Forests play a pivotal role in mitigating global warming as an important carbon sink. Recent global greening trends reflect a positive influence of elevated atmospheric CO₂ on terrestrial carbon uptake. However, increasingly frequent and intense drought events endanger the carbon sequestration function of forests. This review integrates previous studies across scales to identify potential global trends in forest responses to drought and elevated CO₂ as well as to identify data needs in this important research field. The inconsistent responses of ecosystem respiration to drought contributes to the change of forest net CO₂ exchange, which depends on the balance of opposite effects of warming and water stress on respiration. Whether CO₂ fertilization can offset the effects of drought remains controversial, however, we found a potential overestimation of global CO₂ fertilization effects because of increasing water stress and other limitations such as light and nutrients (N, P) as well as the possibility of photosynthetic acclimation.

The rapid development of cities has brought a lot of carbon emissions and water consumption, leading to disasters, such as the greenhouse effect and drought. However, there is a lack of systematic review on the key nodes for the water–carbon nexus and the key points of water-saving and emission reduction improvement in the urban systems. This study reviewed the definition of the water–carbon nexus, analyzed its changing mechanism in different urban systems, and concluded the major methodologies applied in the nexus studies. The main findings are as follows: (1) the water/wastewater treatment in water systems and the structure transformation in energy systems are the key nodes for the water–carbon nexus. (2) From the perspective of methodologies, the research focus has gradually changed from single system and process analysis to multi-system and whole process analysis. (3) There is a tendency to sacrifice large water consumption in exchange for carbon reduction in the urban systems, calling for explorations in the water–carbon balance. (4) More comprehensive methods, systematic data support, and accurate definition of system boundaries are required to quantify the nexus. This study systematically reviewed the existing achievements on the water–carbon nexus, providing support for urban metabolism and related policy makings.