Earth's equator-to-pole temperature gradient drives westerly mid-latitude jet streams through thermal wind balance(1). In the upper atmosphere, anthropogenic climate change is strengthening this meridional temperature gradient by cooling the polar lower stratosphere(2,3) and warming the tropical upper troposphere(4-6), acting to strengthen the upper-level jet stream(7). In contrast, in the lower atmosphere, Arctic amplification of global warming is weakening the meridional temperature gradient(8-10), acting to weaken the upper-level jet stream. Therefore, trends in the speed of the upper-level jet stream(11-13) represent a closely balanced tugof-war between two competing effects at different altitudes(14). It is possible to isolate one of the competing effects by analysing the vertical shear-the change in wind speed with height-instead of the wind speed, but this approach has not previously been taken. Here we show that, although the zonal wind speed in the North Atlantic polar jet stream at 250 hectopascals has not changed since the start of the observational satellite era in 1979, the vertical shear has increased by 15 per cent (with a range of 11-17 per cent) according to three different reanalysis datasets(15-17) . We further show that this trend is attributable to the thermal wind response to the enhanced upper-level meridional temperature gradient. Our results indicate that climate change may be having a larger impact on the North Atlantic jet stream than previously thought. The increased vertical shear is consistent with the intensification of shear-driven clear-air turbulence expected from climate change(18-20), which will affect aviation in the busy transatlantic flight corridor by creating a more turbulent flying environment for aircraft. We conclude that the effects of climate change and variability on the upper-level jet stream are being partly obscured by the traditional focus on wind speed rather than wind shear.