August 6, 2013
Greenhouse World: Are Our Oceans Headed For The Same Fate?
April Flowers for redOrbit.com - Your Universe Online
A new study, led by Scripps Institution of Oceanography, reveals the look of the oceans will change drastically in the future as the coming greenhouse world alters marine food webs and gives certain species advantages over others, if history's closest analog is any indication.Richard Norris, paleobiologist at Scripps Institution of Oceanography, UC San Diego, worked with an international team of scientists to show the ancient greenhouse world had few large reefs, a poorly oxygenated ocean, tropical surface waters like a hot tub, and food webs that did not sustain the abundance of large sharks, whales, seabirds, and seals of the modern ocean. If greenhouse gases continue to rise at the current accelerating rates, aspects of this greenhouse ocean could reappear.
The study, published in Science, is based on what is known about the "greenhouse world" of 50 million years ago. During this time, the levels of greenhouse gases were much higher than those that have been present at any point during human history.
Atmospheric CO2 concentrations have never exceeded 280 parts per million (ppm) during the past million years. Industrialization, forest clearing, agriculture, and other human activities, however, have caused rapid increases in CO2 and other gases known to create a "greenhouse" effect that traps heat in the atmosphere. For the first time in human history, CO2 levels exceeded 400 ppm in May 2013, although those numbers have since been revised. In the coming decades, that milestone could be left well behind. The researchers say at its current pace, Earth could recreate the CO2 content of the atmosphere in the greenhouse world in just 80 years.
Fossils from the greenhouse world period indicate CO2 concentrations reached 800-1,000 parts per million during that time. Other extreme measurements include tropical ocean temperatures reaching 95 degrees F, and polar oceans that were around 50 degrees F, similar to the ocean temperatures off the coast of San Francisco currently. The greenhouse world had no polar ice sheets. Between 42 and 57 million years ago, scientists have identified a "reef gap" in which complex coral reefs largely disappeared and the seabed was dominated by piles of pebble-like single-celled organisms called foraminifera.
"The 'rainforests-of-the-sea' reefs were replaced by the 'gravel parking lots' of the greenhouse world," said Norris.
Differences in the ocean food web with large parts of the tropical and subtropical ocean ecosystems supported by minute picoplankton instead of the larger diatoms typically found in highly productive ecosystems today marked the greenhouse world as well. In the cold oceans of recent geologic times, large marine animals such as sharks, tuna, whales, seals and even seabirds became abundant when algae became large enough to support top predators.
"The tiny algae of the greenhouse world were just too small to support big animals," said Norris. "It's like trying to keep lions happy on mice instead of antelope; lions can't get by on only tiny snacks."
Rapid warming events within the greenhouse world resemble our projected future. One such event, the well-studied Paleocene-Eocene Thermal Maximum (PETM), happened 56 million years ago and serves as a guide to predicting what may happen under current climate trends.
The PETM lasted approximately 200,000 years and warmed the planet by 9-16 degrees F, causing massive migrations of animals and plants and shifts in climate zones. The extinction of species was remarkably light, other than a mass extinction in the rapidly warming deep ocean, despite the disruption to the planet's ecosystems.
"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," said Norris. "Unfortunately, the PETM also shows that ecological disruption can last tens of thousands of years."
Continuing the current fossil fuel economy even for decades magnifies the period of climate instability, Norris added. Abruptly halting fossil fuel use at current levels could limit future climate instability to less than 1,000 years before Earth's climate would largely return to pre-industrial norms. If, however, our fossil fuel consumption remains on its current trajectory until the end of this century, projections show the climate effects beginning to resemble those of the PETM. Major ecological changes could last 20,000 years or more, with a recognizable human "fingerprint" on Earth's climate lasting for 100,000 years.