|1.4 Greenhouse effect on other planets?|
A planet's climate is decided by its mass, its distance from the sun and the composition of its atmosphere.
Mars is too small to keep a thick atmosphere.
Its atmosphere consists mainly of carbon dioxide, but the atmosphere is very thin.
- The atmosphere of the Earth is a hundred times thicker than the Mars atmosphere.
Most of Mars' carbon dioxide is frozen in the ground.
Mars' average surface temperature is about -50°C.
Venus has almost the same mass as Earth but a thicker atmosphere, composed of 96% carbon dioxide.
The surface temperature on Venus is +460°C.
- Earth's atmosphere consists of 78% nitrogen, 21% oxygen, and 1% other gases.
Carbon dioxide accounts for just 0.03 - 0.04%. The total amount of greenhouse gases in the earth atmosphere is thus usually less than 1%.
Water vapour, varying in amount from 0 to 2%, carbon dioxide and some other minor gases present in the atmosphere absorb some of the thermal radiation leaving the surface and emit radiation from much higher and colder levels out to space.
- These radiatively active gases are known as greenhouse gases because they act as a partial blanket for the thermal radiation from the surface and enable it to be substantially warmer than it would otherwise be, analogous to the effect of a greenhouse or a blanket.
This 'blanketing' is known as the natural greenhouse effect.
Without the greenhouse gases, Earth's average temperature would be roughly -20°C.
- The climates on Mars and Venus are very different, but very stable and highly predictable.
The Earth's climate is unstable and rather unpredictable as compared with that of the other two planets.
- Some billions years ago it is likely that Venus did have oceans and a very different atmosphere compared to the present.
However, over millions of years solar activity gradually increased, releasing much carbon from the ground to the atmosphere. Increasing amounts of water evaporated from the oceans.
This caused a runaway greenhouse effect, where the oceans evaporated and disappeared into space, while the atmosphere turned into CO2.
Worst case scenario for the Earth is that a similar development could happen to our planet.
- You may have heard that recently the temperature on Mars have increased.
This is no proof of an increased greenhouse effect on Mars. The atmosphere is far too thin for that.
The tiny rise in temperature is due to that the planet has moved slightly closer to the sun, while the polar area tilts somewhat more towards the sun. This is called the Milankovich effect, and occurs at regular intervals with most planets, Earth included.
In the same time period Saturn has turned colder.
- The cyclic Milankovich effects going over 20-30 thousand years, are the natural climate forcing mechanisms, giving slight changes in the solar input to the planets.
Over time, the relatively small forcing can cause significant climate changes, such as ice ages or mild periods through various feedback systems, such as changes in albedo and in the atmospheric chemistry.
Usually such changes happen over centuries or even millennia.
A slight change in solar input can thus cause a slight change in albedo, which in turn, maybe centuries later, can cause a change in the amount of greenhouse gases in the atmosphere. The delay is due to that it takes long time before the slight positive radiative forcing warms the oceans sufficiently to start releasing gases to the atmosphere
The rule of thumb in a geological perspective is thus that the main forcing factors giving a change in climate are a change in net solar input or a change in albedo e.g. due to big volcanic eruptions.
Explosive volcanic eruptions are mainly cooling because of the aerosols thrown into the atmosphere, while non-explosive volcanoes looking like big cracks in the Earth's surface are warming because of the emissions of CO2 and CH4.
The subsequent change in greenhouse gases has usually been a feedback mechanism, enforcing the trend. In other words: a forcing factor is usually quite small, but nevertheless triggers a small change in climate. The feedback mechanisms decide how significant and how rapid the climate change will be.
With the human input of carbon to the atmosphere, we might be changing this rule, making the GHGs a primary forcing factor rather than a secondary feedback mechanism.
Graphic: UNEP/GRID-Arendal: Vital Climate Graphics, 2001