Page 67 - Earth's Climate Past and Future

CHAPTER 3

CO and Long-Term

2
Climate

Why is Earth habitable? The answer to this question at first seems obvious:
Earth is just the right distance from the Sun for solar heating to keep it
comfortable for life. With a mean temperature of 15°C (59°F) and a relatively
small range of geographic variation around that average, life can flourish almost
everywhere on this planet.
But this answer proves insufficient when we ask why Earth has remained
habitable for most of the 4.55 Byr of its existence. Over that immense interval
of time, our Sun has slowly increased in strength by 25% to 30%, a trend that
should have produced a very large climatic warming. Yet somehow Earth’s
climate has varied only within relatively narrow limits. Our planet’s continuing
habitability seems to require some kind of natural thermostat that allows its
climate to warm up but not overheat during greenhouse eras (times when no
ice sheets are present) and to cool off but not freeze solid during icehouse
eras (times like today when ice sheets are present). This chapter describes the
search for Earth’s thermostat.