Introduction

If there is one thing we depend on, it is the assurance that the Sun will shine day after day, year after year, constantly and dependably. We base this sense of certainty on the collective human experience of a constant Sun, and indeed, the concern or even terror that total solar eclipses brought on was a strong motivation for building eclipse predictors such as, possible, Stonehenge (Chapter 2). And yet there is strong evidence from the record of climate, from observing other stars, and from the physics of nuclear fusion, that the Sun has not really shined with constant output over time. Indeed, when the Sun was young, it almost certainly had a lower output than today by a significant amount, leading to what is called the “faint early Sun” or “faint young Sun” problem. This chapter explores the physics of that variation and the implications for Earth's ancient climate.

The case for an equable climate in the Archean

There is ample evidence that the Archean Earth possessed liquid water. The existence of metamorphosed sedimentary rocks from this period, as discussed in Chapter 11, require erosion by liquid water and deposition in a lake or marine environment. The presence of life itself, recorded through isotopic signatures and fossil evidence, also implies liquid water. As discussed in Chapter 12, we know of no living thing today that can get by without water. Many don't require oxygen (and are poisoned by it), but all require liquid water.