The Greenhouse Effect

Short-wavelength radiation, which comes straight from the sun, is partially reflected by the upper atmosphere and clouds. The rest passes through the atmosphere and heats the Earth’s surface: continents, oceans, ice and so on. This heated surface in turn emits long-wavelenght radiation, most of which is absorbed by the molecules of some of the gases in the atmosphere (carbon dioxide, methane, and so on) and also by water vapour. This phenomenon, known as the Greenhouse Effect, raises the temperature of the Earth surface by as much as 33°C, taking it from -18°C to 15°C (average temperature of the Earth). The greenhouse effect is therefore a natural phenomenon in origin, which contributes to the Earth being " habitable".

Since the beginning of the industrial era, an increase in the presence of greenhouse gases has become apparent. The latest scientific results in the field of climate modification by man show that there is some evidence that Man may have a perceptible effect on climate.

The time constants at stake in these phenomena are in terms of centuries: a century to stabilise the CO2 concentration, then another century to stabilise the temperature, then another century to stabilise the level of the oceans. The temperature increase currently forecast is from 1 to 3.5°C. The consequences are difficult to assess. Scenarios leading to stabilisation of greenhouse gas emissions are based on the one hand on a reduction in current levels of consumption of the industrialised nations (thus saving energy, although one sector remains difficult to solve, i.e. transport), and on the other hand, on the control of growth in developing nations and an increase in the use of non-polluting sources of energy from the point of view of the greenhouse effect, such as nuclear power.

Carbon dioxide is responsible for 55 % of the greenhouse effect. The chart below shows CO₂ production in metric tons per electric GW hour for different energy production methods.

CO₂ production for different energy production methods

• Coal produces 1000 times more CO₂ than nuclear power (fission or fusion)

• 50% of the CO₂ emission from nuclear power is attributable to power plant construction and fuel management (extraction, enrichment and reprocessing). Fusion has a less complex fuel cycle (no enrichment, or reprocessing for example), which explains lower values than in fission.

• Hydraulic energy generates little CO₂ emission. However, peat decomposition may lead to significant emission of methane (around 60 metric tonnes of equivalent CO₂ / GW/h noted in certain Finnish stocks ).

• CO₂ emission from solar power (photovoltaic) depends on two factors: the manufacturing process of sensors, which consumes a lot of electrical energy, and the CO₂ content in the kWh used for manufacture (kWh from a coal power station or a nuclear power station).

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