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The causes and effects of wildfires and how people prepare for and respond to them

Page history last edited by K J Hutchinson 12 years, 8 months ago

Lesson 1: What are the causes of wildfires?


Learning objectives:

To be able to define the term 'wildfire'.

To be able to describe the location of areas in which wildfires are common and explain why they happen here.


To be able to give and explain at least 5 causes of wildfires.


Wildfires are unplanned, unwanted wild land fires, including unauthorised human-induced fires. They are sometimes known as bushfires in Australia and brushfires in the USA.




Wildfires are a common occurence in some parts of the world. They occur on every continent except Antarctica. They happen most frequently in hot areas where there are extended periods of drought. Fires need heat, fuel (eg. dried out vegetation) and oxygen to continue to burn.  High temperatures can preheat fuels in the fire's path so that they burn more readily. Strong winds can fan the flames, pushing them towards new fuel sources as well as transferring burning embers and sparks which can start spot-fires. During the day, the sunlight heats the ground and warm air rises. When hot air currents travel up sloped landscapes they can cause fires to start.


Forest fuels consist of ground, surface and aerial materials.

Ground fuels lie below the earth's surface eg. tree roots.

Surface fuels include twigs, grasses, wood and needles.

Aerial fuels include tree crowns, branches and hanging mosses.


The map below shows the global distribution of wildfires in August and February 2008. The maps show different patterns for the two times of year. In AUgust, the fires were concentrated in two areas - just north of the Black Sea in Romania and the Ukraine and in a band across the African continent at about 15-20 degrees south of the equator.  The map for February shows a concentration in Venezuala (in the north of South America) and in a band across the African continent at about 10-15 degrees north of the equator.



Why do wildfires happen?


The causes of wildfires can be divided into 2 categories - natural and human. WIldfires are not like the other hazards that we have studied because, unlike earthquakes/volcanoes/storms, people can cause wildfires. Experts have suggested that 90% of wildfires are actually caused by people.


Natural causes of wildfires


Lightning is the single biggest natural cause of wildfires. Most fires started by lightning are small and burn out quickly but if the conditions are right then fires started by lightning can spread very rapidly. About 8 million lightning strikes hit the earth every day!


Spontaneous heating is where material becomes heated to the point at which it catches fire without a spark. This is common wherelots of leaves and branches have fallen to the ground and not been cleared away - the flow of air is restricted and often leads to fire.


Volcanic eruptions give out red hot lava and ash whch can start wildfires.


Wildfires are more common on south-facing slopes because the sun dries out the vegetation. Steep slopes also help fires to spread as the flames hit the upslope vegetation.


Human causes of wildfires


There are many human causes of wildfires including...


Arson - deliberate fire-starting. This is illegal.

Sparks for train wheels or from machinery

Military training

Household chimneys

BBQs and camp fires which haven't been extinguished properly

Broken bottles acting as a magnifying glass and concentrating the sun's rays

Slash and burn farming techniques (particualrly common in Brazil)

Discarded cigarettes

Children playing with matches

Electricity pylons falling down in high winds


Exam questions


A very obvious question for the examiner to ask is whether you think the natural or human causes of wildfires are more signficant. Your answer needs to be backed up with evidence (you could use a case study to help you, such as the California wildfires 2007 in lesson 3, section D). Remember, the human causes of wildfires will be much more important in densely populated areas than they are in areas where very few people live / work / visit.


Useful links:

Interactive about the sources of fire

US active fires - updapted in real time

How Stuff Works - Causes of wildfires


Lesson 2: What are the effects of wildfires?


Learning objectives:

To be able to classify the effects of wildfires into social, economic and environmental.

To be able to classify the effects of wildfires into primary and secondary effects.


The effects of wildfires can be classified using either the primary/secondary categories or the social/economic/environmental classification. Make sure that you can use both systems. Remember, a good geographer must open his/her eyes to SEE the world (social, economic, environmental). Social effects are those affecting people. Economic effects are those affecting money and business. Environmental (physical) effects are those affecting natural and built materials.


Primary effects of wildfires


Loss of life and injury to people and animals who are caught in the flames.

Destruction of property and possessions (this is getting more severe as more and more people live in the urban-rural fringe)

Burning of vegetation and crops.

Huge amounts of smoke released.


Secondary effects of wildfires


Health problems for people as a result of the smoke and ash.

Loss of jobs and incomes for agricultural workers whose animals/crops are destroyed.

Decline in the tourist industry, leading to loss of jobs.


Insurance premiums rise.

Access to recreational areas is restricted.

Soil erosion and landslides because there is less vegetation to bind the soil together.

Loss of habitats for animals may lead to extinction. 


Lesson 3: Case study - The California wildfires of October 2007


Learning objectives:

To develop a case study of the California wildfires of October 2007 including the main causes and effects of the fires.

