Title: The role of the land surface in the global carbon and water cycles
Description: The global continental carbon and water cycles are intimately linked through stomatal regulation during vegetation photosynthesis and biosphere-atmosphere interactions. Therefore, to have a complete understanding of both present and future climate, these cycles must be studied as an interconnected system. This thesis presents three studies that aim to better explain these interactions and provide a direction forward for improved model projections of climate. The first study shows that biosphere-atmosphere feedbacks can contribute up to 30% of climate and weather variability in certain regions that help determine the net CO2 balance of the biosphere. It demonstrates that Earth System Models are under-estimating these contributions, mainly due to the underestimation of the biosphere response to radiation and water availability. It emphasizes the importance of correctly capturing these feedbacks in models for accurate subseasonal to seasonal climate predictions. The second demonstrates that changes in soil moisture (both short-term variability and long-term trends) strongly limit the ability of the continents to act as a carbon sink, with overall effects on the same order of magnitude as the land sink itself.

Photosynthesis rates tend to be reduced when soil moisture is depleted, leading to decreased carbon uptake. Additionally, respiration rates increase due to increased temperature through land-atmosphere feedbacks. These carbon losses are not compensated for during wet anomalies due to the nonlinear response of vegetation activity (both respiration and photosynthesis) to soil moisture. This suggests that the increasing trend in carbon uptake rate may not be sustained past the middle of the century and could result in accelerated atmospheric CO2 growth. The third decouples the effects of atmospheric dryness (vapor pressure deficit) and soil dryness on vegetation activity in the largest terrestrial carbon sink: the tropics. Understanding vegetation response to environmental drivers and stressors in the tropics is essential to accurately modeling these ecosystems and predicting whether they will remain carbon sinks in the future. The study finds that in regions that are water limited, vegetation is driven by precipitation and radiation while being limited by high vapor pressure deficit.

Conversely, in the wettest regions that are light limited, increases in vapor pressure deficit accompany increasing rates of photosynthesis. These three studies contribute to our understanding of land-atmosphere and biosphere-atmosphere feedbacks and the coupling of the continental carbon and water cycles. They identify model process representations, such as soil moisture and vegetation water-stress, that are hindering our ability to make accurate forecasts. By improving our knowledge of these mechanisms and evaluating the ability of models to reproduce them, we pave the way forward for improved climate and weather projections. Better predictions can be used not only to protect society in the present, but also to appropriately shape climate policy to protect society in the future.