Asymmetric effects of daytime and night-time warming on Northern Hemisphere vegetation

摘要

模型和观测研究都表明，气候变暖可能会增强 北半球陆地生态系统的植被生长，增加碳存 储。然而，在过去50年，全球陆地表面温度在 夜晚一直比在白天上升快，而我们对于这种不 对称性对植物生产力的潜在影响却知之甚少。 这项研究中所提供的来自多方面的证据(包括 关于植被绿色程度的28年时间的卫星数据以及 对大气CO_2浓度的广泛测定结果）表明，白天 和夜晚变暖对北半球的植被生产力有不同影 响。白天变暖与北方湿冷地区生产力增加有 关，但与干燥温带地区生产力降有关。相比 之下，夜晚变暖降低北方的植被生长，但在干 燥温带地区对植被的影响却是有增有降。%Temperature data over the past five decades show faster warming of the global land surface during the night than during the day. This asymmetric warming is expected to affect carbon assimilation and consumption in plants, because photosynthesis in most plants occurs during daytime and is more sensitive to the maximum daily temperature, T_(max), whereas plant respiration occurs throughout the day and is therefore influenced by both T_(max) and the minimum daily temperature, T_(min). Most studies of the response of terrestrial ecosystems to climate warming, however, ignore this asymmetric forcing effect on vegetation growth and carbon dioxide (CO_2) fluxes. Here we analyse the interannual covariations of the satellite-derived normalized difference vegetation index (NDVI, an indicator of vegetation greenness) with T_(max) and T_(min) over the Northern Hemisphere. After removing the correlation between T_(max) and T_(min), we find that the partial correlation between T_(max) and NDVI is positive in most wet and cool ecosystems over boreal regions, but negative in dry temperate regions. In contrast, the partial correlation between T_(min) and NDVI is negative in boreal regions, and exhibits a more complex behaviour in dry temperate regions. We detect similar patterns in terrestrial net CO_2 exchange maps obtained from a global atmospheric inversion model. Additional analysis of the long-term atmospheric CO_2 concentration record of the station Point Barrow in Alaska suggests that the peak-to-peak amplitude of CO_2 increased by 23 11% for a +1 anomaly in T_(max) from May to September over lands north of 51 N, but decreased by 28 14%fora+1 anomaly in T_(min). These lines of evidence suggest that asymmetric diurnal warming, a process that is currently not taken into account in many global carbon cycle models, leads to a divergent response of Northern Hemisphere vegetation growth and carbon sequestration to rising temperatures.