The dramatic temperature increases that thawed the last ice age followed spikes in carbon dioxide levels in the air, a new study finds. Researchers say that further strengthens the scientific case explaining current man-made global warming.
In the new study, scientists show the atmospheric concentration of that heat-trapping greenhouse gas jumped more than 40 percent. Then global temperatures went up about 6 degrees Fahrenheit (3.5 degrees Celsius).
What is remarkable is that when the two are plotted they rise, plateau and rise again in a striking similar way with a slight lag. The warming over 6,000 years follows the greenhouse gas increase, just as scientific theory has long held.
This is important because, until this study, the two curves weren't quite so in sync. At some points, it seemed that the temperatures warmed before the carbon dioxide levels increased, something that climate skeptics seized upon.
How could carbon dioxide cause warming if the temperatures warmed first, argue skeptics, who are in the scientific minority.
Earlier studies had looked at carbon dioxide levels and temperature readings from Antarctica, not the entire world. A study published Wednesday in the journal Nature estimated global temperatures using 80 different proxies _ ice and mud samples from dozens of places around the world _ and found that globally, temperatures clearly went up only after carbon dioxide jumped.
"You end up with something that looks remarkably similar to the pattern of rising carbon dioxide through time," study lead author Jeremy Shankun of Harvard University said. "This, to me, seems like pretty powerful proof of theory of the connection between greenhouse gases and global warming."
There are two main sources of carbon dioxide, the chief greenhouse gas. The natural source comes mostly from dead plants and animals and that amplified the ice age thaw. In modern day, emissions from burning coal, oil and other fossil fuels add greatly to that natural carbon dioxide.
The ice age warming in Antarctica still appears to come before the carbon dioxide increases, which are calculated using an 800,000 year old Antarctic ice core, but there's good reason, Shankun said.
Temperature records and other ocean data paint a complicated picture of just how the last ice age thawed. It's almost like a Rube Goldberg machine, with one step leading to another and another. When the last ice age peaked about 25,000 years ago, the ice sheet extended to Iowa and New York City, Shankun said.
The ice sheet was actually so large that it was unstable, said study co-author Peter Clark of Oregon State University.
The initial trigger to the melt: A small and predictable wobble in Earth's orbit around the sun. That tiny wobble meant a tilt toward the sun that brought more sunlight in the Northern Hemisphere, causing ice sheets to melt and sending whopping levels of fresh water into the world's oceans.
That caused the global circulation of the oceans to stop, which in turn warmed the southern oceans, melting southern ice sheets over areas where more of the world's carbon dioxide is trapped, Shankun said.
That released massive amounts of the greenhouse gas, which then amplified the global temperature spike, Shankun said.
By 11,000 years ago, the ice age was history and greenhouse gas and temperature levels had stabilized. That changed with the industrial age and the increased use of fossil fuels.
Carbon dioxide levels have jumped roughly the same amount in the last century as they did over 6,000 years to get out of the ice age, Shankun said.
Penn State University professor Richard Alley and others called this a significant advance in studies about past climate change and carbon dioxide, saying "this may be of help in explaining things out in the sound-bite world."
Seth Borenstein be followed at http://twitter.com/borenbears