To consider whether the California wildfires of October 2007 were predominantly a natural or a human hazard.


The October 2007 California wildfires were a series of wildfires that began burning across Southern California on October 20. They covered the area from Santa Barbara County to the U.S.–Mexico border (almost 400km in length). The raging fires were visible from space. The last fire was fully contained on November 9, 2007, 19 days after the series of fires started.


The two largest fires were in the San Diego area where 640,000 poeple had to evacuate their homes.




The fires forced approximately 1,000,000 people to evacuate their homes, the largest evacuation in California's history.

Over 1800 homes were destroyed.

Over 2,000 km² of land burned.

Nine people died as a direct result of the fires. 85 others were injured, including at least 61 firefighters.

Damage to property estimated at US$1.6 billion.

Air pollution levels raised across the area to 3 times their normal level - this causes breathing difficulties. People were advised to stay indoors.

Tourist trade was damaged. San Diego Zoo and Sea World were closed.

Drinking water in some cities was pollutd and people were advised to drink only bottled water.

Crops were destroyed, either by the fire itself or because they weren't watered as the farm workers had been evacuated. Food prices rose.

Many species of plants and animals were killed.


The table below shows some of the effects on San Diego County


Fire nameDate / time startedArea burnedStructures destroyedInjuriesContainment
Witch (Creek) October 21 at 11:00 a.m. 197,990 acres (801 km2) 1040 homes
414 outbuildings
239 vehicles
70 homes damaged
25 outbuildings damaged
2 deaths
39 firefighters
2 civilians
Harris October 21 at 9:30 a.m. 90,440 acres (366 km2) 206 homes
252 outbuildings
253 structures damaged
5 deaths
34 firefighters
21 civilians
Poomacha (Palomar Mountain/Valley Center) October 23 at 3:13 a.m. 50,176 acres (203.06 km2) 143 homes
77 outbuildings
21 firefighters 100%
Horno/Ammo October 24 21,084 acres (85.32 km2)   6 firefighters 100%
Rice October 22 at 4:16 a.m. 9,000 acres (36.4 km2) 206 homes
2 commercial properties
40 outbuildings
5 firefighters 100%
McCoy October 21 300 acres (1.21 km2) 1 residence
1 outbuilding
Coronado Hills October 22 at 1:50 a.m. 250 acres (1.01 km2) 2 outbuildings   100% on Oct. 22
Wilcox October 23 100 acres (0.40 km2)     100%





Over 6,000 firefighters worked to fight the blazes; they were helped by units of the United States Armed Forces,United States National Guard, almost 3,000 prisoners convicted of non-violent crimes, and 60 firefighters from the Mexican cities of Tijuana and Tecate.

California Governor Arnold Schwarzenegger declared a state of emergency in seven California counties where fires were burning.

President George W. Bush agreed, and ordered federal aid to supplement state and local response efforts.


Natural causes

Major contributing factors to the extreme fire conditions were drought in Southern California, hot weather, and the strong Santa Ana winds with gusts reaching 85 mph (140 km/h).

The fires occurred at the end of a dry summer and were exacerbated by the seasonal Santa Ana winds.

Southern California was in the midst of an unusual drought; in Los Angeles it was the driest year on record. The combination of wind, heat, and dryness turned the chaparral into fire fuel. Officials believed that some of the fires generated their own winds.


Human causes

Increasing number of homes built in canyons and on hillsides surrounded by brush and forest.

Several were triggered by power lines damaged by the high winds.

One fire started when a semi-truck overturned.

Another was suspected as having been deliberately caused; the suspect was shot and killed in flight by state authorities.

A 10-year-old boy admitted that he accidentally started the Buckweed Fire playing with matches.

Causes of the remaining fires remain under investigation.  


Useful links:

BBC News article from 23rd October 2007 including a useful map

Excellent animations from the GIS Force Group showing the spread of the San Diego fires and the direction of the winds


Lesson 4: Reducing the damaging effects of wildfires


Learning objectives:

To be able to describe the strategies and methods used to reduce the damaging effects of wildfires including evacuation, education and the role of the emergency services.


What links these images?



Google have hired a herd of 200 goats to help them reduce the risk of wildfires at their headquarters! The goats graze on the land around the building, chomping their way through vegetation which might otherwise act as a fuel for wildfires. (I'm not kidding!). You can learn more about the Google Goats by clicking on the link at the bottom of this lesson.


This is just one of a variety of ways in which people can reduce the damaging effects of wildfires. The strategies and methods that people use fall into two main categories:

(a) preventing the wildfires from happening in the first place, and

(b) stopping the fires from spreading and causing damage once they are alight.


