400 parts per what?

It was refreshing to see New Zealand climate scientist Jim Salinger writing in the Herald this week about carbon dioxide reaching 400 parts per million in the atmosphere for the first time in human history. That's not a particularly headline-grabbing figure – which is probably why the Herald titled the opinion piece 'Climate hurtling towards a hothouse Earth'.

As drastic as it sounds, that headline is probably pretty accurate. The 400ppm figure basically means that for every million molecules in the air, 400 of them are now the climate-warming CO₂. Back in 1750, when humans first started burning fossil fuels in earnest, that figure was more like 280ppm.

We know this because scientists have for years now been braving the cold of Antarctica, drilling out cores of ice that were laid down centuries ago, and testing the tiny trapped bubbles of air inside. They're also manning atmosphere testing stations high on the sides of mountains to work out what's happening in the air today (the 440ppm figure was recorded on the flanks of one of Hawaii's volcanoes).

It's becoming more and more clear that the whole question over whether global warming is a) happening, and b) caused by people has been answered. And the answers are a) yes, and b) we're afraid so.

That didn't stop the Herald from running an opinion piece the next day from climate change sceptic Chris de Freitas. That's all very well and good, and in the spirit of journalistic balance. But as one letter-writer noted, when the science community is agreed on the reality of human-caused climate change by about 30-1, then shouldn't the Herald run 30 more articles on the dangers of climate change to truly be balanced?

You can read the articles here and here. Tell us what you think?