The Portuguese wildfires in 2017 as multivariate high-impact weather event

Compound weather and climate events: the analysis of the October 2017 wildfires in Portugal and implications for end user guidance

In climate science, multivariate events (known as compound events) can be (a) two or more extreme events occurring simultaneously or successively, (b) combinations of extreme events that amplify the impact of the events, or (c) combinations of events that are not extreme themselves but lead to an extreme event or impact when combined [1]. Such events can overwhelm the capacity of natural and human systems to handle the resulting socio-economic and ecological impacts.

New research by Bart et al, [2] takes a step towards end user guidance by providing examples where compound event thinking should be considered in disaster risk management. Disaster management includes timely forecasts, early warning systems, infrastructure improvement projects, or damage recovery programs  

The motivation to incorporate compound event thinking often comes from real-world extreme events. An example of such an analysis is the study by Ramos et al. [3] on the 2017 wildfires in Portugal (Figure 1). Two events occurring in June and October of that year were unprecedented in terms of socio-economic impacts, with ~540,000 ha of burnt area and 114 fatalities. The local insurance sector declared it as the costliest natural disaster in Portugal, with payouts exceeding USD295 million.

Figure 1: A Firefighter battles with a fire in Pampilhosa da Serra, central Portugal, 18 June 2017. Source: LUSA,

The October event (with 200,000 ha of burnt area and 50 fatalities), reveals that the main long-term climate driver was the enduring drought that led to intense cumulative stress of vegetation prior to the event. On the 15th October 2017, the passage of hurricane Ophelia off the Coast of Portugal (Figure 2, [4]) was responsible for exceptional meteorological conditions, with extremely high wind speeds, high temperature and low relative humidity. The impacts were worsened by negligent human activity, which led to an extremely elevated number of fire ignitions mostly associated with agricultural practices.


Figure 2: Evolution of Hurricane Ophelia to an intense post-tropical cyclone, seen by Aqua MODIS true colour image on 15/10/2017. Red dots indicate fire hotspot detection,

This disastrous combination of enduring vegetation stress, the exceptional meteorological conditions and the elevated number of ignitions led to uncontrolled wildfires. In fact, in terms of Fire Weather Index the 15th October 2017 was by larger the record day within the 1980-2020 period (Figure 3). This event is a clear example on the urgent need to enhance civil protection measures that can be put into place if such a compound event is impending.

The remnants of hurricane Ophelia made landfall over Ireland resulting in observed wind speeds of up to 156 km/h and caused major power outages, lifted roofs, countless trees falls, and coastal flooding.

Figure 3: Fire Weather Index as a combination of the Spread Index (ISI, incorporates wind speed and rates the initial fire spread), the Build Up Index (BUI, rates the total amount of fuel available for combustion and pre-conditioning of the vegetation)[3].



[1] IPCC, 2012 – Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (Eds.) Available from Cambridge University Press, The Edinburgh Building, Shaftesbury Road, Cambridge CB2 8RU ENGLAND, 582 pp. Available from June 2012

[2] Bart, et al.  (2023) Consideration of compound drivers and impacts in the disaster risk reduction cycle. iScience, 106030.

[3] Ramos, et al. (2023) The compound event that triggered the destructive fires of October 2017 in Portugal. iScience, 106141.

[4] Moore, D.P. (2019) The October 2017 red sun phenomenon over the UK. Weather, 74: 348-353.

Alexandre M. Ramos, Patrick Ludwig and Julia Mömken, AG “Regional Climate and Weather Hazards”