During the mid-Cretaceous period, 90 to 120 million years ago, the Earth was much warmer than today. Often referred to as Greenhouse World, the Earth's warmth extended even to the high (polar) latitudes. This evidence is supported by the abundance of fossil records of plants and animals at polar locations that are found only in warm environments. For instance, corals were discovered far from the equator. Warm-water animals and plants have also been found in polar locations. Scientists have determined that breadfruit trees, a species of tropical vegetation native to the Malay Peninsula and western Pacific islands, grew even in Greenland.
In scientists' efforts to reconstruct past continental positions and ocean currents, they have determined that the Earth's geography and ocean currents were different during the Hothouse Earth period. They have also determined that CO2 levels were much higher—up to four times higher than today. This Hothouse Earth followed the Snowball Earth conditions mentioned previously, so the shift was due to both the rising atmospheric CO2 concentration and the melting of the vast ice sheets.
According to geologists at the University of North Carolina, this period of intense warming triggered the greatest mass extinction in the Earth's history about 250 million years ago, as opposed to the theory that the mass extinction was caused when a meteor hit the Earth. Instead, they claim that volcanic eruptions and global warming were the causes of the mass extinctions during the Permian. As a result of their research, they have determined that the climate turned exceedingly hot because
Following Snowball Earth, the Earth warmed up dramatically and became much warmer than it is today during the Mid-Cretaceous period 90 to 120 million years ago. Vegetation even grew in the polar regions. (Nature's Images)
of a large increase in volcanic activity that released huge amounts of CO2 and methane into the atmosphere, thereby causing rapid global warming. Also harmful to life on the planet was the enormous amount of hydrogen sulphide that entered the atmosphere, which damaged the ozone layer and killed the majority of life-forms.
According to Christopher Poulsen, a paleoclimatologist at the University of Michigan, as the ice melted from the ice-covered areas and the atmospheric CO2 concentration rose, the Tropics became much more arid, the vegetation became stressed, and the ecosystem was replaced by desert. One of the most important things he learned from his research is that the Tropics are very susceptible to large climate changes.
Another episode occurred 55 million years ago when methane was released from wetlands and turned the Earth into another hothouse. Methane is a very powerful greenhouse gas; it is 23 times more effective
global warming trends than CO2. This event is referred to as the Paleocene-Eocene Thermal Maximum (PETM). During this time, the Earth's surface warmed 9°F (5.4°C) in a few hundred years. The Earth warmed up so much that the Arctic Ocean even reached a temperature of 73°F (23°C). Sea surface temperatures rose 8.3-13°F (5-8°C). Scientists support the theory that the entire ocean depths heated up, not just the surface. In addition, the chemical composition had also changed and become harmful. Oxygen content was drastically reduced, causing major die-offs of deep-sea foraminifera.
Climatologists consider this period one of the most significant examples of Earth's sudden global climate change. The period is an example of what an extreme, rapid global warming event causes. It also coincides with a major extinction of both ocean and land species. This major global warming period lasted for about 200,000 years. Life on land was replaced with animals that could survive the extreme temperatures. These animals were mainly smaller versions of the mammal groups that exist today.
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