Is runaway climate change possible? Hansen’s take

February 4, 2010

in Canada, Economics, Geek stuff, Politics, Science, The environment

Back in 2008, I wrote about whether ‘runaway’ climate change might be possible on Earth. At one point, Venus had liquid water on its surface. Then, the sun grew brighter and Venus warmed. Its oceans evaporated and huge amounts of carbon dioxide (CO2) got baked out of the crust. The heat made the water break up into hydrogen and oxygen: the oxygen bonded with carbon to make more CO2, and much of the hydrogen escaped into space. Venus became permanently hostile to life, with surface temperatures of 450°C.

Could burning all of Earth’s fossil fuels produce the same outcome?

Some people take comfort from the fact that there have been times in the history of the planet when greenhouse gas concentrations were much higher than now. The world was very different, but there was no runaway greenhouse and life endured. James Hansen devotes the entire tenth chapter of Storms of My Grandchildren to considering whether this assessment is valid. Three things give him pause:

  1. The sun is brighter now than it was during past periods with very high greenhouse gas concentrations. The 2% additional brightness corresponds to a forcing of about 4 watts per square metre and is akin to a doubling of CO2 concentrations.
  2. For various reasons, the greenhouse gas concentrations in past hot periods may not have been as high as we thought.
  3. We are introducing greenhouse gases into the atmosphere far more quickly than natural processes ever did. This might cause fast (positive) feedback effects to manifest themselves forcefully, before slower (negative) feedback effects can get going.

He also explains that the sharp warming that took place during the Paleocene–Eocene Thermal Maximum (PETM) were not caused by fossil fuels (which remained underground), but rather by the release of methane from permafrost and clathrates. If human emissions warm the planet enough to release that methane again, it could add a PETM-level warming on top of the warming caused by human beings.

Hansen’s conclusions are, frankly, terrifying:

The paleoclimate record does not provide a case with a climate forcing of the magnitude and speed that will occur if fossil fuels are all burned. Models are nowhere near the stage at which they can predict reliably when major ice sheet disintegration will begin. Nor can we say how close we are to methane hydrate instability. But these are questions of when, not if. If we burn all the fossil fuels, the ice sheets almost surely will melt entirely, with the final sea level rise about 75 meters (250 feet), with most of that possibly occurring within a time scale of centuries. Methane hydrates are likely to be more extensive and vulnerable now than they were in the early Cenozoic. It is difficult to imagine how the methane clathrates could survive, once the ocean has had time to warm. In that event a PETM-like warming could be added on top of the fossil fuel warming.

After the ice is gone, would Earth proceed to the Venus syndrome, a runaway greenhouse effect that would destroy all life on the planet, perhaps permanently? While that is difficult to say based on present information, I’ve come to conclude that if we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty.

To re-emphasize the point, averting catastrophic or runaway climate change is the most important ethical and political task for those alive now, even if most politicians don’t yet realize it or don’t yet understand what that involves.

That last line also offers something to throw back, next time someone says the billions of dollars of revenue from exploiting the oil sands are simply too valuable to not collect.

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{ 43 comments… read them below or add one }

Milan February 4, 2010 at 5:18 pm

All this makes me feel a new clarity about where we should be concentrating our efforts.

They key battlegrounds are coal burning and unconventional oil and gas. If we can keep enough of that stuff in the ground, we can prevent catastrophic climate change. If we burn it all, disaster is all-but-inevitable.

richard pauli February 4, 2010 at 11:47 pm

Milan, your blog is the most spot on target.

The science exploration should be completely aimed toward determining how bad the situation is. You come close to describing it http://www.sindark.com/2008/02/27/ethics-among-the-doomed/

We need to know whether we should go for adaptation, mitigation or endgame preparation. Big decisions ahead. Burning any carbon is foolish.

R.K. February 7, 2010 at 1:18 pm

If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty.

One partial consolation is that tar shale requires more energy to extract than it contains. As long as that remains true, the chances of burning a lot of it are pretty low.

Of course, if we start approving nuclear reactors to steam the stuff out and upgrade it…

Milan February 16, 2010 at 7:25 pm

To me, it seems notable that the only three times ‘runaway climate change’ has been mentioned on the website of The Economist are times when it has been brought up by me.

They really aren’t anywhere close to appreciating what climate change could do to their entire worldview.

. March 4, 2010 at 4:18 pm

Study Says Undersea Release of Methane Is Under Way
By CORNELIA DEAN
Published: March 4, 2010

Climate scientists have long warned that global warming could unlock vast stores of the greenhouse gas methane that are frozen into the Arctic permafrost, setting off potentially significant increases in global warming.

Now researchers at the University of Alaska, Fairbanks, and elsewhere say this change is under way in a little-studied area under the sea, the East Siberian Arctic Shelf, east of the Bering Strait.

Natalia Shakhova, a scientist at the university and a leader of the study, said it was too soon to say whether the findings suggest that a dangerous release of methane looms. In a telephone news conference, she said researchers were only beginning to track the movement of this methane into the atmosphere as the undersea permafrost that traps it degrades.

But climate experts familiar with the new research, reported in Friday’s issue of the journal Science, said that even though it does not suggest imminent climate catastrophe, it is important because of methane’s role as a greenhouse gas. Although carbon dioxide is a far more abundant and persistent in the atmosphere, ton for ton atmospheric methane traps at least 25 times as much heat.

Until recently, undersea permafrost has been little studied, but work so far shows it is already sending surprising amounts of methane into the atmosphere, Dr. Shakhova and other researchers are finding.

. March 4, 2010 at 4:19 pm

In a telephone news conference on Wednesday, Dr. Shakhova said that permafrost in the East Siberian Arctic Shelf, peatland that flooded as sea levels rose after the last ice age, is degrading in part because runoff from rivers that feed the Arctic Ocean is warmer than it has been in the past.

She estimated that annual methane emissions from the East Siberian Arctic Shelf now total about 7 teragrams. (A teragram is 1.1 billion tons.) By some estimates, global methane emissions total about 500 teragrams per year.

Dr. Shakhova, who is also affiliated with the Russian Academy of Sciences, said that undersea methane ordinarily undergoes oxidation as it rises to the surface, where it is released as carbon dioxide. But because water over the shelf at most about 50 meters deep, she said, the gas bubbles to the surface there as methane.

