Paleocene-Eocene Thermal Maximum (PETM)
The Paleocene-Eocene Thermal Maximum (PETM) was a significant period of extreme global warming that occurred approximately fifty-six million years ago. This event is marked by a rapid increase in temperatures, attributed to large amounts of carbon dioxide and methane being released into the atmosphere and oceans, potentially from methane hydrates or volcanic activity. During the PETM, global temperatures rose by as much as sixteen degrees Fahrenheit, leading to severe environmental changes, including droughts and ocean acidification.
The event also prompted evolutionary adaptations as various species migrated to cooler climates or evolved smaller body sizes to better dissipate heat. Notably, this period saw the emergence of many ancestors of modern mammals, including early forms of cows, deer, and primates. Scientists today draw parallels between the PETM and contemporary climate change, noting that current carbon emissions are escalating at a faster rate than during the PETM. This situation raises concerns about the potential for a similar environmental crisis if modern greenhouse gas emissions continue unchecked, as they could trigger further warming and habitat shifts.
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Paleocene-Eocene Thermal Maximum (PETM)
The Paleocene-Eocene Thermal Maximum (PETM) refers to a period of an extreme rise in global temperatures that occurred on Earth about fifty-six million years ago. The warming was caused by the relatively sudden release of large amounts of carbon into the atmosphere and oceans. Paleontologists remain unsure as to the source of the increased carbon levels, but they do know the event caused worldwide temperatures to rise by as much as sixteen degrees Fahrenheit. The PETM led to droughts, acidic oceans, and evolutionary changes as life adapted to the warmer conditions. Scientists compare the event with the effects of modern climate change, although they caution that the amount of carbon in today's atmosphere is rising at a faster rate than during the PETM.
Background
Over the course of its four billion-year history, Earth has undergone numerous periods of global cooling and warming. At times, polar ice reached all the way to the equator, while in other eras, tropical conditions extended into the higher latitudes. The Paleocene epoch began about sixty-five million years ago with the extinction of the dinosaurs. By the Paleocene-Eocene Thermal Maximum, Earth was already in the middle of a warming period. The polar regions were mostly free of ice, and open land routes stretched from modern-day Europe into North America. The surface of Earth was also in transition and had just begun to resemble the familiar seven-continent formation that exists today.
Then, about fifty-six million years ago, massive amounts of carbon in the form of carbon dioxide and methane gases were vented into the planet's atmosphere and oceans. Carbon dioxide and methane are greenhouse gases, substances that act like a blanket in the atmosphere, trapping infrared radiation from the sun and causing temperatures to rise. Some evidence suggests this carbon release occurred in two separate phases, each only a few thousand years long—a very short period in geological terms.
Scientists believe that at its peak, the PETM deposited an estimated 2.2 billion tons of carbon into the atmosphere a year. Global temperatures rose quickly over a period of about six to twenty thousand years. Some regions of the planet saw average temperatures spike at least nine degrees Fahrenheit, while other estimates place the figure at sixteen degrees. Sea surface temperatures were estimated to have warmed by as much as eleven degrees Fahrenheit.
While scientists do not know the exact trigger of the PETM, they suspect it was caused by the release of methane hydrates from the ocean floor. Methane hydrates are methane gases trapped in a form of ice and contained in sediments where certain temperature and pressure conditions exist. An unknown event, such as volcanic eruptions, geological activity, or warmer ocean currents caused by the shifting continents, caused these methane hydrates to thaw and methane gas to be released. Once in the atmosphere, the methane would have acted as a powerful greenhouse gas and trapped the sun's heat. Over time, the methane would have reacted with oxygen to form carbon dioxide. The hotter temperatures caused by the additional gases in the atmosphere would have caused more methane hydrates to be released and accelerated the warming process. Other possible causes of the carbon release may have been large-scale volcanic activity or the decomposition of organic material on land. The PETM lasted for more than one hundred thousand years before carbon levels and temperatures gradually returned to normal.
