The discovery of a 60-million-years-old fossil snake from northeastern Colombia, South America, whose size makes today’s anacondas and pythons seem like garter snakes is being hailed for the light it sheds on ancient climates, but let’s be honest here: the attention it’s getting has more to do with its mammoth measurements. At 13 meters (42.7 feet) from nose to tail, tipping the scales at an estimated 1,135 kilograms (2,500 pounds) and with a girth that would reach the hips of a grown man who had the misfortune to be around when the snake was slithering by, Titanoboa cerrejonensis (it’s from the Greek titan, for giant) was the largest snake the world has ever seen. 

To support a cold-blooded body of that size, estimate scientists led by paleobotanist Carlos Jaramillo of the Smithsonian Tropical Research Institute and paleontologist Jason Head of the University of Toronto, its tropical home would have had to have a minimum mean annual temperature of 30 to 34 degrees Celsius (86 to 93 degrees Fahrenheit), way hotter than today’s tropics. That calls into question the longstanding belief that the planet has a thermostat that keeps tropical temps in check. As they describe in tomorrow’s issue of Nature, it has long been known that there is a rough correlation between an era’s temperature and the size of its cold-blooded animals: warmer temps, bigger beasts. But the average temperature at which the giant snake thrived was 5 degrees warmer than the maximum for tropical rainforests today.

If tropical temps 60 million years ago were really that high, it undercuts the idea that the planet has a thermostat that keeps a tight lid on them. That’s important for three reasons, climatologist Matthew Huber of Purdue University points out in an accompanying article. The tropics, from 30° N to 30° S, make up half of Earth’s surface area and therefore strongly influence the sensitivity of global temperatures to greenhouse-gas concentrations and other climate-altering forcing. Also, the tropics have long been considered “stable, safe havens for fauna and flora compared with the more variable high latitudes,” says Huber; if their temperature can soar, they might not be such safe arks in a greenhouse world after all. Finally, temperature gradients drive atmospheric circulation, so a super-hot tropics would alter the general circulation.

Using snake size to infer temperatures is novel, to say the least, and might be wrong. But if the claims are right, says Huber, “there is no tropical thermostat: although negative feedbacks may slow or inhibit tropical warming, they do not provide a hard limit, and theories that predict the existence of thermostats are invalid.”