Today’s XKCD cartoon was interesting (as well as the usual funny and clever).

The humour is contrasting the ephemeral world of Hollywood with the eventual fate of the world. My initial reaction, beyond enjoyment of the cartoon, was “what, only 800 million years?”.

Two things about this. First, only a geological training could make me say such a thing. Secondly, that’s really soon! In my head, I had the end of the earth as being 5 billion years away. Given that’s longer than the earth already existed, it was over my geological event horizon. The 800 million year figure is, this being XKCD, entirely accurate, and I have rocks sitting on my desk at work that are older than 800 million years.

The difference between the two numbers is because they are talking about different things. Long before it becomes a red giant (5Ga) the sun will hugely increase its output and burn off the thin blue/green layer on the earth’s surface (800Ma). This I did not know. What surprises me is how much this changes the way I think about things.

It’s not about people. The concept of the Anthropocene is interesting and useful in lots of ways and one of them is the way it jams together the geological and human timescales. We know now we have messed up the earth enough to leave a trace for a very long time – humans are the most remarkable thing to ever happen to this planet – but we know that whatever is around in 10s of millions years time, there won’t be any humans, or if ‘we’ avoid extinction, we won’t be human anymore.

So people and 800Ma are things that don’t match. But, somewhere in my brain was a view of the earth as a system, with a long past and a long future. What I know now is that the future will ‘soon’ be very different. You don’t need to like the Gaia hypothesis to be impressed with the way life and earth have cooperated to moderate temperatures and keep liquid water on the surface of the earth for 4 billion years, during which time the solar luminosity has varied a lot. It seems that the sun will, within less than a billion years, break this nice balance and sterilise the planet. Life, having spent ?3 billion years failing to become leave much of a trace has done some remarkable things in the last 540Ma. Does it really only have 800Ma more years to go? What about plate tectonics? Having chugged away for ?3Ga, will it survive a lack of lubricating water, will subduction zones gum up leading to plume tectonics and massive volcanoes?

We’ll never know of course as we’ll be dead. Ah well.

Anyway, thank you Mr Munroe for expanding my mind. My brain does hurt though. Some white wine I think.