As the latest instalment of climate negotiations gets under way in Doha it is important to ask just how far we need to cut emissions of greenhouse gases. A recent UNEP report indicates that there is a substantial gap between the commitments that countries have made to reduce emissions and what is required to avoid global average temperature rising by 2^oC this century.

The ‘business-as-usual’ scenario indicates emissions equivalent to 58 billion tonnes of CO₂ in 2020. To have a ‘likely’ (66%) chance of avoiding crossing the 2^oC line we need to limit emissions to 44 billion tonnes in that year. This is a substantial gap, and while the report indicates that it is possible, many people believe that it is not at all probable.

Into this debate come Negative Emission Technologies (NETs). These are proposed techniques which, if successful, would remove CO₂ from the atmosphere. To some people even discussion of such concepts creates a so-called ‘moral hazard’ – a potential ‘get-out-of-jail-free card’ that undermines the will to cut emissions.

On the other hand, such proposed techniques are written into many of the scenarios that hold global average temperature rises below 2^oC. The UNEP report considers a number of scenarios. All those which achieve the 2^oC target either have NETs or have to have very steep reductions in emissions.

What this means is that even with NETs, a gargantuan effort will be required to hold temperatures down. Without NETs it is well nigh impossible.

So could NETs work on anything like the scale required? The simple answer is that we do not know. As the report states “To achieve such negative emissions is simple in analytical models but in real life implies a need to apply new and often unproven technologies or technology combinations at significant scale.” Quite apart from questions about the technical feasibility of the proposed techniques, there are also issues about scalability, cost and side-effects. For example, large-scale deployment of BioCCS (using plants to remove CO₂ from the air and then sequestering it away deep underground) would – deployed at a scale required to make a material impact on the level of CO₂ in the atmosphere – have a material impact on food availability, water resources and biodiversity.

Given the level of uncertainty surrounding such proposed techniques, should we be using such scenarios to inform us as to what level of emission cuts may be required? I would suggest not – at least not until we have done some research that can demonstrate whether these techniques are actually deployable at the scale indicated.

But omitting all scenarios that exclude NETs would make the 2^oC target unrealistic. And if the target is perceived to be unrealistic, then it undermines the will to suffer the pain of emission cuts – a ‘morale hazard’.

What is the right amount of hope? If people are led to believe that the emission cuts required are small or could be delayed for a number of years, then they are likely to be complacent and delay action. If on the other hand, the cuts required are so large that they are not considered realistic then despair sets in.

The right amount of hope must not be guided by what negotiators sincerely believe is what is required to get the necessary result. The right amount of hope is the amount of hope that rigorous evidence-based research indicates is appropriate.

To apply different levels of rigour to factors that might prove inconvenient to your line of argument is to undermine trust in the messenger and the message – and in that there lies no hope.

• Read the United Nations Environment Programme (UNEP) Report, November 2012
• More about the United Nations Climate Change Conference, Doha 2012

This opinion piece reflects the views of the author, and does not necessarily reflect the position of the Oxford Martin School or the University of Oxford. Any errors or omissions are those of the author.