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Only the total elimination of industrial emissions will succeed in limiting climate change to a 2°C rise in temperatures, according to computer analysis of climate change. Anything above this target has been identified as "dangerous" by some scientists, and the limit has been adopted by many policymakers.
The researchers say their study highlights the shortcomings of governmental plans to limit climate change.
A warming of 2°C above pre-industrial temperatures is frequently cited as the limit beyond which the world will face "dangerous" climate change. Beyond this level, analysis suggests the continents will cease to absorb more carbon dioxide than they produce. As the tundra and other regions of permafrost thaw, they will spew more gas into the atmosphere, adding to the warming effect of human emissions.
The end result will be dramatic ecological changes, including widespread coastal flooding, reduced food production, and widespread species extinction.
Established model
In January 2007, the European Commission issued a communication stating that "the European Union's objective is to limit global average temperature increase to less than 2°C compared to pre-industrial levels".
Andrew Weaver and colleagues at the University of Victoria in Canada say this means going well beyond the reduction of industrial emissions discussed in international negotiations.
Weaver's team used a computer model to determine how much emissions must be limited in order to avoid exceeding a 2°C increase. The model is an established tool for analysing future climate change and was used in studies cited in the IPCC's reports on climate change.
They modelled the reduction of industrial emissions below 2006 levels by between 20% and 100% by 2050. Only when emissions were entirely eliminated did the temperature increase remain below 2°C.
A 100% reduction of emissions saw temperature change stabilise at 1.5°C above the pre-industrial figure. With a 90% reduction by 2050, Weaver's model predicted that temperature change will eventually exceed 2°C compared to pre-industrial temperatures but then plateau.
Stark contrast
The researchers conclude that governments should consider reducing emissions to 90% below current levels and remove what is left in the atmosphere by capturing and storing carbon (see Chemical 'sponge' could filter CO2 from air).
There is a stark contrast between this proposal and the measures currently being considered. Under the UN's Kyoto protocol, most developed nations have agreed to limit their emissions to a minimum of 5% below 1990 levels by 2012. What happens beyond this date is the subject of ongoing debate and negotiation.
The European Union nations have agreed to limit their emissions to 20% below 1990 levels by 2020, and support dropping global emissions to 50% below 1990 levels by 2050.
"There is a disconnect between the European Union arguing for a 2°C threshold and calling for 50% cuts at 2050 - you can't have it both ways," says Weaver, who adds: "If you're going to talk about 2°C you have got to be talking 90% emissions cuts."
Vanishing point
Tim Lenton, a climatologist at the University of East Anglia in the UK, agrees that even the most ambitious climate change policies so far proposed by governments may not go far enough. "It is overly simplistic assume we can take emissions down to 50% at 2050 and just hold them there. We already know that that's not going to work," he says.
Even with emissions halved, Lenton says carbon dioxide will continue building up in the atmosphere and temperatures will continue to rise. For temperature change to stabilise, he says industrial carbon emissions must not exceed what can be absorbed by Earth's vegetation, soil and oceans.
At the moment, about half of industrial emissions are absorbed by ocean and land carbon "sinks". But simply cutting emissions by half will not solve the problem, Lenton says, because these sinks also grow and shrink as CO2 emissions change.
"People are easily misled into thinking that 50% by 2050 is all we have to do when in fact have to continue reducing emissions afterwards, all the way down to zero," Lenton says.
Journal reference: Geophysical Research Letters (DOI: 10.1029/2007GL031018)
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