How far would we have to raise the Earth’s overall average temperature (on the surface) for the process to spiral out of control? A long time ago a friend of mine said that if the average temperature of the Earth was raised accidentaly by man too far (I think he said +5 degrees F) it would reach a point where the process spiraled out of control and would make our planet unlivable for most land life. He mentioned co2 trapping in heat. He even compared the results to Venus although I know that the atmospheres ect. are different. Does such a point exist? Thanks for any info.
I’m not a meteorologist, but I doubt you’ll get a definitive answer on this one… from what I have read in the past few years, scientists are far from unanimous in their opinions of how the global climate works. Some would say that a change of only a few degrees would send the Earth spinning into a viscious cycle of global warming, others would say that the change would have to be MUCH more sever… and there are feedback processes in play that are poorly understood, as well.
Consider the opposite case: during the ice-age (as a grossly simplified example), the Earth was colder. Therefore more snow fell, and the snow stayed around longer. Therefore more of the sun’s heat was reflected back into space, and not absorbed by the ground (white snow reflects heat & light better than bare ground). Therefore the Earth should have gotten progressively colder and colder until it was a frozen ball shivering in space… but that did not happen! As we all know, Earth gradually warmed up and the ice-age ended (why? I dunno… maybe someone will be along eventually who can enlighten us… 
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Natural scientists still have a very hard time telling us exactly why something happens… my father is a scientist (entomology), and even discovered a new insect which he named after our family! (OK, everyone off your knees and stop worshiping him. Douse the candles! Stop the chanting! He discovered the insect during a feild-trip in college where the students were all instructed to capture a bug for later identification… his was, as yet, undiscovered…as were the bugs that 2 of his classmates caught… still, it’s kinda cool to have a bug named after your family… 
) Anyhoo, I get some inside scoop about natural scientist stuff from him, and he tells me that MANY things that are studied in this area have a problem with simple lack of data. IE: no one has studied these things long enough to be sure of anything. How long has climate been seriously studied? Maybe a couple of hundred years… out of a record of a few billion years!
To make a long-winded post short (too late!): I think you’ll find that the only answer you’re going to get is “Insufficient data, captain!”
It’s certainly possible that the Earth has experienced gross changes in climate in the past - BBC2’s Horizon has a programme scheduled for tomorrow about the ice-locked Earth scenario described by Astroboy14, arguing that this actually happened, maybe a billion years ago. Looking around, I guess we recovered from it. Regrettably, my time machine’s broken, so I can’t tell you what the programme’s like.
WAG: if the Earth can go from a deep-frozen state to the present day’s conditions, there’s no reason it couldn’t reover from an overheated condition… depending on how these climatic feedback processes work. Which is not yet known.
Of course, this is not to say that climate change is not a cause for concern. If the Earth were to go into a deep-frozen state, the fact that it would return to normal in a mere hundred million years or so would not be much of a consolation to those of us freezing on the surface. Similarly with global warming. I doubt that the Earth would ever get like Venus - the two planets are different in a number of ways - but it’s comparatively easy to develop a scenario where it gets far too hot to be comfortable for, or maybe even habitable by, humans.
I’m inclined to concur: further information is needed.
It’s just not known at what point the greenhouse effect becomes “runaway”. Hopefully, atmospheric scientists will soon better understand how it all works. But a big problem is in the non-linearity (tough to predict) and feedbacks (positive and negative) of a global climate/weather system.
Until, then humanity’s dumping of greenhouse gases into the atmosphere is a huge uncontrolled experiment. Smart, eh?
My WAG is that the breaking point is more than 5 degrees F, but then again, 5 degrees is a big change when taken as a global average.
It’s my guess (and only a guess) that the oceans are what save us from ever getting as hot as Venus. The thermal capacity of the oceans is quite large (to say the least), and you’ll never boil them all off, or even come close (at least not without the Sun getting significantly hotter). Furthermore, I think you’d stimulate cloud formation, and eventually you’d get to a point where the Earth was reflecting enough light to stabilize surface temperatures. As long as the temperature in the troposphere (below 30,000 feet-ish) dips below freezing I think that’d be true.
