Is it also the case that the other planets are also warming? This has already been said. Much depends on the tilt of the Earth’s axis. (climate) But how can this happen simultaneously with all planets? Or is this simply because the sun is still in its warming phase?
Answer
Two elements play a role in global warming, and also for other planets:
– the extent to which the amount of energy incident on it (ie from the Sun) increases;
– the extent to which the planet develops a greenhouse to retain heat.
The sun has been in the most stable phase of its existence for 4.5 billion years and for another 6 billion years. But that does not alter the fact that its energy output also increases (slowly) in that phase. Already, the Sun is about 20 percent brighter than when it formed, and the increase in brightness continues at about the same rate. Beware: this does not mean that the Sun itself is warming (on the outside); she’s getting bigger. Important in this context is not so much the temperature of the Sun, but the total amount of energy it radiates.
The effect of the Sun therefore always causes a (slow) increase in the temperature on a planet, but the evolution of the greenhouse effect in the atmospheres of planets is more complex. The more greenhouse gases in a planet’s atmosphere, the more that planet retains the heat it gets from the Sun. If we dump greenhouse gases into the atmosphere now, we’ll make our planet warmer. But to compensate for the effect of the Sun, we need to phase out the greenhouse gases sooner! And this is indeed what the Earth has been doing all along: the primitive Earth had a much stronger greenhouse effect than the present one, because the atmosphere was mostly made up of CO2. It is the biosphere that has slowly broken down that carbon dioxide atmosphere, acting as a thermostat that has kept the ambient temperature more or less constant.
On Venus and Mars there is no biosphere that changes the composition, so you would expect that only the effect of the Sun plays there, so that the planet warms up. This is probably the case for Venus, which is large enough to hold its atmosphere. But Mars is a small planet, struggling to keep its atmosphere bound to itself (by gravity): the density of Mars’ atmosphere is only one hundredth that of ours. Thus, the greenhouse effect on Mars must have greatly diminished over time, and the associated cooling may have outweighed the warming caused by the brightening Sun. By the way, a warmer Mars in the past is necessary if one is to believe that there was once massive liquid water on this planet,
Answered by
Prof. dr. Christopher Waelkens
Astronomy
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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