Back in 2008, the Global Commons Institute issued a paper that reviewed this issue, and it stated:
GCI is committed to on-going research into climate risk assessment.When the IPCC published its Fourth Assessment Report (AR4) in 2007, it included for the first time ‘coupled’ modelling for emissions control scenarios alongside the uncoupled modelling that has been shown in its Assessment Reports since 1994.
Coupled Atmosphere-Ocean General Circulation Models (AOGCMs) used by the UK’s Hadley Centre are the most complex climate models in use, consisting of an Atmosphere General Circulation Model (AGCM) coupled to an Ocean General Circulation Model (OGCM). Some recent models include the biosphere, carbon cycle and atmospheric chemistry as well. AOGCM modelling introduces the effects of positive feedbacks from carbon sinks and can be used for the prediction and rate of change of future climate.
Following detailed investigation of the modelling results in IPCC AR4, GCI was able to confirm with IPCC and Hadley that the new evidence points to the need for zero emissions globally by about 2050 to keep below 450ppmv atmospheric CO2 concentration. This level is the most frequently cited maximum within which it may be possible to arrest the rise in global temperature to within a 2°C increase above pre-industrial levels. These results corroborate the risk-analysis previously carried out by the GCI for the UK All-Party Parliamentary Group on Climate Change (APPGCC), shown in summary in Section 8 below.
GCI also produced an online interactive chart that shows how this works in a coupled scenario and an uncoupled scenario.
In slide 2, it shows the difference between these two scenarios and how the coupled emissions model balances sinks and emissions. It shows how this "sink failure" of the oceans and forests can be slowed with faster contraction of carbon emissions, and therefore the interconnection of the model systems is used to calculate the projected atmospheric carbon. This, unfortunately, cannot be a solution to global carbon emissions because of the increasing ocean acidity and the degradation of forests since human habitation began to expand thousands of years ago, and this has rapidly accelerated beginning with the industrial revolution. The only means of keeping the carbon from crossing the tipping point of 450 ppmv is to reduce all carbon emissions to zero by 2050, according to this model and protecting the planet from "sink failure". This point is emphasized by the David Suzuki Foundation, which highlights the nature of the planet's deteriorating carbon sinks.
In addition to this debate, there is now an added factor that has been documented in the northern arctic area, the collapse of the ice shelves and the release of methane as a result of the warming induced by human activities. This is not yet factored into the climate models that have been used as a basis for developing a global agreement about carbon emissions, thus they are completely flawed to this extent. It appears that we are facing a runaway climate change scenario very soon because of this planetary response to escalating warming, which destroys the very "carbon sinks" that we are relying upon.
It's imperative that the people of this planet move to zero emissions by the target date of 2050, with a rapid contraction of emissions in place by 2020 for all countries.