As a result, she said, atmospheric levels of methane over the Arctic are 1.85 parts per million, almost three times as high as the global average of 0.6 or 0.7 parts per million. Concentrations over the shelf are 2 parts per million or higher.

But, “I am not the person to judge” whether the Arctic findings suggest that estimates of climate change in coming decades should be rewritten, she added.

“I would not go so far as to suggest any implications,” she said. “We are at the very beginning of research.”

Milan March 5, 2010 at 3:56 pm

Clarke’s First Law may also apply to the above: “When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.”

Hansen is 68, and was elected to the National Academy of Sciences in 1996. He has many other honours and awards.

That certainly doesn’t prove that runaway climate change is possible, but perhaps it provides a smattering of extra reason to take his opinion on the matter seriously.

. March 9, 2010 at 2:42 pm

Arctic Methane on the Move?
— david @ 6 March 2010

Methane is like the radical wing of the carbon cycle, in today’s atmosphere a stronger greenhouse gas per molecule than CO2, and an atmospheric concentration that can change more quickly than CO2 can. There has been a lot of press coverage of a new paper in Science this week called “Extensive methane venting to the atmosphere from sediments of the East Siberian Arctic Shelf”, which comes on the heels of a handful of interrelated methane papers in the last year or so. Is now the time to get frightened?

No. CO2 is plenty to be frightened of, while methane is frosting on the cake. Imagine you are in a Toyota on the highway at 60 miles per hour approaching stopped traffic, and you find that the brake pedal is broken. This is CO2. Then you figure out that the accelerator has also jammed, so that by the time you hit the truck in front of you, you will be going 90 miles per hour instead of 60. This is methane. Is now the time to get worried? No, you should already have been worried by the broken brake pedal. Methane sells newspapers, but it’s not the big story, nor does it look to be a game changer to the big story, which is CO2.

For some background on methane hydrates we can refer you here. This weeks’ Science paper is by Shakhova et al, a follow on to a 2005 GRL paper. The observation in 2005 was elevated concentrations of methane in ocean waters on the Siberian shelf, presumably driven by outgassing from the sediments and driving excess methane to the atmosphere. The new paper adds observations of methane spikes in the air over the water, confirming the methane’s escape from the water column, instead of it being oxidized to CO2 in the water, for example. The new data enable the methane flux from this region to the atmosphere to be quantified, and they find that this region rivals the methane flux from the whole rest of the ocean.

. March 11, 2010 at 11:03 am

“In the geologic past through the glacial cycles the climate feedback from methane has been smaller than that from CO2. However, as you note we’re pushing CO2 beyond the limits of what it has been for millions of years. The methane hydrates in the ocean take millions of years to grow. So we may be pushing the hydrates to melt down in the future. We did calculations that if the feedback were too strong you’d see the hydrates melting down spontaneous throughout Earth history. You don’t really see that, so the upper limit we predicted was that the hydrates could ultimately release as much carbon as we burn in fossil fuels. But the time scale for that is thousands of years, so the impact on climate over the next few hundred years would be small compared to that from fossil fuel CO2.

. March 17, 2010 at 10:16 am

Methane May Be Building Under Antarctic Ice

* By Alexandra Witze, Science News Email Author
* March 16, 2010
* 3:39 pm
* Categories: Earth Science

BALTIMORE — Microbes living under ice sheets in Antarctica and Greenland could be churning out large quantities of the greenhouse gas methane, a new study suggests.

In recent years scientists have learned that liquid water lurks under much of Antarctica’s massive ice sheet, and so, they say, the potential microbial habitat in this watery world is huge. If the methane produced by the bacteria gets trapped beneath the ice and builds up over long periods of time — a possibility that is far from certain — it could mean that as ice sheets melt under warmer temperatures, they would release large amounts of heat-trapping methane gas.

Jemma Wadham, a geochemist at the University of Bristol in England, described the little-known role of methane-making microbes, called methanogens, below ice sheets on March 15 at an American Geophysical Union conference on Antarctic lakes.

Her team took samples from one site in Antarctica, the Lower Wright glacier, and one in Greenland, the Russell glacier. Trapped within the ice were high concentrations of methane, Wadham said, as well as methanogens themselves — up to 10 million cells per gram in the Antarctic sample and 100,000 cells per gram in Greenland. That’s comparable to the concentration of methanogens found in deep-ocean sediments, she said. The species of microbes were also similar to those found in other polar environments, such as Arctic peat or tundra.

. June 29, 2010 at 4:53 pm

“Six researchers from Canada, the United States and the Netherlands have announced their findings after probing fossilized wood and the well-preserved remains of prehistoric plants and soil bacteria from Ellesmere’s Beaver Pond site, a paleontological time capsule near the Eureka science station on Canada’s northernmost land mass.

Based on three separate lines of evidence, the team has shown that the High Arctic locale once had a relatively balmy average annual temperature of 0 C – about 19 degrees warmer than today.

Furthermore, the amount of carbon dioxide in the atmosphere during that time, the Pliocene era, was almost identical to the elevated CO2 levels of today’s warmed-up globe – making the Beaver Pond site an unusually accurate “proxy” for the 21st century Arctic, the researchers say.

And while the Ellesmere site’s namesake species – a primitive variety of small beaver – and other extinct mammals, such Pliocene rabbits and three-toed horses, are indicative of a warmer environment than today, scientists have generally estimated that the average annual temperature at Beaver Pond four million years ago was no higher than -5 C.

The new evidence, Rybczynski told Canwest News Service, represents a troubling sign of how high Arctic temperatures could rise if present-day climate trends continue.”

Milan July 26, 2010 at 8:30 pm
. September 23, 2010 at 3:59 pm

“Aside from just enhancing the temperature signal, the existence of feedbacks is really what allows for significant departures in planetary climate evolution from some reference state. It would not be possible, for example, to cover the whole planet with ice down to the tropics or to boil off Venus’ oceans without feedbacks kicking in and rearranging the climate system to be compatible with a completely new state. Although it is not feasible to trigger a runaway greenhouse like Venus even if we burned all the coal today, it should really be kept in mind that there’s nothing unique about our current climate except that we have adapted to it. It is very readily capable of changing fast and ending up in a completely new regime, and ruling such a scenario out cannot be done with high confidence.”

