Two weeks before world leaders gather in Marrakesh, Morocco, for the next major United Nations climate change summit meeting, the UN World Meteorological Organization (WMO) has announced global carbon dioxide levels have passed a symbolic threshold.

Global average CO2 levels are above 400 parts per million (ppm) for the first time in 3 million years, and are 144 percent of pre-industrial levels of 278 ppm, the UN weather agency announced in the annual Greenhouse Gas Bulletin it released Monday.

Scientists have long known CO2 levels would eventually pass this threshold. It wasn’t a question of “if,” but “when.”

Hitting this threshold gives fresh urgency to a process that's already seen significant strides in the past year. But it's also a reminder that the progress among nations needs to continue apace if the global thermostat is going to be reset, say climate scientists.

“If 400 has any real meaning – it’s just a number nature doesn’t know – it’s that climate change is no longer an issue of the future. It’s one of today and now,” Adil Najam, dean of the Frederick S. Pardee School of Global Studies at Boston University, tells The Christian Science Monitor in a phone interview.

“You cannot lull us by saying we will do something tomorrow,” says Dr. Najam, who was a co-author for the Third and Fourth Assessments of the Intergovernmental Panel on Climate Change (IPCC). “Let’s start doing something now.”

In the previous century, by some accounts world leaders were slow to agree on actions to combat environmental threats and climate change. The Montreal Protocol that phased out chlorofluorocarbons (CFCs), for instance, wasn’t signed until 14 years after scientists first linked the chemical to holes in the ozone layer.

The pace at which world nations have acted may have accelerated with the Paris climate agreement in 2015, followed by the Kigali deal nearly 200 nations signed on Oct. 15 to phase out hydrofluorocarbons (HFCs), another greenhouse gas.

Now, though, some climate change experts say CO2 levels passing this 400 ppm threshold further emphasizes the need to turn the ambitions of the Paris deal — to curb the rise in global temperatures — into concrete action in Marrakesh.

The global average concentration of CO2 first reached 400 ppm in 2015, according to WMO, the UN weather agency. The amount of atmospheric CO2 previously reached this level during certain months of the year, and in certain parts of the world. But 2015 marked the first time on record that the global average surpassed this threshold for a whole year. This carbon dioxide level, then, surged in 2016, bolstered by the powerful El Nino event, which also triggered droughts in tropical forests and wildfires.

WMO isn’t the first agency to announce the passing of this threshold. The Mauna Loa Observatory in Hawaii — the world’s premier site for measuring CO2 — wrote in a study published in June that the Earth reached these levels. WMO referred back to Mauna Loa in its announcement, saying the observatory "predicts that carbon dioxide concentrations will stay above 400 ppm for the whole of 2016 and not dip below that level for many generations."

To understand how these CO2 levels could affect the world in the future, Jeremy Shakun, a professor of earth and environmental sciences at Boston College, tells the Monitor we can look back to the climate 3 million years ago in the mid-Pliocene era.

“At that point, it’s a pretty different world,” says Dr. Shakun. Global temperatures were up by as much as 3 degrees C., ice sheets were dramatically smaller, and sea levels were up to 10 feet higher, he says.

“It’s not going to happen overnight. You can’t melt an ice sheet in two days. But once you set the thermostat this high, it’s already committed,” he says. “You’re already locked into reengineering the planet.”

But more than 170 nations came together in 2015 to agree to hold the global temperature rise to no more than 2 degrees Celsius (3.6 degrees F) above pre-industrial levels. Under the Paris deal, nations agreed to implement national laws to curb global temperatures. The agreement is scheduled to enter into force on Nov. 4, after dozens of countries have ratified it, formally binding themselves to its terms. These nations include China, the world’s largest greenhouse gas emitter, the United States, second behind China, and the 28-nation European Union.

At the UN summit in Marrakesh that starts on Nov. 7, diplomats hope to create an independent body that will put into force the Paris deal. They plan to create an independent body to monitor, verify, and publicize countries’ pollution levels, according to The New York Times. At Marrakesh, they will seek to hammer out how developed countries will pay for poorer countries to adapt to climate change and develop new clean energy technologies. Under the Paris deal, rich countries voluntarily pledged to spend $100 billion annually by 2020 to help poorer countries.

The WMO's announcement is timely, then, because it highlights the need for countries to "ramp up their ambitions," Anthony Janetos, a professor of earth and the environment at Boston University, tells the Monitor.

“The Paris results don't get the world to previously agreed goals. It’s just a mechanism that everybody proposed,” he says. “That’s a big success.”

But Marrakesh is important, he adds, because diplomats plan to put in measures to ensure countries are on track for their emissions goals. The Paris agreement requires countries come together every five years to evaluate if they should set more ambitious emissions targets, but doesn’t specify how.

But Dr. Najam, also of Boston University, believes that in order for the Paris deal or Marrakesh to have any significance, countries must agree to reduce carbon emissions now, not in the future. Wealthier nations must also help less developed countries already feeling the effects of climate change.

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"They need to start showing real action and make up the losses for those who are suffering from climate change now," he says.

[Editor’s note: A previous version of this article mistakenly reported the percentage increase of global average CO2 levels from pre-industrial levels. It is 144 percent of pre-industrial levels.]