Posts Tagged ‘global change’

Wildfires and global change: the threshold approach

June 3rd, 2021 No comments

To generate wildfires we need some specific components (ignitions, fuel, and right conditions). Traditionally, this has been explained using the triangle approach [1] or the 4-switches approach [2]. We propose a more mechanistic model to explain wildfires, the threshold approach [3]. Under this view, wildfires occur when three thresholds are crossed (ignition, continuous fuel, and drought); and fire weather moves these thresholds to lower values and so it triggers the occurrence and spread of wildfires (Fig. 1). The size and duration of the fire largely depend on how long the fire weather lasts and the extent of the area containing suitable fuel.

Climate change increases the conditions conductive to fire, and thus it also increases the frequency in which some of these thresholds are crossed, extending the fire season and increasing the frequency of dry years. However, climate-related factors do not explain all the complexity of global fire regime changes as human factors are extremely important: humans shifts ignition patterns and modify fuel structure. Humans cause ignitions directly by accident or arson, but also indirectly by altering fuels that can make them more susceptible to ignitions (vegetation openings). Humans also modify fuel continuity, either reducing it (eg fragmentation) or increasing it (eg fire suppression). For instance, in many Mediterranean ecosystems, the drought threshold is crossed annually, and vegetation cover (fuel) is usually high enough for fire spread; thus, ignitions are a key factor. Larger populations of humans in the wildland-urban-interface will likely lead to increased ignition rates, and consequently higher probability of ignitions coinciding with extreme weather events to generate wildfires.

Fig. 1. Probability of fire occurrence vs ignitions; fire spread vs landscape fuel continuity; and, fuel flammability vs drought. In each of the three graphs, vertical lines indicate the thresholds. In all cases, fire weather (strong wind, high temperature, or low humidity) moves the curve (and the threshold) towards lower values (thick red arrow; i.e. , saturation is reached at lower values of the x axis), with the consequence of increasing the probability of an ignition resulting in a fire, the fire spread (for a given landscape configuration), and the flammability of the vegetation (fuel dries out quicker). The flow chart indicates the main factors affecting the fire drivers: growing population (in or near wildlands); fuel changes in the landscape (fragmentation, oldfields, fire exclusion, etc.); and climate change (driven by the increase in greenhouse gases). From [3].



[1] Moritz et al. 2005. Wildfires, complexity, and highly optimized tolerance. P Natl Acad Sci USA 102: 17912–17.

[2] Bradstock RA. 2010. A biogeographic model of fire regimes in Australia: current and future implications. Global Ecol Biogeogr 19: 145–58.

[3] Pausas JG & Keeley JE 2021. Wildfires and global change. Front Ecol Environ. [doi | web | pdf]

Australian fires 2019/20

January 12th, 2020 No comments

Australia is a very flammable continent, and fires have been occurring there for millions of years. As a consequence, many plants and animals have developed adaptations and strategies to cope with recurrent fires. However, the current fire season in eastern Australia is really very severe, including not only very large fires but also high intensity firestorms. SE Australia has suffered other sever fire seasons in the past (an iconic example is the Black Friday bushfires in 1939). Why is this happening now? Here I’ve compiled key figures that help us to understand it.

In the last few years, Australia has been suffering an increase in temperature; on average, each year is hotter than the previous year (Fig. 1). In fact, 2019 was the warmest years, but also the driest year (with the lowest rainfall) ever recorded. December 2019, when most fires started, was climatically an extreme (Fig. 2). During the December heatwave (Fig. 3) some meteorological station (e.g., Penrith, near Sydney) recorded temperatures over 48oC, and the record of highest average maximum temperature for Australia was broken on two consecutive days (40.7 and 41.9oC  on 17 and 18 Dec, respectively). January 4 was Canberra’s hottest day since records began (44oC). In such extreme weather conditions, ignitions easily become a wildfire (in fact, several of the wildfires started from a dry lightning), and fires spread very quickly in a vegetation that has been in a drought for many months. This generates not only huge areas burned (Fig. 4), but also very hot fires and strong uplift air columns that reach the stratosphere (pyrocumulonimbus). These are called firestorms. Firestorms produce there own winds and spread embers and the fire very fast; they even produce lightnings that generate additional wildfires. Firestorms produce extreme fire behaviour that is beyond the capacity of firefighters. In those fires, as it happens in volcanoes, the smoke reaches the stratosphere and circulates at very long distances (e.g., currently smoke from these fires has already reached South America).

