Grasslands and other herbaceous plants store more than one-third of all carbon captured by land ecosystems worldwide. However, as global temperatures rise, these ecosystems risk losing stored carbon through increased soil breathing (respiration). Using a decade of satellite and atmospheric data (2015-2024), we discovered that plant and soil respiration respond very differently to warming temperatures. Soil microbe respiration peaks around 15-22°C then decreases, while plant respiration keeps rising until suddenly dropping at 26-28°C. This creates a “decoupling effect” where moderate warming initially helps carbon storage, but crossing critical temperature limits triggers rapid carbon loss. We found striking regional differences across the globe. Cold northern regions remain relatively stable as carbon stores. Tropical areas are actually gaining carbon because soil respiration is being suppressed by high temperatures while plants continue photosynthesis. However, mid-latitude regions - including many agricultural areas - are losing significant amounts of carbon as soil respiration accelerates under optimal warming conditions. Current climate models miss these temperature “breaking points” and assume steady responses to warming. Our findings suggest that continued global warming could fundamentally alter where Earth stores carbon, potentially turning productive mid-latitude grasslands into carbon sources rather than sinks, accelerating climate change beyond current predictions.