. November 5, 2010 at 1:28 pm

The extreme warm end of a feedback scenario, popularly known as the runaway greenhouse, can arise when the atmosphere is composed of a greenhouse gas that is in vapor pressure equilibrium with a large, surface volatile reservoir. When you add greenhouse gases to the atmosphere, warm surface emission is preferentially replaced by emission from high, cold regions of the atmosphere. Since the water vapor feedback means the IR opacity is dependent on the temperature itself, eventually the emission to space can be decoupled from the surface temperature completely.

As the specific humidity continues to climb, the limiting infrared cooling that can occur for a planet comes at a threshold known as the Simpson-Kombayasi-Ingersoll (SKI) limit. If the incoming absorbed solar radiation exceeds this radiation threshold then the surface temperature must rise until the oceans are either depleted or the body becomes hot enough to radiate in significantly shorter wavelengths to which the air is rather transparent. The SKI limit therefore sets the inner edge of the habitable zone (where a planet can evolve with liquid water). This standard scenario is very important for understanding the evolution of other climates (in particular Venus, or exoplanets close to their host star). The runaway greenhouse scenario is likely the harsh fate Earth will encounter in the geologically distant future, as the sun gradually brightens over time.

TrueBlue July 7, 2011 at 4:14 am

The idea of RGH effect on Earth is clearly fanciful thinking. Hansen just dismisses 4billion years of history and assumes many things that can’t be verified ie the Sun is 2% brighter now and never was in the past. I would like to see a reasonable explanation with some evidence that when past CO2 levels where much higher than predicted the RGH effect didn’t occur. If you cant explain why an ice age starts or finishes to assume you can predict RHGE is simply ignorance and arrogance.
Climate change is inevitable with or without fossil fuel burning but clearly it has a major impact in the short (10,000 years) that being said the doomsday forecast just hurt the argument.
The reality here is the Earth will be just fine if we burn the lot, it is Humans that will disappear (not such a bad thing for the greater diversity’s good). The Earth has endured much worse and bounced back it will again just without us.
It is what we do with this knowledge, either change from an energy based world to a sustainable one with the immediate effect of driving 6 billion people below the poverty line (there is no substitute at present for burning fossil fuels if we need to continue to use energy at the present rate that is fact). We could immediately set in motion plans to reduce the worlds population to 500mill in 3 generations that would certainly be a real solution and focus on renewable s. We could not worry at all or take half heated efforts and let nature take its course and continue to enjoy high living standards for the next say 200 to 300 years with ever increasing temperatures.
See no doomsday, humanity will either invent cold fusion or something else and have inexhaustible supplies of energy with no externalities and survive or they will reduce population dramatically (90%) and become agrarian again or they we will continue as is an die out. The Earth and life however will continue.

david f July 9, 2011 at 4:49 pm

I think it silly for smeone to say a runaway greenhouse effect will occur on earth due to our co2 emmisions. He assumes all feedbacks are postive when most are negative. co2 has been much higher in the past (over 3000 ppm) and the temp didnt exceed 35c. This is too hot for life (this coresponds with the permian extinction) but no way near enough to boil the oceans. Actually its very hard to get a runaway greenhouse effect. As the temperature climbs the atmosphere gets more astaurated and more cloud covered, reducing solar radiation. Also the pressure increases but this raises the boiling point of water to upto 200c (double its present level). A temp of this level is required even through cloud cover for a runaway greenhouse effect to truely occur.
However, i did read something that high temperatures of over 50c can shut down plate tectonics. This leads to a huge surge on volcanic activity which releases co2, so2, h2so4. This extra co2 increases the pressure still mosre to the point that the crust casn bake. This is where the threat truely comes from but i dont seem man made warming pushjing temperatures up to 50c really.

Peer reviewed science! July 14, 2011 at 9:46 pm

Archer, D., B. Buffett and V. Brovkin, 2009, Ocean hydrates as a slow tipping point in the global carbon cycle, PNAS, 106 (49), 20596-20601.

The ocean methane hydrate reservoir can be considered a very slow but irreversible tipping point in the Earth’s carbon cycle. The warming effect potentially caused by methane release from this source is likely to be very gradual and of moderate magnitude, but it would last for millennia.

Archer et al. simulate the spatial distribution of ocean methane hydrates and assess their sensitivity to changing climate. They estimate the current total inventory of methane in ocean hydrates is between 1600-2000 Pg of C. When the sediment column warms and hydrates melt, methane bubbles are produced. If the volume of these methane bubbles exceeds a critical value, the sediment column releases methane. A critical question for the future is how much methane from melting hydrates will potentially escape from the seafloor to reach the ocean or the atmosphere. The authors find that, in response to a 3°C ocean warming and assuming a 10% critical bubble fraction for gas escape, only 2% of the methane inventory (about 30 Pg of C) would potentially escape. However, if the critical bubble fraction is 2.5%, about 50% of the methane (940 Pg of C) could escape. When the hydrate model was embedded in a global climate model forced by two fossil fuel CO2 emission scenarios (1000 Pg of C and 5000 Pg of C) they found that methane is released over a time period of several thousand years. Even for high methane emission responses, the atmosphere only warms by about 0.4-0.5°C. Thus, the potential warming effect of melting hydrates on the atmosphere would be quite moderate and would take millennia to manifest. However, the induced warming was shown to persist for at least 10-kyr.

Summary courtesy of Environment Canada

. August 5, 2011 at 4:22 pm

By analysing the isotopic composition of hydrocarbon molecules from plant waxes of the period, he found what looks like a spike in the amount of recently non-biological carbon (which has a distinctive ratio of light isotopes to heavy ones), lasting between 10,000 and 20,000 years. He thinks the liberation of methane stored at the bottom of the ocean in structures called clathrates is the most likely culprit. The alternative, that the carbon came from the volcanoes, is unlikely because the spike is much shorter than the period of volcanic activity. Methane is a greenhouse gas far stronger than carbon dioxide, so the consequence would have been a rapid warming of the climate—a phenomenon that the rocks suggest did actually happen.

This is not the first time a methane burp has been blamed for an extinction. Though the Cretaceous asteroid cleared the stage, mammals did not really get going until 10m years later, in the Eocene epoch. The preceding Palaeocene epoch was also brought to an end, the rocks suggest, by a sudden release of methane.

The burp could, of course, have been provoked by the eruptions, so the volcanoes are not off the hook completely. But, for those of a nervous disposition, the tying of an ancient greenhouse warming to an ancient mass extinction might suggest lessons for the future.

. January 4, 2012 at 7:36 pm

Could there be a methane runaway feedback?

