2.4. The Human Impact

Question 1

Greenhouse Effect definition

Answer 1

Natural process where greenhouse gases - mainly water vapour, absorbs / retains long wave radiation emitted from the Earth - making the Earth warmer

Question 2

Enhanced Greenhouse Effect

Answer 2

The increased greenhouse effect caused by anthropogenic (human) inputs - mainly CO2

Question 3

Global Warming

Answer 3

The recorded increase in global temperatures - most likely linked to the enhanced greenhouse effect

Question 4

Greenhouse Effect process

Answer 4

1) Light energy from the sun travels through our atmosphere and reaches the Earth’s surface
2) The light energy then changes into heat (infra-red energy)
3) Some of this heat energy then travels back into the atmosphere
4) Some heat energy escapes into space
5) However some gases in the atmosphere can trap escaping heat energy, causing some of it to pass back to the surface
6) These gases are called Greenhouse gases
7) This trapping of heat energy is called the greenhouse effect

Question 5

Causes of the enhanced greenhouse effect

Answer 5

1) Power plants and transport (Carbon dioxide)
2) Rice farming and Cattle Farming (Methane)
4) Deforestation (Carbon dioxide)

Question 6

Power Plants and Transporrt (causes of the enhanced greenhouse effect)

Answer 6

• Oil, coal and natural gas are fossil fuels that were created mainly by the decay of plants and animals that lived millions of years ago. These fossil fuels contain carbon.
• Power plants, motor vehicles and jet aircraft burn coal, oil and natural gas. As a result, we are adding billion of tons of extra carbon dioxide into the atmosphere

Question 7

Rice Farming and Cattle (causes of the enhanced greenhouse effect)

Answer 7

• Methane is released from bacteria in flooded rice fields. As farming increases around the world, large amounts of methane gas are added to the atmosphere
• Bacteria in the intestines of cattle convert some of the food that they eat into methane gas. The methane is released when the cattle burp and ‘fart’. As the cattle population increases, large amounts of methane are added to the atmosphere

Question 8

Deforestation (causes of the enhanced greenhouse effect)

Answer 8

Forests remove and store vast amounts of Carbon dioxide from the atmosphere and store it as Carbon. They are carbon sinks. Burning forests releases Carbon dioxide and cutting down forests means less carbon is stored from the atmosphere.

Question 9

Greenhouse Gases

Answer 9

1) Carbon dioxide - Highest atmospheric concentration - 335ppmv
2) Methane
3) Nitrous Oxides - longest lifetime - 132 years
4) CFCs - highest direct global warming potential - 3400GWP and 7100GWP

Question 10

Impacts of the Greenhouse Effect

Answer 10

• Rise in sea levels, low-lying areas like Netherlands, Egypt and Bangladesh will be flooded
• Increase in storm activity, such as more frequent intense hurricanes
• 4 billion people suffering from water shortage if temperature increase by 2 degrees celsius
• extinction of up to 40% species of wildlife if world temperatures increase by 2 degrees
• 35% drop in crop yields across Africa and Middle East if temperature increase by 3 degrees celsius
• 200 million more people could be exposed to hunger if world temperatures increase by 3 degrees

Question 11

Urban climates

Answer 11

The changes in temperature, humidity, wind patterns, precipitation and air pressure that are noticeable over large urban areas during high-pressure conditions

Question 12

Urban Heat Island

Answer 12

Urban areas are warmer compared to their rural surroundings, which is on average 1–2°C warmer per year than its surrounding rural areas

Question 13

When are Urban Temperatures their highest

Answer 13

Urban temperatures are at their highest in the mid-afternoon over the the CBD.

Question 14

What is Anthropogenic Heat and how does it contribute to the urban heat island (UHI)?

Answer 14

This the heat released by human activity. Metabolic heat (body heat) is negligible, but the heat released from vehicles, central heating (alternatively, in some warmer areas, from air conditioning systems) and industrial activities is substantial. All of these inject heat into the canopy layer and the upper boundary layer.

Question 15

How does height and arrangement of buildings contribute to the urban heat island (UHI)?

