The Kyoto Protocol calls for rich countries to cut greenhouse gas emissions by 5.2% compared to 1990 levels. Britain, South Australia and Victoria have gone for a 60% reduction by 2050, and California proposes a cut of 80%.
Typically, environment groups have gone for the biggest official goal on offer. The Climate Action Network Australia, representing 30 environment groups, echoes the British government with a target of "at least 60% below 1990 levels by 2050". Recently, Friends of the Earth UK and journalist George Monbiot have recommended going further again with an average 90% cut for rich countries by 2030.
With several "tough" targets on offer, which should we choose? To decide, we have to go back to basics.
We want to sustain people and other species, and to protect them the target has to actually do the job. We have to choose a prudent risk level. You wouldn't fly in a plane that had more than a 1% chance of crashing. We should be at least as careful with the planet.
Even with the greenhouse gas in the air now — 430-490 parts per million (ppm) CO2 equivalent — ice sheets and glaciers are melting globally, there is serious drought, and extreme weather events and fires have been triggered.
The most vulnerable — other species and poor people in underdeveloped countries — are struggling with the impacts right now. And this is with a warming of "only" 0.8ºC over pre-industrial temperatures.
Even if no more CO2 is emitted, the current gases will cause at least a further 0.5ºC warming.
Biologists are worried that, based on damage already seen, a 1.5ºC warming will be really damaging for nature.
Climate systems are surprisingly unstable and the world is on the brink of runaway heating because of "positive feedbacks". As things heat, less light is reflected into space, more methane and CO2 is released into the air (from permafrost bogs, peat bogs, ordinary soils, and drying and burning bushland) and less CO2 can be absorbed by the oceans and the land. The result — more heating.
A 3-4ºC warming is likely to trigger runaway greenhouse heating, which could keeping going until the globe is 8ºC warmer.
Such warming has not been experienced for millions of years. Under these conditions, most species could become extinct and most people would die.
The big impacts from climate change and CO2 acidification of the oceans come from the impact on ecosystems, extensive desertification and sea-level rise (possibly as fast as 1 metre per 20 years if Greenland, then the west Antarctic ice sheets, are destabilised).
Earth's climate system is complex and it often doesn't respond in simple ways, and despite growing knowledge, there is still uncertainty. In this situation it is necessary to talk about the probabilities of an event occurring rather than saying that this is what will happen when X, Y or Z occurs.
So, applying these ideas, what greenhouse reduction target emerges?
Starting with the needs of other species, the British government's target of keeping CO2 equivalent levels at or under 550 ppm is too high because when greenhouse gas levels approach this threshold, marine ecosystems will be destroyed through acidification.
The European Union target of staying at or under 2ºC global warming is too high by at least half a degree. With the greenhouse gases in the air now we have a 50-100% chance of exceeding 1.5ºC warming.
So to give adequate protection to nature we can see that the current level of greenhouse gases in the air gives an unacceptably high risk of warming and thus of damage.
Focusing now on runaway greenhouse heating, this is the planetary equivalent of crashing a plane. It simply has to be avoided. The risk must be kept well below 1%.
Using the risk data favoured by the Stern Review, sourced from the British defence department's Hadley Centre, there is, at a minimum, a 24% chance of triggering runaway greenhouse heating at 550 ppm CO2 equivalent, at least an 11% chance at 500 ppm , at least a 3% chance at 450 ppm and at least a 1% chance at 400 ppm. (All four ppm numbers are "CO2 equivalents".)
But note, the atmosphere is now already well over 400 ppm.
James Hansen, head of the NASA's Goddard Institute, the leading US climate research organisation, estimates that we have no more than 10 years to physically make the changes to the economy so that the business-as-usual scenario, that triggers runaway greenhouse heating, does not occur.
If the business-as-usual warming occurs, we have close to a 100% chance of "crashing" the planet.
Taking all this together, the greenhouse gas levels in the air now pose an unacceptably high risk of damage to nature and an unacceptably high risk of triggering runaway heating. The only way to bring the risk down to an acceptable level is to cut greenhouse gas emissions to zero, to take the excess CO2 out of the air as fast as possible, and to find environmentally acceptable ways to cool the atmosphere.
The transformation of the economy from a business-as-usual structure to a sustaining structure must be physically accomplished within 10 years.
It is now clear that rich and poor alike must adopt the zero-minus-fast goal if we are to be practical about how we care for people and other species.
[Reprinted with permission from Chain Reaction, published by Friends of the Earth Australia. Philip Sutton is the convener of the Greenleap Strategic Institute.]