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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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What is the net feedback from clouds?

What the science says...

Select a level... Basic Intermediate

Evidence is building that net cloud feedback is likely positive and unlikely to be strongly negative.

Climate Myth...

Clouds provide negative feedback

"Climate models used by the International Panel on Climate Change (IPCC) assume that clouds provide a large positive feedback, greatly amplifying the small warming effect of increasing CO2 content in air. Clouds have made fools of climate modelers. A detailed analysis of cloud behavior from satellite data by Dr. Roy Spencer of the University of Alabama in Huntsville shows that clouds actually provide a strong negative feedback, the opposite of that assumed by the climate modelers. The modelers confused cause and effect, thereby getting the feedback in the wrong direction." (Ken Gregory)

At a glance

What part do clouds play in your life? You might not think about that consciously, but without clouds, Earth's land masses would all be lifeless deserts. Fortunately, the laws of physics prevent such things from being the case. Clouds play that vital role of transporting water from the oceans to land. And there's plenty of them: NASA estimates that around two-thirds of the planet has cloud cover.

Clouds form when water vapour condenses and coalesces around tiny particles called aerosols. Aerosols come in many forms: common examples include dust, smoke and sulphuric acid. At low altitudes, clouds consist of minute water droplets, but high clouds form from ice crystals. Low and high clouds have different roles in regulating Earth's climate. How?

If you've ever been in the position to look down upon low cloud-tops, perhaps from a plane or a mountain-top, you'll have noticed they are a brilliant white. That whiteness is sunlight being reflected off them. In being reflected, that sunlight cannot reach Earth's surface - which is why the temperature falls when clouds roll in to replace blue skies. Under a continuous low cloud-deck, only around 30-60% of the sunlight is getting through. Low clouds literally provide a sunshade.

Not all clouds are such good sunshades. Wispy high clouds are poor reflectors of sunlight but they are very effective traps for heat coming up from below, so their net effect is to aid and abet global warming.

Cloud formation processes often take place on a localised scale. That means their detailed study involves much higher-resolution modelling than the larger-scale global climate models. Fourteen years on, since Ken Gregory of the dubiously-named Big Oil part-funded Canadian group, 'Friends of Science', opined on the matter (see myth box), big advances have been made in such modelling. Today, we far better understand the net effects of clouds in Earth's changing climate system. Confidence is now growing that changes to clouds are likely to amplify, rather than offset, human-caused global warming in the future.

Two important processes have been detected through observation and simulation of cloud behaviour in a warming world. Firstly, just like wildlife, low clouds are migrating polewards as the planet heats up. Why is that bad news? Because the subtropical and tropical regions receive the lion's share of sunshine on Earth. So less cloud in these areas means a lot more energy getting through to the surface. Secondly, we are detecting an increase in the height of the highest cloud-tops at all latitudes. That maintains their efficiency at trapping the heat coming up from below.

There's another effect we need to consider too. Our aerosol emissions have gone up massively since pre-Industrial times. This has caused cloud droplets to become both smaller and more numerous, making them even better reflectors of sunlight. Aerosols released by human activities have therefore had a cooling effect, acting as a counter-balance to a significant portion of the warming caused by greenhouse gas emissions.

But industrial aerosols are also pollutants that adversely affect human health. Having realised this, we are reducing such emissions. That in turn is lowering the reflectivity of low cloud-tops, reducing their cooling effect and therefore amplifying global warming due to rising levels of greenhouse gases.

It sometimes feels as if we are between a rock and a hard place. We'd have been better off not treating our atmosphere as a dustbin to begin with. But there's still a way to fix this and that is by reducing all emissions.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!


Further details

The IPCC's Sixth Assessment Report eloquently sums up where we are in our understanding of how clouds will affect us in a changing climate:

"One of the biggest challenges in climate science has been to predict how clouds will change in a warming world and whether those changes will amplify or partially offset the warming caused by increasing concentrations of greenhouse gases and other human activities. Scientists have made significant progress over the past decade and are now more confident that changes in clouds will amplify, rather than offset, global warming in the future."

The mistake made by our myth-provider, writing in 2009, was to leap to a conclusion without the information needed in order to do so. He is suggesting that clouds are inadequately represented in climate models, so they must have a negative effect on temperatures. Instead of making such leaps of faith, however, the specialists in cloud behaviour have recognised the challenges and met them square-on. We now know a lot more about clouds as a result.

In the at-a-glance section, we explain the important difference between high clouds and low clouds, agents of warming and cooling respectively. Careful examination of older satellite records has detected large-scale patterns of cloud change between the 1980s and 2000s (Norris et al. 2016). Observed and simulated cloud change patterns are consistent with the poleward retreat of midlatitude storm tracks, the expansion of subtropical dry zones and increasing height of the highest cloud tops at all latitudes. The main drivers of these cloud changes appear to be twofold: increasing greenhouse gas concentrations and recovery from volcanic radiative cooling. As a result, the cloud changes most consistently predicted by global climate models have indeed been occurring in nature.

