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Carbon dioxide levels are typically at their lowest in September because plants in the Northern Hemisphere absorb more CO2 throughout the summer. But levels have remained above 400 ppm.

It is “almost impossible” that October will “yield a lower monthly value than September and dip below 400 ppm, said the scientist who runs the Scripps Institute for Oceanography’s carbon dioxide monitoring program.

“Brief excursions toward lower values are still possible,” Ralph Keeling wrote in a blog post. “But it already seems safe to conclude that we won’t be seeing a monthly value below 400 ppm this year – or ever again for the indefinite future.”

Snyder based her reconstruction on 61 different sea surface temperature proxies from across the globe, such as ratios between magnesium and calcium, species makeup and acidity. But the further the study goes back in time, especially after half a million years, the fewer of those proxies are available, making the estimates less certain, she said.

These are rough estimates with large margins of errors, she said. But she also found that the temperature changes correlated well to carbon dioxide levels.

Temperatures averaged out over the most recent 5,000 years — which includes the last 125 years or so of industrial emissions of heat-trapping gases — are generally warmer than they have been since about 120,000 years ago or so, Snyder found. And two interglacial time periods, the one 120,000 years ago and another just about 2 million years ago, were the warmest Snyder tracked. They were about 2 degrees Celsius warmer than the current 5,000-year average.