That’s not to say humans could live on Earth at the eventual temperature it stabilizes at, but I think it would stabilize, and eventually return to “normal”. This is part of the reason planetary scientists are so interested in figuring out where Venus’ water went.
Hi Andy! Did you get any scope time in Chile?
oops…stay on topic!
I agree. I think this is the prime examples of a “negative feedback” (result of a warming trend that reduces warming) that I mentioned in my previous post.
side thought…another “negative feedback” could be global warming wiping out humanity, thereby stopping the increasing CO2 levels and allowing things to go back to normal. :), I mean :eek:
No reason you couldn’t eventually boil them off. We don’t know that Venus didn’t have oceans early on. As evaporation increases, there’s less water to dissolve CO2 into, and H2O is a pretty efficient greenhouse gas. Water being a light molecule, escapes from the atmosphere as it heats up. As for clouds reflecting heat away, they also hold heat in pretty well. The albedo of Venus’ cloud deck is something like 85%.
I’m not claiming we’re headed for this anytime soon, just that there probably is a point where we get beyond the buffer…
Regardless of whether this overheating will occur or not, it will happen too slowly for anyone to notice. A lifetime for us, is a mere second to this big ball of rock we live on.
We talk about the “Ice Age” like it was the “Blizzard of '77”…but in all reality probably lasted millions of years…and didn’t just happen overnight.
However, over the past couple decades we’re noticing an increased occurance of severe weather (i.e. tornados, hurricanes, flooding, drought, extreme temperature ranges) which may point to a change in global climate. But it’s like measuring the distance to the sun with a ruler…it’ll take alot more data than we’ve captured in the last hundred or so years before we fully understand the cycles our planet goes through, and what the future holds for us.
There are two key differences that guarantee that the Earth won’t turn into Venus anytime soon:
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Incoming solar radiation - On this value all other calculations depend. The “solar constant,” as it’s usually called, varies for each planet depending on its distance from the Sun. The current value for Earth is 1370 Wm[sup]-2[/sup]; for Venus, it’s 2618 Wm[sup]-2[/sup]. So the amount of solar radiation (i.e. heat from the Sun) that Venus receives is nearly twice that of the Earth, and has been since the beginning of the solar system. The higher levels of heat input early in Venus’s history is likely responsible for driving off most of the available water vapor, if indeed it ever accumulated very much.
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Atmospheric composition - This determines the level of the greenhouse effect for a given planet. Venus’s atmosphere is composed of carbon dioxide (96%), nitrogen (3.5%), and trace amounts of other gases (carbon monoxide, argon, sulfur dioxide, water vapor totalling less than 1%). In contrast, Earth’s atmosphere is composed of nitrogen (78%), oxygen (21%), and traces of other gases (e.g., argon, carbon dioxide, and water vapor [0-4%, depending on local conditions]). While water vapor is a more efficient greenhouse gas than carbon dioxide, Venus has a far greater quantity of CO2 than Earth has of equivalent greenhouse capacity in water vapor, so its atmosphere on the whole is capable of retaining heat much more efficiently than Earth’s atmosphere. A planet with Venus’s atmospheric composition in Earth’s orbit would still be warmer than Earth.
So, if you combine higher incoming levels of solar radiation with an atmosphere capable of holding in lots of heat, you’re gonna get a planet that truly sizzles. Earth just doesn’t have what it takes to get to that point right now. Some 4 billion or so years down the road, when the aging Sun enters the red giant phase and expands, we’ll have to reassess this issue.