The “runaway greenhouse effect” that planetary scientists and climatologists usually call by that name involves water vapor. A runaway greenhouse effect involving methane release (such as invoked here) is conceptually possible, but to get a spike of methane concentration in the air it would have to released more quickly than the 10-year lifetime of methane in the atmosphere. Otherwise what you’re talking about is elevated methane concentrations, reflecting the increased source, plus the radiative forcing of that accumulating CO2. It wouldn’t be a methane runaway greenhouse effect, it would be more akin to any other carbon release as CO2 to the atmosphere. This sounds like semantics, but it puts the methane system into the context of the CO2 system, where it belongs and where we can scale it.

So maybe by the end of the century in some reasonable scenario, perhaps 2000 Gton C could be released by human activity under some sort of business-as-usual scenario, and another 1000 Gton C could come from soil and methane hydrate release, as a worst case. We set up a model of the methane runaway greenhouse effect scenario, in which the methane hydrate inventory in the ocean responds to changing ocean temperature on some time scale, and the temperature responds to greenhouse gas concentrations in the air with another time scale (of about a millennium) (Archer and Buffett, 2005). If the hydrates released too much carbon, say two carbons from hydrates for every one carbon from fossil fuels, on a time scale that was too fast (say 1000 years instead of 10,000 years), the system could run away in the CO2 greenhouse mode described above. It wouldn’t matter too much if the carbon reached the atmosphere as methane or if it just oxidized to CO2 in the ocean and then partially degassed into the atmosphere a few centuries later.

The fact that the ice core records do not seem full of methane spikes due to high-latitude sources makes it seem like the real world is not as sensitive as we were able to set the model up to be. This is where my guess about a worst-case 1000 Gton from hydrates after 2000 Gton C from fossil fuels in the last paragraph comes from.

On the other hand, the deep ocean could ultimately (after a thousand years or so) warm up by several degrees in a business-as-usual scenario, which would make it warmer than it has been in millions of years. Since it takes millions of years to grow the hydrates, they have had time to grow in response to Earth’s relative cold of the past 10 million years or so. Also, the climate forcing from CO2 release is stronger now than it was millions of years ago when CO2 levels were higher, because of the band saturation effect of CO2 as a greenhouse gas. In short, if there was ever a good time to provoke a hydrate meltdown it would be now. But “now” in a geological sense, over thousands of years in the future, not really “now” in a human sense. The methane hydrates in the ocean, in cahoots with permafrost peats (which never get enough respect), could be a significant multiplier of the long tail of the CO2, but will probably not be a huge player in climate change in the coming century.

. January 11, 2012 at 6:58 pm

An Arctic methane worst-case scenario

Or, trying to “correct” for the different lifetimes of the gases using Global Warming Potentials, over a 100-year time horizon (which still way under-represents the lifetime of the CO2), you get that the methane would be equivalent to increasing CO2 to about 500 ppm, lower than 750 because the CO2 forcing lasts longer than the methane, which the GWP calculation tries in its own myopic way to account for.

But the methane worst case does not suddenly spell the extinction of human life on Earth. It does not lead to a runaway greenhouse. The worst-case methane scenario stands comparable to what CO2 can do. What CO2 will do, under business-as-usual, not in a wild blow-the-doors-off unpleasant surprise, but just in the absence of any pleasant surprises (like emission controls). At worst comparable to CO2 except that CO2 lasts essentially forever.

. May 27, 2012 at 1:51 pm

Did Ancient Mars Have a Runaway Greenhouse?

Cosmic impacts that once bombed Mars might have sent temperatures skyrocketing upward on the Red Planet in ancient times, enough to set warming of the surface on a runaway course, researchers say.

According to scientists, these findings could potentially help explain how this cold, dry world might have once sustained liquid water, conditions potentially friendly for life.

The largest craters still visible on Mars were created about 3.7 billion to 4.1 billion years ago. For instance, the Argyre basin is thought to be 3.8 billion to 3.9 billion years old, a crater about 710 miles (1,140 kilometers) wide potentially generated by a comet or asteroid 60 to 120 miles (100 to 200 kilometers) in diameter.

The origin of these immense craters roughly coincides with when many branching Martian river valley networks apparently formed. The impact that created Argyre basin would have released an extraordinary amount of energy, far more than any bomb made by humanity, or even the meteor suspected of ending the Age of Dinosaurs — it would have been an explosion with an energy on the order of 10^26 joules, or 100 billion megatons of TNT. Altogether, scientists had calculated these giant collisions would have raised surface temperatures on Mars by hundreds of degrees.

Now these researchers find this heating might not have been fleeting. Instead, this warming might have gone on a runaway course, pushing Mars into a long-term stable warm state.

The idea of runaway warming is most commonly associated with Venus. Scientists think that planet’s close proximity to the Sun heated its water, causing it to build up in its atmosphere as steam. Water is a greenhouse gas, trapping heat from the Sun that would have vaporized still more water, leading to a runaway greenhouse effect that apparently boiled all the oceans off Venus. Ultraviolet light would have then eventually split this atmospheric water into hydrogen and oxygen — the hydrogen escaped into space, the oxygen became trapped in the rocks of the planet, and the end-result was a bone-dry Venus.

The researchers note the many giant impacts Mars experienced might have heated the planet enough to send vast amounts of the the greenhouse gases water and carbon dioxide into the air. Their computer models suggest that there might have been enough of these gas in the Martian atmosphere to trigger a long-lasting runaway greenhouse effect. The impact that created the Argyre basin might have by itself been large enough to trigger such a chain reaction. Other impacts that might have pushed Mars toward a runaway greenhouse include the ones that created the Isidis and Hellas basins.

Milan June 17, 2012 at 10:24 pm

In this video, Stephen Hawking says: “We don’t know where the global warming will stop, but the worst case scenario is that Earth would become like its sister planet Venus…”

Byron Smith June 18, 2012 at 12:44 am

As eminent as they both are, I’m not sure I’m going to go to Sagan or Hawking for my climate science.

Here is a relevant article from a couple of months ago, published in the Philosophical Transactions of the Royal Society, which concludes that based on our present understanding, such an outcome is extremely unlikely (though can’t be entirely ruled out).

http://arxiv.org/abs/1201.1593

Milan June 21, 2012 at 6:06 pm

I will keep an eye on this question and post new information to this thread as I find it.

richard pauli June 21, 2012 at 6:19 pm

I am not sure I would trust philosophers to deliver good climate science.

