The Keeling Curve:Brief Reprieve from 400 PPM Era May Be Thanks to a Hurricane

Hurricane Madeline could be behind an unexpected dip in carbon dioxide concentrations measured at Mauna Loa below 400 parts per million (ppm) this week. The reading reported on Tuesday was 399.86 ppm.

The steady rise caused by human fossil fuel use had made sub-400 ppm readings rare before the El Niño winter of 2015-16. In fact, last year, the rate of increase set a record, rising 3.15 ppm, an acceleration over the average annual pace of 2.1 ppm observed over the full 58-year Keeling Curve history. Because El Niños tend to prod CO2 levels upward, Keeling and other researchers had thought that levels below 400 ppm would not be seen again once the effects of El Niño dissipated this summer.
Keeling said there is evidence to suggest Hurricane Madeline’s approach might have been a source of a temporary drop in CO2 levels.
“A working hypothesis is that the low values are caused by the hurricane bringing in air from much further north, where we do expect to see sub-400 ppm values,” said Keeling. “Earlier this summer, we also saw values that were unseasonably low when a hurricane was looming.”

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