If history's closest analog（模拟） is any indication, 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. Scripps Institution of Oceanography, UC San Diego, paleobiologist Richard Norris and colleagues show that 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. Aspects of this greenhouse ocean could reappear in the future if greenhouse gases continue to rise at current accelerating rates.

The researchers base their projections on what is known about the "greenhouse world" of 50 million years ago when levels of greenhouse gases in the atmosphere were much higher than those that have been present during human history. Their review article appears in an Aug. 2 special edition of the journal Science titled "Natural Systems in Changing Climates."

For the past million years, atmospheric CO2 concentrations have never exceeded 280 parts per million, but industrialization, forest clearing, agriculture, and other human activities have rapidly increased concentrations of CO2 and other gases known to create a "greenhouse" effect that traps heat in the atmosphere. For several days in May 2013, CO2 levels exceeded 400 parts per million for the first time in human history and that milestone could be left well behind in the next decades. At its current pace, Earth could recreate the CO2 content of the atmosphere in the greenhouse world in just 80 years.

In the greenhouse world, fossils indicate that CO2 concentrations reached 800-1,000 parts per million. Tropical ocean temperatures reached 35º C (95º F), and the polar oceans reached 12°C (53°F) -- similar to current ocean temperatures offshore San Francisco. There were no polar ice sheets. Scientists have identified a "reef gap" between 42 and 57 million years ago in which complex coral reefs largely disappeared and the seabed was dominated by piles of pebble-like single-celled organisms called foraminifera（有孔虫类）.

"The 'rainforests-of-the-sea' reefs were replaced by the 'gravel parking lots' of the greenhouse world," said Norris.

The greenhouse world was also marked by 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. Indeed, large marine animals -- sharks, tunas, whales, seals, even seabirds -- mostly became abundant when algae became large enough to support top predators in the cold oceans of recent geologic（地质的） times.

"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."