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There are many Global Warming deniers that claim that our current carbon dioxide levels (387 ppm)were not that unusual, and happened quite often. Now researchers have determined that you would have to go back 15 to 20 million years to have carbon dioxide levels this high. When these concentrations were sustained the last time "global temperatures were 5 to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75 to 120 feet higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland."
Scientists have established a close correlation between CO2 levels and temperature. Researchers were able to determine CO2 levels going back 800,000 years by analyzing gas bubbles trapped in ancient ice core samples. To go back farther in time, scientists at the University of Cambridge developed a new technique to assess carbon dioxide levels in the much more distant past — by studying the ratio of the chemical element boron to calcium in the shells of ancient single-celled marine algae.
The findings from the shells were corroborated by comparing them to the analysis of gases trapped in ancient ice.
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