He notes that during the 10,000 years from the end of the last ice age to the beginning of the current “interglacial” climate, atmospheric CO2 concentrations rose 40 percent, from 180 molecules per million in the atmosphere to 260 parts per million. During the past 100 years, concentrations have risen 34 percent, from 292 ppm to 392 ppm – and continue to rise.
“Clearly, it's not a small amount,” says Dr. Shakun, referring to the increases during the past century. “Rising CO2 at the end of the last ice age had a huge effect on global climate. We've raised it as much in the last century.”
That doesn't mean the full impact of these increases will appear during the course of this century, he explains. It takes much longer for the climate system to fully respond. The oceans are intercepting much of the current warming and additional CO2 humans have added to the atmosphere through burning fossil fuels and land-use changes. And Earth still hosts enormous ice sheets in Greenland and over Antarctica to keep things relatively cool.
“It will take many centuries and beyond to fully feel the effects,” Shakun says.
The international team, led by Shakun and Oregon State University paleoclimatologist Peter Clark, based its work on the chemical makeup of air samples contained in bubbles trapped in ice cores. Ice records from Antarctica go back some 800,000 years.
But the researchers also drew temperature information from 80 locations around the globe, spanning northern and southern hemispheres. Sources to track temperature changes over time ranged from microfossils in deep-ocean sediments to pollen trapped in sediments in freshwater lakes.
The team's results show that the initial trigger for warming to end the last ice age was a periodic change in the angle of Earth's tilt and in the orientation of its axis. This brought more sunlight to warm northern latitudes. As mile-thick ice sheets covering vast areas of the northern hemisphere's continents began to melt, fresh water poured into the oceans, particularly into the North Atlantic, changing mechanisms that governed the climate.