Carbon dioxide – the gas scientists say is most responsible for global warming – peaked again at record levels last month, scientists announced Tuesday.

Levels at Hawaii’s Mauna Loa Observatory averaged 414.8 parts per million in May, surging past yet another climate milestone. This level hasn't been seen in human history and is also higher than at any point in millions of years.

The concentration of carbon dioxide (CO2) in the atmosphere increases every year, and the rate of increase is accelerating, scientists from the Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration announced.

This is the highest seasonal peak recorded in 61 years of observations on top of Hawaii’s largest volcano, and the seventh consecutive year of steep global increases in concentrations of CO2. The 2019 peak value was 3.5 parts per million higher than the 411.3 ppm peak reached in May 2018; this is the 2nd-highest annual jump on record.

While 414 parts per million may not sound like a huge amount, scientists have known for decades that even trace amounts in the atmosphere can raise temperatures around the world.

The burning of fossil fuels such as coal, oil and gas releases greenhouse gases such as carbon dioxide and methane into Earth's atmosphere and oceans. That extra carbon has caused temperatures to rise to levels over the past century and a half that cannot be explained by natural factors, scientists say.

In the past 20 years, the world's temperature has risen about two-thirds of a degree Fahrenheit, NOAA said.

“Many proposals have been made to mitigate global warming, but without a rapid decrease of CO2 emissions from fossil fuels, they are pretty much futile,” said Pieter Tans, senior scientist with NOAA’s global monitoring division.

Carbon dioxide is called a greenhouse gas for its ability to trap solar radiation and keep it confined to the atmosphere.

It is invisible, odorless and colorless, yet is responsible for 63% of the warming attributable to all greenhouse gases, according to NOAA's Earth System Research Laboratory in Boulder, Colorado.

Levels of carbon dioxide go up and down each year, reaching their highest levels in May and then going back down in the fall as plants absorb the gas.

The increase in gases such as carbon dioxide, methane and nitrous oxide is fueling climate change and making "the planet more dangerous and inhospitable for future generations," the World Meteorological Organization said.