We finally did it, we are through the atmospheric door.

After years of dire prognostications that we were reaching an historic tipping point, El Niño nudged us through the gate and we achieved what most scientists and environmentalists have been warning against: In 2015, carbon dioxide levels in the atmosphere were at 400 parts per million (ppm) on average across the year as a whole. This according to the annual greenhouse gas bulletin of the the World Meteorological Organisation’s (WMO).

Artist: Cornelia Konrads

The more carbon dioxide in the atmosphere, the warmer it gets. Prior to the Industrial Revolution, the CO2 levels averaged at 278 ppm. The planet was a cooler place.

Furthermore, it’s not considered likely that the carbon dioxide level will dip below the new level for many (human) generations.

There are likely those who will point to the lack of immediate disaster as a sign that the crossing of this threshold isn’t a big deal, or that carbon dioxide levels aren’t the only factor in determining global climate. And those are good points. But considering the general trend in rising temperature, now is the time to take the matter seriously. In human terms, if someone is seriously ill, you don’t wait for the fever to spike dangerously before taking action on the off chance the problem will just go away.

WMO secretary-general, Petteri Taalas is quoted in The Guardian as saying that “the year 2015 ushered in a new era of optimism and climate action with the Paris climate change agreement. But it will also make history as marking a new era of climate change reality with record high greenhouse gas concentrations. The El Niño event has disappeared. Climate change has not.”

What is our next threshold for action?