With respect to the cooling low clouds, one particularly important area of study has involved marine stratocumulus cloud-decks. These are extensive, low-lying clouds with tops mostly below 2 km (7,000 ft) altitude and they are the most common cloud type on Earth. Over the oceans, stratocumulus often forms nearly unbroken decks, extending over thousands of square kilometres. Such clouds cover about 20% of the tropical oceans between 30°S and 30°N and they are especially common off the western coasts of North and South America and Africa (fig.1). That's because the surface waters of the oceans are pushed away from the western margins of continents due to the eastwards direction of Earth's spin on its axis. Taking the place of these displaced surface waters are upwelling, relatively cool waters from the ocean depths. The cool waters serve to chill the moist air above, making its water vapour content condense out into cloud-forming droplets.

 Satellite image of stratocumulus clouds.

Fig. 1: visible satellite image of part of an extensive marine stratocumulus deck off the western seaboard of North America, with Baja California easily recognisable on the right. Image: NASA.

With their highly reflective tops that block a lot of the incoming sunlight, the marine stratocumulus clouds have a very important role as climate regulators. It has long been known that increasing the area of the oceans covered by such clouds, even by just a few percent, can lead to substantial global cooling. Conversely, decreasing the area they cover can lead to substantial global warming.

Although many cloud-types are produced by convection, driven by the heated land or ocean surface, marine stratocumulus clouds are different. They are formed and maintained by turbulent overturning circulations, driven by radiative cooling at the cloud tops. It works as follows: cold air sinks, so that radiatively-cooled air makes its way down to the sea surface, picks up moisture and then brings that moisture back up, nourishing and sustaining the clouds.

Stratocumulus decks can and do break up, though. This happens when that radiative cooling at the cloud tops becomes too weak to send colder air sinking down to the surface. It can also occur when the turbulence that can entrain dry and warm air, from above the clouds into the cloud layer, becomes too strong.

The importance of such processes has been further investigated recently, using an ultra-high resolution model with a 50-metre grid size. (Schneider et al. 2019). Global climate models typically have grid sizes of tens of kilometres. At that resolution, they cannot detail such fine-scale processes. This model, by contrast, is able to resolve the individual stratocumulus updraughts and downdraughts.

 Results of modelling of marine stratocumulus behaviour.

Fig. 2: results of modelling of marine stratocumulus behaviour in a high-CO2 world. This one compares conditions at 400 ppm (present) and 1600 ppm (hopefully never, but relevant to the Palaeocene and Eocene when a super-Hothouse climate prevailed). Redrawn from Schneider et al. 2019.

The modelling shows how oceanic stratocumulus decks become unstable and break up into scattered cumulus clouds. That occurs at greenhouse gas levels of around 1,200 ppm (fig. 2). When that happens, the ocean surface below the clouds warms abruptly because the cloud shading is so diminished. In the model, the extra solar energy absorbed as stratocumulus decks break up, over an area estimated to cover about 6.5% of the globe, is enough to cause a further ~8oC of global warming. After the stratocumulus decks have broken up, they only re-form once CO2 levels have dropped substantially below the level at which the instability first occurred.

These results point to the possibility that there is a previously undiscovered, potentially strong and nonlinear feedback, lurking within the climate system. These findings may well help to solve certain palaeoclimatic problems, such as the super-Hothouse climate of the Palaeocene-Eocene, some 50 million years ago. It's been hard to fully explain that event, given that estimates of CO2 levels at the time do not exceed 2,000 ppm. Present climate models do not reach that level of warmth with that amount of CO2. But the fossil record presents hard evidence for near-tropical conditions in which crocodilians thrived - in the Arctic. Something brought about that climate shift!

The quantitative aspects of stratocumulus cloud-deck instability remain under investigation. However the phenomenon appears to be robust for the physical reasons described by Schneider and co-authors. Closer to the present, the recent acceleration of global warming may be partly due to a decrease in aerosols. Aerosols produce smaller and more numerous cloud droplets. These have the effect of increasing the reflectivity and hence albedo of low cloud-tops (fig. 3). It follows that if aerosol levels decrease, the reverse will be the case. Of considerable relevance here are the limits on the sulphur content of ship fuels, imposed by the International Maritime Organization in early 2015. These regulations were further tightened in 2020. An ongoing fall in aerosol pollution, right under the marine stratocumulus decks, would be expected to occur. As a consequence, the size and amount of cloud droplets would change, cloud top albedo would decrease and there would be increased absorption of solar energy by Earth. That would be on top of the existing greenhouse gas-caused global warming. James Hansen discussed this in a recent communication (PDF) here.

Cloud effects on Earth's radiation.

Fig. 3: NASA graphic depicting the relationship between clouds, incoming Solar radiation and long-wave Infrared (IR) radiation. High clouds composed of ice crystals reflect little sunlight but absorb and emit a significant amount of IR. Conversely low clouds, composed of water droplets, reflect a great deal of sunlight and also absorb and emit IR. Any mechanism that reduces low cloud-top albedo will therefore increase the sunlight reaching the surface, causing additional warming.

