell fossil stories from the Mesozoic, but there are rich fossil records of that warm world in many places.
Cenozoic Era Climates (the past 65.5 million years)
The Cenozoic Era – the "Age of Mammals" – was a time of climatic transition. The early Cenozoic Era was a time of "greenhouse" climates like those the dinosaurs experienced. By 34 million years ago, permanent ice sheets were present at the South Pole ushering in "icehouse" conditions. Climate warmed during the Miocene (about 20 million years ago) as mammal populations reached their greatest diversity. Climate then cooled again. In North America, the lush "greenhouse" fossil forests (there were palm trees in Wyoming and banana trees in Oregon) were replaced by open grasslands. Grassland ecosystems are better suited for a cooler, drier "icehouse" climate. By 2 million years ago, Earth's climate was cold enough to support large ice sheets on both poles that were poised to advance and retreat during the "ice ages." Ice advanced to Chicago and would subsequently retreat to Greenland. For the past 800,000 years, this cycle occurred about every 100,000 years. The last great ice advance was about 20,000 years ago. Six national parks were established to tell the fossil stories of the Cenozoic Era: (Nebraska), (South Dakota), (Colorado), (Wyoming), (Idaho), and (Oregon). Numerous other parks preserve significant Cenozoic fossils.
Landscapes and life also changed
Climate was not the only thing changing over the past 4.5 billion years of Earth history. The location and configuration of the continents, mountain ranges, and oceans was also changing. Life was changing and evolving. Climate, landforms and life, are the three major components of ecosystems. All living things are adapted to a particular ecosystem "comfort zone" regarding the land they live on, the climate they experience, and the other living things (communities) they interact with. As landforms change, climates change, and the communities change, animals and plants are forced to migrate to more favorable conditions, adapt to the changing conditions, or go extinct. Fossils record these changes over time and provide clues to understanding .
1. A History of Climate Change: http://www.astrobio.net/topic/solar-system/earth/climate/a-history-of-climate-change/
2. Climate has Changed Throughout Earth's History: http://nature.nps.gov/geology/nationalfossilday/climate_change_earth_history.cfm
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