Global warming may have triggered worst mass extinction
mongabay.com, National Center for Atmospheric Research release
August 29, 2005
SUMMARY: A dramatic rise in carbon dioxide 250 million years ago may have caused global temperatures to soar and result in Earth's greatest mass extinction, according to a study published in the September issue of Geology. Global warming, which may have produced temperatures 10 to 30 degrees Celsius higher than today, would have had a significant impact both on oceans, where about 95% of lifeforms became extinct, and on land, where almost 75% of species died out.
the Permian extinction. It is based on a computer simulation generated by the Community
Climate System Model at NCAR. (Illustration courtesy Jeff Kiehl, NCAR.)
Climate Model Links Higher Temperatures to Prehistoric Extinction
August 24, 2005
Original release from the National Center for Atmospheric Research
"The results demonstrate how rapidly rising temperatures in the atmosphere can affect ocean circulation, cutting off oxygen to lower depths and extinguishing most life," says NCAR scientist Jeffrey Kiehl, the lead author.
Kiehl and coauthor Christine Shields focused on the dramatic events at the end of the Permian Era, when an estimated 90 to 95% of all marine species, as well as about 70% of all terrestrial species, became extinct. At the time of the event, higher-latitude temperatures were 18 to 54 degrees Fahrenheit (10 to 30 degrees Celsius) higher than today, and extensive volcanic activity had released large amounts of carbon dioxide and sulfur dioxide into the atmosphere over a 700,000-year period.
The CCSM indicated that ocean waters warmed significantly at higher latitudes because of rising atmospheric levels of carbon dioxide (CO2), a greenhouse gas. The warming reached a depth of about 10,000 feet (4,000 meters), interfering with the normal circulation process in which colder surface water descends, taking oxygen and nutrients deep into the ocean.
As a result, ocean waters became stratified with little oxygen, a condition that proved deadly to marine life. This in turn accelerated the warming, since marine organisms were no longer removing carbon dioxide from the atmosphere.
"The implication of our study is that elevated CO2 is sufficient to lead to inhospitable conditions for marine life and excessively high temperatures over land would contribute to the demise of terrestrial life," the authors concluded in the article.
The CCSM's simulations showed that ocean circulation was even more stagnant than previously thought. In addition, the research demonstrated the extent to which computer models can successfully simulate past climate events. The CCSM appeared to correctly capture key details of the late Permian, including increased ocean salinity and sea surface temperatures in the high latitudes that paleontologists believe were 14 degrees Fahrenheit (8 degrees Celsius) higher than present.
The modeling presented unique challenges because of limited data and significant geographic differences between the Permian and present-day Earth. The researchers had to estimate such variables as the chemical composition of the atmosphere, the amount of sunlight reflected by Earth's surface back into the atmosphere, and the movement of heat and salinity in the oceans at a time when all the continents were consolidated into the giant land mass known as Pangaea.
"These results demonstrate the importance of treating Earth's climate as a system involving physical, chemical , and biological processes in the atmosphere, oceans, and land surface, all acting in an interactive manner," says Jay Fein, director of NSF's climate dynamics program, which funded the research. "Other studies have reached similar conclusions. What's new here is the application of a detailed version of one of the world's premier climate system models, the CCSM, to understand how rising levels of atmospheric carbon dioxide affected conditions in the world's oceans and land surfaces enough to trigger a massive extinction hundreds of millions of years ago."
Original release from the National Center for Atmospheric Research
Bankground information on the Permian-Triassic extinction event
From Wikipedia, the free encyclopedia.
The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 252 million years ago (mya), forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct. For some time after the event, fungal species were the dominant form of terrestrial life.
At one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 MYA (both numbers ±300,000 years), a very brief period of time in geological terms. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was a sudden event, not a gradual change. New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 million years before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.
Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.
The P-T boundary was marked with many volcanic eruptions. In the Siberian Traps, now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic effect on Earth, lasted for millions of years.
