The carbon dioxide level (CO2) in Earth's atmosphere crossed 400 parts per million (ppm), a symbolic milestone which experts believe is impossible to reverse for centuries or ever after.

First time ever in centuries, the weekly and daily CO2 levels in the atmosphere has remained at 400 ppm in September, the month that typically sees a drop in values. Usually, a low point is experienced due to the transition from summer to fall — where the CO2 intake by vegetation drops and CO2 is released from the soil.

The Scripps Institution of Oceanography noted that there is no possibility for the monthly values to drop below 400 ppm during the month of October as well. In the past two decades, the monthly values of October dipped below September only during 2002, 2008, 2009, and 2012, just four out of over 20 years.

Furthermore, the values fell at most 0.45 ppm from September to October, which is not sufficient enough to push the CO2 level below 400 ppm this year. Therefore, October will experience values around 400 ppm or it may see an increase of up to 401 ppm. It is also noted that November marks the rising half of the CO2 cycle and, therefore, a peaking raise of up to 410 ppm could be seen.

Ever since the concept of industrial revolution emerged, CO2 in Earth's atmosphere has been building up consistently. However, this is the first time the CO2 level has crossed 400 ppm mark on Earth for millions of years.

According to David Black, associate professor in the School of Marine and Atmospheric Sciences at Stony Brook University in New York, the Earth crossed the 400 ppm mark about 3.5 million years ago when the climate of the planet was different altogether. For instance, the global sea level was about 15 to 90 feet higher and Arctic was warmer than what it is today.

Black also noted that it took millions of years for the earth's atmosphere to cross the 400 ppm mark and also to dip to 280 ppm before the so-called industrial revolution began.

"One of the things that really concerns climate scientists is we as humans have taken only a few centuries to do what nature took millions of years, and most of that change was just in the last 50-60 years," said Black.

However, Scripps scientist Ralph Keeling, concluded in the report that it is less likely for the CO2 levels to drop lower than 400 ppm for the "indefinite future."

Photo: Climate and Ecosystems Change Adaptation Research | Flickr