Global warming is directly related to greenhouse gases emissions from human activities, essentially burning of fossil fuel, with carbon dioxide being the dominant greenhouse gas. Recent studies have demonstrated that the sum over time of all CO2 emissions (cumulative emissions) largely determines global mean surface warming. Any given level of warming is associated with a specific range of cumulative CO2 emissions, larger cumulative emissions inducing larger warming. Keeping global warming below 1.5 degree Celsius (1.5C) above pre-industrial level will hence require future cumulative CO2 emissions to be below a limited budget. Knowing that remaining carbon budget with as little uncertainty as possible is key for decision on greenhouse gases mitigation policies. It will inform us on how soon and how strong CO2 emissions reduction needs to be implemented.
In it's latest assessment, the intergovernmental Panel on Climate Change reported that limiting the warming below 1.5C since the pre-industrial would require cumulative CO2 emissions to stay below 615GtC. Given that, up to now, humanity already emitted about 555GtC, was is left for the future is only about 60GtC. That would be 6 years from now as current global CO2 emissions are around 10GtC per year.
We argue that this IPCC estimate might not be realistic. It is hardly consistent with the observed warming and associated emissions over the 20th century. For present-day, historical emissions (555GtC) contributed to a warming of about 1C. How could the next 60GtC contribute to 0.5C? We think that this estimate of the remaining carbon budget (60GtC) is quite certainly an underestimate, which has important policy consequences for efforts to meet a 1.5C goal. It could negatively affect the mitigation policy process, giving the impression that the 1.5C is not achievable any more.
In this project we will investigate the main reasons that could explain why the remaining CO2 budget, as given in IPCC, might be biased.
First, natural climate variability is important. The models used to derive this estimate fail to fully simulate the observed warming and carbon cycle. In particular, the majority of these models did not simulate the lower than average rate of warming over the last 15 years. The models overestimate the current warming, some of them having a present-day warming already near 1.5C, while the observed warming is about 1C above pre-industrial. The implication is that those models will significantly underestimate the remaining CO2 emissions before they would reach 1.5C.
Second, although the cumulative budget is about CO2 only, non-CO2 agents such as methane or aerosols also play a role in the estimate of the remaining budget. Assuming large reduction of aerosols in the future, as in the scenario used by IPCC for example would lead to a significant warming, leaving then less room for CO2 emissions before we reach 1.5C. The IPCC estimate is based on one single future scenario (called RCP8.5), which is representative of a world without any climate mitigation, warming being well above 2C by 2100, but with significant air quality policies. Such scenario is probably not ideal for the estimate of CO2 emissions consistent with a 1.5C world.
Third, related to the two points above, the numerous climate and carbon cycle feedbacks that operate in the Earth System need to be fully diagnosed in specific scenarios aiming to remain below 1.5C where global temperature and atmospheric CO2 stabilize, not from unmitigated scenarios with exponential increase in atmospheric CO2 and increasing climate change.
Our project will investigate these sources of potential biases in order to provide a robust assessment of the maximum amount of CO2 humankind could potentially emit in the future while limiting global warming to 1.5C.

Potential Impact:
The main beneficiary from our research will be the IPCC Special Report on "the impacts of global warming of 1.5C above pre-industrial levels and related global greenhouse gas emission pathways". We will publish our results in publications, citable by IPCC. We will make sure publications are being accepted before the cut-off date for the IPCC special report (anticipated to be March 2018). This is the main motivation for us submitting to open access journals with a discussion stage allowing early citations in IPCC. All publications (submission and acceptance stage) will be sent to the IPCC special report co-chairs and lead authors of relevant chapters.

We are proposing to invite all projects funded under this call to join us and present their main findings to the IPCC co-chairs. We took the liberty to contact Valerie Masson-Delmotte, IPCC WG1 co-chair and Jim Skea, IPCC WG3 co-chair. We propose to organise a one-day outreach meeting in a central location (most likely London) in September 2017. Both IPCC co-chairs supported this initiative, Jim Skea offering to host it at Imperial College. Given the focus of our proposal on cumulative carbon budget, we only contacted WG1 and WG3 co-chairs so far, but, if other projects were covering climate impact sciences, we would extend the invitation to Hans-Otter Pörtner, WG2 co-chair.
As this NERC programme also aims to inform the Committee on Climate Change, we also contacted Steve Smith (CCC secretary) and Corinne LeQuéré (CCC Board) to inform them about this potential IPCC outreach event, and both agreed to come. Other parties could be involved (IPCC special report lead authors, representatives of the Department for
Business, Energy & Industrial Strategy, etc). Of course if NERC was already planning to organise such an event, we would be happy to come and contribute to his success.