Think of climate rise as your body's temperature rising: when you're 1°C over normal, you're sick but that's manageable. When you're at +2°C you're really sick; 4°C over normal your life is at stake.
Or think about this: 20000 years ago, Scandinavia, Canada were under 2 or 3 km of ice (like Greenland today). The oceans were 120 meters lower than today. Most of today's temperate countries (Europe, USA, China) were a cold tundra with reindeer, wholly rhinos, bison and mammoths, cave bears and cave lions. In these harsh conditions, only a few hundred thousand humans could scrape by; and the average globe temperature was only 4 to 5°C lower than in 1850 (let that sink in).
The 4°C temperature rise that brought us the neolithic revolution, agriculture and the first population explosion, was a 4000 years affair. Now we're talking of bringing a similar change over the course of 100 years, on a crowded planet of 8 billion people. The question isn't "what could go wrong?" at this point, but can we imagine ways out of a looming catastrophe?