Climate change scenarios for environmental health risk assessment and communication

English Abstract:

This paper addresses how climate science modeling scenarios, employed in projections and carbon budgeting, can be used to gauge environmental health risks of future impacts. It combines the IPCC AR4 (SRES) with the AR5 (RCP) scenarios, and includes the model uncertainty of large positive amplifying feedbacks in the scenarios. Our simple conclusion is that only the best-case IPCC AR5 emissions scenario (RCP2.6) is policy relevant, making it our priority for climate change science communication. The AR4 SRES projections (still valid, according to AR5) include a large upper range that accommodates positive amplifying feedbacks. The RCP scenarios include carbon cycle models but, we find, not the risk of the large uncertainty of the largest sources of positive feedbacks. The AR5 impacts are given for up to 2100, while for environmental health risk, climate change and ocean acidification last over 1000 years. The carbon budget is a useful tool for communication, however, we find it provides the maximum possible estimate of remaining emissions rather than the lowest risk of remaining emissions for policy making. Another policy-relevant environmental health risk communication tool is what we call "total" committed climate change, which the AR5 says it does not assess. This includes all unavoidable sources of warming. We find that the 2ºC policy target is a global environmental health catastrophic risk and the 1.5ºC option is still a disastrous risk. Only the best-case IPCC AR5 RCP2.6 scenario can avoid a warming above 2ºC at and beyond 2100. RCP2.6 could limit warming to 1.5ºC, which we find might avoid catastrophic large amplifying feedbacks, though not their large environmental health risk. The IPCC does not make recommendations. It is therefore of the greatest urgency for science organizations to promote the best-case AR5 RCP2.6 scenario, which we find is not yet happening.