Sir, Your editorial on the Keystone pipeline (November 25) seriously misconstrues the issues regarding Canada's oil sands and the Keystone pipeline, albeit in a typical way.

You state: "Long-term growth in global demand for oil, however, is very likely to make oil sands development profitable even if the crude has to be moved out of Alberta by other pipelines . . . or by rail." In a world of "deep decarbonisation", however, this is not at all clear. If the world agrees to serious climate agreement in Paris in December 2015 consistent with the globally agreed limit of 2C warming, it is quite possible that there will be no place for Canada's oil sands or other high-cost, carbon-intensive unconventional oils. At this point, we simply don't know, though we have ample reason to doubt that high-carbon, unconventional oil fits economically within the 2C global "carbon budget". Neither the US nor Canada has yet tabled a long-term plan for deep decarbonisation, though happily the US has agreed to that very concept in its path-breaking announcement with China on November 11.

Your editorial rightly states that "US climate policy is of vital importance". With that in mind, the first step is a serious carbon mitigation strategy for North America from both the US and Canada. After that - and only after that - can we truly assess the merits of the Keystone Pipeline project.

Jeffrey D Sachs

Director, Earth Institute,

Columbia University, US