Dr Mark Cresswell

Impacts: Extreme Weather

69EG6517 – Impacts & Models of Climate Change

Topics• A Definition of Extreme Weather

• Storm events

• Fires

• Floods and Drought

• Oceans

• Effects on ENSO

• Societal Impacts

• Summary

A Definition• Extreme weather may be regarded as an

anomalously high magnitude weather event leading to deleterious effects upon natural or human systems

• Extreme weather events are a potential response to climate change

• These events, when they occur over land, will often lead to damage to crops and buildings or loss of life and economic hardship

Storm Events• Of all the extreme weather events – storm

events cause greatest loss of life and damage to property

• Hurricanes – centres of intense low pressure that feed of heat and moisture over warm seas and land

• Tropical cyclones – primarily the result of above normal SSTs

• Intense hailstorms and associated high winds and rainfall

Storm Events

Storm Events

Storm Events• Magnitude and frequency of large-scale

storms largely depends on SST anomalies

• Purely continental storm systems rely upon intense surface heating from solar energy – a process enhanced by deforestation and subsequent changes to surface albedo

Fires• During periods of prolonged drought natural

vegetation will begin to die back and dry

• Environments of this type will be highly susceptible to either accidental or naturally occurring fire events

• Some ecosystems rely upon fire events to allow seeds to germinate and release nutrients into the soil

• Climate change may enhance both the magnitude and frequency of fire events

Fires

Floods and Drought• Floods may be caused by excessive rainfall

or changes to the natural hydrology leading to saturation of soils

• Apart from damage to property, crops and loss of life, flood events may damage the landscape – making it more vulnerable to wind erosion

• Floods will become more common in areas where climate change generates enhanced precipitation

Floods and Drought• Drought is caused both by diminished

precipitation due to climate change or human activities

• Droughts can make environments more vulnerable – affecting wind erosion, soil moisture, crop/food security and human health

• Prolonged droughts may lead to a higher incidence of fires

Floods and Drought

Floods and Drought

Floods and Drought

Oceans• One particular concern with climate change

is the effects on ocean surface water temperatures

• Apart from the boundary forcing an SST has on atmospheric characteristics, the biological effects can be significant

• Algal blooms can form off warm water coasts – depleting the waters of oxygen

• Changes to nutrient supply can affect fish stocks (esp. in El Niño years)

Oceans

Effects on ENSO• Climate simulations performed with AGCMs

using prescribed background SSTs and ENSO-like SST anomalies indicate:

1) No change in the frequency of El Niños2) Greater differences in precipitation and soil moisture

between El Niño and La Niña years3) Very different patterns of change at sea-level and upper

tropospheric pressures outside the tropics in response to El Niños – compared to the changes that occur under present conditions

Harvey, 2000

Effects on ENSO• Wet and dry extremes will tend to be

intensified as climate warms because:

1) Non-linearity of the Clausius-Clapeyron equation which governs the relationship between the temperature of a liquid and its vapor pressure

Effects on ENSO• Wet and dry extremes will tend to be

intensified as climate warms because:

2) A given change in SST will have a larger effect on evaporation the warmer the base climate

3) Enhanced evaporation over the eastern Pacific when an El Niño occurs in a warmer climate will lead to a greater increase in precipitation in this region, greater rising motion and greater compensating subsidence in the western Pacific

4) Similar changes, but with the opposite geographical pattern shown in (3) will occur in La Niña events

Societal Impacts

Amount of evidence (observations, model output and theory)

Lev

el o

f ag

reem

ent/

cons

ensu

s

LOW

HIGH

HIGH

Established but incomplete•Increased vulnerability of infrastructure to urban flooding and landslides•Tropical cyclones more destructive•Fire danger to urban/rural fringe increased•Sea-level rise increases cost/vulnerability•Water supplies more vulnerable

Well-Established•Sea-level rise increases cost/vulnerability of coastal infrastructure•Energy demand sensitive; parts of energy supply vulnerable•Local capacity critical to successful adaptation•Infrastructure in permafrost regions vulnerable

Speculative•Fire damage to key resources increased•More hail and windstorm damage

Competing Explanations•Agroindustry and fisheries vulnerable•Heat-waves more serious for human health•Heat island effects increase summer energy demand and reduce winter energy demand•Increased air and water quality problems

From: IPCC – Impacts, adaptation and vulnerability

Summary• Extreme weather events are likely to

become more serious under a warmer climate

• Impacts on natural ecosystems with be great – with events such as fires, floods, drought and algal blooms

• Impacts on human society will induce greater hardship, loss of life and economic losses

• READ THE IPCC REPORTS!!