A couple of years ago I read Malcolm Gladwell's The Tipping Point: How Little Things Can Make a Big Difference. For those who haven't read it, a "tipping point" is the point where a trend goes from rarity to ubiquity, where everything changes. Gladwell focused on sociology and marketing, and used murder and suicide rates and fashion trends as his examples. However, there's also a similar concept in chaos theory, of bifurcation, where an insignificant change in a value leads to phase or state change in a system (as illustrated in this diagram of the long-term behaviour of the logistic equation, or by the Lorenz attractor).
The relevance of this is that the global climate is a chaotic system - and scientists are becoming increasingly concerned about tipping points which could lead to irreversible climate change.
We've known that there are such tipping points for some time. A study of our planet's past climate shows that there have been sudden phase changes in the climate - and not just on a geological timescale, but a human one. According to The Weather Makers, ice cores from Greenland have shown that "spectacular shifts in the North Atlantic climate [have] occurred over just five annual ice-layers". They also show repeated shutdowns in the Gulf Stream (caused by large flows of fresh water into the North Atlantic due to melting ice) as the Earth came out of the last glacial period, leading to sudden coolings of up to 5 degrees for as little as two hundred years. In short, the changes can be sudden, jagged, and against the overall climate trend.
The Washington Post article linked above focuses on three possible tipping points:
widespread coral bleaching that could damage the world's fisheries within three decades; dramatic sea level rise by the end of the century that would take tens of thousands of years to reverse; and, within 200 years, a shutdown of the ocean current that moderates temperatures in northern Europe.
Flannery focuses on the Gulf Stream and two others: the desertification of the Amazon rainforest (due to higher CO2 reducing transpiration, and hence rainfall), and the release of methane from deep-sea clathrate beds (due to an increase in deep-sea temperatures). The latter two would produce massive positive feedback, accelerating existing warming trends and making them far, far worse. The former would paradoxically freeze Northern Europe and North America precisely at a time the world was warming overall. However, it would also lead to massive climate shifts in tropical regions due to heat being trapped at the equator.
The problem is that while we know of these tipping points and positive feedback loops in the global climate, we don't know exactly when they will occur - and therefore what level of CO2 emissions is safe (or at least manageable). Worse, we may already be past the tipping point, and not know it. The inertia in the global climate means that the full effects of today's emissions will not be felt until around 2050. We are already physically committed to at least a degree of climate change - and we have no idea yet whether that will be too much.
That alone should encourage us to adopt a precautionary approach, rather than simply leaving it for future generations to solve.