Thread by @froeltho

Amazing new paper of my (former) PhD student Friedrich Burger (now postdoc), co-authored by @JensTerhaar and @froeltho on compound #marineheatwaves and ocean acidity extremes. A thread (1/n)
ift.tt/8tyFCDp

#Compound events in two or more ecosystem stressors are considered as a major concern for marine life. This study quantifies the frequency of compound MHW-OAX events, during which marine heatwaves (MHWs) co-occur with ocean acidity extremes (OAX) (i.e., extremely high [H+]).(2/n)

We show based on surface open ocean observations that compound MHW-OAX events occur globally more often than expected from chance. Such compound events occur relatively often in the subtropical oceans and relatively rarely in the high latitudes and the tropical Pacific (3/n)

Our results suggest that some of the observed marine heatwaves in the subtropical oceans over the recent past may well have been also compound MHW-OAX events (i.e, the Blob). (4/n)

Higher water temperature during MHWs increases [H+] (temp effect). But carbon concentration is often reduced in warmer waters, promoting a decrease in [H+] (carbon effect). The # of events is thus high where temp effect is stronger than carbon effect (e.g, subtropics) (5/n)

Using large ensemble simulations of an ESM we show that the # of MHW-OAX events is expected to increase in almost all oceans, due to the ongoing ocean warming and ocean acidification. From 12 days per year under pre-industrial conditions to 265 days at 2 °C global warming.(6/n)

Even when long-term trends are removed an increase in [H+] variability leads to a 60% increase in the # of MHW-OAX days under 2°C warming. Compound event frequency is also altered by changes in the correlation between temp and [H+], but this effect is comparably small (7/n)

Although there is evidence that marine organisms can be harmed by co-occurring warm and acidic seawater conditions, relatively little is known about the impacts of compound marine heatwave and ocean acidity extremes, calling for further research. (8/n)

Many thanks to @snsf_ch and @COMFORTH2020 for funding!