Changes in climate variability are arguably more important for society and ecosystems than changes in mean climate, especially if they translate into altered extremes. There is a common perception and growing concern that human-induced climate change will lead to more volatile and extreme weather. Certain types of extreme weather have increased in frequency and/or severity, in part because of a shift in mean climate but also because of changing variability. In spite of mean climate warming, an ostensibly large number of high-impact cold extremes have occurred in the Northern Hemisphere mid-latitudes over the past decade. One explanation is that Arctic amplification--the greater warming of the Arctic compared with lower latitudes associated with diminishing sea ice and snow cover--is altering the polar jet stream and increasing temperature variability. This study shows, however, that subseasonal cold-season temperature variability has significantly decreased over the mid- to high-latitude Northern Hemisphere in recent decades. This is partly because northerly winds and associated cold days are warming more rapidly than southerly winds and warm days, and so Arctic amplification acts to reduce subseasonal temperature variance. Previous hypotheses linking Arctic amplification to increased weather extremes invoke dynamical changes in atmospheric circulation, which are hard to detect in present observations and highly uncertain in the future. In contrast, decreases in subseasonal cold-season temperature variability, in accordance with the mechanism proposed here, are detectable in the observational record and are highly robust in twenty-first-century climate model simulations.