7.9 - Global Warming

The greenhouse effect and global warming

• The Earth's temperature is a balance between the energy it receives from the Sun and the energy it radiates back out into space.
• Light energy from the Sun passes through the atmosphere and warms the surface, which then radiates energy back outwards.
• Greenhouse gases in the atmosphere absorb most of this outgoing energy and re-radiate it in all directions, including back towards the Earth.
• They act like an insulating layer, so heat stays near the surface for longer and the planet is kept at a warm, stable temperature.
• This natural greenhouse effect is essential for life - without it the Earth would be far too cold.

The main greenhouse gases

• Carbon dioxide and methane are the main greenhouse gases.
• Water vapour, nitrous oxide, and CFCs (chlorofluorocarbons) also act as greenhouse gases.

The enhanced greenhouse effect and climate change

• When levels of greenhouse gases rise, the effect is strengthened: more outgoing energy is trapped, so less heat escapes and the Earth gradually heats up.
• This strengthening of the natural effect by extra greenhouse gases is called the enhanced greenhouse effect.
• The resulting rise in the Earth's temperature is called global warming.
• Global warming is one type of climate change - a long-term shift in the planet's temperature and weather patterns.

How human activity increases global warming

• Human activity is increasing the levels of greenhouse gases in the atmosphere.
• As these gases build up, the greenhouse effect is strengthened - this is the enhanced greenhouse effect - and the Earth gradually heats up.
• The faster the levels rise, the faster the planet warms, so reducing emissions from these activities is central to limiting global warming.

Human sources of greenhouse gases

• Carbon dioxide - Burning fossil fuels releases it directly through combustion. Cutting down forests (deforestation) removes trees that would otherwise take carbon dioxide out of the air by photosynthesis.
• Methane - Released by rearing cattle and by growing rice in flooded paddies.
• Nitrous oxide - Raised by the use of fertilisers, together with vehicles and industry.
• CFCs - Leak from old aerosols and fridges.

Biological consequences of global warming

• The rise in temperature is driven by rising carbon dioxide, mainly from the combustion of fossil fuels and from deforestation.
• Global warming then has several biological consequences: changes in distribution of organisms, rising sea levels, more extreme weather, and falling biodiversity.

Changes in the distribution of organisms

• As temperatures and rainfall patterns change, organisms shift where they can live:
  • Organisms that need warmer conditions spread into new areas
  • Organisms that need cooler conditions occupy smaller ranges
  • Some species migrate towards the poles
• Some organisms shift to new regions, but others cannot move or adapt quickly enough, so their ranges shrink and some are lost.
• Evidence for these shifts in distribution links the environmental change to its effect on living things.

Rising sea levels and extreme weather

• Higher temperatures make seawater expand and the ice caps melt, so sea levels rise.
• This increases flooding in low-lying coastal areas and the loss of habitats.
• Extreme weather events - Droughts, storms, and floods - become more frequent and more severe.
• Habitats are lost as a result - for example, wetlands dry out or are submerged.

Falling biodiversity

• Together these changes reduce the variety of organisms an area can support, so biodiversity is likely to fall.
• This is because some species cannot adapt or migrate quickly enough and may become extinct.