Temperatures in the deep crust and upper mantle have been most commonly based on geothermal heat flow measurements, but there are now eight constraints that I describe for the northern Canadian and Alaska Cordillera, along with some consequences. The constraints are: (1) Heat flow measurements, (2) Temperature-dependent upper mantle velocities, (3) Temperature-dependent topographic elevations, thermal isostasy, (4) Depth and temperature of the seismic lithosphere-asthenosphere boundary (LAB), (5) Origin temperature and depth of craton kimberlite xenoliths, (6) Geochemically inferred source temperature and depth of recent volcanic rocks, (7) Depth to the magnetic Curie temperature, (8) Depth extent of seismicity. The backarc lithosphere is everywhere thin, 50-85 km and 1325+/-25C. Moho temperatures are 850+/-50C compared to cool cratonic areas of 400-500C. Some consequences include: (1) Thin and weak backarc lithosphere accommodates pervasive tectonic deformation indicated by wide-spread seismicity and GPS-defined motions, in contrast to the stable cratonic regions, (2) Very weak backarc lower crust flattens the Moho and allows detachment and thrusting of the upper crust over the cold strong Arctic Alaska Terrane and Canadian Shield. This work provides a model for how to estimate deep temperatures from multiple constraints.