You’d be forgiven for thinking that 'ppm' was some texting abbreviation that you hadn’t quite grasped yet. In fact, it stands for ‘parts per million’ and it refers to the measurement of a chemical mass in the atmosphere. 

In the climate change debate, you’ll often see ppm attributed to carbon dioxide (CO2) - the most worrying of greenhouse gases. Here’s the breakdown – if we can limit the concentration of CO2 to 450ppm, we’d only have a 50:50 chance of keeping the planet from warming by more than 2 degrees – the level at which scientists agree would be catastrophic for humanity. Basically, it’s a bit of a gamble on the climate. 

A concentration of 350ppm is considered the safer option. At a level of 350ppm, disastrous weather events such as hurricanes and rising sea levels would be avoided and the dangerous and irreversible effects of climate change would be limited. 350ppm would keep atmospheric levels to a point that life on Earth and the planet itself is accustomed to.

So, where are we now? Bang in the middle, at 400ppm. That means for every million atmospheric molecules, there are 400 CO2 ones. To put that into perspective, pre-industrial concentrations of CO2 were 275ppm.

 At a level of 350ppm, disastrous weather events such as hurricanes and rising sea levels would be avoided and the dangerous and irreversible effects of climate change would be limited

The last time we were at 400ppm, humans weren’t even around. Somewhere between 800,000 and 15 million years ago (depending on which expert you ask) atmospheric CO2 levels were at the same level. This was a warmer world where sea levels were much higher than they are now.

You can understand why scientists are worried that our atmosphere is heading in a similar direction – the prognosis isn’t good. If the world’s atmospheric CO2 was divided equally amongst the seven billion inhabitants on Earth, we’d each have a 446 tonne slice. 

We’re adding about 2ppm of CO2 to the atmosphere each year, which means at this rate we’ll be hitting the dangerous 450ppm mark within 25 years, with many marking 2036 as the danger threshold – or the point of no return.

We’ve been on the wrong side of 350ppm since the late 80s and even though we’ve surpassed the designated safe-zone, it’s still possible to reverse it.

It’ll take some doing though; scientists say that global emissions need to be slashed by at least 50 per cent to stop atmospheric CO2 from rising. Ppm then, you could say, is part of the formula for the 2 degrees warming threshold. The higher the ppm of CO2 in the atmosphere, the more likely global warming will exceed that target, and therefore increase the chances of devastating climatic effects.

If you want to keep track of the world’s ppm levels, you can keep an eye on Earth’s atmospheric CO2 here, as measured from the Mauna Loa Observatory in Hawaii.

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Read more from our content series as we explore everything you need to know about climate change (but were too afraid to ask) in the run up to Paris 2015!

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