What conditions cause large wildfires in Portugal and Spain?

Short- and long-term effects

Despite being episodic events, large wildfires account for the bulk of burned area in Spain and Portugal. These wildfires, with burned areas > 500 hectares, can be characterized by the climate and weather conditions that cause them. Climatic conditions over a time scale of seasons to years determine the amount of fuel that feed these fires. Weather conditions in the days or weeks prior to the onset of a fire change the fuel moisture state of especially the fine fuel and litter, and hence their flammability. On a shorter time scale, hourly and daily meteorological variables control fire ignition and propagation (26).

Four types of large wildfires

For the Iberian Peninsula (Portugal and Spain), large wildfires during the period 2001 - 2015 have been characterized by looking at the climate and weather conditions at different time scales that coincided with these events. Knowledge on the causal relations between these conditions and the onset of large wildfires can be used for early warning and preparation. A distinction is made in four types of large wildfires, labelled: ‘heat-driven’, ‘heat wave’, ‘seasonal drought’ and ‘wind-driven' (26):

• ‘Heat-driven’ large wildfires are driven by temperature and dryness of fuels. They are associated with calm winds and sustained hot and dry conditions conducive to low fuel moisture.
• ‘Heat wave’ wildfires are also driven by temperature and dryness of fine fuels, and associated with calm winds. Contrary to the ‘heat-driven’ type, extremely high temperatures are driving these fires, causing a rapid desiccation of fine fuels and litter, whilst the moisture content of heavy fuels is not low.
• ‘Seasonal drought’ wildfires are associated with extended drought periods, leading to dry heavy fuels. For this type of wildfire, the spread of the fire is not inhibited by the presence of moist fine fuels. It is the low moisture content of the dry heavy fuels, a result of prolonged drought periods, and the wind that drive the fires.
• ‘Wind-driven’ wildfires occur on days with sudden warm conditions and very strong winds. These wildfires are especially associated with a sudden drop in relative humidity and the emergence of strong winds during the day of ignition. Temperatures over a period of 30 days prior to the fire also play a role, however.

‘Heat-driven’ wildfires seem to be the dominant type of large wildfires in Spain and Portugal. Second in importance ranks the ‘heat wave’ type. Exceptional heat and drought conditions in 2003 led to one of the worst fire seasons recorded in Portugal.

Wildfire hotspot: halfway Portugal

During the last decades, the bulk of large fires started halfway Portugal near the border with Spain. All four types of large wildfires are common there. Clearly, it is the combination of heat, drought and wind that promotes larger fires. While annual or seasonal long-term climatic variables influence fuel amount, short-term weather in the previous days or weeks changes fuel moisture state, and hourly and daily meteorological variables control fire ignition and propagation (27).

Fire-fighting strategies

This knowledge on causal factors driving large wildfires can be used for strategies to combat them. The contribution of weather factors may change throughout the landscape promoting different dominant fire spread patterns (28). Where fuel moisture content holds a leading role, fuel treatments might be promoted to diminish hazard. Where wind speed is driving fire spread then early warning systems and fire-fighting arrangement might have to be prioritized (29). The recent decadal increases in fuel load and continuity across Mediterranean landscapes (30) have led to propose shifts in management focus from suppression to fire-, fuel-managed, more resilient landscapes (31).

Vulnerabilities - Changes in the past - Mediterranean

In the last decades, there has been a noticeable increase in the number of wildfires in the Mediterranean countries (1). The total area burnt also increased in Spain (2), and in the other Mediterranean countries of the European Union (3). However, the increase in the total area burnt is not as sustained as the increase in the fire frequency, but there are periods of low annual area burnt punctuated with years of extraordinary fire activity, like 2003 in Portugal or 1994 in Eastern Spain (4).

An increase in the area burnt is not necessarily related to an increase in the ignition frequency. Results from research have been reported that show that the number of ignitions had very little effect on the total area burnt. The results indicate that the total area burnt will be more or less the same despite any effort to reduce it by reducing the number of ignitions, extinguishing fires or by using prescribed burning. This annual area burnt appears to be determined by the characteristics of the vegetation (maximum biomass and rate of the accumulation of biomass) and by the meteorological characteristics (average and standard deviation of the meteorological conditions). Furthermore, reducing the number of ignitions had the perverse effect of increasing the proportion of area burnt in large fires: there were less fires, but these were larger (4).

