A school bus goes up a hill, then down. The hilltop is the tipping point. Earth is the bus, but with no driver, just quarreling teenagers, called governments. And no brakes.
Source: the late Dr. Stephen Schneider, Professor and climate scientist at Stanford University, from a CD recorded from his address at the Climate One at the Commonwealth Club in San Francisco on November 3, 2009. http://www.climate-one.org/
Notes: Full quote:
"What is a tipping point? A school bus with your kids on it. It goes up to the top of a hill and now it goes down. The top of a hill is a tipping point because gravity is going to make that bus go down. Why don't we worry? We assume the school board did due diligence with hiring drivers who are well trained and they are constantly inspecting the brakes, which they do.
However, we are now talking about our planet in this metaphor where there is no driver, just a bunch of quarreling teenagers, called our governments, and we have no idea if the planet has brakes. So, what we are doing is going headlong into increasing our warming (by burning fossil fuels and releasing carbon dioxide) and changing the climate outside of the range in which we have no modern precedent, in which modern human civilization evolved. We are not there yet (of nasty consequences), but we are getting there. What we are saying though is 'Let's hope we are lucky (with climate change).'"