The fire season has not ended yet. The ecological effects of these fires will depend on many factors (spatial variability of fire intensity, previous fires, species, etc…). The size and intensity of these fires suggest that they can have some negative consequences, but it is too early for any quantitative evaluation. Many plants are starting to resprout just few days after the fire, even under those drought conditions; some animals are leaving their hiding places, exploring the burned area, and carcasses are locally abundant suggesting patches of high animal mortality. We’ll see when will the rain come, and how plants and animals will respond. For humans, the consequences are catastrophic (fatalities, destruction of many infrastructures, smoke problems, etc.).

Fig. 1. In Australia, each year is hotter than the previous year, on average. From Australian Bureau of Meteorology
Fig. 2. December 2019 was climatically an extreme, unprecedented in relation to rainfall and temperature. Elaborated with data from Australian Bureau of Meteorology
Fig. 3. Global temperature in December 18th, 2019, as shown by Windy. Note also that part of the differences in temperature are due to the different time zones; i.e., middle of the day in Australia, night time in South America, and early morning in Africa.
Fig. 4. Major fires in south-east Australia by January 10th, 2020 (5,634,000 ha). From @eforestal [update Jan 18th: 6 millions ha]

More information:

Australian Bureau of Meteorology  | @eforestal maphub  | NSW fire service | VIC emergency | Desinformation |

Update (4/2020): For a map of the time-since-fire and fire severity across NSW fires, see: Bradstock et al. 2020, Global Change Biol., doi:10.1111/gcb.15111 (spoiler: most fires burned at relatively low severity!)

Cambio climático, políticas medioambientales y gestión del territorio

April 15th, 2019 1 comment

No hay duda que el planeta está sometido a un calentamiento global causado por al incremento de CO2. Para evitar una pérdida de calidad de vida y de biodiversidad, se necesitan políticas de reducción de CO2, que se deben realizar a todos los niveles, tanto personal, como a nivel municipal, regional y global. Pero además de estas políticas de CO2, existen otras políticas locales relacionadas con la gestión del territorio, que pueden ser muy relevantes para luchar contra el cambio climático. En nuestro clima mediterráneo existen dos factores clave, el régimen de incendios y el régimen de sequías, que actualmente están siendo perturbados. En un artículo reciente [1] se propone que existen principalmente tres factores locales que determinan la actual dinámica de la vegetación y que afectan a los incendios y las sequías en nuestros ecosistemas:

a) El abandono rural en un entorno depauperado de herbívoros salvajes. Este aumenta las áreas de monte (áreas forestales), pero también la abundancia y continuidad de la vegetación, que es el combustible que alimentan los incendios forestales. Ello incrementa la probabilidad de incendios grandes e intensos y de incendios en la interfaz agrícola-forestal.

b) El incremento de la población urbana viviendo o visitando zonas semiurbanas (por ejemplo, incremento de viviendas en la interfaz urbano-forestal). La consecuencia es un incremento de: 1) la degradación de la biodiversidad (por ejemplo, incrementa la introducción de especies exóticas, la contaminación lumínica, el uso ineficiente del agua, etc.); 2) la probabilidad de incendios (más igniciones tanto accidentales como provocadas); y 3) la vulnerabilidad de la sociedad a los incendios (se ponen en peligro vidas e infraestructuras).

c) La degradación costera, que aumenta la sequía a través de procesos de retroalimentación negativa; es decir, la desecación de las marismas costeras, la deforestación para la agricultura y, más recientemente, la explosiva urbanización costera, han reducido drásticamente los ecosistemas originales (y su evapotranspiración) y, por lo tanto, el agua disponible para la brisa marina que en el pasado alimentaba las lluvias en las montañas de la costa [1].

