I was wanting to briefly discuss a paper I ran across that looks at What would it take to achieve the Paris temperature targets? The basic idea is to develop emission pathways that depend on three basic parameters; the time at which emission reductions start, the time it takes to get halfway between the initial and final emission levels, and the target emission level.I thought I would just shows some of the results. The figure on the left illustrates the temperature thresholds for emission reductions starting in 2015 and 2020. The y-axis is the emissions in 2030, and the x-axis is the time of emission neutrality (anthropogenic sources and sinks are net zero). The yellow region is likely below 3oC, the green region is likely below 2oC, and the blue is likely below 1.5oC. The horizontal lines are RCP8.5 emissions in 2030 (red), current policy (yellow), unconditional INDCs (orange), and conditional INDCs (green).
There are few interesting things in the figure. For example, if we have emissions in 2030 of around 50GtCO2, then we could likely keep warming below 2oC if we get to emission neutrality between 2050 and 2070. However, the white contours indicate the final emission level. They show that the earlier we reach emission neutrality, the lower (more negative) the final emission level needs to be. This is a little counter-intuitive, but I think the reason why is illustrated in the figures below, which are only for emission reductions starting in 2015. The slower the initial emission reductions, the sooner we need to reach emission neutrality and the more negative the final emissions need to be.
The green and blue dashed contours also indicate regions where we can have temporary temperature overshoots. Given that these pathways include the possibility of negative emissions, it becomes possible to overshoot a target, and then use net negative emissions to bring temperatures back to the target level. Example emission pathways are illustrated in the right-hand panels of the figure below.
What I think is interesting is that negative emissions and temperature overshoots seem to now becoming part of the narrative. One obvious reason for this is that we’re on the verge of leaving it too late to achieve these targets without them. We could still do so, but it would probably require drastic emissions reductions starting now: the figure above indicates that achieving these targets without negative emissions, and without a temperature overshoot, would require 50% reduction in emissions by around 2030. So, I guess we have to hope that negative emissions of the level required are actually possible, or that – if we don’t achieve these targets – the impacts will be less severe than might be the case.
In a sense, I’m reasonably optimistic. I think it’s at least becoming clearer that emission reductions are necessary, and it seems as though this is being taken seriously. On the other hand, I don’t have a good sense of whether or not negative emissions of the level required are viable or not. Some of what has been suggested (BECCS, for example) seems unrealistic, but maybe there are other possibilities, and we can’t discount our natural ingenuity. Maybe someone who knows more about this than I do, could comment.