We're unsure how humans might affect the climate long-term, but it's certain that nature alone can produce sudden, extreme climatic shifts. New findings suggest these abrupt changes might be completely unpredictable.
Earth's climate more or less fits into a repetitive cycle - long stretches of Ice Age followed by relatively brief periods of warmer climates like what we're experiencing now. The underlying causes of this climate cycle are complex, but they're partially determined by long-term fluctuations in the Earth's orbit around the Sun.
But within these periods of generally warmer or cooler climate are sudden spikes in temperature, in which the average temperature can jump 10 to 15 degrees in less than a decade. Analysis of ice cores in Greenland have revealed several such spikes occurred in the last Ice Age, and each temperature shift lasted for about 1,000 years until suddenly reverting back to the original average temperature.
Known as Dansgaard-Oeschger events after their discoverers, these events have no obvious cause, and modern climatologists still don't know how to fit them into their climate models. That's a problem, particularly if there's a chance another such spike of increased temperature could happen today without warning. But now Peter Ditlevsen, a climate researcher at the Niels Bohr Institute, thinks he's made a breakthrough in understanding these events.
He believes there are two possibilities, and each can be described by a reasonably simple metaphor. The first is a seesaw tipped to one side, in which climate remains in one basic state until too much weight is placed on the other side. That tips the seesaw to the other side and causes the spike. We don't know what the "weight" is in this analogy - it might be the accumulation of carbon dioxide in the atmosphere, but it could just as easily be something else - but the basic takeaway is that, in this scenario, these Dansgaard-Oeschger events are fundamentally predictable, and we just need to know how much "weight" has to be put on the "seesaw" to cause one to start.
But there's another possibility, and he describes it as a ball in a trench that will, with sufficient push and pull from outside forces, roll out of the trench and into another one nearby. (This is admittedly a bit of a weird metaphor, but, at the risk of a terrible pun, roll with it.) In this case, the outside forces, which include storms, heat waves, heavy rainfall and ice sheets melting, will eventually push the ball out of one trench and into the other. But this is a chaotic system, and there's a huge amount of random chance in which of these climatic events will occur when and which event will finally have enough force to push the ball from one climate state to the other, spiked state.
Ditlevsen says he's modeled both possibilities, and the evidence seems to conclusively show that chaos-dynamical fluctuations were the cause of these temperature spikes. That means it's the ball in the trench metaphor, not the seesaw, and it also means such climate shifts are almost impossible to predict. While he stresses that his results only pertain to the temperature swings in the last Ice Age, he points out that this could mean a dramatic temperature swing could happen today at virtually any time, assuming the same forces are still in play.
"Today we have a different situation than during the ice age. The Earth has not had such a high CO2 content in the atmosphere since more than 15 million years ago, when the climate was very warm and alligators lived in England. So we have already started tilting the seesaw and at the same time the ball is perhaps getting kicked more and could jump over into the other trench. This could mean that the climate might not just slowly gets warmer over the next 1000 years, but that major climate changes theoretically could happen within a few decades."