Zero carbon in the first 1990 IPCC assessment for 350 ppm CO2
​Interestingly the very first 1990 assessment reported that, with atmospheric CO2 at 353 ppm, to keep CO2 below 350 ppm could only be done with zero carbon emissions. So here we are 25 years later with atmospheric at over 400 ppm, knowing it must be reduced to 350 ppm or less. There is universal agreement (2015) that in addition to dropping our CO2 emissions down to virtual zero, some CO2 must be removed, though there is no present present capacity for that.  'The availability and potential of BECCS Bioenergy carbon capture and storage, afforestation, and other CDR technologies and methods are uncertain and CDR technologies and methods are, to varying degrees, associated with challenges and risks. There is uncertainty about the potential for large-scale deployment of BECCS, large-scale afforestation, and other CDR technologies and methods' (IPCC 2014 WG 3, TS, p. 52) 

Zero carbon in the 2014 IPCC AR5 assessment

The ​​​most recent 2014 IPCC AR5 assessment was very clear on zero carbon (near/virt​ual zero carbon, this being the best we can achieve by replacing all fossil fuel energy with clean renewable energy. It was widely reported by the media. The IPCC AR5 correctly only uses CO2 equivalent (That includes methane and nitrous oxide) in its mitigation calculations for policy makers (SPM).  

'Con​​centrations by 2100 are characterized by ... emissions levels near zero GtCO2eq or below in 2100'. (IPCC AR5 WG3 SPM)

'The CO2eq emissions are brought to near or below zero by 2100 in the majority of the scenarios reaching concentrations of ​about 450 ppm CO2eq by 2100'. (IPCCAR5 Ch.6 Exec Summary). The 2014 Emissions scenarios are called RCP scenarios. The best case scenario RCP2.6 is the only one not
​above 2.0C of warming by 2100.

​​Th​ese pathways [for global temperature stabilization] ​would require substantial emissions reductions over the next few decades and near zero emissions of carbon dioxide and other long-lived greenhouse gases by the end of the century'(IPCC 2014 AR5 Synthesis Headline Statements)

Emissions reductions of this magnitude will require large-scale transformations in human societies, from the way that we produce and consume energy to how we use the land surface. The more ambitious the stabilization goal, the more rapid this transformation must occur. (AR5 Ch6.1)

​Bringing energy system CO2 emissions down toward zero, as is ultimately required for meeting any concentration goal,
requires a switch from carbon-intensive (e. g., direct use of coal, oil, and natural gas) to low-carbon energy carriers (most prominently electricity,but also heat and hydrogen) in the end-use sectors in the long run. ​​(IPCC AR5 CH6 6.3.4.)

Stabilization of global temperatures requires eventually the stabilization of atmospheric concentrations. This requires decreasing emissions to near-zero (Jones et al., 2006; Meehl et al., 2007b; Weaver et al.,2007; Matthews and Caldeira, 2008; Plattner et al., 2008; Allen et al.,2009; Matthews et al., 2009; Meinshausen et al., 2009; Zickfeld et al.,2009; Friedlingstein et al., 2011; Gillett et al., 2011; Roeckner et al.,2011; Knutti and Plattner, 2012; Matsuno et al., 2012a). (AR5 Ch.12 12 .5.4.2.)

Zero carbon in the 3rd TAR IPCC assessment
​The third assessment published a very useful graphic that shows the long lag times of climate system inertia. It also shows what happens when CO2 emissions are cut to near/virtual zero (95% cut
​in this case).

​1000 years after starting to cut carbon emissions to virtual zero:

  • the rate of sea rise does not drop 
  • ​​​the global temperature has not stabilized 
​​atmospheric CO2 stabilized some decades after emissions reached virtual zero and remains at​ a constant increased level 
and so ocean acidification also increases after stabilization of atmospheric CO2 and remains ​at a constant increased level 

​​​​Zero carbon in the 2007 AR4 IPCC assessment 

​​Here is the quote from the Technical Science report of the 2007 Intergovernmental Panel on Climate Change's (IPCC) Fourth Assessment (AR4) that could not be more clear and definite that only zero CO2 emissions can stop global warming - increasing.

​​IPCC 2007 Assessment WG 1 Science FAQs. Question 10.3
If emissions of Greenhouse gases are reduced, how quickly do their concentrations in the atmosphere decrease?

​​While more than half of the CO2 emitted is currently removed from the atmosphere within a century, some fraction (about 20%) of emitted CO2 remains in the atmosphere for millennia.

Because of the slow removal process, atmospheric CO2 will continue to increase in the long term even if its' emission is substantially reduced from its present levels.

In fact, only in the case of essentially complete elimination of emissions can the atmospheric concentration of CO2 ultimately be stabilized at a constant level.

​​Here is the IPCC 2007 mitigation report showing emissions to stabilize atmospheric carbon dioxide at 350-400​ ppm. This does not only take zero carbon emissions, it requires the capacity for negative carbon emissions. carbon dioxide has been extracted direct from the air and secured.


More on zero carbon from the IPCC 2014 AR5 assessment (RCP scenarios)

​​​​For the ​2014 AR5 assessment the scenarios have been changed and are called RCPs. The best case RCP is RCP 2.6 (or RCP 3 PD) and the global warming is limited to about 1.5C by 2100 and over the very long term declines to 1C. CO2 emissions decline to reach zero in about 50 years and then go negative. This is obviously what the world should be planning for. 

​​Replacing all fossil energy with clean zero carbon (everlasting energy) is feasible and the only future we have.