A shocking increase.
Mercury pollution levels in the Earth's atmosphere have spiked by seven times since the modern era began around 1500 CE, new research shows.
The work, published as a study in the journal Geophysical Research Letters, found that the atmosphere once contained about 580 metric tons of mercury before human emissions of the substance began.
Today, those atmospheric mercury levels are as high as 4,000 metric tons, according to a 2015 study cited by the researchers. That's a huge gap, and some of the best evidence yet of the extent of mercury pollution by humans.
"Understanding the natural mercury cycle driven by volcanic emissions sets a baseline goal for policies aimed at reducing mercury emissions and allows us to understand the full impact of human activities on the environment," said lead author Elsie M. Sunderland, an environmental scientist at Harvard University, in a statement about the work.
Mercury, and in particular a form of it called methylmercury, is highly toxic to humans. Almost all of us are exposed to it to some degree, and where we really get a significant dose of the stuff is through eating a lot of fish and seafood, in which the metal "bioaccumulates."
We've long known that volcanoes are nature's biggest emitters of the metal, being the huge smokestacks that they are. But until now, measuring the amount they emit into the atmosphere has proved near impossible.
According to the researchers, less than a nanogram of mercury is typically found in a cubic meter of air — a trace so minuscule that it can't even be detected by satellites.
Instead, they turned to another volcanic emission: the smelly and easily visible sulfur. By carefully calculating the ratio between the two elements in a volcanic plume, the researchers could retroactively estimate the amount of mercury emitted into the atmosphere by past eruptions.
"The nice thing about sulfur dioxide is that it's really easy to see using satellites," said co-author Benjamin Geyman, an Earth scientist at Harvard, in the statement. "Using sulfur dioxide as a proxy for mercury allows us to understand where and when volcanic mercury emissions are occurring."
Far and Wide
With the help of an atmospheric model, the researchers could also track the movements of the plumes, confirming that mercury can travel far past its injection site.
Despite this mobility, the researchers say that volcanic emissions are only a small part of ground-level accumulations of the metal in most areas of the globe. Some regions with nearby active volcanoes are exceptions, however, and their emissions make it difficult to track humans' impact. That kink is something the researchers hope their new methodology will be able to iron out.
"It's important to be able to correct for natural variability in the volcanic influence in places where we think that influence may not be negligible," Geyman said.
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