Nestled high on the slope of an active volcano on the Island of Hawai’i, the Mauna Loa Observatory has been continuously detecting atmospheric concentrations of carbon dioxide for nearly 60 years. This September, the scientists who monitor those levels recorded some unprecedented bad news.
Over the last few years, global levels of carbon dioxide in the atmosphere have fluctuated above and below 400 parts per million (ppm); this number usually decreases each fall because of natural cycles of absorption. But this September, the level stayed above 400 ppm globally—making that number the new baseline.
“It was a big deal to hit 400 ppm,” says climate scientist Astrid Caldas of the Union of Concerned Scientists. “But now we’re staying above that level year-round, even with seasonal fluctuations.”
For reference, climatologist and activist James Hanson posited the following in a 2008 peer-reviewed paper in Open Atmospheric Science Journal:
If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm.
Levels unseen by humans
The last time carbon dioxide made up 400 ppm in our atmosphere was 3.5 million years ago, during the Pliocene era—before homo sapiens evolved. Over many more millions of years, levels dropped as carbon dioxide was reabsorbed into oceans and vegetation. By the dawn of the industrial revolution in the 18th century, carbon dioxide made up 280 ppm in Earth’s atmosphere. But since then, the release of emissions from burning fossil fuels has caused a precipitous rise in carbon dioxide concentration over an alarmingly short time span.
Caldas says that 400 ppm is here to stay.
“We know that we can stop emissions,” she says, “but we’re not going to stop all emissions tomorrow. And even if we did, we already have a lot of carbon dioxide that natural processes cannot eliminate from the atmosphere so quickly. That means we’re stuck with this amount for a while.”
The consequences of the new normal are clear–and not just to climate scientists such as Caldas: warming (the first six months of 2016 have been the hottest in recorded history); sea level rise as ice sheets melt; ocean acidification; and global ‘weirding.’
A global response gains momentum
Caldas says she remains hopeful, especially since the Paris Climate Agreement, which aims to hold the global temperature increase to under 2 degrees Celsius, was ratified by UN member nations in early October.
“I always say I wouldn’t be doing this work if I didn’t think there was something we could do,” she says. “If we keep pushing for reduction of emissions, then the carbon dioxide levels in the atmosphere will eventually drop.”
A global effort to do just that is yielding significant results. Signaling their firm commitment to major investments in renewable energy, nations around the world approved the ratification of the Paris Agreement much sooner than expected. The European Union ratified the agreement in the last days of September; India on October 2. By the time the next UN summit on climate change is held in Marrakesh on November 7, the agreement will have entered into force—solidifying nations’ commitments for at least four years.
Given the current unprecedented levels of carbon dioxide in the atmosphere, Caldas confirms that some global warming trends are certain and irreversible. But she says rapid reductions in emissions can bend the curve.
“I’m not predicting doomsday,” she says. “That’s why we’re working so hard to make quick and extensive emissions reductions, and to build up our carbon sinks, like forests. These strategies give us more time to deal with the warming that is already locked in.”