Steinacher, Joos and Stocker write in Nature about how "Allowable Carbon Emissions Lowered by Multiple Climate Targets", that hitting a single target, for example 2 C requires a lot less effort than limiting harm from multiple limits. Essentially the same tautology as given natural variability on
top of a rising base, then the hotter extremes are going to get hotter
and you don't need a degree in meteorology or statistics to figure that
out. The authors selected a number of global metrics, mean sea level rise, steric sea level rise, Aragonite undersaturation in the Southern Ocean, global loss of aragonite saturated waters, cropland loss and soil carbon loss.
Meeting the multi-target 1 is very unlikely (<10 360="" eff="" exceed="" if="" u="">+10>40 GtC mean and maximum-minimum range from RFNC scenario uncertainty), although it becomes likely to meet the 1.5 C target (which is a part of set 1) at this range of emissions (Fig. 4). Similarly it is unlikely that multi-target 2 can be met if Eff exceed 470 + 80 GtC while it is still likely to meet the 2 C target if they stay below.
Now certainly bunnies can argue about the choice of metrics, the emission scenarios and the models used to play what if, but the central point remain, it is easy to mislead policy makers if one focuses on a single metric.
Worse, fitting a single globally and temporally averaged metric, such as temperature anomaly is an exercise in making von Neumann's elephant wiggle his trunk without a dose of physics and chemistry. Further, anyone who uses global metrics fall into the trap one anonymouse saw with another study
It's the creeping statistical hints between the lines of this paper that really bother me. Long before or even if we never see broad areas permanently enter a existentially threatening torrid regime, what about excursions? For instance, Pakistan this year has seen record temperatures approaching 54 degrees C in places where many people live, fortunately with lower humidity and only for handful of days but what about when/if such aberrations extend to a handful of weeks and are accompanied by inexorably increasing humidity? The resulting disaster would cause migrations. The worst-case scenario in Sherwood and Huber would not have to happen before we effectively lose major swathes of territory for year-round habitability.Eli is an optimist of course.