The extent to which terrestrial ecosystems slow climate change by sequestering carbon hinges in part on nutrient limitation. We used a coupled carbon–climate model that accounts for the carbon cost to plants of supporting nitrogen-acquiring microbial symbionts to explore how nitrogen limitation affects global climate. The carbon costs of supporting symbiotic nitrogen uptake reduced net primary production, with the largest absolute effects occurring at low-latitudes and the largest relative changes occurring at high-latitudes. The largest impact occurred in high-latitude ecosystems, where such costs were estimated to increase temperature by 1.0 °C and precipitation by 9 mm yr −1 . Globally, our model predicted that nitrogen limitation enhances temperature and decreases precipitation; as such, our results suggest that carbon expenditures to support nitrogen-acquiring microbial symbionts have critical consequences for Earth’s climate, and that carbon–climate models that omit these processes will over-predict the land carbon sink and under-predict climate change.