Years of inaction mean that time is fast running out for the world to reduce greenhouse gas emissions enough to avoid dangerous global climate change, say researchers from the UNSW Climate Change Research Centre.
In a commentary article in the US journal Proceedings of the National Academy of Sciences, the researchers note that by 1994 all major industrialized nations, including the United States, had ratified the United Nations Framework Convention on Climate Change yet - 15 years on -policymakers still debate how best to formulate emissions legislation.
"Unfortunately, the past 15 years of inaction has very real policy implications today," say Professor Matthew England, Dr Alex Sen Gupta and Professor Andy Pitman.
To avoid the worst impacts of climate change, those implications must be taken into account at the crucial UN climate change conference in Copenhagen in December, they say.
Their commentary is based on a new framework for setting targets to reduce emissions - presented in the journal by climate scientists from the University of Victoria, Canada - that offers a more practical way of addressing the problem by setting down the total amount of allowable future emissions that would avoid a 2C temperature increase. This new way of looking at the problem is like planning for expenditure against a net income or setting a catch quota to maintain a sustainable fishery, the team notes.
"In such cases, the available resource is fundamentally limited in a cumulative sense; harvest or spend too much and things become unsustainable. For the global harvesting of fossil fuels the message becomes clear: burn beyond a cumulative cap and you commit the planet to a high risk of dangerous anthropogenic climate change."
They note that the oceans and terrestrial biosphere have so far absorbed more than half the carbon emissions released by humans in the post-industrial era, but that their capacity to keep doing so is waning.
"Fifty years ago, natural carbon sinks removed 600 kg of every ton of CO2 emitted to the atmosphere. Today, these sinks remove only 550 kg per ton emitted and this amount is expected to continue to fall. There are also risks of abrupt change in the terrestrial carbon sink: for example, climate change could trigger Amazon forest die-back and permafrost melt could expose northern peatlands to large releases of carbon.
"In short, the carbon sinks that have served us so well are by no means stable: they are changing and unfortunately changing in the wrong sense."
The new model shows that to have a decent chance - that is a probability of 66% - of stabilizing warming to 2 °C above pre-industrial, net carbon emissions accumulated from the year 2000 must not exceed 590 billion tonnes.
"Alarmingly, in the first 10 years since 2000, net carbon emissions will reach 100 billion tonnes, approximately a sixth of the cumulative emissions budgeted for above.
"If policymakers seek even greater certainty to avoid crossing the 2 °C threshold, say moving into the 'very likely' range [a probability of 90%], then it is estimated we need to cap post-2000 emissions at only 170 billion tonnes." With no reduction in emissions, that quota would be reached by 2017.
The team notes that a global mean warming of 2 °C "could still have devastating impacts on climate, ecosystems, human health, and infrastructure".
"This level of warming, for example, is likely to significantly reduce food productivity over the tropics, substantially increase the risk of extinction for 20-30% of species worldwide, bleach most of the world's coral reefs, and increase the likelihood of severe weather and extreme climate events.
"Global warming to 2.7 °C could additionally trigger a gradual but irreversible disintegration
of the West Antarctic Ice Sheet, with still lower thresholds thought to apply to the Greenland Ice Sheet, ultimately raising sea level by 10 metres and displacing hundreds of millions of people worldwide.
"Two degrees Celsius warming should thus not be seen as a mere aspirational target: it surely has to be the maximum stabilization target for global warming, with recognition that even this carries significant global-scale risks.
"Worryingly, this once-modest target is becoming increasingly difficult to achieve, requiring deep emissions cuts over a confronting time frame, something that must be secured in Copenhagen this year."
Bob Beale UNSW Faculty of Science 0411 705 435 firstname.lastname@example.org