In October this year, Mora et al. presented a new indicator of climate change in a Nature article. Instead of describing how much climatic variables change in different scenarios by a given year, say 2100, they measure how fast the change is. Their indicator, climate departure, points to the year when a given variable shifts completely beyond past variability. They explore projections of 39 earth system models under two emission scenarios. The results reveal that we are likely to experience completely unprecedented climatic conditions within only a few decades – before I need to renew my driving license, for example. These are sobering figures. It is useful to illustrate the rate and variability of the projected.  

Moving outside past variation means different things depending on the breadth of the variation in the baseline, and the variables in question. Furthermore, it can mean very different things for biodiversity and society. Relative and absolute changes play different roles here. In our journal club discussion, we were hoping Mora et al. had discussed these aspects more.

A main conclusion of the paper was that climate departure will occur earlier in tropical regions than in high latitudes, mainly because the range of past variability is much narrower in the tropics. As they point out, this can mean severe impacts on biodiversity which has evolved in a very stable environment. However, we were wondering whether the departure per se would be such a problem for human societies. That does not mean we would question the vulnerability of societies in tropical regions – on the contrary. However, we think that Mora et al. should have gone deeper in describing why and when it is the rate of change relative to past rather than the absolute change that determines how biodiversity and society are affected. We were not convinced by the way they used GDP as a proxy for adaptive capacity. While the two can be linked, GDP completely disregards adaptive action at the grassroots level. Such action does not necessarily depend on money or government interventions. Biodiversity conservation scientists are developing ways to measure inherent sensitivity of species and communities to the exposure from climate change (Foden et al. 2013). Similar research is carried out in social sciences as well, and complementary measures are available (see Lung et al. 2013 for example).

Once shocked by how fast climate is changing in a business as usual and worst case scenarios, we would have been keen to know how much of this dramatic change can be avoided under a 2 degrees climate change scenario. This is what countries have committed to under the UN Convention on Climate Change. Disappointingly, Mora et al. do not include a 2 degrees climate scenario in their study. They justify this by saying that “the implicit mitigation effort is considered currently unfeasible” and cite a study from van Vliet et al. (2009), assessing feasibility of alternative stabilization targets under delayed participation. Van Vliet et al. do conclude that the most ambitious mitigation target becomes unfeasible in a “delayed participation” scenario, where delays in global participation extend to 2030 and 2050. Such a scenario seems rather pessimistic compared to the global commitment to have a global agreement enter into force by 2020. In other words, van Vliet et al. do not consider a 2 degrees scenario unfeasible as such.

Looking at emission trends, it indeed does not seem like the world is entering a zero carbon era any time soon. However, the technical and economic potential for such mitigation efforts exist – and so do political commitments, although the actions are lagging far behind. If other researchers join Mora et al. in not including this scenario in studies, they are effectively limiting the options that the broader society has to choose from. If the low-end scenarios are taken off the table, how can their costs and benefits then be assessed? In my opinion, scientists do not have a mandate to do so. We should provide information about the required actions and impacts of the globally agreed targets, and the impacts of missing these targets.