For almost as long as scientists have known about the presence of different kinds of gasses in Earth's atmosphere, they have known that carbon dioxide (CO2) is a "greenhouse gas". CO2 keeps some of our planet's heat from escaping into space. If there were not some CO2 in the atmosphere, Earth would be frozen. But if there is too much CO2, then the planet will get too hot. That is a very basic and unquestioned principle of chemistry and physics.

How much is normal?
For the 10,000 years of human history until about 200 years ago, our atmosphere contained about 275 parts per million of carbon dioxide. Parts per million is simply a way of measuring the concentration of different gases, and means the ratio of the number of carbon dioxide molecules per million other molecules in the atmosphere. 275 ppm CO2 is a useful amount. It is the level that has created the conditions for humanity to thrive, and for all of the creatures now on Earth to prosper.

Through Earth's long history, the levels of carbon dioxide have changed -- sometimes higher than that 275 ppm recent average, and often lower. By studying air trapped in ice sheets, scientists have tracked CO2 levels for 800,000 years. That span of time includes ice ages and very warm periods, and in that whole time, the CO2 concentrations sometimes have dropped slightly below as 200 ppm, and occasionally they have been as high as 300 ppm (the dotted line on the graph). In 800,000 years, though, there has never been more than 300 ppm -- until about 1920.

Humans burning fossil fuels (coal and oil) have added more carbon dioxide into the atmosphere so rapidly that natural systems haven't been able to keep a healthy balance. In 1880, the CO2 levels were about 290 ppm. In 1920, the level had edged up to 300 ppm, the long-term high level. By 1950, it was a bit over 310 ppm. In 1990, there was 350 ppm of CO2 in the atmosphere. Less than 20 years later, we're at almost 390 ppm -- rising fast into levels that our species' home has never experienced. Today's levels of CO2 are not "normal", and they are not safe.

The best estimates for where we'll be at the end of the century, if dramatic actions are not taken to reduce the emissions of greenhouse gasses, are frightening -- ranging from 550 to 900 parts per million. That is two to three times as much CO2 as the highest levels in the 800,000 year record.

How much is safe?
350 parts per million is what many scientists, climate experts, and progressive national governments are now saying is the safe upper limit for CO2 in our atmosphere. We are already above the safe zone at our current 386 ppm.

As James Hansen of America's National Aeronautics and Space Administration, the first scientist to warn about global warming more than two decades ago, wrote recently, "If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm."

That will be a hard task, but not impossible. We need to stop taking carbon out of the ground and putting it into the air. Above all, that means we need to stop burning so much coal -- and start using solar and wind energy and other such sources of renewable energy -- while ensuring the Global South a fair chance to develop. If we do, then the earth’s soils and forests will slowly cycle some of that extra carbon out of the atmosphere, and eventually CO2 concentrations will return to a safe level. By decreasing use of other fossil fuels, and improving agricultural and forestry practices around the world, scientists believe we could get back to 350 by mid-century. But the longer we remain in the danger zone -- above 350 -- the more likely that we will see disastrous and irreversible climate impacts.