2.4 The human impact

Global warming

Revised

The enhanced greenhouse effect

The greenhouse effect is both natural and good – without it there would be no human life on Earth. On the other hand, there are concerns about the enhanced greenhouse effect.

The enhanced greenhouse effect is a build up of certain greenhouse gases as a result of human activity. Studies of cores taken from ice packs in Antarctica and Greenland show that the level of CO 2 between 10,000 years ago and the midnineteenth century was stable at about 270 ppm. By 1957 the concentration of CO 2 atmosphere was 315 ppm. It has since risen to about 360 ppm and is expected to reach 600 ppm by 2050. The increase is due to human activities – primarily the burning of fossil fuels (coal, oil and natural gas) and deforestation. Deforestation of the tropical rainforest also increases atmospheric CO 2 levels because it removes the trees that convert CO 2 into oxygen. Table 2.2 Properties of key greenhouse gases

Average atmosphereic concentration (ppmv) Rate of change (% per annum)

CO 2 Methane

Nitrous oxide CFC-11 CFC-12

355 1.72 0.31

0.000255 0.000453 0.5 0.6–0.75 0.2–0.3

4 4

Direct global warming potential (GWP)

1 11 270

Lifetime (years)

120 10.5 132

3400 7100 55 116

Type of indirect effect

None Positive Uncertain

Negative Negative

Climate change

Climate change is a very complex issue for a number of reasons: l It involves interactions between the atmosphere, oceans and land masses. l It includes natural as well as anthropogenic forces. l There are feedback mechanisms, not all of which are fully understood. l Many of the processes are long term and so the impact of changes may not yet have occurred.

The effects of increased global temperature

The effects of global warming are very varied. Much depends on the scale of the changes. For example, some impacts could include: l a rise in sea levels, causing flooding in low-lying areas such as the Netherlands,

Egypt and Bangladesh – up to 200 million people could be displaced l an increase in storm activity, such as more frequent and intense hurricanes (owing to more atmospheric energy) l 4 billion people suffering from water shortage if temperatures rise by 2°C l 35% drop in crop yields across Africa and the Middle East if temperatures rise by 3°C l 200 million more people could be exposed to hunger if world temperatures rise by 2°C, 550 million if temperatures rise by 3°C l extinction of up to 40% of species of wildlife if temperatures rise by 2°C

Greenhouse gases, such as water vapour, CO 2 , methane, ozone, nitrous oxides and chlorofluorocarbons (CFCs), like the glass on a greenhouse, allow short-wave radiation from the Sun to pass through, but they trap outgoing long-wave radiation, thereby raising the temperature of the lower atmosphere.

Expert tip

There are many causes of global climate change. Natural causes include: l variations in the Earth’s orbit around the Sun l variations in the tilt of the Earth’s axis l variations in solar output (sunspot activity) l changes in the amount of dust in the atmosphere (partly due to volcanic activity) l changes in the Earth’s ocean currents as a result of continental drift All of these have helped cause climate change, and may still be doing so, despite anthropogenic (humangenerated) forces.

The Stern Report (2006) is a report by Sir Nicholas Stern analysing the financial implications of climate change. The report has a simple message: l Climate change is fundamentally altering the planet. l The risks of inaction are high. l Time is running out.

The effects of climate change vary with the degree of temperature change (Figure 2.10). According to the Stern Report, global warming could deliver an economic blow of between 5% and 20% of GDP to world economies. Dealing with the problem, by comparison, will cost just 1% of GDP, equivalent to £184 billion.

Food

Falling crop yields in many areas, particularly LEDCs

Possible rising yields in some high latitude regions Falling yields in many MEDCs

Water

Small mountain glaciers disappear – water supplies threatened in several areas Significant decreases in water availability in many areas, including Mediterranean and Southern Africa Sea level rise threatens major cities

Ecosystems

Extensive damage to coral reefs Rising number of species face extinction

Extreme weather events

Rising intensity of storms, forest fires, droughts, flooding and heatwaves

Risk of abrupt and major irreversible changes

0 °C 1 °C 2 °C Increasing risk of dangerous feedbacks and abrupt, large-scale shifts in the climate system

3 °C

Global temperature change (relative to pre-industrial)

4 °C 5 °C

Figure 2.10 Projected impacts of climate change, according to the Stern Report

Now test yourself

Tested

Figure 2.10 shows some of the projected impacts related to global warming. 31 Describe the potential changes of a 4°C rise in temperature. 32 Explain why there is an increased risk of hazards in coastal cities. 33 Outline the ways in which it is possible to manage the impacts of global warming.

Answers on p.214

Typical mistake

The Kyoto Protocol (1997) did not cover all countries. It gave all MEDCs legally binding targets for cuts in emissions from the 1990 level by 2008–2012. This was extended to 2017. The EU agreed to cut emissions by 8% and Japan by 7%.

