Impact of Human Population Density
on the Frequency of Wildfires
Wolfgang Knorr
Department of Physical Geography and Ecosystem Science, University of Lund, Sweden
Queensland
Bush Fire
http://myaclicks.blogspot.c
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Western Peloponnese, August 2007
Q1: What are the main determinants
of wildfire occurrence?
Q2: What is the dominant impact of
humans on wildfires?
Fires in Greece
[Koutsias et al., Int. J. Wildland Fire, 2013]
Fires in Greece
[Koutsias et al., Int. J. Wildland Fire, 2013]
2007 heat wave
Temperature [°C]
Fires in Greece
[Met. Station of National Observatory of Athens]
[Koutsias et al., Int. J. Wildland Fire, 2013]
Fires in Greece
850 hPa
(~1600m)
temperature &
geopotontial
height
Y
Y
X
Geostrophic Flow
°C
Skiron forecast 0.2°x0.2° for 25 August 2007, 12h UTC
[Knorr et al., Comp. Ecol. & Softw. 2011]
Fires in Greece
Synchronous Skiron
10m wind forecast
no fires (dry)
AVHRR pseudo-color image from 25 August 2007, 9:48h
Red: Channel 3 (MIR). Green: Channel 2 (NIR); Blue: Channel (VIS)
[Knorr et al., Comp. Ecol. & Softw. 2011]
Fires in Greece
21 Aug 2007
FAPAR
1 Sep 2007
What are the main determinants of
wildfire occurrence?
• “fire weather” (high temperature, strong
wind)
• vegetation density
Fires in Greece
[World Bank, UN DESA]
[Koutsias et al., Int. J. Wildland Fire, 2013]
Fires in Spain
[Pausas & Fernández-Muñoz, Climatic Change, 2012]
What are the main determinants of
wildfire occurrence?
• “fire weather” (high temperature, strong
wind)
• vegetation density
• land abandonment
Example of anthropogenic influence
on fires
Road network in Russia
Number of fires / km2
total 2011
[courtesy of Mikhail Sofiev, Finnish Met. Institute]
Fires in California
Wildland-Urban Interface
[Syphard et al., Ecol. Appl., 2007]
Fires in California
[Syphard et al.,
Ecol. Appl., 2007]
What are the main determinants of
wildfire occurrence?
• “fire weather” (high temperature, strong
wind)
• vegetation density
• land abandonment
• intrusion by humans
Fire Frequency vs. Fire Density
“Fire density”
• counts fires per area, time
• arbitrary low-size cutoff
“Fire frequency”
• fractional burned area per time
• related to probability of a point on Earth being
affected by fire
• more closely related to ecology, hazards, emissions,
climate effects
Fire Density in the Mediterranean
fire density
[Syphard et al.,
Cons. Biol., 2009]
proporation of fires
Fire Density in the Mediterranean
[Syphard et al.,
Cons. Biol., 2009]
population density [1/km2]
Fire Frequency vs. Fire Density
Canada 1959-1997
Lightnings caused 85% of burned
area, but only 70% of fires.
 Lightning-caused fires are 2.4
times larger in extent than humancaused fires.
[Stocks et al., JGR 2003]
[Archibald et al., Int. J. Wildland Fire, 2010]
Fire Frequency vs. Fire Density
[Pausas & Fernández-Muñoz, Climatic Change, 2012]
Fire Frequency vs. Fire Density
fire frequency
fire density
[Archibald et al., Global
Change Biol., 2009]
study area
[Archibald et al., Int. J. Wildland Fire, 2010]
What is the dominant impact of
humans on wildfires?
Basic approach
Empirical burned area model based on key variables
Fractional area burned
(per year y)
Landcover type (MODIS), socio-economic region
Population density
AB(y) = a(L) Fb Nmax(y)c logit (d + ep)
fPAR
Annual maximum Nesterov index
(for drought conditions)
• logit(x) = 1 / (1 + exp(-x)) is the s-shaped logistic
function.
• 5 tunable parameters a-e at a given point.
• Optimised against GFED 3.1, MODIS MCD45, L3JRC.
8 Land Cover Regions
90°N
45°N
0°
45°S
180°W
90°W
0°
90°E
1
Cropland/urban/natural vegetation mosaic
5
Shrubland
2
Broadleaf forest
6
Savanna or Grassland
3
Needleleaf forest
7
Tundra
4
Mixed forest
8
Barren or Sparsely Vegetated
>50% cropland (excluded from optimisation)
180°E
Fire Model Results
Fire
Frequency
(per year)
GFED3
model
0.001
0.003
0.01
0.03
0.1
0.3
1
Fire Model Results
Fire
Frequency
(per year)
GFED3
up to 1
person per
km2
model
0.001
0.003
0.01
0.03
0.1
0.3
1
Fire Model Results
Fire
Frequency
(per year)
GFED3
up to 0.1
persons per
km2
model
0.001
0.003
0.01
0.03
0.1
0.3
1
Fire Model Results
Fire
Frequency
(per year)
L3JRC
model
0.001
0.003
0.01
0.03
0.1
0.3
1
Fire and Population
Population Effect on
Burned Area
[Knorr et al. GRL under review]
Fire and Population
Change in burned area relative to 2005
[Marlon et al. NatureGeo 2008]
modelled burned area at
constant climate
[Knorr et al. GRL under review]
Application to Climate Change
Number of extreme fire years in 2071-2100
defined by 1/30yr event in 1971-2000
KNMI RCM
SRES A1B
UN Medium
population
projection
[Results from FUME EU FP7 Project]
Climate vs. Population Effect
climate
population
change in frequency
of extreme fire years
[Results from FUME EU FP7 Project]
Fire and Population
Population Effect on
Burned Area
[Knorr et al. GRL under review]
Fire Emissions
GCM RCP
[Results from PEGASOS EU FP7 Project]
Fire Emissions
[Results from PEGASOS EU FP7 Project]
Summary (1)
• Most fires are caused by humans
• Lightning important in remote areas
• When population density is low, human
activity is correlated with fire density
• Fire “frequency” more relevant than “density”
• Humans reduce fire size through more
ignitions
Summary (2)
• Human-impact paradox: even if most fires are
started by humans, more humans
predominantly means “less fire”
• Similarity of finding to charcoal record
• Increase of fire risk in Eastern (population)
and Southern Europe (climate+population)
• Future reduction of fire emissions for Africa