Carbon dioxide in the atmosphere increased a record amount from 2015 to 2016, leaving the air laden with a concentration of the potent greenhouse gas not seen for at least the last 800,000 years, the period for which we have direct measurements from ice cores. The increase essentially guarantees that in the absence of rapid and dramatic cuts to emissions, catastrophic temperature increases “well above” those the Paris agreement sought to avoid will become a reality by end of the century, according to Petteri Taalas, the head of the World Meteorological Organization.

According to a report released by the international climate observing body on Monday (Oct 30), the concentration of CO2 was at 403.3 parts per million as of 2016, up from 400 parts per million a year earlier. That 3.3 ppm rise is 50% more than the average rate over the past decade. Over the last 70 years, the rate of increase of carbon in the atmosphere has been “nearly 100 times larger than at the end of the last ice age,” the last time the Earth transitioned to a much warmer world, the WMO writes. As far as the global scientific community can tell, “such abrupt changes in the atmospheric levels of CO2 have never before been seen.”

Such rapid increases in greenhouse gases “have the potential to initiate unpredictable changes in the climate system, because of strong positive feedbacks, leading to severe ecological and economic disruptions,” according to the report.

The last time the Earth experienced these levels of CO2 in the atmosphere was roughly 4 million years ago, during the mid-Pliocene, according to the WMO. The climate back then was 2-3 °C (3.6-5.4 °F) warmer than it is today, and the Greenland and West Antarctic ice sheets melted entirely, causing sea levels to rise 10-20 meters (33-66 feet) higher than those today.

The paper also reported that concentrations of methane, a greenhouse gas with greater short-term potency than CO2, continues to rise rapidly, particularly from tropical zones, a phenomena for which climate scientists do not have clear answers. Some experts fear it signals a “feedback loop” in which methane levels rise, warming the air and triggering more releases of methane ordinarily locked away in natural sinks.

“This was not expected in the Paris agreement,” Euan Nisbet, a climatologist at the Royal Holloway University of London told BBC News. “The carbon isotopes in the methane show that growth is not being driven by fossil fuels. We do not understand why methane is rising. It may be a climate change feedback. It is very worrying.”

News of the rise comes just as countries are preparing to meet at the next United Nations climate talks in Bonn next week.