In any case, boiling oceans is so far into a lifeless future as to make the question moot.

But we are required to assemble scenarios from various scientists – cryo, atmosphere, ocean, climate, even computational modeling. So combine it: 2 degrees C of warming is inevitable, 4 degrees likely, 6 degrees possible. This puts us into a very stressful world – with mass migration for survival.

We argue about Venus only because the horrors that are inevitable are hard to contemplate.

Milan June 21, 2012 at 6:24 pm

I don’t think the Venus possibility is morally irrelevant, no matter how far off in time it might be.

As far as we know, the Earth is the only planet in the universe that supports like. Venus-style runaway climate change wouldn’t just destroy human civilization, but all the life we know about in the universe.

Milan June 21, 2012 at 6:28 pm

I also find it worrisome that one eminent climate scientist (Hansen) says that a Venus scenario is likely if we burn all the coal, unconventional oil, and unconventional gas while at the same time other eminent climate scientists say it is impossible or extremely unlikely.

I admit, it’s hard for scientists to comment on something that has never happened before (on Earth). Still, it almost seems more worrisome to have disagreement on the subject of whether a Venus outcome is possible than it would be to have scientists largely in agreement that we could produce that scenario by pushing the climate enough.

richard pauli June 21, 2012 at 6:38 pm

Hansen explains why, and he has done the math. I have not heard explanations for why it is impossible or unlikely.

Heck, we orbit between Mars and Venus – it would be absurd to say either condition is impossible.

Milan June 21, 2012 at 6:42 pm

I don’t think it’s absurd to say that it’s impossible for humanity to cause a Venus-style runaway warming scenario on Earth.

There is an important negative feedback to consider, for one thing. The hotter the planet gets, the more readily it can shed heat into space. That may put an upper limit on how much warming can occur.

There is also a finite amount of radiative forcing humanity can create. Even if we devoted 100% of the effort of civilization to burning everything containing carbon, venting methane, and painting everything black, it is at least logically possible that we could not create enough radiative forcing to warm the planet enough to boil the oceans and create a Venus greenhouse.

Milan June 21, 2012 at 6:45 pm

Personally, I would very much like to know (a) whether we could cause a Venus scenario with that sort of all-out intentional climate modification and (b) whether we could do it accidentally by burning conventional and unconventional fossil fuel reserves.

Byron Smith June 22, 2012 at 7:54 am

Yes, you are right that there are important moral considerations raised by even the plausible outside possibility of a Venus-scenario, though I would agree with Richard that the almost certain destruction of human civilisation is already an overwhelmingly strong moral consideration in the absence of a Venus possibility.

BTW, Richard, if you were implying that my link above was irrelevant, it might be worth checking out just what the Philosophical Transactions of the Royal Society is, namely, the oldest continuous scientific journal in the world and still one of the most highly regarded. It was established in 1665, when “philosophical” was intended in a now archaic sense, of “natural philosophy” which is the old term for what are now called the natural sciences. Thus, if you are not aware of explanations for why it is impossible, then the publication I mentioned is relevant.

richard pauli June 22, 2012 at 3:45 pm

Byron, Indeed a very respectable organization … and a quick scan of the paper looks interesting. Although the sub-issue of geoengineering is so fraught with risk and untested science as to be serious ethical subject these days. Thanks for steering me back to your link.

. August 26, 2012 at 12:59 pm

Another concern, that thawing Arctic permafrost could release vast quantities of carbon dioxide and methane, looms larger. That, too, has happened before, around 55m years ago, leading to a global temperature increase of 5°C in a few thousand years.

. August 26, 2012 at 1:02 pm

Roughly a quarter of the northern hemisphere, including most of the Arctic land, is covered by this layer of frozen rock, soil and organic carbon. Formed over millennia, it varies in depth from a few centimetres to up to 1,500 metres in Siberia. Much of the Arctic’s shallow continental shelf is also covered by permafrost. According to an estimate made in 2009, terrestrial permafrost holds about 1.7 trillion tonnes of carbon, roughly twice as much as the atmosphere. By another estimate subsea permafrost stores an additional 0.5 trillion tonnes. And underlying it there may be another 0.8 trillion tonnes in the form of methane hydrates, an icy white material discovered in the 1960s.

Though tricky to get at, methane hydrates could be a massive energy source. Globally they are estimated to contain more energy than all known deposits of fossil fuels. Yet if even a small portion of the methane contained in them were to be abruptly emitted, the warming effect could be catastrophic. Methane is a short-lived greenhouse gas—it stays in the atmosphere for 6-10 years before being oxidised—but it is 25 times more efficient than carbon dioxide at trapping heat. And no one is sure how stable the hydrates are.

Given the scale of these risks, it is extraordinary how little research has been done on permafrost. “There are a lot of white spots in our knowledge,” admits Leonid Yurganov, a permafrost expert at the University of Maryland—Baltimore County. But a lot has recently been learned, which suggests that an explosive methane release is very unlikely. Ice cores going back 800,000 years show no trace of such an event. Nonetheless, the release of permafrost or subsea carbon could be gradual and still cause a lot of warming, and that does seem likely.

. September 17, 2012 at 11:41 am

Permafrost is permanently frozen soil, sediment or rock. It’s estimated that about 18.8 million sq km of northern soils hold about 1,700 billion tonnes of organic carbon, or frozen compost—the remains of plants and animals that have accumulated over thousands of years. That’s about four times more than all the carbon emitted by human activity in modern times, and twice as much than is currently in the atmosphere.

The new study predicts that by the end of this century, permafrost could release between 68 and 508 billion additional tonnes of carbon into the atmosphere, raising global temperatures by an average of 0.4 to 0.8°C. When combined with observed warming since pre-industrial times and committed warming in response to existing greenhouse gas levels, this suggests the planet is heading toward a 1.8 to 2.3°C rise in temperature—even if we start reducing emissions immediately.

Milan April 16, 2013 at 10:08 pm

A detailed elaboration and update from James Hansen:

Making Things Clearer: Exaggeration, Jumping the Gun, and The Venus Syndrome

15 April 2013

James Hansen

. April 16, 2013 at 10:24 pm

“I get questions from the public about the Venus Syndrome: is there a danger of “runaway” greenhouse warming on Earth leading to Venus-like conditions? Related questions concern specific positive (amplifying) feedbacks such as methane hydrates: as warming thaws tundra and destabilizes methane hydrates on continental shelves, thus releasing methane, won’t this cause more warming, thus more methane release, thus more warming — a runaway warming?

