The last survivors on Earth

The impact of astronomy can be felt across a huge range of scientific fields, but there are some questions which come around time and again, and of course one that everyone wants to know the answer to is whether there might be life on other planets. Dominic Ford spoke to Jack O'Malley James from the University of St. Andrews about his work using models of the sun to work out what the last living life forms on Earth might look like. This could give us an insight into what life might look like on other planets as well. He started by asking how the sun's light will affect the Earth in the future.

Jack - As the sun ages, it starts burning through its reserve of fuel and as that happens, the core of sun contracts, because it gets contracted, it heats up more and the reaction speeds up. And so, the net result is you get a more luminous star, a hotter and brighter sun. Then this has knock-on effects for the planets orbiting it, including our planet. So, the Earth is going to heat up as the sun throws out more energy towards it. So, surface temperature start increasing. So, over the next billion years or so, we're going up in luminosity by about 0.1 of the current value. But then once we get to the end of that billion years, we get to a tipping point where rapid ocean evaporation sets in. Because water vapour is a greenhouse gas, this sets in motion a runaway greenhouse effect and then temperatures really rapidly start climbing, the oceans really start evaporating and over a further billion years from that, we lose all the water vapour from the atmosphere to space and then we're left with a very dry planet.

Dominic - So, how do you go about modelling the effect that an increase in the sun's luminosity has on the climate of the Earth?

Jack - So, because of the timescales that we're looking at, it almost doesn't pay to have a very complex model in place. So we have a very simple surface temperature climate model which takes into account greenhouse gas values and the changing luminosity of the sun. So, it puts all these together and calculates the temperature at different points over the surface of the Earth. So, different points in latitude and this gives a rough idea of average temperatures in these particular zones, and then we sort of extrapolate that upwards as well and work out the temperature profile upwards in the atmosphere which gives us an idea of temperature change with altitude as well. Then we can start using those temperatures to work out when we start losing liquid water in certain habitats and what kind of microbes could and could not live there.

Dominic - Now, I know you've been researching what impact that will have on the sort of life forms we see on Earth. Are we going to go back to an era like when the dinosaurs were around when the earth is a warmer place with more cold blooded creatures, do you think?

Jack - Something like that is possible within the very early stages before we get this rapid ocean evaporation taking place. So, the other effects in terms of life that's coming into play is the increase in silica weathering. So, if you have more water vapour in the atmosphere, you have more rain and rain draws down carbon dioxide from the atmosphere when it interacts with silicate rocks and minerals in the surface. And so, if you got more rain, it would be taking away more carbon dioxide. Take away carbon dioxide, it's not really good situation for plants. So, once we start lowering carbon dioxide levels, plants starts to die off and the animals that depend on the plants also start dying off. And the most vulnerable animals are also the ones that have most recently evolved. So, the larger mammals and things like that will probably be the first to go. And so, cold blooded reptiles would have a better chance of surviving for a little bit longer, but it's a very short window of time. So, plants and animals would all be on a rapid decline into extinction.

Dominic - So, if you lose the plants and you lose the animals, does that mean you're just left with bacteria and fungi?

Jack - Yes, you'd end up with at first probably, a fairly diverse microbial world, similar to the early Earth in way. So, the early Earth was solely inhabited by microbes for a good couple of billion years and it would be a return to that. But then because conditions become more extreme, temperatures increase. Only the really hardy extremophile microbes will be the ones that can really survive. It's likely there'll be similarities between these and the sort of extremophiles we see on Earth today in places like Yellowstone and other sort of volcanic springs and things like that. But it's also thought that these were the very first life forms to evolve. So, the early Earth was a very hot, hostile environment and so, it's thought that when life first emerged, it emerged to fit these conditions. So, these kind of microbes were possibly the first microbes on Earth and it's possible they'll be the last as well.

Dominic - I guess for search for life on Mars has in recent years become focused on what might be beneath its surface where it's thought there might be a more protected environment. Would a similar thing happen on the Earth, that if the sun was getting warmer, then life might thrive better beneath the surface?

Jack - It is definitely possible. Because we lose the plants and we lose oxygen, we also lose ozone which forms a protective layer around the atmosphere that protects life from UV radiation. So, you'd expect to see a movement underground just for extra UV protection. The situation gets a bit complicated because the general trend, if you were to look at the temperature profile of the crust of the earth, as you go deeper down, temperature goes up. So, if you've very hot temperatures on the surface, you'd get very hot temperatures underground as well. So, if it's too hot for life on the surface, it will be too hot beneath the ground. But this is just a general trend and you see on Earth today, I think the deepest they found life is something like 5 km down and it depends on the particular rock type. So it's very hard to put a final lifetime on life underground, to the point where life could actually have its final refuge underground.

Dominic - But I guess it's very timely because we're discovering lowest numbers of extra solar planets and it's very interesting to think about whether those might have life on them.

Jack - One of the reasons we started doing this work is because if you were to look at an Earth-like planet, so you imagine an exact copy of the Earth around another star and we were to take a snapshot of this in time, either its early lifetime when life first started in the far future when conditions are not very good for quite a lot of life. You're more likely to come across microbial life on the surface than you are, the sort of rich diverse biosphere that we've got today. So, it's very useful to have an idea of what kind of tell-tale signatures this microbial world would leave behind that's different to the signatures we'd see from life as we know it on Earth today. So, it helps in the search for life in different scenarios.