"If we continue on the business as usual trajectory, there will be a tipping point that we cannot avert," says John P. Holdren, science advisor to President Obama. "We will indeed drive the car over the cliff."
Scientists predict that by 2020, global catastrophes may well begin to accelerate. The human population is expected to explode and animal species may be dying off at a rapid rate. As the world becomes more chaotic, the costs of mending it would grow more and more daunting. By 2030, gradually rising temperatures may have shifted rainfall patterns around the globe, and many experts warn much of the world may face serious shortages of our most basic need -- water.
"By 2030, two-thirds of the world's population will be under water stress," says Janine Benyus, science writer and founder of the Biomimicry Institute.
Some cities will have the forethought to plan ahead. Starting in 2009, San Diego began building huge desalination plants to turn ocean water into an abundance of fresh water. But in the middle of the country, people may be running out -- and there may well not be funds to transport it from the coast. If and when a place like Tucson, Ariz., runs dry, people will panic.
"Something that will catch people's attention is the first rich city that just runs out of water," says Homer-Dixon.
Americans may well meet these challenges with resourcefulness and work hard to keep the threats at bay. But even as things stabilize on the home front, experts predict hundreds of thousands of environmental refugees may begin streaming through Europe, fleeing droughts and famines. Millions of Latin Americans could align on the U.S. border seeking entry, and some could encounter violent resistance.
"I can't imagine the horrors that will take place on the border as millions of refugees try to get into the United States," says Klare.
In the history of Earth, there have been five mass extinctions in which at least half the species on the planet disappeared. Scientists believe the extinctions were brought on by natural disasters -- massive volcanic eruptions, rapid climate changes and meteors hitting Earth.
Today, scientists say we are in the middle of a "sixth extinction" -- and for the first time, it's being caused by one species -- us. It seems inconceivable that we could do so much damage to our planet that we actually cause society as we know it to collapse. But historical precedent shows that it is, in fact, a very real possibility.
"Every society that collapsed thought it couldn't happen to them," says Joseph Tainter, an expert in anthropology and societal collapse. "The Roman Empire thought it couldn't happen. The Maya civilization thought it couldn't happen. Everyone thought it couldn't happen to them. But it did."
These populations grew too much and exhausted their resources -- and their climate suddenly changed. People were forced to fight each other for what little was left or face starvation. Entire societies broke down.
"Civilizations in the past have lost the fight," says climatologist Heidi Cullen. "They have collapsed as a result of the inability to deal with several different events going on at once. I think the takeaway is that honestly, we are not that special."