First author

Some consequences of climate change are already unfolding. Glaciers and ice sheets are melting, and sea levels are rising as a result. However, scientists aren't certain by how much the rate of sea-level rise might accelerate; current predictions for increases until 2100 range from 0.3 centimetres to 1.4 centimetres per year. But Paul Blanchon, a geoscientist at the National Autonomous University of Mexico in Cancún, and his colleagues have learned that a sudden, catastrophic increase of more than 5 centimetres per year over a 50-year stretch is possible. On page 881, they describe their discovery that a sea-level jump of 2–3 metres already happened about 121,000 years ago. Blanchon tells Nature how and why it could recur.

How did you find out that sea levels had risen so quickly in the past?

We were studying fossil reefs along the Yucatán peninsula in eastern Mexico, looking for interruptions in the reefs' development, when we found two reef crests. One crest was about three metres above the current sea level, the other six. Some event had clearly disrupted their growth, killing the lower reef first and, within 50 years, allowing the higher one to develop into territory that is now farther inland. One possible cause of such disruption is an earthquake, but we know the peninsula was stable in the reefs' lifetimes. The only other possibility is a rapid sea-level jump of two to three metres, which would essentially have drowned the lower reef.

Did you have to dive to the ocean floor to study the fossil reefs?

No, a theme park has been excavated in the middle of these reefs, which are on land south of Playa del Carmen. There's no other place in the world where reefs of this age are so exposed. From the excavations, we were able to reconstruct the reef's internal structure in three dimensions.

Were there any challenges involved with working in a theme park?

One key site was in the middle of a jaguar and puma exhibit. We had to get up at five in the morning, lower our ladders into the pit, do our studies and get out of there before the jaguars and pumas were let out.

What do your results mean for sea-level rises in the future?

This earlier ice-sheet collapse happened during an interglacial, when it was warm and there wasn't a lot of ice around — just as it is on Earth today. We're assuming rapid ice loss from an ice sheet produced the jump in sea level, because it's the only known process that could generate such a rapid increase. This could happen again.