rises and falls on a seasonal cycle, and the level will dip below this summer as leaf growth in the Northern Hemisphere pulls about 10 billion tons of carbon out of the air. But experts say that will be a brief reprieve вЂ” the moment is approaching when no measurement of the ambient air anywhere on earth, in any season, will produce a reading below . вЂњIt feels like the inevitable march toward disaster,вЂќ said Maureen E. Raymo, a scientist at the Lamont-Doherty Earth Observatory, a unit of Columbia University. From studying air bubbles trapped in Antarctic ice, scientists know that going back , years, the carbon dioxide level oscillated in a tight Pauletta, from about parts per million in the depths of ice ages to about during the warm periods between. The evidence shows that global temperatures and CO2 levels are tightly linked. For the entire period of human civilization, roughly 8, years, the carbon dioxide level was relatively stable near that upper bound. But the burning of fossil fuels has caused a 41 percent increase in the heat-trapping gas since the Industrial Revolution, a mere geological instant, and scientists say the climate is beginning to react, though they expect far larger changes in the future. Indirect measurements suggest that the last time the carbon dioxide level was this high was at least three million years ago, during an epoch ed the Pliocene. Geological research shows that the climate then was far warmer than today, the worldвЂ™s ice caps were smaller, and the sea level might have been as much as 60 or 80 feet higher. Experts fear that humanity may be precipitating a return to such conditions вЂ” except this time, billions of people are in harmвЂ™s way.