Only the total elimination of industrial emissions will succeed in limiting climate change to a 2掳C rise in temperatures, according to computer analysis of climate change. Anything above this target has been identified as 鈥渄angerous鈥� 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掳C above pre-industrial temperatures is frequently cited as the limit beyond which the world will face 鈥渄angerous鈥� 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.
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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 鈥渢he European Union鈥檚 objective is to limit global average temperature increase to less than 2掳C compared to pre-industrial levels鈥�.
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鈥檚 team used a computer model to determine how much emissions must be limited in order to avoid exceeding a 2掳C increase. The model is an established tool for analysing future climate change and was used in studies cited in the IPCC鈥檚 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掳C.
A 100% reduction of emissions saw temperature change stabilise at 1.5掳C above the pre-industrial figure. With a 90% reduction by 2050, Weaver鈥檚 model predicted that temperature change will eventually exceed 2掳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 鈥榮ponge鈥� could filter CO2 from air).
There is a stark contrast between this proposal and the measures currently being considered. Under the UN鈥檚 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.
鈥淭here is a disconnect between the European Union arguing for a 2掳C threshold and calling for 50% cuts at 2050 鈥� you can鈥檛 have it both ways,鈥� says Weaver, who adds: 鈥淚f you鈥檙e going to talk about 2掳C you have got to be talking 90% emissions cuts.鈥�
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. 鈥淚t is overly simplistic assume we can take emissions down to 50% at 2050 and just hold them there. We already know that that鈥檚 not going to work,鈥� 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鈥檚 vegetation, soil and oceans.
At the moment, about half of industrial emissions are absorbed by ocean and land carbon 鈥渟inks鈥�. But simply cutting emissions by half will not solve the problem, Lenton says, because these sinks also grow and shrink as CO2 emissions change.
鈥淧eople 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,鈥� Lenton says.
Journal reference: (DOI: 10.1029/2007GL031018)
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