Hot-dry compound events in the European Alps: Multi-century assessment (1600-2099 CE) indicates the need for fast adaptation

Compound hot–dry events have recently led to severe consequences globally, often triggering cascading impacts across ecological and socio-economic systems. Currently, most analyses of hot–dry extremes rely on short observational records or projections, limiting evaluation against pre-industrial variability—the climatic range to which many natural and human systems adapted over centuries. This makes it difficult to place impacts of the increased intensity and frequency of compound events in an appropriate context for examining adaptation needs.

Here we leverage a unique data coverage in the Swiss Alps to quantify changes in summer mean climate and in compound hot–dry extremes and their associated return periods from 1600 to 2099 CE. Data used include multi-century temperature and atmospheric drought reconstructions from tree rings going back to 1600 CE, instrumental station records, and local-scale climate projections for 1981-2099.

Copula-based modelling shows that summers classified as extreme in pre-industrial conditions have become common in today's climate and are expected to correspond to cold and wet conditions by the end of the century. Our analysis further shows that the hot–dry conditions witnessed in summer 2003—characterized by simultaneous positive temperature and vapor pressure deficit (VPD) anomalies of 5.3°C and 2.6 hPa relative to the pre-industrial mean, respectively—were unprecedented over at least the past 400 years and are projected to remain rare until the end of the century under RCP2.6. By contrast, they are likely to occur every 2-3 years under RCP4.5 and even to become colder and wetter than average by 2070-2099 under RCP8.5, since in the latter case, temperature and VPD anomalies are projected to exceed pre-industrial conditions by 10.4°C and 8.1 hPa in the extreme case (30-year return period).

Without countermeasures, the consequences of these changes will include, among other things, dramatic losses in agricultural production and undesirable changes in forest ecosystem dynamics. Ultimately, our analysis suggests that rapid adaptation is necessary to avoid facing more frequent extreme heat and drought conditions than those observed under pre-industrial conditions. Under RCP8.5, in particular, socio-ecological systems will need to continuously adapt within 15 years to changes in the average climate to avoid facing high-impact hot-dry compound event frequencies higher than those experienced at any time over the past 400 years. Given that adaptation in mountain regions is currently not keeping up with the realized and projected climate impacts, as pointed out in several studies, we argue that the required speed of adaptation can pose substantial challenges for alpine societies.