Frequent periods of intense global warming took place in the distant past, say researchers at UC San Diego's Scripps Institution of Oceanography.
Professor of geology Richard Norris said that these 'hyperthermal' events were probably triggered by releases of carbon dioxide sequestered in the deep oceans. Most raised average global temperatures between 3.6 and 5.4 degrees Farenheit - comparable to conservative estimates of temperature rises over the next few years from manmade global warming.
Most hyperthermals lasted about 40,000 years before temperatures returned to normal. They took place roughly every 400,000 years during a warm period of Earth history around 50 million years ago.
The strongest coincided with the Paleocene-Eocene Thermal Maximum, in which global temperatures rose between 7.2 and 12.6 degrees Farenheit. It took 200,000 years to return to normal.
The events stopped taking place around 40 million years ago, when the planet entered a cooling phase. Nothing similar's been detected in the geological record since then.
The team examined sediment cores from off the South American coast. Warm periods showed up as bands of gray sediment layered within pale greenish mud. The sediment contained increased amounts of clay from the calcareous shells of microscopic organisms - consistent with ocean acidification triggered by large-scale releases of carbon dioxide.
The carbon dioxide could have built upin the deep oceans because of slowing or stopping of circulation in ocean basins that prevented carbon dioxide release.
Norris says the hyperthermals give a clue as to the future effects of the widespread use of fossil fuels, which has increased carbon dioxide in the atmosphere by nearly 50 percent since the beginning of the Industrial Revolution.
The research could help produce a range of estimates for how long it will take for temperatures to fully revert to historical norms depending on how much warming human activities cause.