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.

A group of researchers led by Jeremy Hoffman at Oregon State University compiled a large array of temperature records from seafloor sediment cores around the world to calculate the best possible average.

They collected 104 individual sea surface temperature records from 83 different sediment cores. Each record is based on a chemical proxy that is known to relate to water temperature, like the ratio of magnesium to calcium in the shells of tiny plankton called foraminifera. A lot of the hard work here is precisely lining up each core record onto one timeline—synced to ice core records—and carefully calculating the uncertainties to get the error bars right.

The researchers compared these numbers to the UK Met Office database of sea surface temperature measurements going back to the 1870s. They used averages of two time periods: 1870-1889 and 1995-2014. In each case, sediment core records were compared to instrumental measurements from the same location.

Sea surface temperatures during the last interglacial period peaked 125,000 years ago, and these were 0.5±0.3°C warmer than the late 1800s. However, temperatures from 1995-2014 were indistinguishable—0.1±0.3°C cooler. In other words, it appears that temperatures today are now equivalent to the warmest from the last interglacial period, when sea level was much higher.

(Note that these are sea surface temperatures; the atmosphere has warmed more quickly.)

So if the planet stopped warming and stayed where it is today, the past gives us an idea of how much sea level rise we could expect to see in the long-term future. It would take much more than a century to complete that rise, but we are looking at well over five meters of elevation in the ocean.

The regional pattern of temperatures during the last interglacial period was different because the cause of the warmth—orbital changes—was different. Then, Earth’s tilt and orbit brought stronger sunshine to the poles; the greenhouse gases driving today's warming are globally distributed. Compared to the late 1800s, the tropics were cooler and the mid- to high-latitudes were warmer 125,000 years ago. Analysis like this is also useful for climate modelers, who use simulations of past climate changes to test how well their models are representing all the physics of Earth’s complex climate system.

Humans are currently turning up the planet’s thermostat much more quickly than natural processes have done in the past. Still, we learn a lot by uncovering the way the planet has responded to different set points on the Earth's thermostat. We know that we’ve already warmed the globe enough to significantly raise sea levels. The more complicated question that researchers keep working to answer is “Just how quickly will it rise?”

Science, 2017. DOI: 10.1126/science.aai8464 (About DOIs).