Doyle Rice

USA TODAY

Carbon dioxide — the gas scientists say is most responsible for global warming — reached a significant symbolic milestone in our atmosphere this month, the Scripps Institution of Oceanography said.

For the first time, daily and weekly values of carbon dioxide in our planet's atmosphere have remained above 400 parts per million, said Scripps scientist Ralph Keeling, keeper of his father's famed "Keeling Curve," the longest continuous record of atmospheric carbon dioxide on Earth.

Ralph Keeling and his late father Charles David Keeling have kept CO2 measurements at the Mauna Loa Observatory in Hawaii since 1958.

"We won’t be seeing a monthly value below 400 ppm this year — or ever again for the indefinite future," Keeling wrote in a recent blog post.

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 has said.

Increasing amounts of carbon dioxide and other gases are enhancing the planet's natural "greenhouse effect."

Based on paleoclimatic evidence, the last time carbon dioxide reached 400 ppm was millions of years ago, according to the journal Nature Geoscience. A 2009 report in the journal found evidence of CO2 levels of 365 ppm to 415 ppm roughly 4.5 million years ago.

CO2 levels were around 280 ppm prior to the Industrial Revolution in the late 1800s, when large amounts of greenhouse gases began to be released by the burning of fossil fuels.

The burning of the oil, gas and coal for energy releases greenhouse gases such as carbon dioxide and methane. These gases have caused the Earth's temperature to rise over the past century to levels that cannot be explained by natural variability.

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

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.

"By November, we will be marching up the rising half of the cycle, pushing toward new highs and perhaps even breaking the 410 ppm barrier," Keeling said.