Amplifying feedbacks. Let’s consider a positive climate forcing (say a solar irradiance increase or CO2 increase) that causes a unit of warming. Let’s ask how this unit warming will be amplified by a very strong feedback, one that increases the initial warming by 50%. The added warming of 0.5 induces more feedback, by 0.5×0.5 = 0.25, and so on, the final response being 1 + 0.5 + 0.25 + 0.125 + … = 2. So this very strong feedback causes the final warming to be twice as large as it would have been without the feedback. But it is not a runaway effect.

The strongest feedback that we observe on Earth today, from water vapor, is almost as strong as this example. Other feedbacks are occurring at the same time, some amplifying and some diminishing (negative). The net effect of all fast feedbacks can be assessed by comparing different well-characterized climate states in Earth’s history, as described in our paper, treating slow changes such as ice sheet size as specified boundary conditions. It turns out that the net effect of fast feedbacks is to amplify the global temperature response by about a factor of 2-3.

Other feedbacks become important on longer time scales. As the planet becomes warmer the ice sheet area tends to decrease, exposing a darker surface that absorbs more sunlight. And as the planet warms the ocean and land release long-lived greenhouse gases, mainly CO2 and CH4 (methane). Thus Earth’s climate is dominated by amplifying feedbacks on time scales of 10-100,000 years and less. For this reason, Earth can be whipsawed between glacial and 6 interglacial conditions by the small climate forcings caused by perturbations of Earth’s orbit.

The dominance of amplifying feedbacks and the resulting high climate sensitivity make Earth susceptible to what we can call a mini-runaway. By mini-runaway, I refer to a case with an amplifying feedback large enough that the total feedback reaches runaway (the infinite series above does not converge), but eventually that process runs out of fuel. Evidence of such behavior is provided by hyperthermal events in Earth’s history, sudden rapid warmings that occurred during periods of more gradual warming.

The most studied hyperthermal is the PETM (Paleocene Eocene Thermal Maximum), which occurred in the middle of a 10 million year period of gradual warming. A rapid warming spike occurred in conjunction with injection of a large amount of CO2 into the climate system on a time scale of the order of a millennium. The source of the rapid CO2 increase is most commonly suggested to have been the melting of methane hydrates due to a warming ocean, with an alternative suggestion being incineration of large peat deposits, especially on Antarctica.

Whatever the CO2 source, global temperature increased about 6°C over several millennia. The continental weathering process provided a negative feedback, as a pumped-up hydrologic cycle drew down atmospheric CO2 and deposited it as carbonate on the ocean floor. However, this feedback requires tens of thousands of years, so the rapid warming stopped only when the fuel source was depleted.

Are hyperthermals relevant now, as a possible amplification of fossil fuel warming? Unfortunately, they may be. Burning all fossil fuels would produce such large ocean warming, which would continue to exist for centuries, that ignition of a hyperthermal amplification of global warming is a possibility. Consequences are unclear. Carbon release in prior hyperthermals occurred over a millennium or more, at a rate up to ~ 5 GtC/year. This can be compared with the present global rate of fossil fuel burning, which is ~ 9 GtC/year.

It is instructive to consider the task of dealing with such continuing carbon release, in the event that we did set it off. Humanity could defuse a continuous release of 5 GtC/year, thus avoiding hyperthermal warming, by capturing and sequestering the carbon. The American Physical Society estimates the cost of capture and sequestration as ~ $2 trillion per GtC. Given that the United States is responsible for 26% of the fossil fuel CO2 in the air today, the U.S. cost share for removing 5 GtC/year would be ~$2.6 trillion each year. Technology development might be able to lower that cost, but fundamental energy constraints imply that cost reduction at most will be a factor of a few.

We had better be sure to avoid a mini-runaway. If we phase out fossil fuels rapidly and 8 move to a clean energy future in accord with a scenario that my colleagues and I have described , we could be reasonably confident of avoiding that situation. We know that prior interglacial periods were moderately warmer than the current (Holocene) interglacial. A fossil fuel emissions scenario similar to the one we have defined is needed for other reasons, especially for the purpose of maintaining reasonably stable shorelines, i.e., avoiding sea level rise of many meters, which would destroy thousands of coastal cities all around the world.

In contrast, if we burn all the fossil fuels it is certain that sea level would eventually rise by tens of meters. The only argument is how soon the rise of several meters needed to destroy habitability of all coastal cities would occur. It is also possible that burning all fossil fuels would eventually set off a hyperthermal event, a mini-runaway. Is it conceivable that we could get a runaway leading all the way to the Venus Syndrome, Venus-like conditions on Earth?

Runaway Greenhouse. Venus today has a surface pressure of about 90 bars, compared with 1 bar on Earth. The Venus atmosphere is mostly CO2. The huge atmospheric depth and CO2 amount are the reason Venus has a surface temperature of nearly 500°C.

Venus and Earth probably had similar early atmospheric compositions, but on Earth the carbon is mostly in Earth’s crust, not in the atmosphere. As long as Earth has an ocean most of the carbon will continue to be in the crust, because, although volcanoes inject crustal carbon into the atmosphere as CO2, the weathering process removes CO2 from the air and deposits in on the ocean floor as carbonates. Venus once had an ocean, but being closer to the Sun, its atmosphere became hot enough that hydrogen could escape from the upper atmosphere, as confirmed today by the extreme depletion on Venus of normal hydrogen relative to heavy hydrogen (deuterium), the lighter hydrogen being able to escape the gravitational field of Venus more readily.

Earth can “achieve” Venus-like conditions, in the sense of ~90 bar surface pressure, only after first getting rid of its ocean via escape of hydrogen to space. This is conceivable if the atmosphere warms enough that the troposphere expands into the present stratosphere, thus eliminating the tropopause (see Fig. 7 in our paper in press), causing water vapor to be transported more rapidly to the upper atmosphere, where it can be dissociated and the hydrogen can then escape to space. Thus extreme warming of the lower atmosphere with elimination of the cold-trap tropopause seems to be the essential physical process required for transition from Earth-like to Venus-like conditions.

