How can you tell humans cause climate change?

Listener Peter emailed us asking for us to set the record straight over climate change. Climate scientist Ella Gilbert breaks down the differences between natural change and the human contribution...

Ella - It's a tough one. And I suppose we use a lot of different types of evidence and there's a few different ways of doing it. So when you think about natural climate change, I mean the climate has changed over so many different timescales as a result of natural factors. So things like the amount of oxygen in the planet's atmosphere or the tilt of the Earth's orbit or the way that the earth moves around the sun, all these things can affect the climate on the, on the earth surface over, you know, hundreds of thousands or millions or billions of years.

Ella - So you have to think about which kind of timescale you're thinking about, if you think about much more contemporary climate change, which I suspect the question is probably more interested in, then things like solar cycles or volcanic eruptions. Those things have either very defined injection points - like a volcanic eruption for instance - or you can extract a kind of a cycle. So with solar activity activity, for instance.

Chris - I don't want to put words into Peter's mouth, but I think where he's coming from is from the perspective that if we wind the clock back, say 30 million years or so, the world was so warm, there was not a trace of ice anywhere. The polar ice caps are completely gone. And so there were no humans around then to have caused climate change. So we know that the temperature does have these radical departures. We know the planet does warm and cool, and it does it naturally. So what is different this time and, and why do we think this time is any worse than all the previous times?

Ella - Yeah, you're completely right. There's periods in our history where there's been no ice. There's periods when the entire planet has been covered in ice for, you know, 12 million years. So it really varies between these huge extremes. But what distinguishes modern climate change from all of that kind of climate change is that the rate at which it's happening is completely unprecedented. If we look at evidence such as ice cores, which we take in Antarctica predominantly, you can see that the amount that the atmosphere and the temperature has increased in the last, you know, a century or two centuries, is completely unprecedented in up to, you know, a million years or so.

Chris - So do you think that this is grounds to really worry or do you think that in the same way that the planet has always bounced back, there is still life on Earth, that we're resourceful enough, there won't be a major problem. We'll just carry on business as usual. It'll be fine?

Ella - I think that really depends on whether you're a glass half full or glass half empty kind of person. For me, I think about it in the sense that it's a terrifying prospect, but we must use that fear in a positive way because that should motivate us to actually implement action. And I mean, what's the worst that could happen? We create a world that's sustainable and environmentally friendly for nothing. Oh no. It'd be much better. It'd be much nicer. Genuinely, there's a whole bunch of extra benefits that go along with tackling climate change.

Chris - As one person put it to me. You shouldn't regard a positive future in terms of environment as necessarily one which is a kind of rough future, which where we have enormous sacrifice. Actually, there'll be enormous benefits to come with such as a cleaner environment, better air to breathe and so on. So, indeed.

Ella - Yeah. No one is suggesting a return to medieval times!

Chris - No, not at the moment anyway.