News•February 28, 2013

Wet Times Are Masking New York's Real Drought Risk

The past several decades have been the wettest in nearly five centuries for the watershed serving the nation's largest city, New York, according to a new study. But that wet period is deceiving because it is masking the city's real drought history and may be lulling water managers into a sense of complacency, which could hurt the city when the next severe drought strikes.

Credit: Flickr/hjjanisch.

The study, led by a group of researchers at Columbia University’s Tree Ring Laboratory and published in the Journal of Climate, used tree ring and instrument records to reveal the New York City watershed's multicentury drought history. The study shows that severe droughts have been nonexistent in the New York City watershed since 1970, especially when compared to intense and long-duration drought events that occurred in this region during the 16th and 17th centuries. For example, a 23-year drought struck the area beginning in 1555, and it came just one year after the end of a 5-year drought.

The study raises the possibility that if the current “water boom” comes to an abrupt end as historical records show is not only possible, but perhaps likely, then the city’s water supply could be severely stressed, due in part to the population growth that has occurred since the last major drought. The five boroughs of New York City are expected to be home to more than 9 million people by 2030, up from the roughly 8 million today.

Despite the lack of a historically severe Northeast drought in recent years, the New York metropolitan region has still endured six water warnings and emergencies in the past few decades, most recently in 2002. Yet the study indicates that water managers born after 1966 have never experienced a truly significant New York drought in terms of both intensity and duration.

This is what 500-years of drought history looks like. Here are five records of hydroclimate variability across the eastern United States. Each line is a 20-yr spline of the each annual record. The dashed line is the mean of each record’s mean. Note the large spike near the end of the records, showing the sharp increase in precipitation since 1970.
Click on the image for a larger version. Credit: Pederson et al.

“When you have the 500-year perspective . . . then it just makes you wonder if we’re having water emergencies during this relatively wet period, then what’s going to happen when a drought like the early 1800s or the 1600s megadrought [occurs]?” said lead author Neil Pederson, an ecologist at Columbia’s Lamont-Doherty Earth Observatory. “Droughts can get worse than what we’ve known in most of our lifetimes.”

Pederson said the 43-year wet period in the New York region stands out so starkly from the rest of the area’s 500-year climate history that he didn’t believe his results at first, and went back and triple-checked all the data before sharing it with his peers.

New York City has been improving its water conservation efforts, with the average New Yorker consuming about 300 liters less of water per day now compared to 1988 levels.

“New York City has done a great job of reducing water consumption,” Pederson said.

But even with the efficiency gains, water managers will need to be more mindful of the region’s drought history, and implement flexible measures to save water during this wet period in case drier times return, the study said. In other words, New York shouldn’t give up some of its water rights to downstream states because it has a water surplus.

“We argue that a reallocation of New York City’s water rights may well put metropolitan water supply at risk,” the study said.

The researchers extracted tree ring samples from a dozen tree species, including four species that had never been used for reconstructing historical climates. The species diversity increases the confidence in the results, Pederson said.

The researchers said that neither known mechanisms of natural climate variability nor manmade global warming can fully account for the trend toward much wetter conditions in the Northeast. Climate change projections do show the eastern U.S. getting wetter as the world warms in response to increased amounts of greenhouse gases in the air, but the models don’t show as sharp of an upward trend as what has actually taken place.

Richard Seager, a professor at Lamont and co-author of the study, said the lack of an explanation for recent precipitation trends means that scientists don’t have much confidence that the wet conditions will continue. This, he said, should cause some discomfort for water managers in particular.

“Some things we just don’t know. It’s kind of sobering, that some of this long-term variation remains unexplained,” Seager said.