Human contributions to climate change

Climate expert James Renwick explains how detective chemistry provides the evidence that humans are responsible for climate change.

Questions for discussion:

• How much have greenhouse gas levels increased from the pre-industrial level?

• What does James mean by the phrase ‘detective chemistry’?

Transcript

DREW BINGHAM

It’s very clear that humans are causing climate change right now. Just the weight of evidence we’ve accumulated over the years show that it is from humans.

PROFESSOR JAMES RENWICK

We do know that it’s caused by humans, by us. And that’s what people mean by climate change now. So what’s happening today is that the amount of greenhouse gas in the air is going up dramatically, and we measure that very carefully as well, and we know that greenhouse gas levels, carbon dioxide especially, has gone up nearly 50% in the last couple of hundred years from what we call the pre-industrial level.

And then, we can look at the chemistry of that carbon dioxide and methane and so on, and we can tell that most of the increase comes from what we call fossil carbon. Human activity, industrial activity releases carbon dioxide into the air – we burn fossil fuels. There are radioactive isotopes, there’s a chemical signature to the carbon dioxide that we breathe out every day, and there’s a chemical signature to the carbon that comes from a source that’s been underground for a very long time such as coal and oil. So if you dig up that old carbon and burn it and put carbon dioxide in the air, what you see is these little trace bits of radioactivity going down, so the carbon in the atmosphere is becoming less radioactive. It’s a tiny fraction, but it’s there, we can measure it.

It’s a bit of detective chemistry really, but the numbers all add up. We know that the amount that’s going into the atmosphere and the oceans matches up with the amount that we burn every year. So it’s pretty conclusive that it’s human burning of fossil fuels that’s mostly changing the climate.

Acknowledgements
Drew Bingham, Ministry for the Environment
Professor James Renwick, Victoria University of Wellington
Baring Head, and Principal Technician Gordon Brailsford collecting and analysing air samples, NIWA
Mass spectrometry screengrabs, Gordon Brailsford, NIWA
Graph CO₂ concentrations at Baring Head, NIWA