Urban climates

Urban climates occur as a result of extra sources of heat released from industrial, commercial and residential buildings as well as from vehicles. In addition, concrete, glass, bricks and tarmac all act very differently from soil and vegetation. For example, the albedo (reflectivity) of tarmac is about 5–10% while that of concrete is 17–27%. In contrast, that of grass is 20–30%. Some of these materials – notably dark bricks – absorb large quantities of heat and release them slowly by night. In addition, the release of pollutants helps trap radiation in urban areas. Consequently, urban microclimates can be very different from rural ones. Greater amounts of dust mean increasing concentration of hygroscopic particles. There is less water vapour, but more CO 2 and higher proportions of noxious fumes owing to combustion of imported fuels. Discharge of waste gases by industry is also significant.

Urban heat budgets differ from rural ones. By day the major source of heat is solar energy; in urban areas brick, concrete and stone have high heat capacities. The extensive surfaces of these materials in urban areas allow a greater area to be heated.

Revised

Urban climates refer to the changes in temperature, humidity, wind patterns, precipitation and air pressure that are noticeable over large urban areas during high-pressure conditions.

In urban areas there is relative lack of moisture. This is due to: l lack of vegetation l high drainage density (sewers and drains), which remove water

Thus, there are decreases in relative humidity in inner cities due to the lack of available moisture and higher temperatures there. Nevertheless, there are more intense storms, particularly during hot summer evenings and nights, owing to greater instability and stronger convection above build-up areas. There is a higher incidence of thunder, but less snow fall. At night the ground radiates heat and cools; in urban areas the release of heat by buildings offsets the cooling process, in addition, some industries, commercial activities and transport networks continue to release heat throughout the night. There is higher incidence of thicker cloud cover in summer and radiation fogs or smogs in winter because of increased convection and air pollution respectively. The concentration of hygroscopic particles accelerates the onset of condensation.

Table 2.3 Average changes in climate caused by urbanisation

Factor

Radiation

Temperature

Wind speed

Relative humidity

Precipitation

Cloudiness Global Ultraviolet, winter Ultraviolet, summer Sunshine duration Annual mean Sunshine days Greatest difference at night Winter maximum Frost free season Annual mean Gusts Calms Winter Summer Total Number of rain days Snow days Cover Fog, winter Fog, summer Condensation nuclei Gases

Comparison with rural environments

2–10% less 30% less 5% less 5–15% less 1°C more 2–6°C more 11°C more 1.5°C more 2–3 weeks more 10–20% less 10–20% less 5–20% more 2% less 8–10% less 5–30% more 10% more 14% less 5–10% more 100% more 30% more 10 times more 5–25 times more

Daytime temperatures in rural areas are, on average, 0.6°C warmer. This urban heat island effect is noticeable, especially by dawn during anticyclonic conditions. The effect is caused by a number of factors: l heat produced by human activity l buildings having a high thermal capacity in comparison with rural areas – up to six times greater than agricultural land l fewer bodies of open water (therefore less evaporation) and fewer plants (therefore less transpiration)

Expert tip

The contrasts between urban and rural areas are greatest under calm, high-pressure conditions. The typical heat profile of an urban heat island shows the maximum at the city centre, a plateau across the suburbs and a temperature cliff between the suburban and rural area. Small-scale variations within the urban heat island occur, with the distribution of industries, open space, rivers, canals and so on.

Now test yourself

34 Describe the main differences between the climates of urban areas and those of their surrounding rural areas. 35 What is meant by the urban heat island? 36 Describe one effect that atmospheric pollution may have on urban climates. 37 Why are microclimates, such as urban heat islands, best observed during high-pressure (anticyclonic) weather conditions?

Answers on p.214

Tested

l

the composition of the atmosphere, involving the blanketing effect of smog, smoke or haze less thermal energy required for evaporation and evapotranspiration due to the surface character, rapid drainage and generally lower wind speeds

Air flow over an urban area is disrupted – winds are slow and deflected over buildings. Large buildings can produce eddying. Severe gusting and turbulence around tall buildings causes strong local pressure gradients from windward to leeward walls. Deep, narrow streets are much calmer unless aligned with prevailing winds to funnel flows along them – the ‘canyon effect’.

The nature of urban climates is changing, however. With the decline in coal as a source of energy there is less SO 2 pollution, hence fewer hygroscopic nuclei and so less fog.

Exam-style questions

Section A

1 (a) Explain the meaning of the terms insolation and albedo. (b) How would insolation and albedo vary between a polar ice cap and a tropical rainforest? 2 Explain why a coastal area may have a different temperature from a continental area of the same latitude. 3 Using examples, explain how aspect can affect temperature.

Section B

1 (a) Define the terms dry adiabatic lapse rate (DALR) and saturated adiabatic lapse rate (SALR). (b) Briefly explain how conditional instability occurs. 2 Using an annotated diagram, show how an understanding of lapse rates can help explain cloud formation. 3 Explain how atmospheric stability, instability and conditional instability lead to different weather conditions.

[2] [2] [3] [3]

[4] [3] [8] [10]

Exam ready