Global warming was ‘more common 50m yrs ago’Published On: Thu, Mar 17th, 2011 | Climate Change | By BioNews
Periods of intense global warming lasting tens of thousands of years occurred more frequently in the past than previously believed, according to a new study.
A team, led by the Scripps Institution of Oceanography at the University of California, San Diego, said that releases of carbon dioxide sequestered in the deep oceans were the most likely trigger of these ancient ‘hyperthermal’ events.
The hyperthermals took place roughly every 400,000 years during a warm period of Earth”s history that prevailed some 50 million years ago and lasted about 40,000 years before temperatures returned to normal.
The majority of these hyperthermals increased average global temperatures by 3.6 to 5.4 degrees Fahrenheit.
“These hyperthermals seem not to have been rare events. There are lots of ancient examples of global warming on a scale broadly like the expected future warming. We can use these events to examine the impact of global change on marine ecosystems, climate and ocean circulation,” said Richard Norris, a Scripps geology professor who co-authored the study.
The strongest of them coincided with an event known as the Paleocene-Eocene Thermal Maximum, the transition between two geologic epochs in which global temperatures rose between 7.2 and 12.6 degrees Fahrenheit and needed 200,000 years to return to historical norms.
The events stopped taking place around 40 million years ago, when the planet entered a cooling phase. No warming events of the magnitude of these hyperthermals have been detected in the geological record since then.
Norris and lead author Phil Sexton, now at the Open University in the UK, analysed 50-million-year-old ocean sediment cores gathered off the South American coast.
In the cores, evidence of the warm periods presented itself in bands of grey sediment layered within otherwise pale greenish mud.
The grey sediment contained increased amounts of clay left after the calcareous shells of microscopic organisms were dissolved on the sea floor.
These clay-rich intervals are consistent with ocean acidification episodes that would have been triggered by large-scale releases of carbon dioxide.
The authors concluded that a release of carbon dioxide from the deep oceans was a more likely cause of the hyperthermals than other triggering events that have been hypothesized.
Norris noted that the hyperthermals provide historical perspective on what Earth will experience as it continues to warm from widespread use of fossil fuels, which has increased carbon dioxide concentrations in the atmosphere nearly 50 percent since the beginning of the Industrial Revolution.
The study appears in the journal Nature.