End of the world – when the Sun fries the Earth

Here’s how the Sun will evolve, and what it means for life on Earth. I’m afraid there’s some really bad news 1.1 billion years from now. And then it gets worse.

Venus.	(NASA, Pioneer Venus)

If you find yourself wondering  “The doomers say one thing, the Jehovah’s Witnesses say another, but where can I get some solid information about the apocalypse?”  then this is the blog for you. Here, I turn to astrophysics for some answers. The Sun holds the key.

The diagram below shows the life cycle of the Sun, lined up with the timescale of life on Earth. The time sequence is rather simple. The Sun formed about 4.5 billion years ago, and the planets formed with it. The simplest life (prokaryotes) emerged less than a billion years after that. The complex animals, fish, land plants and insects appeared about half a billion years ago in the “Cambrian explosion” ^[1]. That’s when life emerged from the sea onto the land.

↑ Early Life

↑ Complex Life

↑ Photosynthesis

↑ End of land life

↑ Multicellular Life

↑ End of All Life on Earth

←

Duration Of Life On Earth

→

(Image: Life cycle of the Sun, Wikipedia)

That’s the past, but what does the future have in store? The long term prospects for life on Earth are shaped by the evolution of the Sun, and the Sun will gradually get warmer over the next few billion years. Here is part of a lecture by Prof. Richard Pogge of Ohio State University, called “The Once and Future Sun” ^[2], where he goes over the early life of the Sun and explains how things are going to pan out for life on Earth (note: 1 Gyr means one billion years):

A Star is Born,  T=0
Hydrogen ignition occurred 4.5 Gyr ago. There is sufficient hydrogen available in the central core of the Sun to sustain nuclear fusion at a fairly constant rate for almost 11 Gyr. The young Sun started out with slightly different properties than we see today. It was a little smaller (0.90 R_sun ), a little fainter (0.70 L_sun ) and a little cooler (5586 K).

There is some evidence in the fossil record of the different properties of the young Sun, as reflected in how sunlight affected life on Earth. This evidence is still hard to read precisely, but it largely confirms in outline the fact that Sun has steadily, if slowly, evolved over the life of the Earth.

The Sun Today,  T=4.55 Gyr
The Sun today is a middle-aged star, 4.55 billion years old.

“Quiet Adulthood”
The Sun evolves very slowly over this time as it consumes hydrogen in its core. It grows slightly larger, it gets slightly brighter, and its temperature gets slightly hotter then slightly cooler.

Otherwise, very little else happens, so far as the Sun is concerned. For the planets, however, the change in total solar radiation has an impact. For one particular planet, the impact is quite dramatic.

Mid-Life Crisis for the Earth,  T=5.6 Gyr (1.1 Gyr from today):

• Sun will be 10% brighter than today;
• Extra solar energy causes a Moist Greenhouse Effect.

The Earth’s atmosphere will dry out as water vapor is lost to space. Such a situation will probably spell the end of large surface life on Earth. Some types of marine life and simpler life forms will likely survive in the oceans and localized pools of water.

Venus on Earth,  T=8 Gyr (3.5 Gyr from today):

• Sun will be 40% brighter than today;
• Extra solar energy results in a Runaway Greenhouse Effect.

The oceans will evaporate into space, and conditions on the Earth will be like those on Venus today. Such conditions will probably mean the end of all forms of terrestrial life.

R.W. Pogge, The Once and Future Sun ^[2]

Life on Earth has existed for about four billion years, and has three and a half billion years left to go. That is to say, life has more time behind it than ahead of it. Life on land will only exist for one and a half billion years in total, and a half-billion years of that is already gone. The ever-warming Sun will turn the Earth into another Venus in the long run.

So there we have it. Sorry about that.

References

1. The Cambrian Period, University of California Museum of Paleontology  (WebCite cache)
2. The Once and Future Sun, Richard W. Pogge, New Vistas in Astronomy (1997)  (WebCite cache)