The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life. The theory is debated if volcanic activity, over such a long time, could alter the climate enough to kill off 95% of life on Earth. Volcanic activity affects the concentration of atmospheric gases directly, and, indirectly, the oceanic dissolved gases. Increases in carbon dioxide enhance the greenhouse effect and cause global warming, which would reduce the temperature gradient between the equator and the poles. As a result, thermo-haline circulation would slow and eventually stop. The oceans would stagnate, and nutrients would fail to disperse themselves. Many marine ecosystems rely on upwelling and circulation of nutrients, oxygen included; without the regular circulation, organisms would starve or suffocate. In addition, sulfur and particulates contribute to cooling, or volcanic winter, which usually lasts three to six months. Combinations of the two effects could produce a cooling cycle in which the climate alternatively warms then cools. Such temperature fluctuations could cause convective overturn of the oceans, bringing anoxic bottom waters to the surface; in an already oxygen-deprived environment, this would be fatal to many forms of life.
Significant evidence supports this theory. Fluctuations in air and water temperature are evident in the fossil record, and the uranium/thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the Siberian Traps -- the largest known eruption in the history of Earth -- occurred at this time, while no direct evidence of bolide impact has been located.
Atmospheric hydrogen sulfide buildup
In 2005, Dr. Lee R. Kump, a geoscientist from Pennsylvania State University published a theory explaining a cascade of events leading to the Great Extinction. Several massive volcanic eruptions in Siberian Traps, described above, started warming of the atmosphere. The warming itself did not seem to be large enough to cause so massive extinction event. However, it could have interfered with the ocean flow.
Cold water at the poles dissolves atmospheric oxygen, cools even more, and sinks to the bottom, slowly moving to the equator, carrying the dissolved oxygen. The warmer the water is, the less oxygen it can dissolve and the slower it circulates.
The resulting lack of supply of dissolved oxygen would lead to depletion of aerobic marine life. The oceans would then become a realm of bacteria metabolizing sulfates, and producing hydrogen sulfide, which would then get released into the water and the atmosphere, killing oceanic plants and terrestrial life. Once such process gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide.
Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. Increase of concentration of carbon dioxide would not cause extinction of plants, but according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages ozone layer, and fossil spores from the end-Permian era shown deformities that could have been caused by ultraviolet radiation.
Dr. Kump and his colleagues are now looking for biomarkers, indicating presence of green sulfur bacteria in the ocean sediments. Such bacteria indicate lack of oxygen in combination with available sunlight. Such biomarkers were recently found in appropriately dated shallow water sediments by Kliti Grace and her colleagues from Curtin University of Technology, Australia.
Methane hydrate gasification
In 2002, a BBC2 'Horizon' documentary, 'The Day the Earth Almost Died' summarized some recent findings and speculation concerning the Permian extinction event. Paul Wignall examined Permian strata in Greenland, where the rock layers devoid of marine life are tens of meters thick. With such an expanded scale, he could judge the timing of deposition more accurately and ascertained that the entire extinction lasted merely 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost total extinction of marine life. Such a process seemed too long, however, to be accounted for by a meteorite strike. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation for such a spike rotting vegetation seemed insufficient.
Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released by upwellings of frozen methane hydrate from the seabeds. Experiments to assess how large a rise in deep sea temperature would be required to gasify solid methane hydrate suggested that a rise of 5°C would be sufficient. Released from the pressures of the ocean depths, methane hydrate expands to create huge volumes of methane gas, one of the most powerful of the greenhouse gases. The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth.
Sudden release of methane hydrate has also been hypothesized as a cause of the Paleocene-Eocene Thermal Maximum extinction event.
The Permian extinction is unequalled; it is obviously not easy to destroy almost all life on Earth. The difficulty in imagining a single cause of such an event has led to an explanation humorously termed the "Murder on the Orient Express" theory: they all did it. A combination involving some or all of the following is postulated: Continental drift created a non-fatal but precariously balanced global environment, a supernova weakened the ozone layer, and then a large meteor impact triggered the eruption of the Siberian Traps. The resultant global warming eventually was enough to melt the methane hydrate deposits on continental shelves of the world-ocean.
There is no way to calculate the odds of some such combination occurring, but for it to have occurred once in the four billion year history of Earth is not unbelievable.