Why is the Earth so different than Venus? Both planets are roughly the same size, and both exist within what is known as the “Life Zone” in the solar system. (The Life Zone is the region of solar space where it’s possible for liquid water to be found on the surface of a planet.)
I used to ask that question every semester when I got to the section on planetology in an Astronomy course, and I never had a good answer.
The best answer seems to be that life evolved on the surface of the Earth and it didn’t on the surface of Venus. Because of life, much of the carbon CO2 in the early atmosphere, which was created by volcanic outgassing on both planets and was primarily CO2 and methane, both major greenhouse gasses, was sequestered out of the atmosphere and stored in giants rafts of bacteria on the Earth’s surface. As the CO2 was scrubbed, the greenhouse effect diminished and the surface temp dropped. Eventually the temperature dropped to a point where liquid water could collect on the surface, in which even more CO2 could be dissolved, which accelerated the process.
(Key point this. For people who argue that life can not drive global warming (of which the anthropogenic idea is a part), then they have to come up with a different explanation for the primordial cooling of the Earth and otherwise explain the wildly different surface temps between the Earth and Venus.)
Venus, where we have not found life, is still so hot on the surface that the ground has the consistency of stiff oatmeal – it’s not cooled enough to lose its plasticity.
So what happens if we reload the atmosphere with CO2? Temps should rise until the extra CO2 can be absorbed in the water or sequestered by biological processes (like stored in limestone). That assumes that ocean isn’t saturated CO2 (which it appears to be) and can absorb the additional gas. (Bio processes take a very long time, and they can contribute to CO2 just as they can act to sequester it.)
So, if we’re putting more CO2 in the air (which we are) at an increasing rate (which we are), then why isn’t the temperature rising right now? That’s been the biggest criticism of anthropogenic global warming. Temps increased for a while in the early 20th cent. and then started to level off.
A group of scientists think they’ve found the answer.
“”The masking of CO2-induced global warming by short term sulphur emissions is well known – it’s believed that the flattening off of global mean temperatures in the 1950s was due to European and US coal burning, and just such a mechanism could be operating today from Chinese coal,” he told BBC News.
“Other natural fluctuations in the Sun’s output, volcanoes and water vapour have also been proposed for causing the non-warming ‘noughties’, and may have contributed to a degree.
“It needs to be emphasised that any masking is short-lived, and the increased CO2 from the same coal will remain in the atmosphere for many decades and dominate the long-term warming over the next decades.”
Since the end of the study period, in 2007, China’s coal consumption has risen again by about 30%.”
So this is basically a form of the old Nuclear Winter idea that was thought to be a consequence of an unrestrained global nuclear war (back in the days when we were worried about such things). The problem here is that there’s not enough junk being put into the atmosphere to overcome the greenhouse effect longterm. And since we’re not going to mask it, the system will very quickly and exponentially begin to be driven by the greenhouse warming.
We talk about Venus as a place of run-away greenhouse effect. If we don’t get a handle on things here on Earth, at some point, just in the way that the existence of life on the surface of Earth had the long term effect of cooling the planet, so too will life restore it to a closer resemblance to our sister planet.