The Earth’s climate system moves slowly, since the atmosphere and oceans take time to reach new equilibria. As a result, there are some questions about the climate where it’s much easier to provide an answer for a thousand years from now than it is for, say, the next hundred years. When we look at past changes in sea level, for example, the planet’s ice may take thousands of years to come to equilibrium. But we can use those views of the past as a preview of what’s in store for us.

During the previous break between glacial periods, about 120,000 years ago, sea level was around six to nine meters (20 to 30 feet) higher than it is today, as the cycles in Earth’s orbit that drive the “ice ages” were in a particularly warm phase then. But working out precisely how warm that world was compared to the present day has been difficult. Have we already reached that temperature because of human-driven warming, or are we still a few degrees off from that? Estimates based on natural climate records have differed, which makes it hard to say how much sea level rise we’re committing ourselves to.