Due to their high potential impact in terms of human lives, commodities and natural heritage, the Alpine forest fires require a relatively large amount of resources for fire-fighting and prevention: the mountainous environment makes fire fighting very difficult, and a rapid intervention is required because the fires readily endanger human activities and infrastructures. Furthermore, secondary damages via other consecutive natural hazards, such as an enhancement of debris flows, erosion and avalanche danger, may occur, too (3).

The impact of climate change on the fire potential in the Alps in the past and in future scenarios has been evaluated from a multimodel approach (simulations from 7 Regional Climate Models), a detailed Regional Climate Model for the Alps area (COSMO), and data on daily temperature and precipitation observations during 1961–2010. The scenario period is 2031–2050; the RCM projections are based on the SRES scenario A1B. These model confirm previous results (4) that in recent decades the strongest increase of forest fire danger has been observed in the Southern Alps, while north of the Alps no clear trend was observed (3).

Southeastern France 

In southeastern France, most of the large fires (>120ha) occur within the coastal Mediterranean areas whereas the mountains (Pyrenees, Southern Massif Central and Southern Alps) are less affected. In the period 1973–2013 on average 66% of annual burnt area in southeastern France burnt during July and August. Corsica is by far the most fire-prone region of southeastern France (6,32). 

Trends of fire activity and climate across the Mediterranean and mountain ecosystems of south-eastern France over the period 1973–2009 show a general decreasing pattern of fire activity since the early 1990s, specifically during summer in historically burned regions (5). However, extreme conditions in fire weather (fire danger indices) have greatly increased in south-eastern France over the past four decades: the flammability of ecosystems in south-eastern France has probably increased in these recent decades (5,32).

Climate effects: fuel-limited versus drought-driven fire regimes

With respect to climate effects, the situation is even more complicated because climate affects wildfires in two opposing ways. In dry ecosystems wet conditions may be so rare that not enough fuel accumulates to start large fires, and fire activity is limited. These areas have a fuel-limited fire regime. In moist ecosystems on the other hand, dry conditions may be so rare that fuel doesn’t get sufficiently dry to sustain fire spread. These areas have a drought-driven fire regime (7). The alternation of wet and dry conditions, however, favours fires by increasing the amount of fine fuel in litter, grass and shrub layers during wet periods, which burn more intensely in subsequent dry periods (9). In Mediterranean ecosystems both fuel-limited and drought-driven fire regimes are present (10).

Human alterations of fire regimes

Human activities directly modify the fire regime In the Mediterranean Basin by setting or suppressing fires and by changing patterns of vegetation in the landscape (11). Moreover, the fuel build-up following agricultural land abandonment as a result of the rural exodus has created an increasing fire hazard in Mediterranean Europe (12). On the other hand, changes in fire suppression policy over the last few decades have probably induced sharp decreases in fires (13). Hence the functional relationships linking fire to climate have been partially modified by human activities (14), decreasing or increasing the fire activity independently of climate change.

The example of southern France

In southern France three pyroclimates can be discriminated (7): (1) the Mediterranean mountains, characterized by a high seasonality in precipitation and fires (dry summers and wet winters), and the highest burned area fraction, fire season length and the strongest increase in fire danger over the last four decades, (2) the Temperate mountains, characterized by wet and cold conditions, and the shortest fire seasons and the lowest fire activities, and (3) the Mediterranean lowlands, with the driest and warmest climates, and strong and dry winds (the Mistral and Tramontane) that favour fire spread in summer. Human presence and activities decrease from the Mediterranean coast to the rural hinterland, with anthropogenic ignitions accounting for 90% of the number of fires and 96% of the total burned area (15).

During the period 1976-2009 the climatic influence on fires in southern France was not restricted to the occurrence and duration of drought during a particular year: a mixture of drought-driven and fuel-limited fire regimes operated, emphasizing the lagged effects of warm or moist periods on fire (16). Higher fire activity was related to wetter conditions in the last three years. This illustrates that fuel abundance is an important constraint on Euro-Mediterranean (and other) fire regimes, even when drought is the main driver.

With respect to human alterations of fire regimes, contrasting short-term impacts of changes in land-use and fire suppression policy have been found: more fires where fuel biomass is high as a result of land-use change, and less fires in fire-prone ecosystems due to more effective fire suppression (17). Indeed, in the Mediterranean lowlands, which are densely populated and highly susceptible to extreme fire weather due to strong winds (18), both winter and summer fire activity were strongly suppressed, suggesting a gradual increase in the efficiency of fire suppression policy (19).

The assessment shows that the new fire policy is undoubtedly successful (see also 29). The area that is burned on average every 5 years has significantly decreased. For very large fires (> 10 000 ha), for instance those with a return period of 50 years, this is not the case. Apparently, firemen cannot control many large fires. Despite the new suppression-oriented policy set up in 1994, very large fires can still occur in southern France, and especially in the regional hot spots of Corsica and Provence. These fires, that cause most of the accidents or fatalities for fire crews, may belong to a new generation promoted by global changes (25).

Very large fires, generating high human, economic, and ecological damages, are a growing issue in southern Europe and almost worldwide. A combination of climate change, fuel accumulation, and increasing human pressure on ignitions in urbanized areas is the root cause for a new generation of wildfires characterized by extreme behavior (25). These new types of devastative fires are increasingly observed in southern Europe, and many of them are beyond the present suppression capacity (26). They often display eruptive or erratic behaviors. Recent studies in southern France have shown that new combinations of prolonged droughts, heat waves, and windy periods lead to a higher frequency of such extreme fire events (27).

Massive aerial and ground forces for fire suppression may not be sufficient to control extreme wildfire events. Managing landscapes and fuels, limiting unwanted ignitions, and the participation of the public in self-protection (i.e., tackling the root causes of fire risk) are key to leverage fire risk in the long term.