Analogues from Earth's Past Bode Ill for Coral Reefs, Marine Ecosystems

In a bid to anticipate the effects of a warming world, climate scientists around the world are scouring the geological record for modern-day analogues – periods of Earth history when the concentration of carbon dioxide (CO2) in the atmosphere reached or exceeded the 400 parts per million (ppm) we find today.

Marine ecosystems in danger in a greenhouse world

Delving into climate, marine biology, ecosystems and the marine ecology of 42 million to 57 million years ago – encompassing the so-called Paleocene-Eocene Thermal Maximum (PETM) – paleobiologists and colleagues at UC San Diego's Scripps Institution of Oceanography conclude that the human population just 80 years hence may be living in a “greenhouse world,” dependent, in part, on vastly different marine food webs.

Life in a "Greenhouse World"

The level of CO2 in the atmosphere – the primary contributor to the Greenhouse Effect – hasn't exceeded 280 ppm throughout human history – up until modern times that is. Having exceeded 400 ppm for the first time in human history several times this May, annual global greenhouse gas emissions continue to increase at ever greater rates despite best efforts to contain and reduce them.

Reporting in the August 2 special edition of Science, Scripps researchers found indications that atmospheric CO2 concentrations between 42 million and 57 million years ago reached 800-1,000 ppm. Tropical ocean temperatures, moreover, were comparable to that of a hot tub (35º C, 95º F), polar ocean temperatures were similar to those of San Francisco Bay today (12ºC, 53º F), and there were no polar ice sheets.

Moreover, marine “food webs did not sustain the abundance of large sharks, whales, seabirds, and seals of the modern ocean,” Scripps News reports. Coral reefs – the “rainforests of the sea” – largely disappeared. Instead, the researchers found the seabed was dominated by accumulations of the tiny, microscopic shells of foramenifera akin to “gravel parking lots.”

The Scripp's research team project that humans may be living in such a “greenhouse world” in only 80 years, dependent on vastly changed, less rich and less productive marine food webs. Larger diatoms and other plankton typically support the highly productive marine ecosystems and food webs that help us, as well as large marine animals survive and thrive today. The base of the “greenhouse ocean” marine food web of 50 million years ago, in contrast, was characterized by much smaller picoplankton.

“The tiny algae of the greenhouse world were just too small to support big animals,” Scripps Institution, UC San Diego paleobiologist Richard Norris was quoted as saying. “It’s like trying to keep lions happy on mice instead of antelope; lions can’t get by on only tiny snacks.”

Troubling portents of climate change

Rapid warming events similar to those projected today, such as the PETM, occurred during this period of Earth history can serve as indicators for what we can expect should concentrations of atmospheric carbon and other greenhouse gases continue to increase, climate scientists say. Global mean temperatures rose 5-9º C (9-16º F) during the PETM, causing dramatic changes in ecosystems and their productivity, including “massive migrations of animals and plants and shifts in climate zones.”

“Notably, despite the disruption to the Earth's ecosystems, the extinction of species was remarkably light, other than a mass extinction in the rapidly warming ocean,” according to Scripps News' report.

“In many respects the PETM warmed the world more than we project for future climate change, so it should come as some comfort that extinctions were mostly limited to the deep sea,” Norris was quoted. “Unfortunately, the PETM also shows that ecological disruption can last tens of thousands of years.”

In another recently released study, scientists at the University of Colorado, Boulder's Arctic and Alpine Research Center found similarly troubling portents of ecosystems disruption in the Arctic. Sea levels during the Pliocene some 3 million to 5 million years ago – the most recent period of geologic time scientists believe atmospheric CO2 concentrations reached 400 ppm – global mean temperatures were some 3-5ºF (2-5ºC) warmer. What is now Arctic tundra was covered in forest and sea levels were some 65-80 feet (20-24m) higher.

What can be done to avert such relatively rapid and drastic change?

“An abrupt halt to fossil fuel use at current levels would limit the period of future climate instability to less than 1,000 years before climate largely returns to pre-industrial norms,” Scripps News' paraphrased Norris.

Continuing to burn fossil fuels at our current rate in this and coming decades “magnifies the period of climate instability” and bring about a period of major ecological change stretching out some 20,000 years or more and lasting for 100,000 years, according to the report authors, which included researchers from Yale and the UK's University of Bristol.

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