Shrubland carbon sink depends upon winter water availability in the warm deserts of North America

•Chihuahuan, Sonoran and Mojave Deserts vary in precipitation amount and seasonal distribution.•Due to hydrologic losses, ET was a better metric of ecosystem-available water than precipitation.•Ecosystem water use efficiency (GEP/ET) did not differ between winter and summer.•Due to lower respiration, winter seasons were critical for net carbon uptake.•Reduced 21st century winter precipitation reduced the carbon sink ∼6.8TgCyr1 in these 3 deserts.