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<p>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. To do this, we first calculated the reduction in net primary production due to the carbon cost of nitrogen acquisition. We then used a climate model to estimate the impacts of the resulting increase in atmospheric <span class="inline-formula">CO₂</span> on temperature and precipitation regimes. The carbon costs of supporting symbiotic nitrogen uptake reduced net primary production by 8.1&thinsp;Pg&thinsp;C&thinsp;yr<span class="inline-formula">⁻¹</span>, with the largest absolute effects occurring in tropical forest biomes and the largest relative changes occurring in boreal and alpine biomes. Globally, our model predicted relatively small changes in climate due to the carbon cost of nitrogen acquisition with temperature increasing by 0.1&thinsp;<span class="inline-formula">^∘</span>C and precipitation decreasing by 6&thinsp;mm&thinsp;yr<span class="inline-formula">⁻¹</span>. However, there were strong regional impacts, with the largest impact occurring in boreal and alpine ecosystems, where such costs were estimated to increase temperature by 1.0&thinsp;<span class="inline-formula">^∘</span>C and precipitation by 9&thinsp;mm&thinsp;yr<span class="inline-formula">⁻¹</span>. 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 overpredict the land carbon sink and underpredict climate change.</p>