For over a decade, future climate change impacts have been assessed using a number of standardised emission scenarios. These scenarios assume different future states of the world’s population and corresponding greenhouse gas emissions – from really optimistic to really grim. For more information on these scenarios, please see IPCC SRES Scenarios and RCP Scenarios.
It is important to note that under each of these scenarios, there is a range of uncertainty in climate change impacts under different climate model simulations. This blog entry will not attempt to explore this matter further. However, a second level of uncertainty exists. What are the probabilities of each of the scenario events occurring?
Right from the inception of these future emission scenarios, no probability was assigned to the occurrence of any particular scenario. Over time, these scenarios have been updated but this is only to reflect the increased cumulative understanding of emissions and atmospheric forcings.
It seems that gone are the days where policymakers and decision-makers are rejecting climate change as a doom and gloom prophecy, at least in most cases. Impacts of climate change are now being considered to such an extent that a whole new field of “Climate Services” has been established. This generally involves climate “specialists” collaboratively informing policymakers and decision-makers of expected climate change and to make relative vulnerability assessments.
These policymakers and decision-makers are already largely apprehensive of the uncertainty of climate change under any given scenario (Moss et al., 2010). This can only be compounded by the additional complexity of different emission scenarios. Surely it would be beneficial and helpful to understand the probabilities of the particular scenario occurrence? If this were done, could it not better inform decisions?
Furthermore, the assignment of probabilities would lead to a time evolution of climate change risk. By revising our estimates of the probability of scenarios as we progress, the accuracy of climate change forecasts would surely improve which could lead to better adaptation and mitigation strategies. In contrast, by not apportioning probability to the scenarios, we are left basing our decisions on a static state of understanding where decisions made will not be improved with time.
It seems that the general consensus amongst CSAG staff is that we are headed on the RCP8.5 route (worst case scenario). One of the CSAG staff members informed me that he found no supporting evidence of this track in the literature. Risbey (2004) admits that a few studies have identified the lack of scenario probability assignment but this has not been adequately addressed as of yet. As mentioned earlier, these scenarios are not new to the discipline of climate change. Therefore, it would appear almost irresponsible that no one has followed it up.
I am of the strong opinion that the lack of probability assignment to the various scenarios provides a major gap in climate change research and limits the potential for climate change risk management. Without the understanding of where we are headed, I feel that Climate Services are hugely undermined.
Risbey, J.S. 2004. Agency and the Assignment of Probabilities to Greenhouse Emissions Scenarios. Climate Change. 67(1): 37-42.
Moss, R.H, Edmonds, J.A., Hibbard, K.A., Manning, M.R., Rose, S.K., van Vuuren, D.P., Carter , T.R., Emori, S. et al. 2010. The next generation of scenarios for climate change research and assessment. Nature. 463: 747-756.