Increasing tropical water control on interannual CO2 growth rate over the past decades
<p>Terrestrial climate-carbon feedbacks are the leading-order uncertainties in climate projections, hindering&#160;the full assessment of climate mitigation scenarios. Since year-to-year variations of atmospheric carbon dioxide growth rate (CGR) are mostly driven by fluctuations of tropical land carbon fluxes, it provides a &#8220;natural experiment&#8221; to explore the climate drivers of terrestrial carbon cycle. Recently, direct observations of terrestrial water storage confirmed the tight coupling between&#160;the&#160;water and carbon cycles, in addition to the well-documented temperature effects. Here we show that the strength of this relationship between CGR and&#160;the interannual variability of&#160;tropical water has increased substantially&#160;from 1960 to 2018&#160;and has even recently&#160;become stronger than CGR-temperature correlations. We find&#160;that&#160;this increment may be relevant to local drying trends in a warming climate and&#160;that&#160;above-ground&#160;carbon uptake might be a critical underlying ecological process. We also demonstrate that most state-of-the-art&#160;Earth System&#160;models and land surface models do not capture this increasing carbon-water coupling over time. Our results suggest that tropical water availability could increasingly dominates the interannual variability of&#160;the&#160;terrestrial carbon cycle&#160;in the future and&#160;that current&#160;models may not be able to capture this feature.</p>