Holocene climate forcings and lacustrine regime shifts in the Indian summer monsoon realm

Extreme climate events have been identified both in meteorological and long-term proxy records from the Indian summer monsoon (ISM) realm. However, the potential of palaeoclimate data for understanding mechanisms triggering climate extremes over long time scales has not been fully exploited. A distinction between proxies indicating climate change, environment, and ecosystem shift is crucial for enabling a comparison with forcing mechanisms (e.g. El-Niño Southern Oscillation). In this study we decouple these factors using data analysis techniques [multiplex recurrence network (MRN) and principal component analyses (PCA)] on multiproxy data from two lakes located in different climate regions – Lonar Lake (ISM dominated) and the high-altitude Tso Moriri Lake (ISM and westerlies influenced). Our results indicate that (i) MRN analysis, an indicator of changing environmental conditions, is associated with droughts in regions with a single climate driver but provides ambiguous results in regions with multiple climate/environmental drivers; (ii) the lacustrine ecosystem was ‘less sensitive’ to forcings during the early Holocene wetter periods; (iii) archives in climate zones with a single climate driver were most sensitive to regime shifts; (iv) data analyses are successful in identifying the timing of onset of climate change, and distinguishing between extrinsic and intrinsic (lacustrine) regime shifts by comparison with forcing mechanisms. Our results enable development of conceptual models to explain links between forcings and regional climate change that can be tested in climate models to provide an improved understanding of the ISM dynamics and their impact on ecosystems. © 2020 John Wiley & Sons, Ltd.     

Laboratory simulation experiments on the solid-state greenhouse effect in planetary ices

Icy surfaces like the polar caps of Mars, comets, Edgeworth-Kuiper belt objects or the surface areas of many moons in the outer Solar System behave different than rock and soil surfaces when irradiated by solar light. The latter ones absorb and reflect incoming solar radiation immediately at the surface. In contrast, ices are partially transparent in the visible spectral range and opaque in the infrared. Due to this fact it is possible for the solar radiation to reach a certain depth and increase the temperature of the sub-surface layers directly. This internal temperature rise is called “solid-state greenhouse effect,” in analogy to the classical greenhouse effect in an atmosphere. It may play an important role in the energy balance of various icy bodies in the Solar System. Within the scope of a project conducted at the Space Research Institute of the Austrian Academy of Sciences in Graz the solid-state greenhouse effect was investigated experimentally and theoretically. A number of experiments with diverse materials, focussing mainly on layered samples with a surface cover consisting of transparent H₂O-ice, were performed. The samples were irradiated under cryo-vacuum conditions by a solar simulator. The temperature distributions inside the samples were measured and compared with the results of numerical model calculations. We found that the predicted sub-surface temperature maximum is very clearly measurable in glass beads samples with various particle size distributions, but can also be detected in transparent compact surface ice layers. However, in the latter case it is less distinct than originally expected. Measuring the effect by laboratory methods turned out to be a difficult task due to the shallow depth where the temperature maximum occurs.     

Seasonal surface frost at low latitudes on Mars

During the past 4 Mars years, Mars Orbiter Camera imaging capabilities have been used to document occurrence of seasonal patches of frost at latitudes as low as 33° S, and even 24° S. Monitoring reveals bright patches on pole-facing slopes; these appear in early southern winter and disappear in mid winter. The frost forms annually. Thermal Emission Spectrometer and daytime Thermal Emission Imaging System observations show surface temperatures on and near pole facing slopes reach the condensation temperature of CO₂, indicating the patches consist of carbon dioxide rather than water frost. For several months, temperatures on pole-facing crater walls are so low that even carbon dioxide condenses on them, although the slopes are illuminated by the Sun every day. Thermal model calculations show slopes accumulate a several centimeter thick layer of CO₂ frost. The frost becomes visible only months after it has begun to form, and has an orientational preference which is due to illumination bias at the time of observation. H₂O condenses at higher temperatures and water frost must therefore also be present. Potential opportunities to observe seasonal water frost at low latitudes are also described.