The main determinant of the fire regime is probably the accumulation of fuel in the vegetation. Processes that allow the natural accumulation of fuel, like reducing the number of ignitions or increasing the fire fighting capacity, permit the creation of continuous areas with a high fuel load, which will burn under adverse meteorological conditions and produce large fires. On the contrary, processes that eliminate patches of fuel, like a high number of ignitions, non-fire suppression, or prescribed burning, create a patchy mosaic in the vegetation that can behave as fuel-breaks under some circumstances, and reduce the extension of some fires that, otherwise, would become large fires (4).

Vulnerabilities - Changes in the past - Spain

The majority of wildfires are caused by people. Over the last 30 years some 400,000 wildfires have occurred in Spain. During 1991-2002 each year 0.55% of the total forest area of Spain burnt (5).

The analysis of fire records suggested a clear increase in the annual number of fires and area burned during the last century in the eastern Iberian Peninsula; however, in the last three decades the number of fires also increased but the area burned did not show a clear trend (6). In fact, a study on the Mediterranean region on fire trends in Portugal, Spain, southern France, Italy, and Greece in the period 1985-2011 revealed a general decreasing trend of the total annual burned area in all countries, with the exception of Portugal (25).

For the last three decades the inter-annual variability in area burned was significantly related to the summer rainfall, that is, in wet summers the area burned was lower than in dry summers. Furthermore, summer rainfall was significantly cross-correlated with summer area burned for a time-lag of 2 years, suggesting that high rainfall may increase fuel loads that burn 2 years later (6).

Aragón

Over the past 30 years, wild land fires in Aragón (NE Spain) have became more extreme, however, with fire behaviour more and more often exceeding fire fighting capabilities (18), and fire agencies experience difficulties in suppressing extreme-behaviour fires while providing safety for both fire-fighters and citizens (19).

In the Aragón region in Northeastern Spain, high temperature days and large wildland fires are correlated: the five largest wildland fires on record in Aragón did develop under high temperature days. High temperature days are days with high air temperatures and low relative humidity, commonly correlated with heat waves. In a study on wildfires in Aragón, so-called main wildland fires are defined as those over 60 ha. These large fires, 2.5 % of the total number of fires, accounted for 85 % of the total area burned in NE Spain in the 1978–2011 period (24).

In Aragón, the total annual number of main wildland fires and the annual area burned declined during 1978–2011 for fires during days when temperatures were not high. This illustrates that in the last years, fire suppression resources have improved in technology and training and. For high temperature days, however, the total annual number of main wildland fires and the annual area burned did not decline. From 1978 to 2011, the ratio of main wildland fires under high temperature days compared with fires the rest of the year rose from 0.30 in 1981 to 0.65 in 2008. In a similar way, the ratio of area burned under high temperature days versus total area burned also increased from 0.49 in 1981 to 0.85 in 2007 (24).

High temperature days more and more determine the origin of large wildland fires in Aragón. The annual number of days with these high temperatures increases. The authors of this study warn that in the future we might be expecting larger fires and could face extreme-behavior wildland fires beyond our firefighting capacity. Wildland fires are mostly happening from June to September. Fire season seems to be starting earlier and last longer than in former years: the number of high temperature days in June is already increasing (24).

Valencia

The role of climate and human-driven fuel changes on the fire regime change over the last 130 years in the Valencia province of Spain has been evaluated based on contemporary statistics plus old forest administration dossiers, newspapers, rural population and climatic data (15). The result suggest that there was a major fire regime shift around the early 1970s in such a way that fires increased in annual frequency (doubled) and area burned (by about an order of magnitude). It was concluded that the main driver of this shift was the increase in fuel amount and continuity due to rural depopulation (vegetation and fuel build-up after farm abandonment (15,17)) suggesting that fires were fuel-limited during the pre-1970s period. Climatic conditions were poorly related to pre-1970s fires and strongly related to post-1970s fires, suggesting that fires are currently less fuel limited and more drought-driven than before the 1970s.

Vulnerabilities - Projections for the future

Forest fire danger, length of the fire season, and fire frequency and severity are very likely to increase in the Mediterranean (8,9), and will lead to increased dominance of shrubs over trees (10).

Dry weather and damaged ecosystem with accumulation of dead biomass increase the risk of forest fires. Increased climate variability, therefore, will augment the risk of forest fires (11). In addition, forest fires are expected to encourage the spread of invasive species which in turn, have been shown to fuel more frequent and more intense forest fires (12).

Mediterranean

An indication of the forest fire risk under the future climate scenarios has been calculated (12). Under both A2 and B2 (IPPC SRES) scenarios, fire risk is shown to increase nearly everywhere in the Mediterranean region, especially in inland locations. The southern Mediterranean is at risk of forest fire all year round. In the Iberian Peninsula, northern Italy and over the Balkans, the period of extreme fire risk lengthens substantially. The only region that shows little change in fire risk is in the southeastern Mediterranean (12).

