Continentality determines warming or cooling impact of heavy rainfall events on permafrost.

Permafrost thaw can cause an intensification of climate change through the release of carbon as greenhouse gases. While the effect of air temperature on permafrost thaw is well quantified, the effect of rainfall is highly variable and not well understood. Here, we provide a literature review of studies reporting on effects of rainfall on ground temperatures in permafrost environments and use a numerical model to explore the underlying physical mechanisms under different climatic conditions. Both the evaluated body of literature and the model simulations indicate that continental climates are likely to show a warming of the subsoil and hence increased end of season active layer thickness, while maritime climates tend to respond with a slight cooling effect. This suggests that dry regions with warm summers are prone to more rapid permafrost degradation under increased occurrences of heavy rainfall events in the future, which can potentially accelerate the permafrost carbon feedback.

Extremely wet summer events enhance permafrost thaw for multiple years in Siberian tundra.

Permafrost thaw can accelerate climate warming by releasing carbon from previously frozen soil in the form of greenhouse gases. Rainfall extremes have been proposed to increase permafrost thaw, but the magnitude and duration of this effect are poorly understood. Here we present empirical evidence showing that one extremely wet summer (+100 mm; 120% increase relative to average June-August rainfall) enhanced thaw depth by up to 35% in a controlled irrigation experiment in an ice-rich Siberian tundra site. The effect persisted over two subsequent summers, demonstrating a carry-over effect of extremely wet summers. Using soil thermal hydrological modelling, we show that rainfall extremes delayed autumn freeze-up and rainfall-induced increases in thaw were most pronounced for warm summers with mid-summer precipitation rainfall extremes. Our results suggest that, with rainfall and temperature both increasing in the Arctic, permafrost will likely degrade and disappear faster than is currently anticipated based on rising air temperatures alone.

Surveillance for Heartland and Bourbon Viruses in Eastern Kansas, June 2016.

In June 2016, we continued surveillance for tick-borne viruses in eastern Kansas following upon a larger surveillance program initiated in 2015 in response to a fatal human case of Bourbon virus (BRBV) (Family Orthomyxoviridae: Genus Thogotovirus). In 4 d, we collected 14,193 ticks representing four species from four sites. Amblyomma americanum (L.) (Acari: Ixodidae) accounted for nearly all ticks collected (n = 14,116, 99.5%), and the only other species identified were Amblyomma maculatum Koch (Acari: Ixodidae), Dermacentor variabilis (Say) (Acari: Ixodidae) and Ixodes scapularis Say (Acari: Ixodidae). All ticks were tested for both BRBV and Heartland virus (Family Bunyaviridae: Genus Phlebovirus) in 964 pools. Five Heartland virus positive tick pools were detected and confirmed by real-time reverse transcription PCR (rRT-PCR), while all pools tested negative for BRBV. Each Heartland positive pool was composed of 25 A. americanum nymphs with positive pools collected at three different sites in Bourbon County. A. americanum is believed to be the primary vector of both Heartland and BRBVs to humans based upon multiple detections of virus in field-collected ticks, its abundance, and its aggressive feeding behavior on mammals including humans. However, it is possible that A. americanum encounters viremic vertebrate hosts of BRBV less frequently than viremic hosts of Heartland virus, or that BRBV is less efficiently passed among ticks by co-feeding, or less efficiently passed vertically from infected female ticks to their offspring resulting in lower field infection rates.