One would have to reach back at least 15million years to find carbon-dioxide levels on Earth as high as they are today, according to US research which has its roots in a high-accuracy technique developed at Cambridge.
“The last time CO2 levels were apparently as high as they are today – and were sustained at those levels – global temperatures were five to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75-120ft higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland,” said Aradnha Tripati, of UCLA (University of California – Los Angeles), lead author of a paper just published in the respected journal, Science.
“Carbon dioxide is a potent greenhouse gas and geological observations that we now have for the last 20million years lend strong support to the idea that CO2 is an important agent for driving climate change throughout Earth’s history,” said the professor, who is with UCLA’s department of earth and space sciences and its department of atmospheric and oceanic sciences.
Through analysing the chemistry of bubbles of ancient air trapped in Antarctic ice, scientists have been able to determine with confidence the composition of Earth’s atmosphere going back 800,000 years.
Moreover, they have apparently developed a good understanding of how CO2 levels have varied in the atmosphere since that time. But there has been little agreement before the latest UCLA work on how to reconstruct CO2 concentrations prior to 800,000 years ago.
Before joining UCLA, Tripati was a member of a Cambridge team which developed a new technique to assess CO2 levels in the much more distant past – by studying the ratio of the chemical element, boron, to calcium in the shells of ancient single-celled marine algae.
She has since applied this method to determining the amount of CO2 in Earth’s atmosphere as far back as 20million years ago.
“We are able, for the first time, to accurately reproduce the ice-core record for the last 800,000 years – the record of atmospheric CO2 based on measurements of carbon dioxide in gas bubbles in ice,” Tripati said.
“This suggests that the technique we are using is valid. We then applied this technique to study the history of carbon dioxide from 800,000 years ago to 20million years ago.
“We report evidence for a very close coupling between carbon-dioxide levels and climate.
“When there is evidence for the growth of a large ice sheet on Antarctica or on Greenland, or the growth of sea ice in the Arctic Ocean, we see evidence for a dramatic change in carbon-dioxide levels over the last 20million years.
“A slightly shocking finding is that the only time in the last 20million years that we find evidence for CO2 levels similar to the modern level of 387 parts per million was 15-20million years ago, when the planet was dramatically different.”
She added that levels of carbon dioxide have varied only between 180 and 300 parts per million over the last 800,000 years – until recent decades.
It has been known that modern-day levels of carbon dioxide are unprecedented over the last 800,000 years, but the finding that modern levels have not been reached in the last 15million years is new. Prior to the Industrial Revolution of the late-19th and early-20th centuries, the carbon-dioxide level was about 280 parts per million, according to Tripati.
“That figure had changed very little over the previous 1,000 years. But since the Industrial Revolution, the carbon-dioxide level has been rising and is likely to soar unless action is taken to reverse the trend,” she said.
“During the Middle Miocene (the time period approximately 14-20million years ago), carbon-dioxide levels were sustained at about 400 parts per million, which is about where we are today.
“Globally, temperatures were five to 10 degrees Fahrenheit warmer – a huge amount.”
It is claimed that the new chemical technique has an average uncertainty rate of only 14 parts per million.
“We can now have confidence in making statements about how carbon dioxide has varied throughout history,” Tripati added.
In the last 20million years, key features of the climate record include the sudden appearance of ice on Antarctica about 14million years ago and a rise in sea level of about 75-120ft.
“We have shown that this dramatic rise in sea level is associated with an increase in carbon-dioxide levels of about 100 parts per million – a huge change,” Tripati said.
“This record is the first evidence that carbon dioxide may be linked with environmental changes, such as changes in the terrestrial ecosystem, distribution of ice, sea level and monsoon intensity.”
Today, the Arctic Ocean is covered with frozen ice all year long, an ice cap that has been there for about 14million years, though is now under serious threat, according to newly released data.
“Prior to that, there was no permanent sea ice cap in the Arctic,” Tripati said.
Some projections show carbon-dioxide levels rising as high as 600, or even 900, parts per million in the next century if no action is taken to reduce CO2, she added.
“Such levels may have been reached on Earth 50million years ago, or earlier,” said Tripati, who is working to push her data back much further than 20million years and to study the last 20million years in detail.
More than 50million years ago, there were no ice sheets on Earth, and there were expanded deserts in the subtropics, Tripati noted. The planet was radically different.
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