Rising atmospheric CO2 concentrations are driving ongoing climatic changes, leading to agricultural crops increasingly experiencing extreme weather events1. Stomata serve as gatekeepers on plant leaves, regulating both CO2 capture for photosynthesis and the concomitant release of water. At higher CO2 concentrations or higher vapour pressure deficit (VPD), stomatal pores narrow, reducing stomatal conductance to water vapour (gsw) and transpiration (E)2-6. Increasing temperatures and/or nitrogen fertilisation promote an opposite stomatal response, enhancing gsw and E7,8. With atmospheric CO2 concentration, temperature and VPD predicted to rise throughout this century1, it is unclear how crops will modify stomatal gaseous exchanges, particularly under differing N-fertilisation regimes. Here, we show in wheat (Triticum aestivum), that elevated CO2 does not reduce gsw or E during heatwaves when VPD is high, instead plant water usage increases. High-VPD heatwave events also impact stomatal responsiveness to N-fertiliser application, prompting significantly higher gas exchange contributions from abaxial leaf surfaces, irrespective of CO2 growth conditions. Dynamic stomatal responsiveness to light and high CO2 are also attenuated during heatwaves in a CO2-independent manner. Taken together, our data suggests that future wheat crops will use significantly more water during heatwaves than might be expected, which has substantial implications for future global food security.