A study of sediments deposited beneath the Caribbean Sea between 100 million and 60 million years ago is being touted as a glimpse of our climatic future. It’s probably not, but a glimpse of the past can still tell us a lot about where we’re heading.
The study’s team of American and British investigators used neodymium, a rare mineral element preserved in the sediments, to retrace the movements of ocean currents during a natural greenhouse warming episode that occurred during and shortly after the end of the Age of Dinosaurs. Their main conclusion was that global temperature back then was "tightly correlated with changes in ocean circulation." Why is this of interest beyond a narrow circle of specialists? Presumably because future changes in marine circulation are an important part of the modern climate change narrative, though their effects are often overstated in the media.
In theory, global warming and/or abundant meltwater from shrinking Arctic ice sheets can disrupt ocean currents in ways that might slow the transport of tropical heat into the North Atlantic, thereby cooling western Europe. Geologic evidence suggests that this has indeed happened occasionally during the last few hundred thousand years, though its relevance to modern times is questionable.
Although the press release from the University of Missouri states that these new findings "could predict Earth’s future," the original article makes no such claim, and rightly so. Leaving aside various alternative interpretations of the neodymium and other nerdy nit-pickery, fundamental issues make these ancient records only mildly representative of the present.
The key events happened long ago under conditions that differed quite a bit from those of today. Back then, the Atlantic was narrower than it is now, shallow seas occupied much of North America, and Central America did not fully separate the Atlantic from the Pacific. There were no large ice masses back then to pour meltwater into the sea as there are now, and it’s not even clear whether the ocean disturbances were the cause or the effect of the ancient climatic shift.
A study that seems more relevant to our present situation appeared recently in the same journal. The study shows that slow natural warming cycles following the demise of the dinosaurs may have released huge bursts of heat-trapping methane from the sea floor. Because the world is warming again today—albeit due to fossil fuels burning rather than natural cycles (and because modern marine deposits still contain large amounts of frozen and dissolved methane) that example from the past warns us that we may be approaching a tipping point beyond which human-driven greenhouse warming could accelerate dramatically.
So what to make of this new Caribbean study? It’s still important because it presents a new way of reconstructing past ocean circulation and it helps to explain some potentially important changes in marine fossil records. The most valuable lesson it offers is simple. Greenhouse gases really can make our planet a lot warmer than it is today, so the idea of a future hothouse Earth is not just wild-eyed fantasy but hard reality. How do we know? Because it happened before, even without our help.