In its definitive scientific synthesis report (PDF), the Intergovernmental Panel on Climate Change (IPCC) today issued its strongest call for immediate action to save humanity from the deadly consequences of unrestrained greenhouse gas emissions.

This report — signed off by 130 nations including the U.S. and China — slams the door on any argument for delay and makes clear we must under no circumstances listen to those who urge that we wait (who knows how long) to develop as yet non-existent technology [this means you President Bush, Newt Gingrich, Bjorn Lomborg]. As The New York Times put it:

Members of the panel said their review of the data led them to conclude as a group and individually that reductions in greenhouse gasses had to start immediately to avert a global climate disaster that could leave island states submerged and abandoned, African crop yields decreased by 50 percent, and cause over a 5 percent decrease in global gross domestic product.

… this summary was the first to acknowledge that the melting of the Greenland ice sheet from rising temperature [which would raise the oceans 23 feet] could result in sea-level rise over centuries rather than millennia.

Readers of this blog know the IPCC almost certainly underestimates the timing and severity of likely impacts because it ignores or downplays key amplifying feedbacks in the carbon cycle (see “Are scientists overestimating or underestimating climate change,” especially Part II and Part III). Indeed, IPCC head Rajendra Pachauri — a scientist and economist — admitted as much:

He said that since the panel began its work five years ago, scientists have recorded “much stronger trends in climate change,” like a recent melting of polar ice that had not been predicted. “That means you better start with intervention much earlier.”

How much earlier? The normally understated Pachauri warns:

“If there’s no action before 2012, that’s too late. What we do in the next two to three years will determine our future. This is the defining moment.”

In short: time’s up! America, we better pick the right President in 2008.

To balance the bad news, the IPCC and its member governments agree on the good news — action is affordable:

In 2050, global average macro-economic costs for mitigation towards stabilisation between 710 and 445ppm CO2-eq are between a 1% gain and 5.5% decrease of global GDP. This corresponds to slowing average annual global GDP growth by less than 0.12 percentage points.

But how is that possible? How can the world’s leading governments and scientific experts agree that we can avoid catastrophe for such a small cost?

Because that’s what the scientific and economic literature — and real-world experience — says:

Both bottom-up and top-down studies indicate that there is high agreement and much evidence of substantial economic potential for the mitigation of global GHG emissions over the coming decades that could offset the projected growth of global emissions or reduce emissions below current levels.

In fact, the bottom up studies — the ones that look technology by technology, which I believe are more credible — have even better news:

Bottom-up studies suggest that mitigation opportunities with net negative costs have the potential to reduce emissions by around 6 GtCO2-eq/yr in 2030.

Wow! A 20% reduction in global emissions might be possible in a quarter century with net economic benefits! Take that, delayers who oppose rapid, mandatory action and supposedly represent the “pragmatic center on climate and energy” — but who in fact represent the fatal siren song of “wait for new technology, wait for new technology.”

But don’t we need new technologies? Of course, but we don’t need — and can’t afford — to sit on our hands when we have so many cost-effective existing technologies:

There is high agreement and much evidence that all stabilisation levels assessed can be achieved by deployment of a portfolio of technologies that are either currently available or expected to be commercialised in coming decades, assuming appropriate and effective incentives are in place for their development, acquisition, deployment and diffusion and addressing related barriers.

Yes delayers — we need to do two things at once: aggressively deploy existing technology (with carbon prices and government standards) and aggressively finish developing and commercializing key technologies and systems that are in the pipeline. Anyone who argues for just doing the latter is disputing a very broad consensus, and is neither pragmatic nor centrist.

What do we risk if fail to act now?

Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.

Partial loss of ice sheets on polar land could imply metres of sea level rise, major changes in coastlines and inundation of low-lying areas, with greatest effects in river deltas and low-lying islands. Such changes are projected to occur over millennial time scales, but more rapid sea level rise on century time scales cannot be excluded.

In short, we risk that our top climatologists’s warnings on sea level rise prove true. What else?

As global average temperature increase exceeds about 3.5 degrees C, model projections suggest significant extinctions (40-70% of species assessed) around the globe.

IPCC to world: The time to act is now or we risk destroying life on the earth as we know it!

You can listen to the IPCC press conference, download their PPT presentation, and get the entire synthesis report here.