Extreme climatic events down-regulate the grassland biomass response to elevated carbon dioxide

Terrestrial ecosystems are considered as carbon sinks that may mitigate the impacts of increased atmospheric CO2 concentration ([CO2]). However, it is not clear what their carbon sink capacity will be under extreme climatic conditions. In this study, we used long-term (1998–2013) data from a C3 grassland Free Air CO2 Enrichment (FACE) experiment in Germany to study the combined effects of elevated [CO2] and extreme climatic events (ECEs) on aboveground biomass production. CO2 fertilization effect (CFE), which represents the promoted plant photosynthesis and water use efficiency under higher [CO2], was quantiffied by calculating the relative differences in biomass between the plots with [CO2] enrichment and the plots with ambient [CO2]. Down-regulated CFEs were found when ECEs occurred during the growing season, and the CFE decreases were statistically significant with p well below 0.05 (t-test). Of all the observed ECEs, the strongest CFE decreases were associated with intensive and prolonged heat waves. These findings suggest that more frequent ECEs in the future are likely to restrict the mitigatory effects of C3 grassland ecosystems, leading to an accelerated warming trend. To reduce the uncertainties of future projections, the atmosphere-vegetation interactions, especially the ECEs effects, are emphasized and need to be better accounted

Volcanic Influence on European Summer Precipitation through Monsoons: Possible Cause for “Years without Summer”*

Strong tropical volcanic eruptions have significant effects on global and regional temperatures. Their effects on precipitation, however, are less well understood. Analyzing hydroclimatic anomalies after 14 strong eruptions during the last 400 years in climate reconstructions and model simulations, a reduction of the Asian and African summer monsoons and an increase of south-central European summer precipitation in the year following the eruption was found. The simulations provide evidence for a dynamical link between these phenomena. The weaker monsoon circulations weaken the northern branch of the Hadley circulation, alter the atmospheric circulation over the Atlantic–European sector, and increase precipitation over Europe. This mechanism is able to explain, for instance, the wet summer in parts of Europe during the “year without a summer” of 1816, which up to now has not been explained. This study underlines the importance of atmospheric teleconnections between the tropics and midlatitudes to better understand the regional climate response to stratospheric volcanic aerosols

Magnetostratigraphically-calibrated dinoflagellate cyst bioevents for the uppermost Eocene to lowermost Miocene of the western North Atlantic (IODP Expedition 342, Paleogene Newfoundland sediment drifts)

The Oligocene epoch represents a somewhat neglected chapter in paleoclimate and paleoceanographic history, which is at least partially due to the scarcity of complete Oligocene sedimentary archives and poor biostratigraphic age control. Many of the biotic events registered in Oligocene microfossils are strongly diachronous across latitudes as a response to increased global cooling and enhanced meridional temperature gradients. To improve biostratigraphic age control for the Oligocene of the North Atlantic Ocean, we carried out a high-resolution study of dinoflagellate cysts from Integrated Ocean Drilling Program (IODP) Sites U1405, U1406 and U1411 off Newfoundland. Together the sites comprise an apparently complete uppermost Eocene (34.9 Ma) to lowermost Miocene (21.7 Ma) sequence with good magnetostratigraphic age control. This allows us to firmly tie identified dinoflagellate cyst bioevents to the geomagnetic polarity timescale. In the dinoflagellate cyst assemblages studied we have identified and magnetostratigraphically-calibrated ten first and 19 last appearance datums. Our magnetostratigraphically-calibrated dinocyst-based biostratigraphy, which is based on an average sample resolution of a sample every ~ 150 kyrs, will contribute to an improved age framework for future paleoceanographical studies in the higher-latitude North Atlantic