Now, as for natural variability here on Earth… it is true that we don’t fully understand which processes interact to govern positive and negative climate feedbacks, and we could certainly use more data. However, that’s not to say that we have no idea what went on in the past. The geologic record makes it pretty clear that earth has gone through cycles of being rather warm (warm enough to not have any ice sheets) as well as generally cool (past ice ages). For our planet, the biogeochemical cycle of carbon seems to be one of the fundamental regulators on long time scales… and the interesting thing is that life plays a prominent role. (This observation is at the root of the controversial Gaia hypothesis.) Life on Earth has managed to survive through both sorts of swings; seems Earth critters are tough that way. 
That’s a separate issue from whether or not a 5 deg. Celsius increase in temperature over the next century or so would make life uncomfortable for humans; the answer to that is a resounding YES! For discussion on whether or not you need to be worried, I would direct you to some of the past GD threads on global warming so that Manny doesn’t have to move this thread.
[mini rant] For anyone who watches the “Snowball Earth” program about Neoproterozoic extreme glaciations on BBC Horizon tomorrow, or on the Discovery Channel when it airs in late March/early April - You should know that the producers decided to make no effort to present an evenhanded view of the science controversy, and instead focused on personalities. The particular concepts presented should be taken with a large chunk of salt, as they are increasingly NOT supported by the broader scientific community. My apologies for the hijack, but this situation has gotten my goat. [/mini rant]
Fillet–
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correct me if I am wrong, but your solar constant values are off by a factor of 4. I believe that the value for the Earth is 346 Wm-2 and for Venus it is 662Wm-2. This is the value for radiation incident at the top of the atmosphere, however. Atmospheric temperature is related more closely to radiation that reaches the ground, thus you must take into account the albedo. Venus’ albedo is ~0.72 (I’ve seen values ranging from 0.66 to 0.85) and Earth’s albedo is ~0.37. Thus radiation received at the ground is about the same for both planets.
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atmospheric composition and temperature are interrelated. To a first approximation, Venus and Earth have similar amounts of carbon. On Venus, it’s all in the atmosphere and on Earth it’s mostly stored as limestone. One of the key questions implied in this thread is what temp is required to bake the CO2 out of that limestone (I expect that it’s way beyond the few measly degrees we’re discussing here).
Note that the high albedo for Venus is due to the sulfuric acid clouds (SO4 from volcanoes, which doesn’t ppt to surface). If Earth’s atmosphere were to experience some catastrophic greenhouse warming, I expect that you would see the albedo increase as well.
As Phobos said, we’re engaged in a grand climatic experiment, and we really don’t know what the results will be. I would bet a lot of money that we’re not staring down the barrel of a Venusian greenhouse, but I stop short of saying that it could not happen here…
jrepka,
Re solar constant values - we’re actually talking about the same amount of energy, just expressed differently. The values I gave are for the shadow area of each planet (the area of the shadow cast by the planet on a flat plane behind it). If you take the ratio of the global surface area of each planet to the shadow area, then you get a solar constant value 1/4 of that for the shadow area (the values you cite). Either value can be used to get at the blackbody temperature emission temperature for each planet, using the appropriate global average albedo. Using 30% albedo for the Earth, the blackbody temperature (i.e. no greenhouse warming) would be 255 K, or -18.15 deg. C; using 72% albedo for Venus, we get a temperature of just 238 K, or -35.15 deg. C.
So, in terms of solar radiation received at the top of the atmosphere, Venus actually receives a bit less than Earth does today. The difference between Earth and Venus now is the optical depth (or optical thickness) of their respective atmospheres. This characteristic is a measure of atmospheric gases ability to absorb re-emit radiation, and is dependent not only on the amount of incoming radiation but also on the thickness of the atmosphere, the density of the gas and a property of the gas called its absorption coefficient. The optical depth of Venus’ atmosphere far exceeds that of Earth’s in large part because of the huge quantity of carbon dioxide in the atmosphere, so the greenhouse properties of its atmosphere are greater despite the slightly lower amount of radiation reaching the surface today.
