Bubbles of air in the 800,000-year-old ice, drilled in the Antarctic, show levels of carbon dioxide (CO2) changing with the climate. But the present levels are out of the previous range.
On Monday, Eric Wolff, leader of the science team for the European Project for Ice Coring in Antarctica, said: "It is from air bubbles that we know for sure that carbon dioxide has increased by about 35% in the last 200 years."
Wolff, speaking at the British Association Festival of Science in the UK, said: "Before the last 200 years, which man has been influencing, it was pretty steady.
"The natural level of carbon dioxide for most of the past 800,000 years has been 180-300 parts per million by volume (ppmv) of air. But today it is at 380 ppmv.
"The most scary thing is that carbon dioxide today is not just out of the range of what happened in the last 650,000 years but already 100% out of the range."
Carbon dioxide was at levels of 280 ppmv from 1000 CE until 1800 CE, before accelerating towards its present concentration.
Wolff added that measurements of carbon isotopes indicated that the extra carbon dioxide is coming from a fossil source, due to increased human activity.
"There is an urgent need to find innovative technologies to reduce the impact we are having on our climate"
Professor Peter Smith,
University of Nottingham,
The ice core record showed that it used to take about 1,000 years for a carbon dioxide increase of 30 ppmv.
It has risen by that much in the past 17 years alone.
"We really are in a situation where something is happening that we don't have any analogue for in our records. It is an experiment that we don't know the result of," he said.
Peter Smith, of the University of Nottingham in England, said the study showed that more is needed to be done to combat the rising levels.
"There is an urgent need to find innovative technologies to reduce the impact we are having on our climate," he told the science conference.