Climate Change

I am not going to go into details about impacts of climate change, for example where it warms most, where it doesn’t warm much or where a semi-arid climate changes to arid but instead I will just be looking at the big picture. If you want a much fuller description of global warming then this wikipedia article is a good place to start.

The main points are:

• Greenhouse gases like carbon dioxide are being emitted into the atmosphere as pollution from energy production, transport, industrial processes and construction.
• Earth’s energy balance: Greenhouse gases reduce the amount of heat from the surface of the earth that radiates out to space.
• The Earth’s climate has always changed as a result of natural factors. The energy balance change that results from our greenhouse gas emissions currently swamps all natural factors, both positive and negative, and is the reason for recent temperature increases.
• Natural variability can disguise the effects of climate change on a short time scale.
• Climate feedbacks.

Greenhouse gas emissions

The main greenhouse gases emitted by humans are: carbon dioxide, methane, nitrous oxide, and ozone and various halocarbons. Carbon dioxide is by far the most significant of these greenhouse gases. Water vapour is also a greenhouse gas but behaves as a feedback rather than a forcing. The amount of water vapour in the atmosphere is dependent on the temperature of the atmosphere. The warmer the atmosphere the more water it can hold. The reason water is a feedback rather than a forcing is because of the speed at which water vapour equilibrium within the atmosphere is restored. If you removed all water vapour from the atmosphere, then through evaporation the atmosphere would be at 85% of the equilibrium level within 10 days and by 30 days be at 100%. In comparison, the removal of carbon dioxide from the atmosphere is measured in the hundreds of years and methane in the tens of years. As a result, Water Vapour amplifies the effect of any other forcing which is why it is called a feedback.

As of the end of 2006 about 7 gigatons of carbon dioxide (GtC) are emitted into the atmosphere each year. About 4 GtC are taken in by plants and the oceans leaving a net increase of 3 gigatons of carbon dioxide in the atmosphere each year. This net increase shows up in the increase in the parts per million of carbon dioxide in the atmosphere.

It is projected that as temperatures rise, the ocean, plants and soil will not be able to absorb so much carbon dioxide and at some point may even start emitting carbon dioxide. In this scenario the atmosphere becomes the carbon dioxide sink of last resort.

Earth’s Energy Balance

For the Earth to remain at constant temperature it must remain in energy balance. This means that the radiation from the Sun that is absorbed by the Earth must equal the radiation emitted by the Earth. Much of the radiation energy emitted from the Sun is in the visible spectrum, whereas most of the radiation energy emitted by the Earth is infra-red. The description of radiation as infra-red, visible, ultra-violet is based on the energy of the radiation, which can also be defined by its wavelength or frequency. Each descriptive term defines an energy band, for example visible radiation refers to radiation that ranges from red light which is at the low energy end of visible radiation to blue light which is at the high end.

The atmosphere allows most of the radiation from the Sun to reach the surface of the Earth and this solar radiation is reflected back into space or warms the surface of the Earth. However, the infra-red radiation emitted by the Earth is absorbed by the greenhouse gases in the atmosphere. When infra-red radiation is absorbed by greenhouse gas molecules the energy is reemitted as infra-red radiation in any direction or is converted into kinetic energy, adding to the temperature of the atmosphere. If you were looking at the Earth from the moon with your hypothetical infra-red glasses on, the surface of the Earth would appear to be somewhere within the Earth’s atmosphere, the infra-red radiation that you see would not be that emitted from the surface of the Earth but that emitted by greenhouse gas molecules in the atmosphere. (more…)