Have you ever had this experience?  You’re in a conversation about climate change, maybe with someone whom you respect and admire. They turn to you, pause and with a somewhat patronizing tone, they say:

“ Well, you know, the climate has changed before.”

The implication, which usually remains unsaid, is that climate change is part of a natural cycle of heating and cooling of the planet. We didn’t do this and there’s nothing to do about it. Temperatures will swing back naturally.

And there you have it.  The work of tens of thousands of scientists, over the last 80 to 100 years, is dismissed as nothing more than a childish oversight.  Silly scientists!

It’s something I encounter regularly.  Not from random trolls on the internet, but from some of my friends.  From people who I think are smarter than me.

It’s another one of those things where, in the moment, you don’t know what to say. It’s frustrating, so I looked into it.  And now I realize that the key to debunking this common belief is to ask the person to explain just what they mean by “before”:

Do you mean in the history of the planet?

The average global temperature today is about 59˚ F.  Scientists have reconstructed estimates of the average global temperature going back 500 million years.  And yes, there were periods when the earth was much hotter – over 90˚F at times.

Here’s the thing though – humans did not exist then.  

Hominins, the ape-like proto-humans began evolving around 6 million years ago in a period when the planet cooled and the climate became more stable.

So let’s talk about time periods more relevant to humans.

Do you mean in the history of our species?

Our species, Homo sapiens, is thought to have begun evolving around about 800,000 years ago – with the earliest fossil records dating back about 300,000 years.  And yes, average temperatures have varied over this period.  

But it was never hotter than it is today.  

Scientists have concluded that swings in temperature over the last 800,000 years are due to the small cyclical changes in the earth’s orbit axis called Milankovitch cycles. 

These cycles, which play out over periods of 25,000 to 100,000 years, change the way solar energy is distributed over the earth and are the drivers of the several ice ages our planet has experienced.

So yes, since homo sapiens have been around, the climate has changed – there have been several ice ages.  And coming out of those ice ages, average global temperatures increased. The hottest period occurred between 130,000 and 115,000 years ago when it was about 1˚- 2˚C warmer than pre-industrial temperatures.  That is the kind of warming we are experiencing now.

Consider too that these interglacial warming periods occurred over tens of thousands of years.   Those are evolutionary timescapes – plants, animals and ecosystems had time to adapt to the relatively gradual changes in climate.

Do you mean in the history of our civilization?

Modern human civilization, with its permanent agriculture and settlements, has developed over just the past 10,000 years or so, but when people talk about civilization, they’re often referring to the “common era” of the last 2,000 years.

The warming we are experiencing now is taking place over decades – a blink of an eye in planetary time scales.

The other very different thing is that warming is now being driven by the concentration of CO2 and other greenhouse gases in our atmosphere – primarily due to fossil fuel use.  

We know that with higher CO2 levels in the atmosphere, more heat gets trapped and warms the planet.   CO2 and temperature move together in lockstep.

The combustion of fossil fuels since about the 1850s has led to CO2 concentrations in the atmosphere that have never occurred while humans, (or hominins, or mammals, for that matter) have existed.  

If we don’t stop emissions soon, global average temperatures will also reach levels that no human has ever experienced.

So the next time someone says to you that the climate has changed before – I hope you’ll feel confident in saying the following :

“No, it hasn’t – not like this.  The changes in temperature and our atmosphere are unprecedented in all of human history and represent a grave threat to life as we know it.  We all need to get behind measures that will rapidly cut fossil fuel use to prevent the worst effects of climate change.”

Then for bonus points, you can send them this brilliant webcomic.