Overshooting tipping point thresholds in a changing climate

Palaeorecords suggest that the climate system has tipping points, where small changes in forcing cause substantial and irreversible alteration to Earth system components called tipping elements. As atmospheric greenhouse gas concentrations continue to rise as a result of fossil fuel burning, human activity could also trigger tipping, and the impacts would be difficult to adapt to. Previous studies report low global warming thresholds above pre-industrial conditions for key tipping elements such as ice-sheet melt. If so, high contemporary rates of warming imply that exceeding these thresholds is almost inevitable, which is widely assumed to mean that we are now committed to suffering these tipping events. Here we show that this assumption may be flawed, especially for slow-onset tipping elements (such as the collapse of the Atlantic Meridional Overturning Circulation) in our rapidly changing climate. Recently developed theory indicates that a threshold may be temporarily exceeded without prompting a change of system state, if the overshoot time is short compared to the effective timescale of the tipping element. To demonstrate this, we consider transparently simple models of tipping elements with prescribed thresholds, driven by global warming trajectories that peak before returning to stabilize at a global warming level of 1.5 degrees Celsius above the pre-industrial level. These results highlight the importance of accounting for timescales when assessing risks associated with overshooting tipping point thresholds

Sediment removal by prairie filter strips in row-cropped ephemeral watersheds

Twelve small watersheds in central Iowa were used to evaluate the effectiveness of prairie filter strips (PFS) in trapping sediment from agricultural runoff. Four treatments with PFS of different size and location (100% rowcrop, 10% PFS of total watershed area at footslope, 10% PFS at footslope and in contour strips, 20% PFS at footslope and in contour strips) arranged in a balanced incomplete block design were seeded in July 2007. All watersheds were in bromegrass ( L.) for at least 10 yr before treatment establishment. Cropped areas were managed under a no-till, 2-yr corn ( L.)-soybean [ (L.) Merr.] rotation beginning in 2007. About 38 to 85% of the total sediment export from cropland occurred during the early growth stage of rowcrop due to wet field conditions and poor ground cover. The greatest sediment load was observed in 2008 due to the initial soil disturbance and gradually decreased thereafter. The mean annual sediment yield through 2010 was 0.36 and 8.30 Mg ha for the watersheds with and without PFS, respectively, a 96% sediment trapping efficiency for the 4-yr study period. The amount and distribution of PFS had no significant impact on runoff and sediment yield, probably due to the relatively large width (37-78 m) of footslope PFS. The findings suggest that incorporation of PFS at the footslope position of annual rowcrop systems provides an effective approach to reducing sediment loss in runoff from agricultural watersheds under a no-till system

Increases in the temperature seasonal cycle indicate long-term drying trends in Amazonia

Earth System Models project a wide range of rainfall changes in the Amazon rainforest, and hence changes in soil moisture and evapotranspiration. Hydrological changes are heterogeneous, meaning local measurements are too sparse to constrain projections of large-scale hydrological change. Here we show that changes in the amplitude of the temperature seasonal cycle are strongly correlated with annual mean evaporative fraction (surface latent heat flux as a fraction of surface net radiation) changes, across reanalyses and Earth System Model projections. We find an increase in annual temperature amplitude of 1 °C is associated with a reduction in evaporative fraction of up to 0.04. The observed temperature seasonal cycle amplitude increase (0.4 °C) over the last three decades implies Amazon drying, determined in the absence of soil or energy flux measurements, matches Earth System Model simulations of the recent past. Additionally, Earth System Models predict further temperature seasonal cycle amplitude increases, suggesting drying will continue with future climate change

Acceleration of daily land temperature extremes and correlations with surface energy fluxes

Assessment of climate reanalysis data for land (ECMWF Re-Analysis v5; ERA5-Land) covering the last seven decades reveals regions where extreme daily mean temperatures are rising faster than the average rate of temperature rise of the 6 months of highest background warmth. However, such extreme temperature acceleration is very heterogeneous, occurring only in some places including regions of Europe, the western part of North America, parts of southeast Asia and much of South America. An ensemble average of Earth System Models (ESMs) over the same period also shows acceleration across land areas, but this enhancement is much more spatially uniform in the models than it is for ERA5-Land. Examination of projections from now to the end of the 21st Century, with ESMs driven by the highest emissions Shared Socio-economic Pathway scenario (SSP585) of future changes to atmospheric greenhouse gases, also reveals larger warming during extreme days for most land areas. The increase in high-temperature extremes is driven by different processes depending on location. In northern mid-latitudes, a key driver is often a decrease in the evaporative fraction of the available energy, consistent with soil drying. By contrast, the acceleration of high-temperature extremes in tropical Africa is primarily due to increased available energy. These two drivers combine via the surface energy balance to equal the sensible heat flux, which we find is often strongly correlated with the areas where the acceleration of high-temperature extremes is largest

Setdb1-mediated H3K9 methylation is enriched on the inactive X and plays a role in its epigenetic silencing

Background: The presence of histone 3 lysine 9 (H3K9) methylation on the mouse inactive X chromosome has been controversial over the last 15 years, and the functional role of H3K9 methylation in X chromosome inactivation in any species has remained largely unexplored. Results: Here we report the first genomic analysis of H3K9 di- and tri-methylation on the inactive X: we find they are enriched at the intergenic, gene poor regions of the inactive X, interspersed between H3K27 tri-methylation domains found in the gene dense regions. Although H3K9 methylation is predominantly non-genic, we find that depletion of H3K9 methylation via depletion of H3K9 methyltransferase Set domain bifurcated 1 (Setdb1) during the establishment of X inactivation, results in failure of silencing for around 150 genes on the inactive X. By contrast, we find a very minor role for Setdb1-mediated H3K9 methylation once X inactivation is fully established. In addition to failed gene silencing, we observed a specific failure to silence X-linked long-terminal repeat class repetitive elements. Conclusions: Here we have shown that H3K9 methylation clearly marks the murine inactive X chromosome. The role of this mark is most apparent during the establishment phase of gene silencing, with a more muted effect on maintenance of the silent state. Based on our data, we hypothesise that Setdb1-mediated H3K9 methylation plays a role in epigenetic silencing of the inactive X via silencing of the repeats, which itself facilitates gene silencing through alterations to the conformation of the whole inactive X chromosome.Andrew Keniry, Linden J. Gearing, Natasha Jansz, Joy Liu, Aliaksei Z. Holik, Peter F. Hickey, Sarah A. Kinkel, Darcy L. Moore, Kelsey Breslin, Kelan Chen, Ruijie Liu, Catherine Phillips, Miha Pakusch, Christine Biben, Julie M. Sheridan, Benjamin T. Kile, Catherine Carmichael, Matthew E. Ritchie, Douglas J. Hilton and Marnie E. Blewit

Human Health Risk Assessment For Arsenic: A Critical Review

Millions of people are exposed to arsenic resulting in a range of health implications.This paper provides an up-to-date review of the different sources of arsenic (water, soil and food), indicators of human exposure (biomarker assessment of hair, nail, urine and blood), epidemiological and toxicological studies on carcinogenic and non-carcinogenic health outcomes, and risk assessment approaches. The review demonstrates a need for more work evaluating the risks of different arsenic species such as; arsenate, arsenite monomethylarsonic acid, monomethylarsonous acid, dimethylarsinic acid and dimethylarsinous acid as well as a need to better integrate the different exposure sources in risk assessments