Por lo tanto, las políticas de gestión del territorio para luchar contra el cambio climático deben enfocarse a dos objetivos principales: gestionar los incendios, y reducir las sequías; siempre evitando que la gestión afecte negativamente a la biodiversidad.


Figura: La perturbación de los regímenes de incendios naturales y sequías en los paisajes mediterráneos está determinada tanto por factores globales como locales. El aumento de la actividad de incendios se debe a la cantidad de combustible y la homogeneidad del paisaje generada por el abandono rural en un entorno depauperado de herbívoros y con crecientes igniciones (de origen humano) y sequías. El aumento de las condiciones secas es consecuencia del calentamiento global, pero también de las pérdidas por tormentas causadas por la perturbación del ciclo del agua generado por la degradación de los ecosistemas costeros. El incremento de población semi-urbana se refiere al incremento de población urbana viviendo en o visitando zonas de monte, incluyendo zonas de la interfaz urbano-forestal.


Políticas para la gestión de incendios: La tolerancia cero a los incendios no ha funcionado ni en España ni en ningún otro país; al contrario, la extinción total de los incendios genera ecosistemas con gran acumulación de biomasa que cuando arden lo hará con elevada intensidad produciendo incendios de grandes dimensiones (megaincendios). Esto es lo que se llama la paradoja de la extinción de los incendios. Por lo tanto, el reto de la gestión no debería ser eliminar los incendios, sino crear paisajes que generen regímenes de incendios sostenibles tanto ecológica como socialmente [2,3]. Eliminarlos es imposible, antinatural y ecológicamente insostenible [4]. Por lo tanto, en áreas cerca de zonas urbanas debe potenciarse los incendios pequeños, frecuentes, y de baja intensidad, sea aprovechando incendios naturales o realizando quemas prescritas. Además, la introducción de herbívoros, sean nativos (rewilding) o ganado, puede reducir la cantidad de combustible y por lo tanto limitar los incendios y facilitar las quemas prescritas. La gestión de los incendios también implica decisiones tan conflictivas como limitar la interfaz urbano-forestal, es decir, limitar la expansión de urbanizaciones y polígonos industriales en zonas rurales y naturales. Los mecanismos para limitar estas zonas pueden ser diversos, incluyendo la recalificación de terrenos (a no urbanizables), o la implementación de tasas (disuasorias) por construir en áreas con alto riesgo de incendios, entre otros. En las zonas ya urbanizadas, se requiere forzar la realización de planes de evacuación y multar la falta de autoprotección alrededor de las viviendas, de manera que se reparta la responsabilidad entre administración y propietarios.

Políticas para la conservación del clima: Una manera de reducir las sequías es potenciar el ciclo del agua que ha sido perturbado por la degradación de la costa. Esto implica, la conservación, restauración, y ampliación de la mayor zona posible de vegetación nativa en la zona más litoral, y en especial de los marjales litorales. Es decir, la conservación de los marjales litorales son importantes no solo para la conservación de la biodiversidad, sino también para la conservación del clima. Además, en las zonas urbanas y semiurbanas se debería potenciar los parques, jardines y zonas verdes con abundante vegetación, así como plantar árboles en todas las calles posibles. La evapotranspiración que realizaría toda esta vegetación (en sistemas naturales, urbanos y semiurbanos), beneficiaría al ciclo del agua y contribuiría a la conservación del clima. Además, la vegetación en zonas urbanas y semiurbanas también beneficia a la calidad de vida en muchos otros aspectos que no abordamos aquí. Dado que pequeños incrementos de temperatura en la costa tienen implicaciones en toda las montañas vecinas [1], es importante reducir el efecto ‘islas de calor’ que ejercen las zonas urbanas de la costa. Para ello, se podría potenciar el uso de materiales de construcción con elevada reflectividad en superficies horizontales (tejados, calles, patios, etc), cosa que disminuiría el efecto de calor urbano; además, las viviendas construidas con estos materiales requieren un menor uso de la calefacción. En la zona de montaña, se puede potenciar el uso de colectores de niebla para obtener agua para la agricultura o otros usos.

