David Victor and Charles Kennel argue that aiming to keep average global warming within 2 °C of pre-industrial temperatures is neither politically nor scientifically useful (Nature 514, 30–31; 2014). I disagree: global temperature change is the closest thing we have to a metric with a clear link to emissions; it can also be related quantitatively to a range of local climate impacts.
Because global temperature seems to respond linearly to cumulative emissions of carbon dioxide (H. D. Matthews et al. Nature 459, 829–832; 2009), policies to cut emissions should also reduce global temperature change. This offers a simple framework for estimating a global carbon budget that contains warming to within 2 °C.
Policy goals should not have adverse effects on human and environmental welfare. Using global temperature avoids these too, because it seems to be an indicator of the extent of local climate changes (see, for example, M. Markovic et al. Clim. Change 120, 197–210; 2013). Furthermore, the average global temperature over decades relates well to many climate impacts and to Victor and Kennel's 'vital signs' of planetary health (National Research Council Climate Stabilization Targets National Academies Press, 2011).
Now that the international community has finally coalesced around the 2 °C goal, compelling reasons are needed to interrupt this momentum.