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Climate extremes such as heat waves and droughts are projected to occur more frequently with increasing temperature and an intensified hydrological cycle. It is important to understand and quantify how forest carbon fluxes respond to heat and drought stress. In this study, we developed a series of daily indices of sensitivity to heat and drought stress as indicated by air temperature (Ta) and evaporative fraction (EF). Using normalized daily carbon fluxes from the FLUXNET Network for 34 forest sites in North America, the seasonal pattern of sensitivities of net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (RE) in response to Ta and EF anomalies were compared for different forest types. The results showed that warm temperatures in spring had a positive effect on NEP in conifer forests but a negative impact in deciduous forests. GEP in conifer forests increased with higher temperature anomalies in spring but decreased in summer. The drought‐induced decrease in NEP, which mostly occurred in the deciduous forests, was mostly driven by the reduction in GEP. In conifer forests, drought had a similar dampening effect on both GEP and RE, therefore leading to a neutral NEP response. The NEP sensitivity to Ta anomalies increased with increasing mean annual temperature. Drier sites were less sensitive to drought stress in summer. Natural forests with older stand age tended to be more resilient to the climate stresses compared to managed younger forests. The results of the Classification and Regression Tree analysis showed that seasons and ecosystem productivity were the most powerful variables in explaining the variation of forest sensitivity to heat and drought stress. Our results implied that the magnitude and direction of carbon flux changes in response to climate extremes are highly dependent on the seasonal dynamics of forests and the timing of the climate extremes. [ABSTRACT FROM AUTHOR]

Rapid urbanization in China in the past several decades has been shown to affect surface air temperature (SAT) locally, but its global impact is unclear. Using a global climate model, we have investigated the potential impact of urbanization in China on SAT at the global scale. The expansion of urban areas in China from 1985 to 2017 leads to winter surface air warming by as much as 0.8°C in North America (NA), largely caused by warm temperature advection due to global‐scale atmospheric circulation changes. The East Asian jet stream moves northward in eastern and northeastern China where the expansion of urban areas has increased SAT and sensible heat flux, forcing change of the atmospheric stationary waves and so the structure of the jet streams. Plain Language Summary: Rapid urbanization in China has been found to impact surface air temperature (SAT) at local and regional scales significantly through altered land surface properties. However, its impacts on global climate are less clear. Using a state‐of‐the‐art global climate model, we show that tremendous expansion of urban areas in China from 1985 to 2017 has resulted in global‐scale atmospheric circulation changes by altered surface properties and enhanced anthropogenic heating. Anomalous atmospheric circulation engenders changes of SAT by as much as 0.8°C in winter over NA, which is more pronounced than the local SAT changes in China for any season. This study provides a robust conceptual advance in understanding the climate implications of land use change especially on the global impacts of urbanization. Key Points: The fractional coverage of impermeable land surface has increased by six‐fold in China from 1985 to 2017Urbanization in China results in winter surface air warming by as much as 0.8°C in North AmericaThe altered atmospheric stationary waves and East Asian jet streams are responsible for winter warming in North America [ABSTRACT FROM AUTHOR]