En conclusión, además de disminuir la concentración de CO2 en la atmósfera, existe un gran número de acciones que se pueden realizar con políticas locales, y que contribuirían en gran manera a la disminución de los efectos del calentamiento global y al aumento de la calidad de vida.

Más información en:
[1] Pausas J.G. & Millán M.M. 2019. Greening and browning in a climate change hotspot: the Mediterranean Basin. BioScience 96:143-151. [doi | oup | blog | pdf]

[2] Pausas J.G. 2012. Incendios forestales. Catarata-CSIC. [Libro]

[3] Pausas J.G. 2018. Incendios forestales, encrucijada natural y social. En: Ecología de la regeneración de zonas incendiadas (García Novo F., Casal M., Pausas J.G.). Academia de Ciencias Sociales y del Medio Ambiente de Andalucía. pp. 9-14. [pdf]

[4] Pausas J.G. 2017. Acabar con los incendios es antinatural e insostenible. 20minutos (Ciencia para llevar), 13 Julio 2017. [20minutos]


Global change in the Mediterranean basin

January 9th, 2019 2 comments

The paleartic region with mediterranean climate (southern Europe and northern Africa; the Mediterranean Basin; Fig. 1) is a hotspot of biodiversity, a hotspot of climate change (warming of the region is above global average), and a hotspot of human population (a highly populated area and a top tourist and retirement destination). In addition, the Mediterranean Sea is the world’s largest inland sea, and climatic disruptions in the region have consequences in the large catchment area that includes central-eastern Europe (Fig. 1). That is, environmental changes and disruptions of the water cycling in the Mediterranean region have consequences affecting a large human population [1].

Fig. 1. Area with mediterranean climate (green) and limits of the Mediterranean catchment (red).  The European catchment limit based on Cortambert (1870). From [1].

The region, as all the planet, is subject to global warming. In addition there are three main local processes (not directly related to global warming) that are very important in understanding dynamic changes in the region [1]:

a) Rural abandonment in an environment depauperate of native herbivores; this increases wildlands (greening) but also the abundance and continuity of fuels that feed wildfires [2]

b) Increasing the wildland-urban interface; this increases biodiversity degradation (e.g., alien species), fire ignitions, and the vulnerability of the society to fires

c) Coastal degradation enhances drought (browning) through negative feedback processes; that is, the desiccation of coastal marshes, the deforestation for agriculture, and more recently, the explosive coastal urbanization, have drastically reduced the original ecosystems and thus the water available for the sea breeze that was once feeding the rain in the upper part of the mountains [1].

All these mechanisms act in different directions (greening, browning), and the current balance is still towards greening, as land abandonment is buffering the browning drivers; however, it is likely to switch with global warming. The challenge is to mitigate the browning processes. The good news is that the importance of small-scale drivers suggests that local policies and actions can make a difference in reducing overall impact on the landscape and society.

Mechanisms acting at a fine scale, together with global drivers (CO2 enrichment and climatic warming) interact and drive current vegetation changes in Mediterranean landscapes. Any model aiming to predict the future of our vegetation and climate must consider these local mechanisms; and failing to consider them at an appropriate scale is likely to produce inconclusive predictions.

