BOULDER, Colo., Feb. 23 (UPI) -- Over the next century, if the current trend continues, Earth is going to see climate conditions that have not existed in hundreds -- and perhaps thousands -- of millennia. The outcome of this trend will shape the future not only of homo sapiens, but many other species with which we share the planet.
Even if all nations undertake aggressive measures -- and even if those measures are successful -- atmospheric carbon dioxide levels will stabilize at somewhere above 400 parts per million. Without any restrictions, CO2 levels in the atmosphere will reach 1,000 ppm.
"This is an experiment that hasn't been done in a long time," said Dan Schrag, professor of geochemistry at Harvard University. "Atmospheric CO2 has never been higher than 300 ppm in the last 400,000 years, and probably not in the last 30 million years," he said.
What will happen to the world at the end of this inadvertent experiment?
There is no dispute -- even among the most skeptical of climate skeptics -- that CO2 in the atmosphere is rising. In their book, "Man-Made Global Warming: Unravelling a Dogma," European climate skeptics Hans Labohm, Simon Rozendaal and Dick Thoenes write: "Fact No. 1: the concentrations of carbon dioxide and other heat-trapping gases in the atmosphere have risen during the past 100 years and especially in the past decades."
Most -- but not all -- scientists agree the rise in CO2 and temperatures since about 1950 is the result of human activity, in particular the burning of carbon-rich fossil fuels. In contrast to the reliability of global temperature measurements -- about which there is heated but honest disagreement -- no one disputes the CO2 rise.
Moreover, it is rising fast. The government of the United Kingdom recently announced a very ambitious program to reduce its own CO2 emissions by 60 percent by 2050. This goal, if adopted globally, probably would stabilize atmospheric CO2 concentrations at about 450 ppm by the end of the 21st century.
CO2 is the most abundant of several so-called greenhouse gases being pumped into the atmosphere. These gases trap heat from the sun instead of allowing it to be reflected back into space. CO2 is taken up by plants and by the oceans, absorbed and released by various mechanisms on the planet, including human breathing. CO2 stays in the atmosphere for a long time. Green plants need CO2 to live, and they probably first evolved on Earth during periods of CO2 concentration.
Some climate skeptics argue the excessive heat-trapping effects of CO2 anticipated by climate scientists and climate computer models are purely theoretical.
"It is not certain that these physical qualities of the heat-trapping gases result in real warming, as so many scientists, politicians and environmentalists believe," Labohm, Rozendaal and Thoenes wrote. "That is not a fact but a hypothesis and nothing more."
Although the impact of Earth's oceans and other factors on CO2-induced warming makes the exact calculation of temperature difficult, Schrag disagreed with this assessment.
"The link between carbon dioxide and climate is not just a theory," he said. "There is plenty of observational evidence."
The planet Venus, in fact, has an atmosphere consisting of 96 percent CO2 with a temperature of about 470 degrees Fahrenheit at the surface.
Historically, a CO2 concentration of 450 ppm does not hearken back to the Medieval Warm Period, an age of climate warming that occurred about 1,000 years ago and is the current favorite of some skeptics to demonstrate humans can adapt easily to modest warming. The last time atmospheric CO2 rose above 450 ppm was about 55 million years ago, at the beginning of the Eocene, not so long after the disappearance of the dinosaurs.
At that time, according to the National Academy of Sciences report, "The Effects of Past Global Change," the mean annual temperature at the North Pole was about 55 degrees F. There were broad-leafed evergreens, including palm trees, in Greenland, Siberia and as far north as today's Point Barrow, Alaska. Global sea level was near its maximum.
In the southern hemisphere, there were pine forests in Antarctica. Eastern Antarctica had a warm, semi-arid climate and there was a major and sudden oceanic warming.
About 55 million years ago, in the early Eocene, there was a large extinction event, which dramatically reduced species richness. This was not one of the so-called big five extinctions that punctuated Earth's paleohistory. It was a smaller but still significant event, from which Earth required about 8 million years to recover.
According to "Mass Extinctions and Their Aftermath," by Anthony Hallam and Paul Wignall, "The rich record of Paleogene mammals in the non-maritime deposits of western North America allows a more thorough examination of change through time than anywhere else in the world ... The patterns of diversity change are consistent with global warming near the early middle Eocene boundary and global cooling and reduction of climatic equability through the late Eocene and Oligocene."
Primates and hoofed mammals were particularly hard hit by the extinctions. Wignall and Hallam wrote, "Overall, there was steady reduction in generic richness from the middle Eocene to the Oligocene" with high extinctions early in the period.
Now, as any scientist will be quick to say, correlation does not necessarily mean causality -- and analogy does not equal science. Conditions change in 55 million years, and it may be that Earth will respond to these new stresses differently than it has in the past.
Nonetheless, as Schrag notes, the planet's climate is in the throes of a large and unprecedented experiment.
"There's very good evidence that climate change will be worse" than is currently predicted, he said, not less, as the skeptics argue.
David Warrilow, head of science policy for the United Kingdom's Department for Environment, Food and Rural Affairs, said, "Unrestrained climate change presents unacceptable risk. We are looking at huge changes in the world as we know it."
Dan Whipple covers the environment for UPI Science News. E-mail firstname.lastname@example.org