Levels of greenhouse gases in the atmosphere reached a record high in 2014, according to a new report released by the World Meteorological Organization on Monday.

The WMO reported that concentrations of carbon dioxide (CO2) in the atmosphere reached 397.7 parts per million (ppm) in 2014, and in the Northern hemisphere it “crossed the symbolically significant 400 ppm level in 2014 spring, when CO2 is most abundant.” The global average levels of CO2 concentration also passed 400 ppm in spring 2015.

“We can’t see CO2. It is an invisible threat, but a very real one. It means hotter global temperatures, more extreme weather events like heatwaves and floods, melting ice, rising sea levels and increased acidity of the oceans. This is happening now and we are moving into unchartered territory at a frightening speed,” Michel Jarraud, WMO Secretary-General said in a statement.

CO2 and other greenhouse gases trap heat on the Earth’s surface which warms it. The United Nations agency reports that there was a 36% increase in radiative forcing, which is the warming effect on our climate, due to higher levels of greenhouse gases such as CO2, methane (CH4) and nitrous oxide (N2O) from 1990 to 2014.

Scientists say CO2 levels should remain below 400 ppm to avoid long term climate issues, the Washington Post reports. The burning of fossil fuels has lead to a steady increase of CO2 in the atmosphere. The new report is released ahead of the U.N. climate change negotiations in Paris starting November 30.

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