The amount of carbon dioxide in the Earth's atmosphere has reached its annual peak, climbing to 419 parts per million (ppm) in May, according to scientists from Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration (NOAA).

Why it matters: It's the highest CO2 reading since reliable instrument data began 63 years ago, but evidence shows it's also a peak since well before the start of human history.

• The rate of increase showed "no discernible impact" from the pandemic-induced economic slowdown, the scientists found.
• Carbon dioxide is a long-lived greenhouse gas emitted through human activities such as fossil-fuel burning, deforestation and agriculture.

Threat level: Not only is CO2 now at its highest levels in human history, but one would have to go all the way back beyond the beginning of human history — to the Pliocene Epoch, between 4.1 to 4.5 million years ago — to find a time when Earth's atmosphere held a similar amount of carbon.

• Data gleaned from ice core records and other indicators of what Earth was like at that time serve as a stark warning for our future on this planet, scientists say.
• Global average sea levels, for example, were nearly 80 feet higher during the Pliocene than they are today, while the global average temperature was about 7°F above the preindustrial era.
• Studies show that large forests were located in areas of the Arctic that are now home to tundra.

Quick take: The world first passed the 400 ppm threshold in 2013, but took just eight years to climb toward the 420 ppm mark. It's an indication of how countries are failing so far to bend the emissions curve dramatically downward in order to slow, and eventually reverse, global warming.

• The CO2 concentration figures may seem abstract, but they correspond to the risk of tipping points in the climate system that would have profound societal consequences, such as the melting of the West Antarctic Ice Sheet.
• Numerous studies show that the lower CO2 concentrations are stabilized, the greater the chances that climate change will be more manageable for society and the planet's natural systems.

Details: The 1.8 ppm rate of increase from 2020 was slightly slower than other recent years, but within the range of natural variability.

• According to NOAA senior scientist Pieter Tans, the temporary pandemic-related dent in global carbon emissions got drowned out by natural variations that affect the rate at which carbon builds up in the air.
• Tans, as well as Ralph Keeling, who oversees the Mauna Loa observations from his post at Scripps, told Axios they were not surprised the pandemic, which caused a global emissions cut of about 7% in 2020, failed to slow or halt the growth of atmospheric CO2.
• They each said net carbon emissions have not declined significantly and for long enough to be noticeable. "As long as we keep emitting, CO2 will keep going up. And that's what we see. Even if we manage to freeze net emissions," Tans said, emphasizing the need to reach net-zero emissions as soon as possible.

What they're saying:

• Keeling told Axios that 420 ppm, which the planet is almost certain to exceed next year, is "a psychological threshold." He added, "We're moving deeper and deeper into a territory we almost certainly never would have wanted to get to."
• Tans emphasized the long-lived nature of CO2, with each molecule lasting in the air for as long as 1,000 years. "In terms of human civilization, these emissions are forever," he said, endorsing plans to drive emissions down to net zero as soon as possible.