In the following video, Jeffrey T. Kiehl, author of Facing Climate Change: An Integrated Path to the Future, discusses how we can learn about future climate from Earth’s deep past. He offers a warning about the current trajectory we are on in terms of climate change:
“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. We will have committed human civilization to living in a different world for multiple generations.”
In the video, Kiehl focuses on a fundamental question: When was the last time Earth’s atmosphere contained as much carbon dioxide as it may by the end of this century?
If society continues on its current pace of increasing the burning of fossil fuels, atmospheric levels of carbon dioxide are expected to reach about 900 to 1,000 parts per million by the end of this century. That compares with current levels of about 390 parts per million, and pre-industrial levels of about 280 parts per million.
Since carbon dioxide is a greenhouse gas that traps heat in Earth’s atmosphere, it is critical for regulating Earth’s climate. Without carbon dioxide, the planet would freeze over. But as atmospheric levels of the gas rise, which has happened at times in the geologic past, global temperatures increase dramatically and additional greenhouse gases, such as water vapor and methane, enter the atmosphere through processes related to evaporation and thawing. This leads to further heating.
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 degrees F (5-10 degrees C) above present-day temperatures. The polar regions were some 27-36 degrees F (15-20 degrees C) above present-day temperatures.
Drawing on recent studies, Kiehl concludes that “This analysis shows that on longer time scales our planet may be much more sensitive to greenhouse gases than we thought.”