Chapter 14: Droughts and Wildfires

Introduction to Natural Disasters

  • Natural Disasters: Sudden catastrophic events caused by environmental factors leading to significant damage.

Wildfires

  • Definition: An uncontrolled, self-sustaining blaze that consumes forests, grasslands, and human communities.

  • Causes: Often occur during droughts or heatwaves, exacerbated by climate events such as El Niño and human activities.

    • Example: Australia 2019-2020 Wildfires

    • Burnt ~243,000 km²

    • Destroyed >3,000 buildings

    • 34 People died

    • Economic damage estimated between 920 million and 4.65 billion AUD

Drought

  • Definition: An extended period with little or no rain, leading to freshwater shortages.

  • Impacts:

    • Affects vegetation, crops, livestock, groundwater, and human activities.

    • Location-specific; manifests differently based on regional climate conditions.

  • Drought is a relative term that implies a comparison of water availability typical of the local climate.

  • Types of Drought:

    • Meteorological Drought: Decreased precipitation over weeks to years. Compares recorded precipitation over a specific period of time with average precipitation during the same time in other years.

    • Agricultural Drought: Soil moisture drops below required levels for plant growth (e.g., visible mud cracks).

      • Can be a consequence of a meteorological drought or by human activity.

    • Hydrological Drought: Insufficient water in lakes, ponds, or reservoirs.

      • Can be caused by deficit in rainfall or a lack of spring snowmelt.

      • Ex. Lake Chad has shrunk by 90% since the 1960’s

    • Socioeconomic Drought: Shortages impacting goods and services.

      • Can be brought on by the other three types of droughts due to climate change and an increase in population.

    • Mega-droughts: Long droughts that last two decades or more.

Climate Zones

  • Earth has distinct climate zones based on:

    • Temperature, Seasonality, Precipitation

    • Controlled by factors like latitude, elevation, and ocean proximity.

    • Monsoonal and Mediterranean climates are the most sensitive to droughts and have can have the greatest impacts.

Monsoonal Climate:

  • Characterized by distinct wet and dry seasons, often leading to harsh drought conditions impacting crop yields and water supplies.

  • Average temperature of the coldest month is > 18ºC and precipitation in the driest month is > 6cm

  • During a normal year, the rainy season saturates the soil, fill reservoirs and flood streams.

  • Crops fail, water supplies become critically low and livestock perish in droughts

  • Regions with a monsoonal climate include Southern Asia and Africa

    • Ex. India Drought (2009): Rain was 23% below average and was the worst since 1972

  • Heavy monsoon rains fall from thunderstorms that develop along the ITCZ.

  • Factors that play an important role in triggering droughts:

    • Wind Strength: Weaker trade winds carry less moisture into the ITCZ

    • Sea-Surface Temperatures: Decreases in sea-surface temperatures decreases evaporation.

      • During El Nino, evaporation decreases, moisture decreases over South Asia and NE & E Australia.

Mediterranean Climate:

  • Wet winters (not as intense as monsoonal climate); dry summers, with potential drought occurrences linked to mid-latitude cyclone activity.

  • Average temperatures of the warmest month is >= 18ºC and average temperatures of the coldest month is between -3ºC and 18ºC

  • Precipitation distributed throughout the year

  • Areas: North Mediterranean basin and W coast of US, Mexico and Chile; Central South Africa, and South Australia

  • Droughts occur when winters are significantly less wet than normal. When fronts of mid-latitude cyclones pass over those regions, winter storms are less frequent.

  • Droughts develop when a persistent area of high pressure in the NE Pacific Ocean causes the mid-latitude cyclones to track northward and doesn’t generate precipitation across western states.

    • Ex. 2011-2017 California Drought: wiped out 102 million trees, California state passes water conservation laws, and was the worst drought in 1,200 years

Other Temperate Climate Zones

  • Central North America, east of the Rockies, many parts of Europe, Asia and South America have rain year round so they do not classify as Mediterranean climate

  • Drought occur when normally frequent summer rains do not happen.

  • The Bermuda High is a subtropical high pressure centre. Air flows clockwise around this High and carries moisture northward from the ocean, fuelling thunderstorms.

  • Dry summers happen when the Bermuda High shifts eastward and the northern air flow shifts to the east of the Atlantic Coast, and less moisture is carried over eastern and central North America.

Positive Feedbacks that Amplify Droughts:

  • Heat increases the rate of evaporation, causing the ground to dry out faster

  • Ground will have less moisture and humidity will decrease

  • Plants dry out and wither

  • Lack of vegetation decreases shade and increases more solar energy

  • Ground warms even more and becomes drier, making it warmer

Palmer Drought severity Index (PDSI)

  • Measures moisture deficiency, relative to average local moisture conditions, by comparing the supply of water available from precipitation and stores reserves with the depletion of water by evaporation, infiltration and runoff.

