Microscopic Evidence of a Warming World

By Michael D. Lemonick

Much of the skepticism about whether humans are causing climate change would probably go away if only the Neanderthals had invented a good, reliable thermometer. Whenever someone announces that this year or that season was the warmest or the third warmest or whatever on record, “on record” only goes back to the late 1800’s at best. That’s when accurate instrument measurements of temperature began. To obtain readings from before that time, scientists have to rely on proxy measurements — things like tree rings or pollen deposits or oxygen-isotope ratios that change as temperature changes. The famous (or infamous) “Hockey Stick” graph showing temperatures shooting up during the second half of the last century after hundreds of years of relative stability is based almost entirely on proxies — and records from ancient ice cores push the temperature record back hundreds of thousands of years further.

Drawings of different types
of foraminifera. Credit: NOAA.

With a few small exceptions, proxies for the past thousand years or so are in broad agreement regarding temperature trends, and now one more has been reported in the journal Geophysical Research Letters — but there’s a twist.

This proxy involves the shells of tiny sea creatures called foraminifera, or forams, which lived and died in the bottom waters of the Lower St. Lawrence Estuary, in eastern Canada. The shells are made of calcium carbonate, which contains oxygen drawn from the surrounding seawater — and when the temperature warms, the ratio of two different types of oxygen atoms changes (you get more of the heavier isotope oxygen-18 in relation to the lighter oxygen-16).

Benoit Thibodeau and his colleagues from the Montreal-based GEOTOP, a multi-university geosciences institute, found that the oxygen-18 in foram shells remained relatively constant for about a thousand years, and then, over the past century, shot up significantly.

What makes this a little different from other proxies, though, is that the water 400 meters down, where the forams lived, is effectively insulated from the surface. “The bottom water,” says Thibodeau, “barely mixes with the upper layers.”

Now comes the twist: You might think that this couldn’t have anything to do with climate change, since greenhouse gases warm the atmosphere, which in turn warms the land and the sea surface. And indeed, Thibodeau et. al. aren’t ready to make the human link… yet. What they do know is that these bottom waters are fed by a mix of cold water coming down from the north, in the Labrador Current, and warmer water coming up from the south — what’s called the North Atlantic Central Water, which in turn is part of the Gulf Stream. Changes in the temperatures and strength of those feeder currents are presumably what’s warming things up down below, and, says Thibodeau, human-caused climate change “could be warming those water masses at their source.” Could being the operative word: “We know climate history of those water masses, for 50 years or less,” he says, “so it’s hard… we really need to have a better understanding of how they have warmed in recent years.”