Fig. 2. The disruption of the natural fire and drought regimes in Mediterranean landscapes is driven by global and local drivers. Increased fire activity is a response to the fuel amount and landscape homogeneity generated by rural abandonment (fire hazard) in an environment depauperated of herbivores and with increasing human ignitions (fire risk) and droughts (fire weather). The increased dry conditions are the consequence of global warming, but also of storm losses caused by the disruption of the water cycle generated by the coastal degradation. WUI: wildland-urban interface. From [1].


[1] Pausas J.G. & Millán M.M. 2019. Greening and browning in a climate change hotspot: the Mediterranean Basin. BioScience 69: 143-151 [doi | OUP | pdf]  

[2] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf]  

Every time you build anything, a highway, a school, whatever, you are altering the precipitation regime somewhere downwind” Millán M. Millán

Socioeconomics and fire regime in the Mediterranean

August 26th, 2017 No comments

In recent decades, fires in Mediterranean Europe have become larger and more frequent. This trend has been driven mainly by socioeconomic changes that have generated rural depopulation and changes in traditional land use. This has increased the amount and continuity of vegetation (fuel), and thus an increase in the fire size and area burnt [1-3]. In a recent paper [4] we compared fire statistics of the Western Rif (Morocco) with those form Valencia (eastern Spain) to show that current fire regimes in Mediterranean Africa resemble past fire regimes in the Mediterranean Europe when rural activities dominated the landscape. The temporal fire regime shift observed in different countries of the Mediterranean Europe (from small, fuel-limited fires to drought-driven fires) can be identified when moving from the southern to the northern rim of the Basin. That is, most spatial and temporal variability in fire regimes of the Mediterranean Basin is driven by shifts in the amounts of fuel and continuity imposed by changes in socioeconomic drivers (e.g., rural depopulation). In fact, we can use rural population density as an early warning for abrupt fire regime shift. Consequently we can predict future fire regimes in North Africa, based on the trends observed in southern Europe, and we can better understand past fire regimes in Europe based on the current situation in North Africa [4].

Figure 1. Western Rif (northern Morocco) and Valencia (eastern Spain).

Figure 2. Fire-size distribution in Valencia, for the period 1880-1970 (white boxes) and for the period 1975-2014 (grey boxes), and in the western Rif (red symbols, 2008-2015). For details see [4]


[1] Pausas, J.G. 2004. Changes in fire and climate in the eastern Iberian Peninsula (Mediterranean basin). Climatic Change 63: 337-350. [pdf | doi]

[2] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf]

[3] Pausas J.G. & Paula S. 2012. Fuel shapes the fire-climate relationship: evidence from Mediterranean ecosystems. Global Ecol. & Biogeogr. 21: 1074-1082. [doi | pdf | supp]  

[4] Chergui B., Fahd S., Santos X., Pausas J.G. 2018. Socioeconomic factors drive fire-regime variability in the Mediterranean Basin. Ecosystems 21: 619–628 [doi | pdf]


Future fires

November 11th, 2016 No comments

There is a tendency to think that fires will increase in the near future due to global warming. This is because many fire risk prediction are based on climate only. However fire regime changes not only depend on climate [1]; there are other factors, like land-use changes, CO2, plant invasion, fragmentation, etc. that are also important drivers of change in fire activity [1]. Even plant drought stress (and flammability) not only depends on climate [2,3].

A recent simulation study [4] suggests that global burned area is certainly predicted to increase in the following decades when simulations are based on climate only (blue line in the figure below). However, adding the effect increased CO2 reduces the predicted burned area to no increase (green line below). Furthermore, when adding increased population density and urbanization (black and red lines), the model predicts much more area burnt in the last century (black lines 1900-2000) and a reduction of future burned area (red lines). The predicted reduction of fire during 1900-2000 is consistent with global charcoal records [5] and can be explained by increasing agriculture, land use and fragmentation. Overall, this study suggests that global area burned is unlikely to increase in the following decades.