Impact
The rising temperatures of the Paleocene-Eocene Thermal Maximum had a profound environmental impact on Earth. Geological evidence suggests that the planet's ocean circulation reversed, which would have brought warmer water to higher latitudes and added fuel to the warming process. The abundance of carbon dioxide would have also led to an increase in the acidic level of the oceans. Normally, the oceans absorb carbon dioxide from the atmosphere and from calcium on the sea floor. With increased levels of the gas in the atmosphere, however, the ocean would have absorbed an excess of carbon dioxide, raising the acid level of the water. This would have proved deadly to certain forms of marine microorganisms on the sea floor.
Other animals adapted to the changes brought about by drought and warmer temperatures by finding cooler climates. Marine life migrated toward the poles, while animals that had lived in the tropics, such as turtles, began moving northward into North America and Europe. Other creatures evolved to the changing conditions by developing smaller body sizes. Smaller animals can dissipate heat better, making it easier for them to cool down. They can also process the increased carbon dioxide in the atmosphere more easily. As a result, it was during this time that the ancestors of many of today's mammals first appeared—among them were early cows, deer, camels, horses, and primates. The PETM is also considered the end of the Paleocene and beginning of the Eocene epoch, a geological period that lasted until about forty million years ago.
Many scientists say the conditions that existed during the Paleocene-Eocene Thermal Maximum are repeating themselves in the modern era and can be seen in rising global temperatures. Greenhouse gases released by the burning of fossil fuels are believed to be trapping solar radiation and causing the planet's temperatures to increase. The rate of emissions into the atmosphere, however, is much higher than it was fifty-six million years ago. According to the weather forecasting service Weather Underground, modern carbon emissions total more than thirty-six billion tons each year. Some scientists are concerned that in addition to man-made greenhouse gases, the warmer climate may lead to the thawing and release of methane hydrates, just as they suspect happened during the PETM. That, in turn, could create another chain reaction of global warming, resulting in serious environmental impact.
Bibliography
Bender, Michael L. Paleoclimate. Princeton UP, 2013.
Bowen, Gabriel J., et al. "Two Massive, Rapid Releases of Carbon during the Onset of the Palaeocene–Eocene Thermal Maximum." Nature Geoscience, 15 Dec. 2014, www.nature.com/ngeo/journal/v8/n1/full/ngeo2316.html. Accessed 26 Oct. 2016.
Dai, Yuhao, Jimin Yu, and Xuan Ji. "Prolonged Deep-Ocean Carbonate Chemistry Recovery after the Paleocene-Eocene Thermal Maximum." Earth and Planetary Science Letters, vol. 620, 15 Oct. 2023, doi.org/10.1016/j.epsl.2023.118353. Accessed 15 Nov. 2024.
"Geologic Time Scale." University of California Museum of Paleontology, www.ucmp.berkeley.edu/help/timeform.php. Accessed 26 Oct. 2016.
Gornitz, Vivien. Rising Seas: Past, Present, Future. Columbia UP, 2013, 77–81.
Jardine, Phil. "Patterns in Palaeontology: The Paleocene–Eocene Thermal Maximum." Palaeontology Online, 10 Jan. 2011, www.palaeontologyonline.com/articles/2011/the-paleocene-eocene-thermal-maximum/. Accessed 26 Oct. 2016.
Kunzig, Robert. "World without Ice." National Geographic, Oct. 2011, ngm.nationalgeographic.com/2011/10/hothouse-earth/kunzig-text. Accessed 26 Oct. 2016.
Li, Mingsong, et al. "Astrochronology of the Paleocene-Eocene Thermal Maximum on the Atlantic Coastal Plain." Nature Communications, vol. 13, 2022, DOI: 10.1038/s41467-022-33390-x. Accessed 3 Jan. 2023.
Mathez, Edmond A. Climate Change: The Science of Global Warming and Our Energy Future. Columbia UP, 2009.
"PETM: Global Warming, Naturally." Weather Underground, www.wunderground.com/climate/PETM.asp?MR=1. Accessed 26 Oct. 2016.