The impact perspective on extreme seasons
The nature of individual extreme season events can be characterized by their intensity, spatial extent, or substructure. From this perspective, we can shed light on what makes intense extremely hot summer events covering an entire continent, while other equally intense events only cover fractions of such areas. But in addition to the epistemic will to understand the nature of extreme seasons, there lies great interest in how it affects mankind. The impact on infrastructure, economic sectors, our natural environment, or human health is probably the most noticeable aspect of extreme seasons.
Furthermore, there are components of the Earth’s system that are especially prone to seasonal meteorological conditions. For example, the fate of the Greenland Ice Sheet is mainly determined from snowfall in the cold seasons and meltwater runoff in the warm season. Due to several feedback processes, it matters whether most of the snow falls at the beginning or the end of the cold season, and whether a summer heat wave is melting and darkening the ice sheet over four weeks or over two times two weeks.
Since the extremely dry and hot summer 2018 in Europe, we were also reminded of the vulnerability of forest ecosystems to seasonal disturbances. Trees are long-lived organisms and, together with the soil they are rooting in, they have a natural memory. While a thunderstorm can make amends for a weekend without rainfall, the reduction of available water over an entire season can be mortal. The compound effect of heat and dryness is tricky, as forests suffer from both in the same way: high temperatures drive water loss by evapotranspiration and the precipitation deficit reduces the amount of available water in the first place. Due to the dependency of temperature and precipitation, hot and dry extremes are especially likely to co-occur and multiply the stress on forests.
Within the INTEXseas project, we take an impact perspective, but focus on atmospheric contributions to/drivers of extreme season impacts. This part of INTEXseas thus requires collaborations with colleagues from other research fields and thereby enables timely, challenging, novel and relevant research at the intersection between atmospheric and climate impact research.
A Swiss forest that was heavily affected by the summer drought in 2018, showing several signs of tree damage or mortality (Twitter @WSL_research).