Greenhouse effect

The greenhouse effect

The greenhouse effect is the natural warming of the Earth’s atmosphere. Solar radiation enters the atmosphere mainly as light, and some of that radiation is absorbed by the Earth’s surface then changed to heat that is re-radiated into the atmosphere where it is absorbed by greenhouse gases then re-radiated back to Earth again.

How does warming of the Earth’s atmosphere occur?

The greenhouse effect occurs naturally, and without it, the Earth would be -19ºC on average.

Total solar radiation entering the atmosphere is 342 Watts per square metre (W m-²). This is mainly visible light (short wave radiation) but also some ultra-violet radiation and infra-red radiation (heat or long wave radiation).

Of the incoming solar radiation

• 31% is reflected back to space by clouds, aerosols, the Earth’s surface and atmosphere
• about 20% is absorbed by the atmosphere
• 49% is absorbed by the Earth’s surface then transformed to infra-red radiation (heat) and re-radiated into the atmosphere

Some heat is lost to space, while the rest is absorbed by greenhouse gases, warming the Earth’s atmosphere to an average 14ºC.

Which are the greenhouse gases?

The main greenhouse gases are:

• water vapour (H₂0)
• carbon dioxide (C0₂)
• methane (CH₄)
• nitrous oxide (N₂0)
• ozone (O₃)
• fluorocarbons
• chlorofluorocarbons
• sulphur hexafluoride (SF₆)

What makes them greenhouse gases?

These gas molecules, in particular, are all bound loosely enough to absorb heat. Once heat is absorbed, the molecule vibrates and heat is released and absorbed by another molecule.

Do all greenhouse gases have the same effect?

Reflection of solar radiaiton by clouds

Each gas differs in its effectiveness at absorbing heat, the effect it has on other greenhouse gases, and the average time it stays in the atmosphere and consequently its potential for warming over time.

Methane, for instance, is considered to be 23 times more effective at warming than carbon dioxide over 100 years! Although methane lasts about 12 years in the atmosphere, whereas carbon dioxide lasts for 50–200 years, methane indirectly affects ozone and water vapour, making it a more effective greenhouse gas. Methane comes from many sources – in New Zealand, our many cows and sheep are responsible for high methane production. This is the reason why a “fart tax” was proposed. Interestingly, though, most of the methane actually comes from the animals burping, not farting.

Table 1 shows the global warming potential (GWP) of some greenhouse gases.

Changes in atmospheric gas composition

Earth’s atmospheric composition has changed throughout its existence. Initially, there was very little oxygen and the atmosphere consisted mainly of carbon dioxide and nitrogen with a little methane, ammonia and sulphur gas. When blue-green algae evolved several billion years ago, they used sunlight, carbon dioxide and water to produce carbohydrates and released oxygen as a by-product (i.e. photosynthesis). Slowly the atmosphere accumulated enough oxygen for oxygen-breathing organisms to survive.

Today, Earth’s atmosphere is composed of:

• 78% nitrogen
• 21% oxygen
• 0.9% argon
• 0.037% carbon dioxide
• neon, helium, methane, krypton and hydrogen in smaller proportions
• water vapour in varying amounts

What evidence is there that greenhouse gases affect the Earth’s temperature?

It has been learned from ice core data from Antarctica and Greenland that, over the past 400,000 years at least, temperatures were warmer when there were more greenhouse gases in the atmosphere, and vice-versa.

Is global warming the same as the greenhouse effect?

The Earth goes through natural cycles of temperature increase and decrease due to changes in the Earth’s orbit called Milankovitch cycles. However, since 1860, the increase in global temperature by about 0.6°C is considered by the Intergovernmental Panel on Climate Change (IPCC) to result mainly from increased greenhouse gas concentrations. From 1750 to 2001, there was an increase of 31 percent carbon dioxide, 150 percent methane and 16 percent nitrous oxide concentration in the atmosphere. This is due to an increase in the burning of fossil fuel, change in land use and agriculture, and decrease of carbon sinks such as forests through deforestation.