By Jonathan Amos
Science reporter, BBC News, Norwich
Carbon dioxide levels are substantially higher now than at any time in the last 800,000 years, the latest study of ice drilled out of Antarctica confirms.
Epica drills have extracted ice from deep under the surface
The in-depth analysis of air bubbles trapped in a 3.2km-long core of frozen snow shows current greenhouse gas concentrations are unprecedented.
The East Antarctic core is the longest, deepest ice column yet extracted.
Project scientists say its contents indicate humans could be bringing about dangerous climate changes.
"My point would be that there's nothing in the ice core that gives us any cause for comfort," said Dr Eric Wolff from the British Antarctic Survey (BAS).
"There's nothing that suggests that the Earth will take care of the increase in carbon dioxide. The ice core suggests that the increase in carbon dioxide will definitely give us a climate change that will be dangerous," he told BBC News.
The Antarctic researcher was speaking here at the British Association's (BA) Science Festival.
Slice of history
The ice core comes from a region of the White Continent known as Dome Concordia (Dome C). It has been drilled out by the European Project for Ice Coring in Antarctica (Epica), a 10-country consortium.
The column's value to science is the tiny pockets of ancient air that were locked into its millennia of accumulating snowflakes.
Each slice of this now compacted snow records a moment in Earth history, giving researchers a direct measure of past environmental conditions.
Not only can scientists see past concentrations of carbon dioxide and methane - the two principal human-produced gases now blamed for global warming - in the slices, they can also gauge past temperatures from the samples.
This is done by analysing the presence of different types, or isotopes, of hydrogen atom that are found preferentially in precipitating water (snow) when temperatures are relatively warm.
Earlier results from the Epica core were published in 2004 and 2005, detailing the events back to 440,000 years and 650,000 years respectively. Scientists have now gone the full way through the column, back another 150,000 years.
The picture is the same: carbon dioxide and temperature rise and fall in step.
"Ice cores reveal the Earth's natural climate rhythm over the last 800,000 years. When carbon dioxide changed there was always an accompanying climate change. Over the last 200 years human activity has increased carbon dioxide to well outside the natural range," explained Dr Wolff.
Like tiny time capsules, bubbles trap ancient samples of atmosphere
The "scary thing", he added, was the rate of change now occurring in CO2 concentrations. In the core, the fastest increase seen was of the order of 30 parts per million (ppm) by volume over a period of roughly 1,000 years.
"The last 30 ppm of increase has occurred in just 17 years. We really are in the situation where we don't have an analogue in our records," he said.
The plan now is to try to extend the ice-core record even further back in time. Scientists think another location, near to a place known as Dome A (Dome Argus), could allow them to sample atmospheric gases up to a million and a half years ago.
Some of the increases in carbon dioxide will be alleviated by natural "sinks" on the land and in the oceans, such as the countless planktonic organisms that effectively pull carbon out of the atmosphere as they build skeletons and shell coverings.
But Dr Corinne Le Quéré, of the University of East Anglia and BAS, warned the festival that these sinks may become less efficient over time.
We could not rely on them to keep on buffering our emissions, she said.
"For example, we don't know what the effect will be of ocean acidification on marine ecosystems. There is potential for deterioration," she explained.
More CO2 absorbed by the oceans will raise their acidity, and a number of recent studies have concluded that this will eventually disrupt the ability of marine micro-organisms to use the calcium carbonate in the water to produce their hard parts.