The latest UNEP Emissions Gap Report (EGR) has been criticized for (1) maintaining that we can still keep global mean temperature increase below 2 degrees, although actual emissions are still rising, and not peaking by 2020 as considered necessary by science earlier, (2) for making very challenging assumptions for possible future emission reductions, in particular through negative emissions and (3) for making politicians believe in something that is no longer realistic.
What are the arguments?
Moving the goal posts
The first point that is made is that earlier EGRs stated that global emission levels in 2020 could not be higher than 44 GtCO2e (range 38-47), if we would want to keep long-term temperature increase below 2oC, while projected emissions based on pledges and implementation thereof are clearly going to be higher. Nevertheless the 2014 and 2015 reports maintain that the 2oC limit can still be met if global emissions by 2020 are higher than that, provided that by 2030 they are below 42 GtCO2e.
What actually happened? In the 2010, 2011, 2012 and 2013 reports, the scenario set from the IPCC Fourth Assessment (AR4) Report was used that had no scenarios that attempted to keep temperature increase below 2oC with action not starting immediately (2000 or 2010 in this case). From these scenarios, the 44 GtCO2e in 2020 is derived. By 2014 new scenario sets from the IPCC Fifth Assessment (AR5) Report were available that assumed delayed action till 2020 (more or less consistent with the 2010 Cancun pledges that countries made), followed by stringent reductions thereafter. These scenarios were not longer following the “least cost” trajectory as of 2010 that all of the IPCC AR4 Report scenarios did. As a result there were now solutions available that postponed action, as long as emission levels by 2030 would be kept below 42 GtCO2e. So this is clearly a matter of new scientific insights.
The second criticism is that these IPCC AR5 scenarios rely on substantial negative emissions in the second half of the century and that these negative emissions are supposed to be achieved predominantly by using biomass power plants with CO2 Capture and Storage (BECCS) at a very large scale. BECCS is a technology that has only been demonstrated at small scale, is currently quite expensive, requires large amounts of sustainable biomass that may not be available and would therefore never deliver the significant negative emissions that the scenarios assume.
What are the facts? The IPCC AR5’s 2-degree scenarios that are used as a reference in the 2014 and 2015 EGR all follow the Cancun pledges till 2020 and then a least cost reduction path thereafter. They all assume a >66% probability of meeting the 2oC limit. As already two thirds of the available carbon budget for a 2oC temperature increase has been used up and there are limits in how fast emissions can be reduced in the future, all scenarios assume negative emissions in the second half of the century from afforestation and particularly from BECCS. The cumulative amount of CO2 storage in these scenarios ranges from about 400 to 800 GtCO2. The amount of CO2 removal through BECCS is lower. The amount of biomass that is needed for the use of BECCS is in the order of several hundred Exajoules. The combination of biomass as fuel and CCS is not particularly challenging. So two issues need to be considered: the penetration of CCS at scale and the availability of sustainable biomass.
Could power plant CCS penetrate from the current scale (1 million t/yr in the Boundary Dam power plant in Canada) to reach the cumulative scale of at least 400 GtCO2 by the end of the century? A simple exponential growth curve with a growth rate of 15-20% per year can do this in a period of 85 years. Growth rates of solar and wind power have been higher than this over the last 10-15 years, also in a situation where these technologies were significantly more expensive than fossil fuel based energy supply. And China built 2 coal-fired 600 MW power plants per week between 2005 and 2011. Integrated Assessment Models base their assumptions of penetration rates on historic data. The IMAGE model has an upper limit for BECCS of 10 GtCO2/yr by 2050 and 20 GtCO2/yr by 2100, which would allow having a cumulative amount of negative emissions over the century of about 900 GtCO2, much more than what would be needed.
Would sufficient amounts of sustainable biomass be available? The IPCC AR5 estimates the available biomass to be 10- 245 EJ/yr by 2050 and 105-325 EJ/yr by 2100. The AR5 scenarios for staying below 2 degrees assume amounts of available biomass energy that are lower than these maximum values. And biomass is not only used for BECCS, so the amount needed for BECCS will be even lower. Producing all the required biomass of this would require a substantial percentage of agricultural land; a recent assessment indicated that for 100 EJ/yr 30% of cropland and 10% of crop and pasture land combined would be needed. However, waste streams can already deliver significant amounts of biomass (ref), biomass can be grown on marginal lands that not compete with food production (ref) and productivity of biomass production (amount/hectare) can be further increased. Conclusion: the amounts of sustainable biomass that would be needed for large-scale application of BECCS can most likely be delivered in a sustainable manner.
Last but not least, negative emissions can also be realised by enhanced afforestation and reforestation and revegetation of degraded lands. If those options are further developed they would reduce the amounts to be removed through BECCS.
Conclusion on BECCS assumptions in the IPCC AR5 scenarios: plausible, but challenging to realize.
Incentives for postponing action
The third criticism is that the EGR would make it seem easy to still meet the 2oC limit and thus provide an incentive for politicians to postpone action in the short time.
What are the facts? EGRs have consistently warned against delay, based on the risk of locking in carbon intensive infrastructure, higher costs, more serious climate impacts, missed co-benefits for health, energy security and employment and also very specifically the risk of relying on large amounts of negative emissions through technologies that have not yet been demonstrated at scale. So they have certainly not encouraged politicians to delay action.
 See e.g. Revkin,A., The reality gap in the push to close the “Emissions Gap” in Paris, New York Times, November 6, 2015, http://dotearth.blogs.nytimes.com/2015/11/06/the-reality-gap-in-the-push-to-close-the-global-warming-emissions-gap-in-paris/ and Tollefson,J., Is the 2oC world a fantasy?, Nature, November 24, 2015, http://www.nature.com/news/is-the-2-c-world-a-fantasy-1.18868
 Importantly, no attempts were made to model scenarios that limit warming to below 2°C from Cancun pledge levels in 2020 in the absence of BECCS. The absence of such scenarios thus does not imply that they could not be generated.
 This is actually the cumulative amount of emissions avoided through application of CCS, corrected for the fact that only delayed action scenarios are considered; see e.g. Gough,C. and Vaughan,N.E., Synthesising existing knowledge on the feasibility of BECCS. Tyndall Centre Working Paper, February 2015, http://avoid-net-uk.cc.ic.ac.uk/wp-content/uploads/delightful-downloads/2015/07/Synthesising-existing-knowledge-on-the-feasibility-of-BECCS-AVOID-2_WPD1a_v1.pdf
 Van Vuuren et al, The role of negative CO2 emissions for reaching 2oC- insights from integrated assessment Modelling, Climatic Change 118 (2013) 15–27
 See 4
 Kemper,J., Biomass and carbon dioxide capture and storage: A review, International Journal of Greenhouse Gas Control 40 (2015) 401–430
 Ibid 8
 See also Schaeffer,M., et al, Feasibility of limiting warming to 1.5 or 2 oC, http://climateanalytics.org/files/feasibility_1o5c_2c.pdf