Ancient Warming of Earth Not Entirely Explained by Rise in CO2

Scientists have long wondered what caused a dramatic warming of the planet 55 million years ago, when temperatures rose 5 degrees C to 9 degrees C (9 F to 16 F) in 10,000 years. A new study by U.S. researchers says, however, that only about 40 percent of the sharp rise in temperatures can be explained by increasing CO2 levels, meaning that our current understanding of how earth will react to humankind’s massive release of carbon dioxide is incomplete, the researchers say. Studying deep-sea sediments and other evidence of climate change, the researchers calculated that increases in CO2 during the Paleo-Eocene Thermal Maximum should have at most caused global temperature rises of 3.5 degrees C, or 6.3 F. Yet temperatures rose within 10,000 years by as much as 9 degrees C, the researchers reported in the journal Nature Geoscience. One possibility is large releases of other greenhouse gases, such as methane, while another is that the high temperatures during the period — when no ice existed on earth — set in motion other changes that further warmed the planet. “If this additional warming… was caused as a response to CO2 warming, then there is a chance that a future warming could be more intense than people anticipate,” said one of the study’s authors.