Preventing the wildfires from happening in the first place


1. Remove dead leaves and branches from areas at risk of wildfires.

2. Organise controlled burns to remove dead plant material before any fires start.

3. Educate the public about wildfires so they know how to avoid them from startiing.


Smokey Bear


The Smokey Bear character has been used in the USA for over 60 years to educate the public about the risk of wildfires. Smokey Bear's slogan 'Only you can prevent forest fires' was introduced in 1944 and it was updated in 2001 to 'Only you can prevent wildfires'.




Smokey Bear's adverts refer to the human causes of fires. Experts believe that 90% of wildfires are caused by people - some of these are deliberate, some are accidents. Remember: wildfires are very different to the other hazards that we have studied because they so often result from the actions of people whereas earthquakes, volcanoes and tropical storms do not. Have a look at the video below to see how Smokey gets his message across to the public:


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Smokey also has a Facebook account - this is a great way of using technology to get the message about preventing wildfires out to the general public! You can see Smokey's profile by clicking on the Facebook link below.



Reducing the damaging effects of wildfires once they have started


Fire-fighting teams spray water and foam onto the fire in areas where the fire is accessible.

Organise air-drops - water and fire-retardant chemicals are released from aircraft onto the wildfire - particularly used in inaccessible areas (may include using helicopters with large buckets that can be filled from rivers, lakes and the sea.

Spray ahead of wildfires to stop the fire spreading.

Fire-fighters create fire-lines (also called fire breaks) eg. dig trenches or clear areas to remove all sources of fuel for the fire eg. twigs, leaves, bushes.

Burn a backfire - this is an area ahead of the wildfire which is set alight and a controlled burning of the fuel supply takes place ro remove it.

Evacuate the area - this could be a forced or a voluntary evacuation. The Army may be called in to help.


Useful links:

Smokey Bear's website

Smokey Bear's Facebook

Smokey Bear's Twitter account

Smokey Bear activity book (pdf)

California Wildfires on Twitter

Google Goats

Photo story about prescribed burns


Lesson 5: Will the distribution and frequency of wildfires change?


Learning objectives:

To be able to explain why the distribution and frequency of wildfires might change.

To be able to explain why areas in which wildfires occur should be protected.


Look at the located bar-graph below (this means that the bars are located at appropriate points on the map). Try to describe how the number and location of wildfifes has changed over time.




Key points to remember:

The number of wildfires has incrased dramatically in all locations.

The greatest increase in the number of wildfires has been in America.

In Asia, increases have been failry small since 1960.

The only area to experience a reduction in wildfires at any point over the period 1950-2000 was Europe between 1990-2000.


The graph shows that the number of wildfires has increased dramatically. Why might this be? And how might things change in the future? To answer these questions, we need to consider both the natural and human causes of wildfires.


Natural causes of wildfires


Many experts have argued that global warming will increase the number of wildfires and the areas in which they occur.

  • Higher temperatures dry out vegetation so it is more likely to catch fire.
  • Heatwaves are associated with a higher number of fires.
  • We seem to be getting periods of wetter weather followed by periods of drier weather - in wet periods, vegetation grows quickly and in dry periods fires spread very rapidly.
  • We might be getting more storms (see Unit 2, section C) - hotter temperatures lead to more intense evaporation, producing more lightning and leading to more lightning stirkes.
  • Wildfires give out lots of carbon dioxide, which itself intensifies the greenhouse effect and leads to global warming.



The graph below shows a clear link between the average spirng/summer temperature and the number of wildfires in Western US forests between 1970 and 2004.


Human causes of wildfires


People are making greater use of forested areas for recreation - this leads to a greater risk from wildfires as there is a greater chance of matches/cigarettes being dropped, BBQs/campfires not being extinguished properly etc.

More and more people are living in semi-arid areas - this increased pressure on areas likely to suffer from wildfirs has led to an increase in the number and intensity of the fires.


The length of the wildfire season


The length of the wildfire season is also increasing. Indeed, between 1970 and 2007, the US wildfire season increased by 78 days. This is shown in the graph below.





Some scientists have argued that the number and size of wildfires won't continue to increase. They believe that warmer and cooler periods occur naturally (we learnt about this when we studied tropical storms) and they think that when we enter a cooler period, the number of wildfires will drop.


Why should areas at risk from wildfires be protected?


~ People – save lives and injuries (and cost of treating burns victims)…

~ Property – save homes, businesses…

~ Environment – tourism, soil…

~ Economic – jobs...

~ Infrastructure – roads, services, buildings…

~ Food supply – save crops…


Can you think of any other ideas?


Possible exam questions


Explain why the distribution and frequency of wildfires may change (4 marks).

Explain why the damage caused by wildfires is likely to increase (4 marks).

Explain why areas in which wildfires occur should be protected (4 marks).


Useful links:

Boston Globe news article - link between wildfires and global warming

New Scientist article about the link between wildfires andf global warming

USFA wildfires statistics

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