The Greenhouse Effect

The Earth and the moon are the same distance from the sun yet the mean temperature on Earth is 14°C and on the moon is -18°C.

Because the Earth has an atmosphere and the moon doesn’t.

Certain molecules in the Earth’s atmosphere keep the Earth warm by transmitting short-wave radiation from the sun, but blocking long-wave radiation from the Earth.

Since this is the same way that the glass walls of a greenhouse work, this is called the greenhouse effect.

The greenhouse effect has always existed and is essential for life on Earth, as without it the temperature would be 33°C lower and there would be no liquid water.

Several atmospheric gases contribute to the greenhouse effect, mainly carbon dioxide (CO2), methane (CH4), water vapour and ozone (O3).

The molecules of these gases all absorb radiation in the infra-red range, so are called “greenhouse gases”.

The concentrations of the greenhouse gases may be very small, but they are increasing.

The result of these increases is the enhanced greenhouse effect, which is causing more heat to be trapped on Earth, leading to an increase in mean global temperature called global warming.

People have been making direct measurements of temperatures and atmospheric gas concentrations at many locations over the world for over a century, but indirect (orproxy) measurements can also be made going back thousands of years.

Two important techniques are:

• Tree rings. It is well-known that tree trunks have annual rings due to seasonal patterns of growth: big cells in summer followed by small cells in winter. These rings can also tell us about the climate while the tree was growing. Narrow annual rings mean that growth was slow so the temperature was colder, while wider annual rings mean that growth was faster so the temperature was warmer.

• Ice cores. The thick ice in places like Antarctica, Greenland and Siberia has been building up for tens of thousands of years, so by drilling out long ice cores many kilometres deep we can collect frozen ice from the past. These ice cores have annual rings, like trees, reflecting seasonal rainfall, so specimens from the cores can be dated. As each fresh layer ofice freezes tiny bubbles of air are trapped and using modern sensitive techniques we can analyse these ancient air bubbles for gas content. Air moves rapidly around the Earth, so these measurements represent global rather than local effects.

What is causing the rise in greenhouse gases? The main sources are:

• Burning fossil fuels. This releases carbon dioxide that was removed from the atmosphere by plants 300 million years ago during the carboniferous era.
• Deforestation. This increases the carbon dioxide concentration by reducing photosynthesis and through burning or decay of the trees.
• Agriculture. Methane is produced by methanogenic anaerobic respiration by certain bacteria. These bacteria are found in the intestines of ruminant herbivores like cows, and in flooded rice paddy fields, where the mud is anaerobic. Since cow and rice farming have both increased dramatically in recent decades to feed the growing human population, methane emissions have also increased.
• Landfill. Methanogenic bacteria are also found in landfill  sites, where they contribute to increased methane production.

Although greenhouse gases are also affected by natural events, like volcanic eruptions and solar activity, it seems very likely that the increases are mainly due to human activities, since the timing of the increase correlates with increased emissions due to the
industrial revolution and human population increases. 

The changes are therefore called anthropogenic changes, since they are caused by humans.

Although the mean global temperature is increasing, not everywhere will see a temperature increase: some areas may experience a cooling due to climate change or ocean current disruption.

Global warming is leading to a variety of changes in the Earth, collectively called climate change.

One of the biggest effects of global warming is a rise in sea level as the ice sheets on Antarctica, Greenland and Siberia begin to  melt (the Arctic ice is floating, so its melting won’t affect sea levels), coupled with thermal expansion of sea water.

Climate change also includes changes in rainfall patterns and increased frequency of extreme weather events.

Predictions about the effects of global warming aredifficult, but many changes have already been observed that can be attributed to anthropogenic global warming:

Wild animals and plants.

•  As habitats change, many animals and plants, who are adapted to their current environment, will struggle to survive as their environment warms up.
• They have to colonise new habitats with a more suitable climate or become extinct.
• Many species are moving towards the poles as their habitats warm and polar species like polar bears and emperor penguins are declining as their habitats disappear.
• Many birds are starting their annual migrations earlier and butterflies haveshifted their ranges northward by 200km in Europe and North America.
• The white lemuroid possum, only found in the mountain forests of northern Queensland, is the first mammal species to be driven extinct by global warming.

Crops.

• Increases in temperature and carbon dioxide will  both contribute to an increase in growth rate in some crops, but the effects will vary around the globe, partly because the effects of global warming on rainfall are almost impossible to predict.
• Cool and temperate regions may be able to grow crops currently confined to warmer regions, for example barley can now be grown for the first time in Iceland.
• But Mediterranean and tropical regions maybe unable to support their current crops as the
  climate becomes too warm and dry.
•  In particular, African countries may struggle to feed their populations as staple crops fail due to rising temperatures and decreased rainfall.
• Globally, it is predicted that agricultural output is most likely to fall due to global warming.

Pests.

• The geographic range of insect pests is already shifting towards the poles, as mild winters mean that insects can now survive in areas where they couldn’t before.
•  For example, the green shield bug, acommon pest of vegetable crops in Mediterranean countries, has recently been found in southern England for the first time.
• Since insects like mosquitoes are also vectors of human and animal pathogens,
  tropical diseases could soon be found in more northern countries.