If and when the Amazon flips into a drier state, it would have an big effect of weather patterns. The Amazon is basically a big spot of wet tropics. Knock out the trees and lose that moist air, and the regional circulation pattern changes as well. A similar flip could occur in Canada's boreal forests (tipping point number eight). A die-off of these forests would release much of the 50 billion to 100 billion tons of carbon now trapped in permafrost.
The basic weather patterns that we've grown used to on weather maps are also subject to rapid change. Among them is what's called El Niño– Southern Oscillation—the ninth and last of Lenton's tipping points. El Niño involves movement of a blob of warm water on the west side of the Pacific Ocean toward the east, bringing with it moist warm air. When this warm water cools and circulates back westward, El Niño comes to an end and La Niña begins. These two patterns alternate roughly every five years. From observations, scientists have begun to see a more erratic trade-off between these two patterns. They fret that the weather patterns could flip to some different state—perhaps a more frequent switching off between the patterns. That would have a detrimental effect on the Amazon, says Lenton, exacerbating trends that already threaten to destroy the rain forest.
The real nightmare scenario is when all these changes begin to rein- force one another. The Arctic loses its summer sea ice, causing Greenland's ice to melt and encouraging the boreal forests to change as well. The freshwater runoff changes the thermohaline dynamics and affects the jet stream. The El Niño–Southern Oscillation and the Amazon interact in such a way as to reinforce one another, perhaps affecting the monsoon in India and Africa. "It wouldn't be such a silly thing to say that if you meddle with one, you might affect the other," says Lenton. "Which direction the causality would go is not always obvious. We know it's connected, we know it's nonlinear, we know they somehow couple together. When you see one change, you see changes in the other."
"Then we start talking about domino dynamics," says Lenton. "The worse case would be that kind of scenario in which you tip one thing and that encourages the tipping of another. You get these cascading effects."
It would take a perfect storm of climate flips to get us to this particular worst-case scenario. If it does come to pass, however, at least it will happen quickly.