I think to make a fair comparison to Earth, we’d need to consider what conditions were like on Venus before its carbon dioxide-rich atmosphere built up, before it acquired clouds yielding such a high average planetary albedo. I don’t know of any estimates, though, for early Venus atmospheric composition/albedo, nor the time it took to reach its present state, so I suppose we are out of luck at the moment on that score.
I guess we’ve read this thread a bit differently - I read the OP as wondering about anthropogenic greenhouse emissions run amok and producing another Venus that way. (Although baking CO2 out of limestone would not be out of the question once the Sun begins to expand.) To get as much CO2 into Earth’s atmosphere as Venus has, you’d have to boil off the oceans first, which requires a global average temperature in excess of 100 deg. C (currently it is about 10 deg. C), and then bump up temps further to several hundred degrees C to start sublimating CO2 out of limestone. IMHO, we’d have to get pretty far out of hand down here to get to that point anywhere in the next billion years.
The question of what would happen to the Earth’s planetary albedo under (normal) anthropogenic warming (related to CO2 build-up) is an interesting one, and one that has provoked considerable discussion among climate modelers concerned with getting this parameter correct in their simulations. Catastropic warming could increase planetary albedo initially, as water vapor accumulates in cloud form at mid-latitudes and arid (cloudless) regions expand at low latitudes. However, at some point the negative feedbacks should kick in to bring temperatures back down (which ones depends on what time scale you’re considering).* Earth has done a pretty good job of staying on an even keel, climate-wise, so I feel safe in placing my bets on an un-Venus-like Earth in any time frame that humans care about.
*Not that I think greenhouse emissions shouldn’t be controlled to the best of our ability, because even a few degrees C change in the global average temp. will cause a lot of problems to us puny humans.
I have left CaCO[sub]3[/sub] at 540 C for fairly long periods of time (like over a weekend) without any measurable weight loss. 950 C is the temperature usually used to determine loss on ignition for cement, which involves decomposing carbonate. 750 C is the temperature used for loss on ignition of fly ash, which isn’t supposed to decompose carbonate.
How warm would it have to get for all the methane gas hydrates in the oceans to decompose?
It would take a lot less than a Venusian type of runaway greenhouse effect to pose a danger to humans.
Under anything close to the level of warming projected over the next century or two, there would likely be plenty of gas hydrates left. Over the longer term (several hundreds to thousands of years)… Gas hydrates in arctic permafrost regions, on artic continental shelves and at the tops of continental slopes are most vulnerable because they are already fairly close to the upper limits of stability under their present temperature and pressure conditions. However, it would take time for heat on the earth’s surface to diffuse downward and cause destabilization, particularly for the deeper deposits (as much as 2000 meters down).
Gas hydrates at the base of continental slopes and in the abyssal ocean would actually become initially more stable if sea level rose, because of the increased pressure overhead. Getting these deep hydrate deposits to melt would require raising ocean bottom water temperatures significantly to counteract the higher pressures. Current bottom temperatures hover around -1.5 to 1 deg. C; to de-stabilize hydrates at a depth of more than 1000 m, say, would require raising ocean bottom temps to at least 11 deg. C. Such a warming and subsequent methane release does appear to have happened during the Latest Paleocene Thermal Maximum (LPTM), about 55 million years ago, so it’s not out of the realm of possibility that it could happen again. Unfortunately, we don’t know yet what exactly caused the ocean bottom to warm so significantly back then, so it’s hard to say what would cause such an event now. Given the large thermal capacity of the ocean, I’d guess that such warming would need to take place over a couple thousand years, at least.
Note that you don’t need to destabilize gas hydrates to get methane into the atmosphere. Swamps and coastal areas are significant sources of methane even today; such areas could easily expand in a warmer world with higher sea levels.
A couple of interesting sites about gas hydrates for more info:
http://www.hydrate.org/
http://www.radix.net/~bobg/faqs/scq.methane.html