How fast do we need to reduce emissions?
The United Nations Framework Convention on Climate Change urges stabilization of greenhouse gas emissions… "at a level that would prevent dangerous anthropogenic interference with the climate system.”
It's an odd question, what is a "reasonable" definition of "dangerous". If we say 2°C, does this imply that events happening at less than 1.5°C are not dangerous? That the loss of the Arctic floating ice and its implications is not already dangerous? That the destruction of the Great Barrier Reef is not dangerous in all sorts of ways, including maritime species diversity? That a 30% chance of starting the irretrievable melting of the Greenland ice sheet is not dangerous? Its consequences would be catastrophic.
Yet the question as to what would constitute "dangerous anthropogenic interference with the climate system” is commonly answered as a temperature rise of 2°C. This is the target set by the European Union, the IPCC (2001) and the International Climate Change Taskforce.
NASA's James Hansen says that "further global warming of 1°C (above the 2000 temperature of 0.7°C to 1.7°C) defines a critical threshold. Beyond that we will likely see changes that make Earth a different planet than the one we know" (Pearce 2006a). "We conclude that global warming of more than ~1°C, relative to 2000, will constitute "dangerous" climate change as judged from likely effects on sea level and extermination of species" (Hansen et al, 2006). Taking thermal inertia into consideration, we are now effectively just 0.3°C from 1.7°C.
Others have named 1.5°C as the danger mark: "Based on our current understanding of responses of species and ecosystems, we propose that efforts be made to limit the increase in global means surface temperatures to maximally 1.5C above pre-industrial levels" (van Vlliet and Leemans).
Christian Aid and EcoEquity concludes that: "the pace of our response has been profoundly inadequate ... and the science now tells us that we’re pushing beyond mere ‘dangerous anthropogenic interference with the climate system,’ and are rather on the verge of committing to catastrophic interference" (Athanasiou et al 2006).
It is clear that we are already moving into the zone of dangerous climate change, and there is evidence, such as the increasing seismic activity in the Greenland ice sheet as it starts to crack and move at a much-increased rate (Hansen 2006a), that we are already on the edge of triggering catastrophic events such as the melting of that ice sheet. If that goes we are facing sea level rises of 7 metres from Greenland (as quickly as a metre every 20 years) let alone the consequences for Antarctica and climate more generally.
For our part, the evidence is that climate change so far is already serious, and "in the system" rises to 1.4°C will, from the Arctic evidence, reflect "dangerous anthropogenic interference with the climate system". Above 2°C degrees there is a reasonable chance that it is catastrophic.
Analysing data from the 2006 UK government's Stern Review, it can be shown that right now:
In other words it is certain that we will pass 1.5°C and into the zone of "dangerous anthropogenic interference with the climate system", and a high and unacceptable risk of exceeding 2°C and moving towards catastrophic territory.
Runaway greenhouse heating is the planetary equivalent of crashing a plane. It simply has to be avoided. The risk must be kept well below 0.1%. Using the risk data featured by the Stern Review, sourced from the UK Defence Department's Hadley Centre, there is, at a minimum:
So right now, with levels half way between 400 and 450 ppm CO2ewe have an unacceptably high risk of causing runaway heating, of "crashing" the planet.
Another study showed that "at 550 ppm CO2 equivalence (corresponding approximately to a stabilization at 475 ppm CO2 only), the risk of overshooting 2°C is very high, ranging between 68% and 99% for the different climate sensitivity PDFs with a mean of 85%. In other words, the probability that warming will stay below 2°C could be categorized as ‘unlikely’ .... If greenhouse gas concentrations were to be stabilized at 450 ppm CO2e then the risk of exceeding 2°C would be lower, in the range of 26% to 78% (mean 47%), but still significant. In other words, 7 out of the 8 studies analyzed suggest that there is either a “medium likelihood” or “unlikely” chance to stay below 2°C. Only for a stabilization level of 400 ppm CO2eq and below can warming below 2°C be roughly classified as ‘likely’ (risk of overshooting between 2% and 57% with mean 27%). The risk of exceeding 2°C at equilibrium is further reduced, 0% to 31% (mean 8%), if greenhouse gases are stabilized at 350 ppm CO2e" (Meinshausen 2005).