Corals as climate archive: Are El Niños intensifying?

A storm is gathering. Most likely a very big one. El Niño is a natural climate phenomenon occurring every two to seven years, towards the end of the year, but it comes in various different strengths, depending on the prevailing conditions.

El Niños are considered strong if the surface temperature of the water off the west coast of South America is more than 1.5 degrees Celsius above average. In July this year, the US National Oceanic and Atmospheric Administration (NOAA) was already observing temperature increases of greater than three degrees Celsius in the eastern and central Pacific.

The German Meteorological Service believes this "could in all likelihood develop into one of the strongest El Niño events since records began."

But a recent article by the Max Planck Institute for Meteorology states that whether or not strong El Niño events will intensify as a result of climate change "remains the subject of ongoing research."

The problem is that the data on ocean surface temperatures is only truly reliable from the 1980s onward, with the start of satellite observation. And this data doesn't tell us how El Niño behaved before industrialization and the burning of fossil fuels.

US researchers say corals can help to close this knowledge gap. Julia Cole, a climate scientist and paleoclimatologist at the University of Michigan, and her team recently published a study on this subject in the journal Science .

"We've known for a long time that the last 40 or 50 years has really strong El Niño events," says Cole. "It's been hard to say whether the recent strong El Niño that we've had is part of a natural cycle, or if it's something a little bit unusual related to being in a warmer than normal world." The coral analysis points firmly in one direction: Unusual. Not normal.

Cole and her team examined fossilized and living corals around the Galapagos Islands off Ecuador in the eastern Pacific. Corals grow between one and two centimeters per year, and in doing so form a structure similar to growth rings on trees. "We made use of the fact that corals record ocean temperatures as they grow, in the chemical composition of their skeleton," Cole explains.