Earth's Hot Past a Glimpse Into the Future?
The study, by National Center for Atmospheric Research (NCAR) scientist Jeffrey Kiehl, appears as a "Perspectives" article in the journal Science. The work was funded by the National Science Foundation (NSF), NCAR's sponsor.
Building on recent research, the study examines the relationship between global temperatures and high levels of carbon dioxide in the atmosphere tens of millions of years ago. It warns that, if carbon dioxide emissions continue at their current rate through the end of this century, atmospheric concentrations of the greenhouse gas will reach levels that existed about 30 million to 100 million years ago. Global temperatures then averaged about 29 degrees Fahrenheit (16 degrees Celsius) above pre-industrial levels. Kiehl said that global temperatures may take centuries or millennia to fully adjust in response to the higher carbon dioxide levels.
"If we don't start seriously working toward a reduction of carbon emissions, we are putting our planet on a trajectory that the human species has never experienced," says Kiehl, a climate scientist who specializes in studying global climate in Earth's geologic past. "We will have committed human civilization to living in a different world for multiple generations."
The Perspectives article pulls together several recent studies that look at various aspects of the climate system, while adding a mathematical approach by Kiehl to estimate average global temperatures in the distant past. Its analysis of the climate system's response to elevated levels of carbon dioxide is supported by previous studies that Kiehl cites.
"This research shows that squaring the evidence of environmental change in the geologic record with mathematical models of future climate is crucial," says David Verardo, Director of NSF's Paleoclimate Program. "Perhaps Shakespeare's words that 'what's past is prologue' also apply to climate."
Kiehl drew on recently published research that, by analyzing molecular structures in fossilized organic materials, showed that carbon dioxide levels likely reached 900 to 1,000 parts per million about 35 million years ago. At that time, temperatures worldwide were substantially warmer than at present, especially in polar regions--even though the Sun's energy output was slightly weaker. The high levels of carbon dioxide in the ancient atmosphere kept the tropics at about 9-18 F (5-10 C) above present-day temperatures. The polar regions were some 27-36 F (15-20 C) above present-day temperatures. Kiehl applied mathematical formulas to calculate that Earth's average annual temperature 30 to 40 million years ago was about 88 F (31 C) -- substantially higher than the pre-industrial average temperature of about 59 F (15 C).
"This analysis shows that on longer time scales, our planet may be much more sensitive to greenhouse gases than we thought," Kiehl says.
Climate scientists are currently adding more sophisticated depictions of ice sheets and other factors to computer models. As these improvements come on-line, Kiehl believes that the computer models and the paleoclimate record will be in closer agreement, showing that the impacts of carbon dioxide on climate over time will likely be far more substantial than recent research has indicated. Because carbon dioxide is being pumped into the atmosphere at a rate that has never been experienced, Kiehl could not estimate how long it would take for the planet to fully heat up. However, a rapid warm-up would make it especially difficult for societies and ecosystems to adapt, he says. If emissions continue on their current trajectory, "the human species and global ecosystems will be placed in a climate state never before experienced in human history," the paper states.