A Major Climate Threshold Has Been Permanently Crossed

For the first time in human history, atmospheric CO2 concentrations exceeded 400 parts per million (ppm) in 2015. They're expected to do so again this year, and every subsequent year for many generations to come, according to a new report issued by the World Meteorological Organisation. Image: David Stanley/Flickr

Four hundred ppm is one of the most important symbolic thresholds in the climate change conversation, rivalled only by the 2C warming threshold world leaders agreed not to cross when they signed the Paris climate agreement last year. The reason 400 ppm is so significant? It's a nice round number for one, but moreover, it's a level of CO2 our atmosphere has not seen in the last three million years. In other words, for the entirety of our species' existence.

The last time our atmosphere held this much carbon, during the mid Pliocene, sea levels were roughly 20m higher than they are today. Since sea level changes, ocean acidification and other consequences of a carbon-loaded atmosphere take time to catch up, crossing the 400 ppm threshold locks us in to a certain level of climate impacts, unless we can figure out how to recapture that carbon and pump it underground.

If you've been paying attention, you'll know this isn't the first time we've heard that the symbolic 400 ppm threshold is a fait accompli. Back in June, Gizmodo reported that last year's strong El NiƱo event pushed the atmosphere past 400 ppm in 2015. And last month, at a time when atmospheric carbon typically hits its annual low, the Mauna Loa climate observatory failed to drop below 400 ppm, all but ensuring that 2016 would pass that mark, too.

"It's unlikely we'll ever see CO2 below 400 ppm during our lifetime and probably much longer," Pieter Tans, a lead scientist at NOAA's Global Greenhouse Gas Reference Network, said in a statement last month.

With the publication of the WMO's latest annual Greenhouse Gas Bulletin, the reality that we're permanently living in a 400 ppm world seems institutionalised at the highest levels of climate monitoring. Furthermore, the bulletin notes that more potent greenhouse gases, including methane and nitrous oxide, also reached new highs in 2015. Atmospheric methane levels are now 256 per cent above pre-industrial concentrations, and account for nearly 20 per cent of greenhouse warming. Finally, the WMO report warns that forests and oceans which currently absorb roughly half of human carbon emissions, "may become saturated" in the future, accelerating the buildup of CO2 in our atmosphere.

Here is the good news: The Paris climate agreement officially enters force next week. Earlier this month, world leaders agreed to phase out HFCs, a class of refrigerator coolants that are 2000 times as potent as CO2 when it comes to trapping heat in the atmosphere.

[WMO via BBC]

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