The first large-scale study of temperature change where rivers meet the sea has found Australian estuaries are warming twice as fast as either the ocean or land nearby. Although only conducted in part of one country, studies at individual Northern Hemisphere sites suggest a disturbing pattern is emerging, threatening sites of great economic and environmental value.
“Estuaries are very important both for the environment and communities for fishing and aquaculture," the University of Sydney's Dr Elliot Scanes told IFLScience. "They are places where a lot of juvenile fish live, where nutrients are transported from land to the ocean.” Scanes notes the area where the study was conducted includes a major estuarine oyster farming industry.
A century of data collected from a handful of estuaries in North America and Europe shows all are warming rapidly. The sites studied are particularly important given the large numbers of people living nearby, but it's less clear how representative they are. To address this gap Scanes and Professor Pauline Ross studied 166 estuaries along 1,100 kilometers (700 miles) of New South Wales coastline over a 12-year period.
In Nature Communications they report an astonishing 2.16ºC (3.9ºF) rise, almost 0.2ºC per year. "This is evidence that climate change has arrived in Australia; it is not a projection based on modeling, but empirical data from more than a decade of investigation,” Scanes said in a statement. By comparison the state trend for land temperatures was a little over half as fast over the same period, and the Pacific Ocean warmed slower still.
Climate scientists often insist on a 30-year time samples to establish trends. Anything shorter risks the cherry-picking data so beloved of climate change deniers by using an unrepresentative starting point.
“It might be nice to have longer, but I think it would not be a good idea to ignore this effect," Scanes acknowledged to IFLScience. "We were able to cover three El Niños and three La Niñas, which are the main source of inter-yearly variation in weather in south-eastern Australia.”
In addition to the overall trend, the study allowed a comparison between different sorts of estuaries with the variation providing clues to the cause. "Lagoons (shallow water bodies separated from the ocean by barrier islands) and rivers increased in temperature faster than creeks and lakes because they are shallower with more limited ocean exchange," Ross said.
Scanes told IFLScience the shallow nature of most Australian estuaries leads them to warm faster than the open ocean, as they're more exposed to the atmosphere, a pattern that also holds in much of the rest of the world. The study did not explore why temperatures were rising faster than over land, however.
Heat is not the only threat the estuaries face. Their pH levels fell 0.09 units a year, a trend that could lead to dangerous acidity. Meanwhile, rivers became more salty, but reduced flow cut more lagoons off from exchange with the ocean, paradoxically making them less saline.