"All complex systems, such as the earth’s system, are characterized by one important feature: a very small change (“perturbation”) can lead to an extraordinarily large and unpredictable impact that “tips” the system into an entirely different and unpredictable state.Let’s take just one of the tipping points we’re heading for: a rise in global average temperature of above 2 degrees Celsius. There is a politically agreed global target—driven by the Intergovernmental Panel on Climate Change (IPCC)—to limit the global average temperature rise to 2 degrees Celsius. The rationale for this target is that a rise above 2 degrees carries a significant risk of catastrophic climate change that would almost certainly lead to irreversible planetary “tipping points” caused by events such as the melting of the Greenland Ice Shelf, the release of frozen methane deposits from Arctic permafrost, or dieback of the Amazon. But in fact the first two are happening now—at below the 2-degree threshold. As for the third, we’re not waiting for climate change to do this—we’re doing it right now through deforestation. And unfortunately, recent research shows that we look certain to be heading for a larger rise in global average temperature than 2 degrees—a far larger rise. It is now very likely that we are looking at a future global average temperature rise of 4 degrees—and we can’t rule out a rise of 6 degrees. A 4-to 6-degree rise in global average temperature will be absolutely catastrophic. It will lead to runaway climate change, capable of tipping the planet into an entirely different state, rapidly. Earth would become a hellhole. In the decades along the way, we will witness unprecedented extremes in weather, fires, floods, heat waves, loss of crops and forests, water stress, and catastrophic sea-level rises. But even if we’re lucky enough to fall short of anything like a 4-to 6-degree rise in global temperature, there almost certainly won’t be a country called Bangladesh by the end of this century—it will be underwater. Large parts of Africa will become permanent disaster areas. The Amazon could be turned into savannah or even desert. And the entire agricultural system will be faced with an unprecedented threat. More “fortunate” countries such as the United States, the UK, and most of Europe may well look like something approaching militarized countries, with heavily defended border controls designed to prevent millions of people who are on the move from entering, because their own country is no longer habitable, or has insufficient water or food, or is experiencing conflict over increasingly scarce natural resources. These people will be “climate migrants.” The term “climate migrants” is one we will increasingly have to get used to. Indeed, anyone who thinks that the emerging global state of affairs does not have great potential for civil and international conflict is deluding themselves."