If Earth’s lower atmosphere did warm enough to accelerate escape of hydrogen it would still take at least hundreds of millions of years for the ocean to be lost to space. Additional time would be needed for massive amounts of CO2 to accumulate in the atmosphere from volcanoes associated with plate tectonics and convection in Earth’s mantle. So Venus-like conditions in the sense of 90 bar surface pressure and surface temperature of several hundred degrees are only plausible on billion-year time scales.

Is it possible, with the present surface pressure of ~ 1 bar, for Earth’s surface to become so hot that that the planet is practically uninhabitable by humans? That is the situation I depicted in “Storms of My Grandchildren”, which was presumed to be a consequence of burning all fossil fuels over a period of several centuries, with warming further amplified by ignition of PETM-like hyperthermal warming. Support for the possibility of large warming was provided by global climate model simulations indicating an upturn in climate sensitivity at climate forcings ~10 W/m^2 (Fig. 30 in “Storms”10). If other forcings are unchanged, a 10 W/m^2 forcing requires a CO2 increase by a factor of 4-8 times its pre-industrial amount (~280 ppm) — an increase that is possible if all extractable fossil fuels are burned. Other complex global climate models also find an upturn in climate sensitivity or climate model “crash” when CO2 amount reaches such high levels, raising the question of whether such a level of climate forcing is already trending toward a runaway greenhouse effect.

The concept of a runaway greenhouse effect was introduced by considering a highly idealized situation with specified troposphere-stratosphere atmospheric structure, a simple approximation for atmospheric radiation, and no inclusion of how clouds might change as climate changes, as is appropriate for introduction of a concept. More recent studies relax some of the idealizations and are sufficient to show that Earth is not now near a runaway situation, but the idealizations are still sufficient that the studies do not provide a picture of where Earth is headed if all fossil fuels are burned.

An alternative promising approach is to employ the fundamental equations for atmospheric structure and motions, i.e., the conservation equations for energy, momentum, mass, and water, and the ideal gas law. These equations form the core of atmospheric general circulation models and global climate models. However, today’s global models generally contain representations of so many additional physical processes that the models are difficult to use for investigations of extreme climatic situations, because invariably some approximations in the scores of equations become invalid in extreme climates. In contrast, my long-term colleague Gary Russell has developed a global model that solves the fundamental equations with the minimum additional physics needed to investigate climate sensitivity over the full range from snowball Earth to a hothouse uninhabitable planet. The additional physics includes accurate spectral dependence of solar and thermal radiation, convection, and clouds. Although the precision of the results depends on the representation of clouds, we suggest that the simple prescription employed is likely to correctly capture essence of cloud change in response to climate change.

We use the Russell model in our paper to show that the tropopause rises in response to the global warming that occurs with larger and larger CO2 amounts (Fig. 7 in our paper), and cloud cover decreases with increasing CO2. In consequence climate sensitivity initially increases as CO2 increases, consistent with the upturn of sensitivity found in more complex global climate models. With the more realistic physics in the Russell model the runaway water vapor feedback that exists with idealized concepts does not occur. However, the high climate sensitivity has implications for the habitability of the planet, should all fossil fuels actually be burned. Furthermore, we show that the calculated climate sensitivity is consistent with global temperature and CO2 amounts that are estimated to have existed at earlier times in Earth’s history when the planet was ice-free.

One implication is that if we should “succeed” in digging up and burning all fossil fuels, some parts of the planet would become literally uninhabitable, with some time in the year having wet bulb temperature exceeding 35°C. At such temperatures, for reasons of physiology and physics, humans cannot survive, because even under ideal conditions of rest and ventilation, it is physically impossible for the environment to carry away the 100 W of metabolic heat that a human body generates when it is at rest. Thus even a person lying quietly naked in hurricane force winds would be unable to survive. Temperatures even several degrees below this extreme limit would be sufficient to make a region practically uninhabitable for living and working.

The picture that emerges for Earth sometime in the distant future, if we should dig up and burn every fossil fuel, is thus consistent with that depicted in “Storms” — an ice-free Antarctica and a desolate planet without human inhabitants. Although temperatures in the Himalayas may have become seductive, it is doubtful that the many would allow the wealthy few to appropriate this territory to themselves or that humans would survive with the extermination of most other species on the planet. At least one sentence in “Storms” will need to be corrected in the next edition: even with burning of all fossil fuels the tropical ocean does not “boil”. But it is not an exaggeration to suggest, based on best available scientific evidence, that burning all fossil fuels could result in the planet being not only ice-free but human-free.”

. July 30, 2013 at 11:56 am

The “runaway greenhouse” — which is thought to have happened on Venus in the past — is basically a climate change worst-case scenario: We reach a critical point where the atmosphere is so thick with greenhouse gases that no sunlight can escape back into space, the planet heats uncontrollably, the oceans evaporate completely, and things get, well, pretty uncomfortable, to put it mildly.

“Everything is really quite dead at that point,” Colin Goldblatt, a planetary scientist at Canada’s University of Victoria, says in a chipper English accent. Goldblatt has been working to understand whether a runaway greenhouse could ever happen on Earth. Scientists have long believed that even with extreme greenhouse gas concentrations, our sun simply doesn’t heat the planet enough to trigger this effect. But using a series of custom computer programs that model incoming sunlight, greenhouse gas concentration, radiation absorbed by water vapor, and a host of other physical factors, Goldblatt has revised that threshold down, and in a paper published Sunday in Nature Geoscience says that a runaway greenhouse could kick off with the amount of sunlight we get today.

“What we’re seeing now is that if you pump the atmosphere full of carbon dioxide, you could make the planet so hot it would never be habitable again,” he says. Taken to its conclusion, he explains, the runaway greenhouse would produce a new atmosphere with global temperatures around 2,420 degrees F, which would make even the Northeast heat wave of the last couple weeks feel like a vacation into a meat freezer.

