On Monday, scientists at the National Oceanic and Atmospheric Administration’s Mauna Loa observatory in Hawaii told Climate Central that June would be be the third month in a row where, for the entire month, average levels of carbon dioxide were above 400 parts per million (ppm). In other words, that’s the longest time in recorded history that this much carbon dioxide has been in the atmosphere.

The finding is troubling to climate scientists, several of whom told ThinkProgress on Monday that the levels are a reminder that humans are still pumping too much carbon dioxide into the sky. If the trend continues, some said, carbon levels will soon surpass 450 ppm — a level that many scientists agree would create a level of global warming that would be too difficult for some humans to adapt to.

“CO2 levels continue to increase, the amount of heat in the climate system continues to increase, ice continues to melt, and the seas continue to rise,” said Ken Caldeira, an atmospheric scientist at the Carnegie Institution for Science’s Department of Global Ecology. “We will continue to break through threshold after threshold — unless we stop using the sky as a waste dump soon.”

Reaching the 400 ppm level of carbon dioxide has largely been considered symbolic among watchers of climate change — sort of like the temperature reaching 100°F on a hot day. It really isn’t much hotter than 99°F, but somehow it feels more like a landmark of a true scorcher.

For global warming, 400 ppm is like 100°F, except it’s a palpable representation of how quickly humans are pumping carbon dioxide — the main driver of global warming — into the atmosphere.

The first time we reached that 400 ppm threshold was in May 2013. But it was brief — just a small moment in time where the levels hit the landmark. In reality, it doesn’t make much of a difference if carbon is at 400 ppm for a brief period of time. What does make a difference is a constantly-increasing average level of carbon dioxide trapping more heat in the atmosphere. Because there have now been three months in a row of average levels about 400 ppm, climate scientists are worried that the landmark is becoming the norm, which would bring us closer to a level of global warming that will be too difficult to adapt to.

“Within a few years, CO2 levels will remain above 400 ppm for the entire year, and in less than a decade, the average level will be close to 405 ppm,” said Michael Mann, director of the Earth System Science Center at Pennsylvania State University. “Even 405 ppm could potentially commit us to a ‘dangerous’ 2°C (3.5°F) warming of the globe.”

Mann predicts that in the next few years, carbon levels will reach an average of 405 ppm, which he says could cause enough sea level rise, drought, and severe weather to significantly harm human populations across the globe. International climate negotiations generally center around keep warming below 2°C above preindustrial levels, which most scientists agree is the upper limit for allowable global warming. Mann says this could happen at 405 ppm. Others’ estimates are closer to 450 ppm.

To solve the problem, scientists generally agree that concentrated carbon levels in the atmosphere would have to stabilize at 350ppm — a difficult milestone considering our current 400 ppm average. To reduce the average, carbon dioxide not only needs to stop being so rapidly emitted, but also needs to be actively sucked out of the atmosphere. This could be done by growing a lot of long-lived plants that absorb and store a lot of carbon dioxide, or investing in some of the many carbon capture technologies that have been developed over the years.

Either way, it seems undeniable that the amount of carbon in the atmosphere is going to get worse before it gets better, especially since June is a month where carbon levels are supposed to be at their lowest. Jennifer Francis, a research professor at Rutgers University’s Institute of Marine and Coastal Sciences, called the fact that June saw such high levels of carbon “remarkable,” noting that the Northern Hemisphere’s increase in summer vegetation is supposed to soak up a lot of the carbon.

“Even though we’ve been in the primary growing months, the [carbon]concentration has stayed high,” Francis said. “Mother Nature is poking us in the ribs again reminding us that we’re still making the problem worse as we continue to emit CO2 at ever-increasing amounts.”