Note that 1) this is a model, so take it with caution! 2) This model is at the global scale, but changes in different directions are expected in different regions, and this can have biodiversity consequences (even if the global balance is steady); for instance, in the Mediterranean Basin, fire are likely to keep increasing as land abandonment and fuels are increasing [6]. And 3) there is a high uncertainty in some fire drivers. For instance, temperature is likely to keep increasing, however, rainfall and wind changes are very uncertain, and landuse and emissions are subject to uncertain changes in environmental policies in different countries. In any case, this study gives us an idea of the possible sensitivity of different parameters.

Figure: Simulation of global area burned for 1900 to 2100 under different scenarios: a) climate only (blue line); b) climate + CO2 (green); c) climate + CO2 + population & urbanization (black lines; red area for the future predictions). From [4].

[1] Pausas J.G. & Keeley J.E., 2014. Abrupt climate-independent fire regime changes. Ecosystems 17: 1109-1120. [doi | pdf | blog]

[2] De Cáceres M, et al. 2015. Coupling a water balance model with forest inventory data to predict drought stress: the role of forest structural changes vs. climate changes. Agr. For. Meteorol. 213: 77–90. [doi | pdf | suppl. | blog]

[3] Luo, Y. & H. Y. H. Chen. 2015. Climate change-associated tree mortality increases without decreasing water availability. Ecol, Let. 18:1207-1215.

[4] Knorr W, Arneth A, & Jiang L, 2016. Demographic controls of future global fire risk. Nature Clim. Change 6:781-785.

[5] Marlon JR, et al. (2008). Climate and human influences on global biomass burning over the past two millennia. Nature Geosci, 1, 697-702.

[6] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | pdf | blog]


Climate-independent drought stress in plants?

July 14th, 2015 No comments

Climate warming is increasing water stress in many ecosystems, with consequences of increased plant mortality and susceptibility to pests (Figure below). However, there are other mechanisms (climate-independent mechanisms) by which plant drought stress can also increase. In a recent paper [1], we use a water balance model coupled with a vegetation model, to simulate changes in leaf area (LAI) and evapo-transpiration between two forest inventories in NE Spain. The results suggest that during 1980-2010 there was a tendency of increasing in drought stress for most tree species; however, drought stress was not predicted to change when considering that forest structure did not change between the two forest inventories. That is, changes in climate alone did not predict changes in water stress. In contrast, the recent increase in forests (in extension and in tree density) in the study area showed to be the main driver for the drought stress observed in trees. This forest increase is due to the abandonment of land and rural activities durint the recent decades [2,3]. That is land abandonment is not only increasing the fuel in the landscape (amount and continuity [2,3]) but also the flammability of this fuel. Consequently, land abandonment and increased forest is a major driver of drought and fire regime changes [3].

Caution must be taken in extrapolating these results as they are based in a model; i.e., more information from other approaches and places is needed. However, it is clear that climatic change should not be considered the only source of current drought stress in vegetation; there may be changes in drought stress as well changes in fire regime that are climate-independent [1,4] and more related to changes in forest and landscape structure linked to factors like socio-economic (and landuse) changes, changes in herbivores, plant invasion, etc… [4]; and in many cases, these different processes interact. The good thing is that climate-independent processes are easier to manage than climate!


Figure: Plant mortality by drought stress in Calderona, Valencia, Spain (Photo:  P. García-Fayos, 2015).

[1] De Cáceres M.,  Martínez-Vilalta J.,  Coll L., Llorens P., Casals P., Poyatos R., Pausas J.G. and Brotons L. 2015. Coupling a water balance model with forest inventory data to predict drought stress: the role of forest structural changes vs. climate changes. Agric. Forest Meteorol. 213: 77-90. [doi | pdf | suppl.]

[2] Pausas J.G. 2004. Changes in fire and climate in the eastern Iberian Peninsula (Mediterranean basin). Climatic Change 63: 337-350. [pdf | doi]

[3] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf]

[4] Pausas J.G. & Keeley J.E., 2014. Abrupt climate-independent fire regime changes. Ecosystems 17: 1109-1120. [doi | pdf ]