2.4 The human impact

The enhanced greenhouse effect and global warming: the evidence, possible causes and atmospheric impacts.

What does UHi stand for?

Urban heat island

Define urban microclimate :

Human activity chemically and physically alters air and weather characteristics over and around urban areas, making it different from air and weather over rural areas

How do urban climates occur?

Due to human activity, the temperature in an urban microclimate is higher than that of the surrounding areas. Urban areas are said to be urban heat islands as under calm conditions, temperatures are highest in the built-up city centre and decrease towards the suburbs and countryside.

Define microclimate

Climate within a small area that differs significantly from the climate of the surrounding area

How do urban areas influence climate?

Anthropogenic sources: - more `LWR trapped and released at night.

Warmer over urban areas compared to rural areas

Reduced vegetation so less evaporation/evapotranspiration and cooling from it

Urban heat Island effect

More dark surfaces, lower albedo, more long-wave radiation reradiated, heats air above it by conduction.

Asphalt albedo: 5%

Grass albedo: 25%

Tarmac albedo: 5-10%

Concrete albedo: 27%

Little wind and more clouds makes it warmer

Tall buildings absorb high amounts of long wave radiation, as well as slowing windspeed, preventing further mixing of air

Tall buildings act as an obstruction for the wind, forcing it upwards (orographic uplift)

The most obvious temperature differences between rural and urban areas are at night

Buildings warm up and cant cool down as easily so stay warmer for longer than in rural areas

Anthropogenic sources (central heating, AC units, factories)

Drains and gutters means that water does not stay on the surface for long, low evaporation, lower humidity + less vegeatation + less water sources such as a lake.

Pollution creates more condensation nuclei, increasing the number of water droplets and as such creating more clouds

10% more rainfall than rural - conventional rain.

Heat differences are greater on clear, calm and cloudless nights

5-10 degrees warmer then rural areas

Less snowfall: even if it falls as snow, it melts before it reaches the ground, temperatures aren't cold enough for snow

Key points to remember about urban climates:

• They are hotter

• There is less water vapour and lower humidity

• There are more intense storms

• There is reduced visibility

• There are slower winds in general but there can be wind tunneling

• There is less snow

UHI biggest at?

1. Night I before sunrise)

2. In the summer no winds

3. Sky is clear

4. Weather doesn’t change at night

Explain why UHI are hotter

• greater SA - absorb more heat

• Albedo of concrete (10%) tarmac (20%)

• Releases LER at night

• Littler energy for evapotranspiration - more available to heat atmosphere

• Pollutants trap radiation

• High buildings trap insulation

• Low building - heat streets

• 5-10* warmer than rural area

• Extra sources of heat - Anthropogneinc

Explain why winds are slower

Wind tunnelling:

Wind deflected and scattered by building

Obstruction for wind forcing it upwards

Explain why there is reduced visibility

Pollutants acta s extra condensation nuclei (hygroscopic particles)

Increase number of water droplets → clouds

Explain why there is intense storms

• greater instability and stronger convection above urban area

• Lack of humidity

Explain why there is less humidity

• lack of vegetation reduce evapotranspiración

• High drainage density less water available on surface for evaporation

• Fewer bodies of water

What are the anthropogenic sources of:

1. Methane

2. Carbon dioxide

3. CFCs

4. Nitrous oxide

1. Agriculture (animal, beef), natural gas systems, landfills

2. Fossil fuel combustion, coal and crude oil, deforestation

3. CFCs (aerosols, fridges etc)

4. Manafacturing, agricultural soil management (Catalytic converters)

What is the most abundant GHG produced by humans

CO2 - 77%

But least effective at trapping heat in atmosphere

What is radiative forcing?

Ability of gas to warm atmosphere.

Why does CO2 fluctuate each year

Seasonality

Green leaves summer = photosynthesis take in co2

Main causes of enhanced greenhouse effect + arguments against human enhanced

• fossil fuel bruning

• Albedo change: deforestation and urbanisation - more LWR back to atmosphere and trapped = hotter

Arguments against

• Orbital eccentricity

• Axial tilt

• Axial precession

• Solar output variations

• Changes in ocean currents

• Increased dust from volcano

• El Niño : produces hotter years westward surface flow of currents, reverse and warm, water moves east, causing higher temperatures and drought in Australia