Greenhouse gases were the main driver of climate throughout the warmest period of the past 66 million years, providing insight into the drivers behind long-term climate change. Antarctica and Australia separated around the end of the Eocene (56 to 22.9 million years ago), creating a deep water passage between them and changing ocean circulation patterns. Some researchers believe these changes were the drivers of cooling temperatures near the end of the Eocene "hothouse" period, but some think declining levels of carbon dioxide were to blame. If the cooling had been caused by changes in ocean circulation, regions around the equator would have warmed as the polar regions cooled, shifting the distribution of heat on Earth. But changing the concentration of greenhouse gases would affect the total heat trapped in Earth's atmosphere, causing cooling everywhere (including in the tropics), which is what the researchers found. Historically, researchers have had trouble reproducing temperature gradients between the tropics and the poles throughout the Eocene. These new climate models are capable of overcoming most of the issues faced by past models.
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