Reassessment and update of long-term trends in downward surface shortwave radiation over Europe (1939–2012)

This paper presents trends in downward surface shortwave radiation (SSR) over Europe, which are based on the 56 longest series available from the Global Energy Balance Archive that are mainly concentrated in central Europe. Special emphasis has been placed on both ensuring the temporal homogeneity and including the most recent years in the data set. We have generated, for the first time, composite time series for Europe covering the period 1939–2012, which have been studied by means of running trend analysis. The mean annual SSR series shows an increase from the late 1930s to the early 1950s (i.e., early brightening), followed by a reduction until mid-1980s (i.e., global dimming) and a subsequent increase up to the early 2000s (i.e., global brightening).This research was supported by the Swiss National Science Foundation grant 200021 135395 (“Towards an improved understanding of the Global Energy Balance: Absorption of solar radiation”) and the Spanish Ministry of Science and Innovation projects CGL2010-18546 and CGL2011-27574- CO2-02. The first author was supported by a postdoctoral fellowship from the “Secretaria per a Universitats i Recerca del Departament d’Economia i Coneixement, de la Generalitat de Catalunya i del programa Cofund de les Accions Marie Curie del 7è Programa marc d’R+D de la Unió Europea” (2011 BP-B 00078) and the postdoctoral fellowship JCI-2012-12508

Reassessment and update of long-term trends in downward surface shortwave radiation over Europe (1939-2012)

This paper presents trends in downward surface shortwave radiation (SSR) over Europe, which are based on the 56 longest series available from the Global Energy Balance Archive that are mainly concentrated in central Europe. Special emphasis has been placed on both ensuring the temporal homogeneity and including the most recent years in the data set. We have generated, for the first time, composite time series for Europe covering the period 19392012, which have been studied by means of running trend analysis. The mean annual SSR series shows an increase from the late 1930s to the early 1950s (i.e., early brightening), followed by a reduction until mid-1980s (i.e., global dimming) and a subsequent increase up to the early 2000s (i.e., global brightening). The series ends with a tendency of stabilization since the early 21st century, but the short time period is insufficient with regard to establishing whether a change in the trend is actually emerging over Europe. Seasonal and regional variations are also presented, which highlight that similar variations are obtained for most of the seasons and regions across Europe. In fact, due to the strong spatial correlation in the SSR series, few series are enough to capture almost the same interannual and decadal variability as using a dense network of stations. Decadal variations of the SSR are expected to have an impact on the modulation of the temperatures and other processes over Europe linked with changes in the hydrological cycle, agriculture production, or natural ecosystems. For a better dissemination of the time series developed in this study, the data set is freely available for scientific purposesThis research was supported by the Swiss National Science Foundation grant 200021 135395 ("Towards an improved understanding of the Global Energy Balance: Absorption of solar radiation") and the Spanish Ministry of Science and Innovation projects CGL2010-18546 and CGL2011-27574-CO2-02. The first author was supported by a postdoctoral fellowship from the "Secretaria per a Universitats i Recerca del Departament d'Economia i Coneixement, de la Generalitat de Catalunya i del programa Cofund de les Accions Marie Curie del 7e Programa marc d'R+D de la Unio Europea" (2011 BP-B 00078) and the postdoctoral fellowship JCI-2012-12508. The last author was supported by a SCIEX postdoctoral fellowship (9th Call, November 2013, Sciex Nr. 13.155-2) of the Scientific Exchange Programme NMS-CH. We would like to thank Guido Muller, Barbara Trussel, and Mijung Song for the maintenance of the GEBA and Christoph Schar for continuous support. We also thank A. Tsvetkov from the World Radiation Data Center (WRDC) of the Main Geophysical Observatory in St. Petersburg that provides solar radiation data that have been included in the GEB

Partitioning global land evapotranspiration using CMIP5 models constrained by observations

The ratio of plant transpiration to total terrestrial evapotranspiration (T/ET) captures the role of vegetation in surface–atmosphere interactions. However, its magnitude remains highly uncertain at the global scale. Here we apply an emergent constraint approach that integrates CMIP5 Earth system models (ESMs) with 33 field T/ET measurements to re-estimate the global T/ET value. Our observational constraint strongly increases the original ESM estimates (0.41 ± 0.11) and greatly alleviates intermodel discrepancy, which leads to a new global T/ET estimate of 0.62 ± 0.06. For all the ESMs, the leaf area index is identified as the primary driver of spatial variations of T/ET, but to correct its bias generates a larger T/ET underestimation than the original ESM output. We present evidence that the ESM underestimation of T/ET is, instead, attributable to inaccurate representation of canopy light use, interception loss and root water uptake processes in the ESMs. These processes should be prioritized to reduce model uncertainties in the global hydrological cycle

Field-experiment constraints on the enhancement of the terrestrial carbon sink by CO₂ fertilization

Clarifying how increased atmospheric CO₂ concentration (eCO₂) contributes to accelerated land carbon sequestration remains important since this process is the largest negative feedback in the coupled carbon–climate system. Here, we constrain the sensitivity of the terrestrial carbon sink to eCO₂ over the temperate Northern Hemisphere for the past five decades, using 12 terrestrial ecosystem models and data from seven CO₂ enrichment experiments. This constraint uses the heuristic finding that the northern temperate carbon sink sensitivity to eCO₂ is linearly related to the site-scale sensitivity across the models. The emerging data-constrained eCO₂ sensitivity is 0.64 ± 0.28 PgC yr−1 per hundred ppm of eCO₂. Extrapolating worldwide, this northern temperate sensitivity projects the global terrestrial carbon sink to increase by 3.5 ± 1.9 PgC yr−1 for an increase in CO2 of 100 ppm. This value suggests that CO₂ fertilization alone explains most of the observed increase in global land carbon sink since the 1960s. More CO₂ enrichment experiments, particularly in boreal, arctic and tropical ecosystems, are required to explain further the responsible processes