In the Iberian Peninsula, mean burnt areas could be about two to three times larger by 2075  than in the present, taking into account current climate projections for the end of this century, and non-significant changes in other external factors, such as human activity, fire suppression or land use (23).

Projections of forest fire risk in 2030-2060 compared with 1961-1990 suggest that (12):

• The increase is higher during the summer, with maximum increase in August in the North Mediterranean inland;
• Balkans, Maghreb, North Adriatic, Central Spain, and Turkey are the most affected regions;
• The south of France is as strongly affected as Spain, but only in August and September;
• The islands of Crete, Sardinia, Sicily (southernmost Italy too), Peloponnese, and Cyprus see no increase or decrease. Cyprus may even see a small decrease every month;
• There will be 2 to 6 additional weeks of fire risk everywhere, except for the south of Italy and Cyprus. The maximum increase is again inland (Spain, Maghreb, Balkans, North Italy, and Central Turkey), where at least an additional month with risk of fire is expected. A significant proportion of this increase in fire risk is actually extreme fire risk;
• The south of France, Crete, and the coastal area of the rest of Mediterranean Region also show a significant increase in the number of days with fire risk (1-4 weeks), but not in the number of extreme fire risk.

Contrary to the pattern expected in boreal and temperate forests, both the frequency and intensity of fires in subtropical forests will eventually decrease after an initial phase of increase once rainfall has decreased so much that less grass fuel is available to support fires (14).

Catalonia

During the period 1970–2010, the number of fires in Eastern Catalonia (North-East Spain) decreased, indicating that past improvements in management actions have more than counterbalanced the climatic trend. It was shown, however, that for future projections of climate change (A1B scenario) for 2001-2050 compared with 1970-2000, the number of fires may increase in absence of further improvements in fire management. For these projections of climate change, the trend of burned area is non-significant and slightly negative. According to the researchers, this could be associated with the importance of antecedent climate conditions for this variable: warmer conditions can act on the fuel structure by limiting the availability of fine fuel and favouring fuel gaps, thus reducing the spread of large fires. However, the researchers stress that the increasing number of fires might increase the chance of large fires as well because the fuel load and connectivity may increase, for example due to land-use and land-cover changes (21).

For NE Spain, the impacts of climate change and local management options on number of fires, burned area, fraction of area burned in large fires and forest area during the twenty-first century have been assessed by means of a fire regime model (FIRE LADY, climatic projection for the period 2001–2100 generated by global circulation model using the A2 and B1 scenarios). The results show that currently fuel-humidity limited regions could suffer a drastic shift of fire regime with an up to 8 fold increase of annual burned area, due to a combination of fuel accumulation and severe fire weather, which would result in a period of unusually large fires. The impact of climate change on fire regime is predicted to be less pronounced in drier areas, with a gradual increase of burned area (16).

Aragón

It has been stated that the so-called high temperature days, with air temperature higher than 20°C at an altitude in the atmosphere where pressure is 850 hPa (around 1500m up in the atmosphere), provide extreme conditions for fire propagation and difficulties to suppress those fires (17). From an analysis of the effects of high temperature days on large wild land fires (those over 100 ha) during 1978–2010 in Aragón (NE Spain), it was concluded that if days with these temperatures become more frequent and these conditions are able to decrease air humidity and fuel moisture and increase the fire behaviour potential, the area may be facing larger wild land fires in the future (17), and very likely extreme-behaviour fires beyond suppression capacity (19).

Canary Islands

Projections of the impact of climate change on wildfire risk in the Canary Islands for a moderate (RCP 4.5) and high-end scenario (RCP 8.5) of climate change indicate an increase in fire risk, length of the fire season, and the number of days with extreme risk by the end of this century (32).

Adaptation strategies

Fire regime model calculations show that a combination of classical fire prevention, i.e. fire suppression and ignition reduction, and land use management (the creation of agricultural fields in marginal areas) is the most promising strategy to mitigate the effects of climate change on fire regime. With this approach, burned area could be reduced without the increment of fire size that the classical scenarios predict, as the agricultural fields scattered in the flammable area break the fuel continuity. Nevertheless, no realistic management strategy was able to totally offset the effect of climate change (16).

