The level of carbon dioxide (CO2) in the atmosphere is now at a record high. An American weather station in Hawaii recorded the new high on the 9th of May. It found CO2 levels were at 400 parts per million (ppm). The last time CO2 was at this level was five million years ago. This was when there were no humans on the Earth. Scientists say the 400ppm level is a warning sign. They say we must do more to stop this number getting bigger. However, scientist James Butler said he thought CO2 levels would keep rising. He warned: "In eight to nine years, we will probably have seen the last CO2 reading under 400ppm." He told reporters that even at the South Pole, CO2 would be above 400ppm.

Carbon dioxide is one of the gases that create global warming. Just one CO2 molecule can stay in the atmosphere for hundreds of years. Pieter Tans, one of the scientists measuring CO2 levels, said human activity is making the Earth warmer. He said that, "the burning of coal, oil, and natural gas is driving the acceleration". He pointed out that 300 years ago, CO2 levels were around 280ppm. He added that 800,000 years ago, they were at 180ppm. Another scientist, Dr Ralph Keeling, warned that CO2 is warming the Earth to dangerous levels. He is worried about arctic ice melting, causing sea levels to rise. This will put billions of people in danger. He said: "It takes a long time to melt ice, but we're doing it. It's scary."