You don’t need to get nearly that extreme before the greenhouse goes into runaway mode; Venus is what happens after the seas have boiled away and the water photodisassociated into oxygen and the hydrogen ( which drifts into space ). Venus is the end point, not the beginning of a runaway. Earth is towards the inner edge of the habitable zone around the Sun; last I heard we are estimated to be only about a hundred million years from when the natural warming of the Sun will trigger a runaway greenhouse anyway. It wouldn’t take nearly as much as you seem to think to push Earth over the edge.
To highlight the point about climate change instead of actual raw temperature being a big deal, you should look at the graphs on global warming. Almost all of them measure temperature anomaly that is, the variation from mean temperature (the mean temperature in a region over a 30 year period.)
I’s incredibly hard to actually establish the global mean temperature; try finding it, you may be surprised at how fruitless your search is, the truth is the idea of a global mean temperature only exists as a theoretical concept. To get a true idea of global mean temperature you’d have to have temperature monitoring the world over. Unfortunately, most of the world is covered by water where some of the greatest variations in temperature take place and we do not have comprehensive cover at the surface level for recording temperature. Large swathes of Russia and virtually all of Antarctica don’t have monitoring stations. Many monitoring stations, even in the developed world are poorly maintained and in rural areas or in areas that experience local temperature anomaly. So for that reason having a totally accurate idea about what the global average temperature is at surface level is very difficult (although I think it’s estimated to be around 13-16 C.) It’s easier if you want to measure just the low troposphere generally and not sea level, as satellites can measure that, unfortunately satellite data tends to not synch up with measurements at sea level or at surface level, although newer adjustments to the data have gotten the two about 50% closer than they used to be.
But it would certainly take more than you seem to think.
The current atmospheric concentration of CO2 is 383 parts per million. Right before the industrial era started it was around 280 ppm, which is believed to be the *lowest it has been since the beginning of microbiotic life over 1 billion years ago.
CO2 concentrations and current global temperature is high relative to the half million years but well within normal, and not even near the “hot” end of the range when you zoom out to the multiple-million year view. There’s good evidence that earth goes through fairly dramatic climate change on 140 million year cycles (the earth is still actually in a colder period compared to say, the Jurassic or Cambrian eras where global mean temperature was around +22 C.)
100 million years ago CO2 levels were around 2,000 pp, and life was abundant then, and the world was in no danger of turning into Venus. 400 million years ago the CO2 levels were around 6,000 ppm, this was around the time the first land-dwelling vertebrates were moving onto land, plant life was fairly abundant. No threat of turning into Venus then, and that was with CO2 concentrations about 20x what they are now and temperatures much much hotter than we have today.
(Note I’m not saying that CO2 concentrations 15-20x higher than they are now would be “an okay thing.” Just that it wouldn’t result in the Earth turning into Venus.)
And monkeys fly out my butt. We don’t know what conditions would trigger a hypothetical runaway greenhouse effect, or why the atmosphere of Venus is so different from the Earth despite similar size and composition of both bodies. (The predominant theory once held was that Earth’s oversized Moon skimmed away heavy atmosphere, but this has been subsequently abandoned, though it is still possible that the presence or absence of a moon might have in some way indirectly contributed to the displayed differences.) Venus’ rotation is very slow–about 243 days–and it has essentially no magnetic field. It has been proposed that the lack of CO[sup]2[/sup] and presence of larger organic molecules in Earth’s atmosphere is due to the existence of photosynthetic life which binds up carbon and permits a highly oxydizing atmosphere.
As Martin Hyde has noted, natural catastrophies have done degrees of damage that are unparalleled by any manmade nastiness, and the Earth’s climate and ecosystem have recovered, albeit after the loss of many species. It’s also noteworthy, however, that the expansion of new and more complex phyla succeeds such extention events. Does this matter to humanity, as a species? Not in the short term (nor does it excuse heedless pollution and waste of resources) but it does indicate that extinction is a part of the cycle of nature.
As for the long-term benefit to civilization such an event might offer a greater context; right now, we worry about Venice sinking into the sea, or the Neatherland dykes being breeched, as if these entities have existed for all time and should continue to do so. Most post-Industrial social and economic structures are predicated on long-term stability and population growth, neither of which are ultimately sustainable. Confronting the species with its own mortality in the face of an indifferent nature might be one step to moving beyond existing institutions and planning for future sustainment.
