Frédéric Parrenin

A section of an Antarctic ice core shown under polarized light reveals the individual ice crystals.

By Tia Ghose, LiveScience

Rising carbon dioxide levels may have caused Antarctic warming in the past, new research strongly suggests.

The findings, published Thursday in the journal Science, just add to the body of evidence that human-caused greenhouse gas emissions will lead to climate change.

"It's new evidence from the past of the strong role of CO2 [carbon dioxide] in climate variation," said study co-author Frédéric Parrenin, a climate scientist at the CNRS in France.

Past data
Eons of the Earth's climate history are revealed deep within ice sheets in the Arctic and Antarctic. The Antarctic ice traps gas bubbles from the climate that can reveal what the ancient atmosphere looked like, while the ice itself can reveal historical temperatures.

But gas bubbles from a given period get buried deeper than ice of the same period, making it hard to tie past temperatures with atmospheric changes.

In the past, scientists using older techniques found that increases in carbon dioxide happened after global warming, not the reverse. [Images of Melt: Earth's Vanishing Ice]

Past link
But Parrenin and his colleagues wondered whether that was actually the case. To answer that question, the team looked at five ice cores that had been drilled from Antarctica over the last 30 years.

They focused on ice from 20,000 to 10,000 years ago, which encompassed the last period when the planet warmed naturally and glaciers melted.

The team measured the concentration of nitrogen-15 isotopes, or atoms of the same element with different weights, at different depths throughout the ice cores. They compared the depth of that isotope with the ice composition for all the cores to determine the distance between ice bubbles and ice from the same period.

Global warming
The team found that global warming and a carbon dioxide increase happened at virtually the same time — between 18,000 and 11,000 years ago.

"It makes it possible that CO2 was the cause — at least partly — of the temperature increase during the courses of the last glaciation," Parrenin told LiveScience.

And if increased carbon dioxide could lead to rising temperatures in the past, it also can in the present day, he said.

The findings may deflate some climate skeptics, who used the poor dating of ice cores to question the link between carbon dioxide and warming, said Robert Mulvaney, a glaciologist with the British Antarctic Survey, who was not involved in the study.

It also confirmed the view of most climate scientists that in the past, rising temperatures and carbon dioxide were locked in a feedback loop, where high temperatures led to more carbon dioxide being released from the deep oceans, which increased temperatures further, Mulvaney said.

But because predictions of future warming are based on recent carbon dioxide and temperature data, not historical models, "it hasn't really changed anything about our understanding of how climate change will change our modern environment." Mulvaney told LiveScience.

Follow Tia Ghose on Twitter @tiaghoseor LiveScience @livescience. We're also on Facebook & Google+. 

• Image Gallery: One-of-a-Kind Places on Earth
• The Reality of Climate Change: 10 Myths Busted
• 8 Ways Global Warming Is Already Changing the World

Copyright 2013 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

By Alister Doyle
Reuters

The Amazon rain forest is less vulnerable to die off because of global warming than widely believed because the greenhouse gas carbon dioxide also acts as an airborne fertilizer, a study showed on Wednesday.

The boost to growth from CO2, the main gas from burning fossil fuels blamed for causing climate change, was likely to exceed damaging effects of rising temperatures this century such as drought, it said.

"I am no longer so worried about a catastrophic die-back due to CO2-induced climate change," Professor Peter Cox of the University of Exeter in England told Reuters of the study he led in the journal Nature. "In that sense it's good news."

Cox was also the main author of a much-quoted study in 2000 that projected that the Amazon rain forest might dry out from about 2050 and die off because of warming. Others have since suggested fires could transform much the forest into savannah.

Plants soak up carbon dioxide from the atmosphere and use it as an ingredient to grow leaves, branches and roots. Stored carbon gets released back to the atmosphere when plants rot or are burnt.

A retreat of the Amazon forests, releasing vast stores of carbon, could in turn aggravate global warming that is projected to cause more floods, more powerful storms and raise world sea levels by melting ice sheets.

"CO2 fertilization will beat the negative effect of climate change so that forests will continue to accumulate carbon throughout the 21st century," Cox said of the findings with other British-based researchers.

Root and branch
The scientists said the study was a step forward because it used models comparing forest growth with variations in the rising levels of carbon dioxide in the atmosphere.

It estimated that the damaging effects of warming would cause the release of 53 billion tons of carbon stored in lands throughout the tropics, much of it in the Amazon, for every single degree Celsius (1.8F) of temperature rise.

The benefits of CO2 fertilization exceeded those losses in most scenarios, which ranged up to a 319 billion ton net gain of stored carbon over the 21st century. About 500 to 1,000 billion tons of carbon are stored in land in the tropics.

Climate change would be more damaging for the Amazon if greenhouse gases other than CO2, such as ozone or methane which do not have a fertilizing effect, take a bigger role, the study said.

It did not factor in damaging effects from deforestation, mostly burning to clear land for farms, that is blamed for perhaps 17 percent of world greenhouse gas emissions from human activities.

Brazil has sharply reduced forest losses in recent years. But predictions of a die-back in coming decades had led some people to conclude that there was no point safeguarding trees.

"Some people argued bizarrely that it would be better to chop them down and use them now," Cox said, adding that the new findings meant that reasoning was no longer valid.

By underlining the importance of trees for soaking up CO2, the study could also bolster slow-paced efforts to create a market mechanism to reward nations for preserving tropical forests as part of U.N. negotiations on a new treaty to slow climate change, due to be agreed by the end of 2015.