For regions with short fire return intervals, fire suppression leads to fuel build up, which can lead to uncontrollable fires under extreme meteorological conditions. Fuel fragmentation by agricultural fields is an important factor hindering fire spread, as agricultural fields can act as fire breaks, but without the need of specific maintenance; the creation of new fields in marginal areas can increase fuel fragmentation and greatly reduce the burned area (16).

Major funding has also been put into increasing the capacity to combat forest fires in Europe. For example, Italy has Europe’s largest fleet of aircraft and helicopters, and has on several occasions loaned out its planes to France and Spain. The high level of preparedness requires significant resources, but has shown good results: the year 2000 saw 6,600 fires destroy 58,000 hectares of forest, while almost the same number of fires in 2006 only destroyed 16,000 hectares. Protezione Civile considers itself to have a successful organisation with a high level of preparedness and great capacity to handle the effects of climate change (13).

Current policies in all Mediterranean countries, based only in reducing the number of ignitions by suppressing fires, will not reduce the extension of the area burnt in large, catastrophic fires. Firefighters can extinguish most of the fires, but when there is one out of the extinction capacity, it burns larger areas than when fire-exclusion policies were not as powerful as today. Fire-exclusion policies have to be inevitably complemented with fuel-reduction techniques and fire prevention management of the forest. Strategic areas with different fuel loads would probably help in the extinction of large wildfires and might help to prevent catastrophic events to occur as frequently as they have been occurring in the Mediterranean basin in the last 20 years (4).

Catalonia

An increasing effort in fire management, considering both fire prevention and fire extinction, is presumably one of the main causes for the observed reduction in the number of fires in Eastern Catalonia (North-East Spain) during the period 1970–2010 (21). For instance, after the big fires in the 1980s, fire management strategies were improved (22), including: the generation of daily maps of fire risk, combing weather forecasts and vegetation maps obtained by satellite and aerial photography; the presence of fire-guard in areas of risk of fire; the use of specific aerial means (e.g. Canadairs); the increasing awareness of the population and its coordination with fire-fighters.

An increased effort in future fire-management policies is needed, however (21): future projections of climate change (A1B scenario) for 2001-2050 compared with 1970-2000 indicate that the number of fires may increase in absence of further improvements in fire management. Prevention (e.g. increasing efforts to avoid ignitions during those days with adverse weather conditions; reducing fuel load and continuity with prescribed burning and with fuel-breaks) and suppression strategies (in particular investing in the early stage of extinction) should be combined (21).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Spain.

1. Xanthopoulos (2000); Viegas (1998, 2004); Moreno et al. (1998); Piñol et al. (1998) Pausas (2004), all in: Oliveras et al. (2005)
2. Moreno et al. (1998), in: Oliveras et al. (2005)
3. Xanthopoulos (2000); Viegas (2004), both in: Oliveras et al. (2005)
4. Oliveras et al. (2005)
5. Comisión de Coordinación de Políticas de Cambio Climático (2007)
6. Pausas (2004)
7. Piñol et al. (1998)
8. Santos et al. (2002); Pausas (2004); Moreno (2005); Pereira et al. (2005); Moriondo et al. (2006), all in:Alcamo et al. (2007)
9. Oficina Española de Cambio Climático (2008)
10. Mouillot et al., 2002, in: Alcamo et al. (2007)
11. Ministry for the Environment, Land and Sea of Italy (2007)
12. Giannakopoulos et al. (2005)
13. Swedish Commission on Climate and Vulnerability (2007)
14. Fischlin (ed.) (2009)
15. Pausas and Fernández-Muñoz (2012)
16. Loepfe et al. (2012)
17. Cardil et al. (2013)
18. Miralles et al. (2010); Molina et al. (2010), both in: Cardil et al. (2013)
19. Werth et al. (2011, in: Cardil et al. (2013)
20. Molina et al. (2010), in: Cardil et al. (2013)
21. Turco et al. (2014)
22. Turco et al. (2013b); Moreno et al. (2014), both in: Turco et al. (2014)
23. Sousa et al. (2015)
24. Molina-Terrén et al. (2016)
25. Turco et al. (2016), in: Silva et al. (2019)
26. Rodrigues et al. (2020)
27. Ruffault et al. (2016), in: Rodrigues et al. (2020)
28. De Angelis et al. (2015), in: Rodrigues et al. (2020)
29. Cardil et al. (2016), in: Rodrigues et al. (2020)
30. Gouveia et al. (2016); Vega-García and Chuvieco (2006), both in: Rodrigues et al. (2020)
31. Fernandes (2013); Tedim et al. (2013), both in: Rodrigues et al. (2020)
32. Carrillo et al. (2022)