  • The PDSI is cumulative and reflects incremental changes from prior values.

  • Ranges: Extreme Drought > Severe Drought > Moderate Drought > Mid Range > Moderately Moist > Very Moist > Extremely Moist

  • Droughts cannot be predicted. Predictions made from measurements of sea-surface temperatures have not been consistently correct.

Heat Waves

  • Definition: Prolonged periods of abnormally high temperatures ranging from two days to several weeks. Dependant on air temperature relative to the normal range of the region.

  • Health impacts:

    • Humans constantly produce metabolic heat from biochemical reactions in our body that keeps us alive.

    • Normal body temperature: 36.1ºC - 37.3ºC

    • Body temperature regulation failure can lead to…

      • Heat stress: when the body can’t cool itself sufficiently and its core temperatures rises beyond normal range

      • Heat Exhaustion: Internal temperature reaches 39ºC, extreme sweating, weakness, cramps, fast pulse

      • Heat stroke: Core temp rises to 40ºC, fainting, very fast pulse, vomiting, confusion.

    • If temp reaches 42ºC, person can become delirious or comatose. At temperatures of 43ºC brain damage will lead to death

    • Critical thresholds: 39°C (heat exhaustion), 40°C (heat stroke).

  • Heat index:

    • The relationship between human comfort, air temperature, and relative humidity used to convey the threat of heat illness

    • Heat Advisory for defined heat index thresholds over 2 days; Excessive Heat Warning for 4+ days.

  • Chicago 1995: 739 deaths in a period of 5 days

  • Europe 2003: 70,000 deaths from July to August

Anatomy of Fire

  • Fire: Reaction of gases with oxygen producing heat and light; involves various phases:

    • Flames: Visible gaseous components.

    • Smoke: Results from incomplete combustion

    • Soot: Very fine particles of carbon that are products of incomplete burning

    • Ash: Flakey particles of carbon and other materials produced by incomplete burning

    • Fuel: Any flammable material.

Igniting a Wildfire

  • Natural or anthropogenic causes;

    • Lightning is the most common natural ignition source.

    • Human activities cause over 90% of forest fires in the US.

  1. The Preheating Stage: When a heat source raises a fuel’s temperature

  2. Pyrolysis Stage: When a fuel reaches a high enough temperature in which molecules in the fuel decompose to produce gases that rise into the air.

  3. Combustion Stage: Gases ignite

The fire triangle is the three components for a fire to begin: fuel, heat and oxygen.

Types of Wildfires

  • Ground Fires: Occur in roots and organic material; smolder due to limited oxygen.

    • Peat Fire: type of ground fire in which a layer of peat undergoes smouldering combustion

  • Surface Fires: Burn low-lying vegetation (grass and brush).

  • Ladder Fires: Spread from ground to canopy, fuelled by underbrush.

  • Canopy or Crown Fires: Burn through the canopy, often due to intense conditions.

Fire severity represents the percentage of the biomass that burns during the fire

Fire intensity is the measured amount of energy released during burning

Spread of Wildfire

  • Convection: Vertical circulation of heat in air

    • The heated air rises and cooler air rushes towards the perimeter of the fire. This ventilates the fire and maintains the oxygen supply

  • Radiation: Electromagnetic energy

    • Materials within 10-35m of a wildfire can receive enough radiation to ignite

  • Firebrands: Embers and sparks that ignite new fires.

  • Influence of Wind: Increases fire spread and intensity, causing severe damage in vulnerable areas.

Environmental Challenges

  • Fire and Drought Interaction: Severe droughts increase wildfire risk; areas become susceptible due to lack of moisture in vegetation.

  • Impact of Wildfires: Entire neighbourhoods can be destroyed; loss can compromise soil stability, increasing risks for future rain-related disasters.

  • Secondary disasters:

    • Smoke inhalation

    • Slope erosion

Fighting Wildfires

  • Containment: preventing a fire from spreading beyond a certain limit

  • Firebreak: a natural or built barrier prevents the fire from spreading

  • Fire Retardants: Chemical spray to cool the fuel, form a coating that prevents the fuel from ignition, release water or carbon dioxide when burned or inhibit oxidation reactions

  • Backfires: A controlled fire to consume fuel in a wildfires path

Mitigating Wildfire Hazards

  • Wildfire suppression: the assumption that wildfires do more harm than good

  • Prescribed burns: reduce fuel loads and prevent future catastrophic wildfires

  • Wildfire hazard maps: residents can see their risks