. July 30, 2013 at 11:56 am

Low simulated radiation limit for runaway greenhouse climates

The atmospheres of terrestrial planets are expected to be in long-term radiation balance: an increase in the absorption of solar radiation warms the surface and troposphere, which leads to a matching increase in the emission of thermal radiation. Warming a wet planet such as Earth would make the atmosphere moist and optically thick such that only thermal radiation emitted from the upper troposphere can escape to space. Hence, for a hot moist atmosphere, there is an upper limit on the thermal emission that is unrelated to surface temperature. If the solar radiation absorbed exceeds this limit, the planet will heat uncontrollably and the entire ocean will evaporate—the so-called runaway greenhouse. Here we model the solar and thermal radiative transfer in incipient and complete runaway greenhouse atmospheres at line-by-line spectral resolution using a modern spectral database. We find a thermal radiation limit of 282 W m−2 (lower than previously reported) and that 294 W m−2 of solar radiation is absorbed (higher than previously reported). Therefore, a steam atmosphere induced by such a runaway greenhouse may be a stable state for a planet receiving a similar amount of solar radiation as Earth today. Avoiding a runaway greenhouse on Earth requires that the atmosphere is subsaturated with water, and that the albedo effect of clouds exceeds their greenhouse effect. A runaway greenhouse could in theory be triggered by increased greenhouse forcing, but anthropogenic emissions are probably insufficient.

. July 30, 2013 at 3:30 pm

What does your work say about Hansen’s warning?

What my results show is that if you put about ten times as much carbon dioxide in the atmosphere as you would get from burning all the coal, oil, and gas—about 30,000 parts per million—then you could cause a runaway greenhouse today. So burning all the fossil fuels won’t give us a runaway greenhouse. However, the consequences will still be dire. It won’t sterilize the planet, but it might topple Western civilization. There are no theoretical obstacles to that.

http://news.nationalgeographic.com/news/2013/13/130729-runaway-greenhouse-global-warming-venus-ocean-climate-science/

Byron Smith October 8, 2013 at 7:02 am

Hansen’s latest publication (available here) concludes that a Venus-syndrome is not possible until solar radiation has increased significantly, which will take a billion year timescale before sufficiently high enough forcing could be sustained for the 100m to 1bn years it would take for the oceans to boil off. See 7(d), but also the discussion earlier at 5(b).

So Hansen has now rejoined the rest of the discussion in acknowledging that Venus-like runaway warming is not possible from contemporary anthropogenic carbon disruption.

Milan October 14, 2013 at 2:25 pm

In the abstract, it says:

Burning all fossil fuels, we conclude, would make most of the planet uninhabitable by humans, thus calling into question strategies that emphasize adaptation to climate change.

In the full text, it also says:

One implication is that if humans burn most of the fossil fuels, thus injecting into the atmosphere an amount of CO2 at least comparable to that injected during the PETM, the CO2 would stay in the surface carbon reservoirs (atmosphere, ocean, soil, biosphere) for tens of thousands of years, long enough for the atmosphere, ocean and ice sheets to fully respond to the changed atmospheric composition. In addition, there is the potential that global warming from fossil fuel CO2 could spur release of CH4 and CO2 from methane hydrates or permafrost.

Regarding Venus, it says:

This amplifying feedback has been known for centuries and was described remarkably well by Tyndall. Ingersoll discussed the role of water vapours in the ‘runaway greenhouse effect’ that caused the surface of Venus to eventually become so hot that carbon was ‘baked’ from the planet’s crust, creating a hothouse climate with almost 100 bars of CO2 in the air and a surface temperature of about 450◦C, a stable state from which there is no escape. Arrival at this terminal state required passing through a ‘moist greenhouse’ state in which surface water evaporates, water vapour becomes a major constituent of the atmosphere and H2O is dissociated in the upper atmosphere with the hydrogen slowly escaping to space. That Venus had a primordial ocean, with most of the water subsequently lost to space, is confirmed by the present enrichment of deuterium over ordinary hydrogen by a factor of 100, the heavier deuterium being less efficient in escaping gravity to space.

And:

Given the transient nature of a fossil fuel CO2 injection, the continuing forcing required to achieve a terminal Venus-like baked-crust CO2 hothouse must wait until the Sun’s brightness has increased on the billion year time scale. However, the planet could become uninhabitable long before that.

The practical concern for humanity is the high climate sensitivity and the eventual climate response that may be reached if all fossil fuels are burned. Estimates of the carbon content of all fossil fuel reservoirs including unconventional fossil fuels such as tar sands, tar shale and various gas reservoirs that can be tapped with developing technology imply that CO2 conceivably could reach a level as high as 16 times the 1950 atmospheric amount. In that event, figure 7 suggests a global mean warming approaching 25◦C, with much larger warming at high latitudes (see electronic supplementary material, figure S6). The result would be a planet on which humans could work and survive outdoors in the summer only in mountainous regions —and there they would need to contend with the fact that a moist stratosphere would have destroyed the ozone layer.

Milan October 14, 2013 at 2:30 pm

Also:

The runaway greenhouse effect has several meanings ranging from, at the low end, global warming sufficient to induce out-of-control amplifying feedbacks, such as ice sheet disintegration and melting of methane hydrates, to, at the high end, a Venus-like hothouse with crustal carbon baked into the atmosphere and a surface temperature of several hundred degrees, a climate state from which there is no escape. Between these extremes is the moist greenhouse, which occurs if the climate forcing is large enough to make H2O a major atmospheric constituent. In principle, an extreme moist greenhouse might cause an instability with water vapour preventing radiation to space of all absorbed solar energy, resulting in very high surface temperature and evaporation of the ocean. However, the availability of non-radiative means for vertical transport of energy, including small-scale convection and large-scale atmospheric motions, must be accounted for, as is done in our atmospheric general circulation model. Our simulations indicate that no plausible human-made GHG forcing can cause an instability and runaway greenhouse effect as defined by Ingersoll, in agreement with the theoretical analyses of Goldblatt & Watson [128].

On the other hand, conceivable levels of human-made climate forcing could yield the low- end runaway greenhouse. A forcing of 12–16 W m−2 , which would require CO2 to increase by a factor of 8–16 times, if the forcing were due only to CO2 change, would raise the global mean temperature by 16–24◦C with much larger polar warming. Surely that would melt all the ice on the planet, and probably thaw methane hydrates and scorch carbon from global peat deposits and tropical forests. This forcing would not produce the extreme Venus-like baked-crust greenhouse state, which cannot be reached until the ocean is lost to space. A warming of 16–24◦C produces a moderately moist greenhouse, with water vapour increasing to about 1% of the atmosphere’s mass, thus increasing the rate of hydrogen escape to space. However, if the forcing is by fossil fuel CO2, the weathering process would remove the excess atmospheric CO2 on a time scale of 10^4–10^5 years, well before the ocean is significantly depleted. Baked-crust hothouse conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a few tens of per cent, which will take a few billion years.

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