Stranger
It was too near the Sun, that’s what happened. And we have a good idea what would trigger a runaway greenhouse effect; the greenhouse effect simply has to grow large enough that countervailing effects, like the reflectivity of clouds, can’t stop it. AFAIK no one knows exactly what that point is, but it’s clear that it exists.
First, that was in the past; as I said, the Sun is slowly growing warmer, and other things have changed as well, so something that the Earth survived 100,000,000 years ago, it might not survive today ( or it might survive better ). Second, a volcano or asteroid impact is going to make things colder, and is unlikely to cause a runaway greenhouse ( although I do recall some theorizing that a billion or so years ago the opposite nearly happened; a runaway “icehouse” with Earth freezing permanently ). Third, I think you underestimate just how much effect humans have on the world; last time I saw as estimate, for example, we were a natural force up there with wind or water ( but below continental drift ) in terms of moving rock and earth. We are not the trivial thing you believe. Fourth, we have already devastated the Earth, and are at the tail end of a mass extinction that started in the Stone Age; Earth is ripe for the second half of a one-two punch that would do less damage if we hadn’t aready done so much ourselves.
And finally, I never said it was likely; I really don’t know if we could pull off a runaway greenhouse or not, especially by accident ( and it would take quite the demented culture to do so on purpose ). I was simply responding to the claim that it’s impossible for climate change to destroy humanity; that’s simply wrong.
Or it might drive us into a permanent Dark Age, or even one that “only” lasts for a few centuries.
Nitpick: I agree that 280ppm is toward the low end but it is not the lowest…The levels have been down in the range of 180-200ppm during the ice ages (and thus, as recently as ~15,000 years ago). We are now in a “relatively” warm interglacial period where I admit that the relative means that we are on the warm side of what we have been over the last few million years…a period that has generally been on the cool side, as you noted, on the timescales of tens to hundreds of millions of years.
I agree with your larger point though that scientists generally believe that us triggering a runaway greenhouse effect is exceedingly unlikely.
Plus, the simple truth of the matter is climatologists and other scientists aren’t entirely sure the mechanics of climate change on the “grand scale” (ie over the hundreds of millions of years.)
There are several important differences between Earth and Venus. Venus and Earth are relatively close but Venus, at its closest point to the Sun, is much closer to the sun than the Earth is on average. And Earth has long periods where it is significantly farther from the Sun than the Earth would ever be.
Venus also has way more volcanoes than Earth does, with way more active volcanoes at any given time than Earth has. It’s not totally understood the effect that plate tectonics and volcanic eruptions have on climate change over long periods of time, but a big part of why Venus is like it is today could be because of its huge number of volcanoes.
On Earth, a volcanic eruption will, as has been noted, decrease global temperatures in the short term. But in the long term they could easily contribute to global warming as volcanic eruptions send out CO2 and aerosols which can degrade the o-zone layer. Not all volcanic eruptions are created equally. They spew out water vapor, CO2, SO2 as well as other things. The SO2 rises up into the atmosphere and causes the earth’s reflection of radiation from the Sun to increase, in effect causing a cooling effect. However it also contributes to the concentration of greenhouse gases by sending tons of CO2 into the atmosphere.
These are terrestrial volcanoes. Volcanoes on Venus could spew forth tons of CO2 for example but relatively little to no SO2, meaning they wouldn’t really cool Venus down at all while also contributing dramatically to the greenhouse effect on Venus globally as Venus has more volcanoes than Earth. In addition to way more volcanoes, Venus has more “big” volcanoes. It has 167 identified volcanoes that are over 167 km across, the only volcano like this on Earth is Hawaii’s Big Island.
Mass extinction? Humans appear to have wiped out some of the mega-fauna whenever they first came to a new part of the planet (Australia, the Americas). Whether this qualifies as a “mass extinction” is debatable, as that term is usually reserved for events that wipe out a wide variety of species, not just mega-fauna.
And while a number of island species have gone extinct in the last 500 years, birds and mammals on the continents are very difficult to drive to extinction. The main path to extinction is predation by other species, including humans. In the last 500 years, only 3 continental mammal species and 6 continental bird species have gone extinct.
Despite claims in some circles that an average global temperature rise of a degree will cause a third of all species to go extinct, and despite climate change of more than a couple degrees in the last 500 years, there is no documented case of any bird or mammal going extinct from climate change during that time.
Life is incredibly tenacious, and birds and mammals are most ingenious in being able to change their habits to avoid being wiped out. Life changes paths, evolves, modifies its behavior, and generally does amazing things to stay alive. As a result, the odds of a few degrees of warming driving humans extinct are extremely, incredibly, unbelievably small.
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