Answer 15

Both rural and urban areas tend to have low albedos. Hence both areas would absorb heat to a similar degree, all other things being equal. However, the main difference is that urban areas have more vertical surfaces, and this means that radiation will tend to be reflected off many surfaces, each one absorbing some of the energy and warming up in the process.

Question 16

How does the nature of building materials contribute to the urban heat island (UHI)?

Answer 16

Compared to rural surfaces, urban areas heat up more quickly – the specific heat capacity for concrete is one-third that of wet mud, and so for a given input of energy, concrete will warm more rapidly than wet mud. Also, vegetation shades soil, so that it stores less heat. In cities the surface is exposed and the surface area is larger, so it is able to store the heat and release it slowly over the night.

Question 17

How does the presence of water contribute to the urban heat island (UHI)?

Answer 17

The surplus heat can be transmitted as either latent heat or as sensible heat. Latent heat evaporates water. After evapotranspiration has taken place, any heat ‘left over’ can take the form of sensible heat, to raise the temperature of the air – the more sensible heat that enters the atmosphere, the warmer it gets. Latent heat does not raise air temperature. If there is little water at the surface, then less of the surplus heat is used for evaporation (latent heat), and the remainder of the surplus heat will take the form of sensible heat to raise the air temperature.

Question 18

How does the presence of pollutants contribute to the urban heat island (UHI)?

Answer 18

• In terms of the urban heat island, pollutants have both a cooling and heating effect. Smokey, dusty cities can reduce (attenuate) the amount of sunshine that reaches the city surface.
• On the other hand, the pollutant dome can absorb heat and prevent some of it escaping. The buildup of CO2, ozone and particulates helps to absorb longwave radiation. In so doing, the pollutant dome warms up even more, as it was already warmed by the insolation it absorbed in the first place.

Question 19

What are the reasons for the increased patterns of precipitation in urban areas?

Answer 19

• Urban areas have greater cloud cover – some 5%–10% more (Met Office). This may be due to a greater concentration of condensation nuclei, around 100 times more than rural areas. The presence of condensation nuclei encourages cloud formation, since water vapour needs a surface onto which condensation can take place. Also, in cities, heating increases uplift of air by convection, so encouraging cloud formation.
• A second factor is convergence. Due to the low pressure caused by rising air surface winds are drawn in from the surrounding rural fringe. This air then converges as it is forced to rise over the high urban canopy.

Question 20

Why are urban areas prone to more thunderstorms?

Answer 20

Not only are cities wetter, but they are also prone to more thunderstorms. The enhanced convectional uplift from the heat island effect can lead to considerable instability and thermals, giving more storms.

Question 21

What factors lower wind speeds in urban areas?

Answer 21

Urban wind speeds are thought to be lowered by the friction and barrier of the urban canopy. The surface area of cities is uneven because of the varying height of the buildings. Buildings exert a powerful frictional drag on air moving over and around them. This creates turbulence and can cause rapid and abrupt changes in wind direction and speed. However under intense heating strong convectional processes form low pressure thermals over cities that can draw in strong localised winds.

Question 22

What factors speed up wind speeds in urban areas?

Answer 22

In the CBD, wind speeds are far more variable, and around some buildings winds are more gusty and strong. This is due to the layout of the buildings and building height. Straight streets lined with tall buildings can produce ‘urban canyons’, which funnel wind down them, producing high velocities thanks to the venturi effect, which is the effect of ‘squeezing’ the airflow, increased as buildings tend to get taller towards the city centre.

Question 23

How does building shape and spacing all affect the behaviour of the wind in urban areas?

Answer 23

Building shape and spacing all affect the behaviour of the wind. Widely spaced buildings act as single blocks, but where buildings are closer together, the airflow around one interferes with that around the next (see left diagram below). In cities, buildings are densely packed and this allows air to move over the top, leaving the lower urban canopy layer with relatively less turbulence.

Question 24

Factors that control temperature in Urban Heat Islands

Answer 24

anthropogenic heat, height and arrangement of buildings, the nature of the building materials, the presence of water and pollutants