In their Sixth Assessment Report, the IPCC also points out that the concentration of aerosols in the atmosphere has markedly increased since the pre-industrial era. As a consequence, clouds now reflect more incoming Solar energy than before industrial times. In other words, aerosols released by human activities have had a cooling effect. That cooling effect has countered a lot of the warming caused by increases in greenhouse gas emissions over the last century. Nevertheless, they also state that this counter-effect is expected to diminish in the future. As air pollution controls are adopted worldwide, there will be a reduction in the amount of aerosols released into the atmosphere. Therefore, cloud-top albedo is expected to diminish. Hansen merely suggests this albedo-reduction may already be underway.

Last updated on 15 October 2023 by John Mason. View Archives

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Argument Feedback

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Additional viewing

To explore this complex topic further, this is a great TED talk by climate scientist Kate Marvel:

Denial101x video(s)

Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial

Additional video from the MOOC

Expert interview with Steve Sherwood

Comments

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Comments 176 to 200 out of 271:

  1. 174, lancelot,
    So while RW1 has not proposed a mechanism, others appear to have.
    I'm afraid I don't see that. Can you be more specific? The question at hand is: "Given warming that results from anthropogenic CO2, what will be the resulting cloud feedback?" While the work you discuss is interesting, it does not in any way affect the topic at hand -- which is the direction and degree of feedback from clouds in response to warming. That cloud formation is impacted by many factors is obvious. That temperatures are impacted by unexpected but measurable factors such as cosmic rays, through cloud formation, is what is under debate in current science (and on other threads here at SkS) but at the current time is not in any way supported by the evidence available. But in any event, it is not relevant to this thread, which discusses cloud feedbacks, not clouds as a primary forcing.
  2. lancelot, So while RW1 has not proposed a mechanism," The mechanism is the whole of the atmospheric water cycle (ground state water -> evaporation -> water vapor -> clouds -> precipitation -> ground state water), which is the primary way the planet's energy balance and ultimately the globally averaged surface temperature is maintained despite such a large degree of local, regional, seasonal hemispheric and even sometimes globally averaged variability. In general, mechanistically, when clouds are increasing the climate is too warm and trying to cool (more or denser clouds are exposed to space, reflecting more of the Sun's energy) and when clouds are decreasing the climate is too cool and trying to warm (fewer clouds lets in more of the Sun's energy). If water vapor is the primary amplifier of warming, as claimed, what then is the controller? If not clouds via there ability to reflect incoming solar energy and precipitate out the water from the atmosphere, then what? Does anyone think it's just a coincidence that energy from the Sun drives evaporation of water? Is it just coincidence that evaporated water removes heat from the surface, condenses to form clouds and the clouds reflect the sun’s energy? Is it just a coincidence that the water precipitated out of the atmosphere emanates from clouds?
  3. That's a rather medieval style of argumentation RW1 - leading questions and anthropomorphism! So no, the climate isn't "trying" to do anything. It responds to forcings according to the laws of physics. Likewise the answer to your repeated "is it just coincidence?" is, in each case, "no, it's physics". RW1, the "controller" is very well understood. It's temperature. The water vapour content of the atmosphere rises as the atmospheric temperature rises. That is very well understood physics, and has been measured repeatedly in the real world. No leading questions about "coincidences" required! Your "mechanism" lacks both physics and evidence. One really needs to consider the response of the atmosphere in terms of humidity. I'm sure you agree that the absolute humidity of the atmosphere has risen as a result of greenhoue-induced warming. That's uncontestable - we can measure this. How about "relative humidity"? There is some evidence that the water vapour content of the atmosphere might maintain approximately constant relative humidity in a warming world. In such a situation there's no reason to expect the levels of clouds to increase since the atmosphere can maintain increased water vapour without enhanced condensation into clouds. If the water vapour content of the atmosphere rises (i.e. the observed increase in absolute humidity) at a level that is somewhat lower than required to maintain constant relative humidity, then the formation of clouds is more likely to be suppressed. That's consistent with the so far rather limited real world evidence. There's zero real world evidence that clouds increase in a warming world. On the contrary, the limited evidence supports a small positive cloud feedback in a warming world (see the top article of this thread).
  4. 177, RW1, Your "intelligent design" approach which requires that the earth's climate system be constructed and constrained like an electrical engineering project imposes any number of requirements (such as the idea that there be a single controller) which are quite simply fabricated. Your "coincidences" are mere observations of known physics. You imply that because they exist, they must be primary factors in the system, but you provide no evidence of such. You imply that because you haven't bothered to list any other mechanisms here, they cannot exist or must be inconsequential. Your position amounts to argument from ignorance. Your argument boils down to "I can only envision things happening this one way, so how can it be anything else?" Your position amounts to recognizing that you cannot touch the sun but it moves through the sky, therefore it must be the flaming wheel of the sky god's chariot. How could it be anything else?
  5. The climate system's energy balance is extremely dynamic. It's constantly changing spatially and in time - all the time, yet long-term averaged it's very tightly constrained. The global temperature anomaly barely moves by more than a couple tenths of a degree per year despite such variability. This is hardly consistent with net positive feedback on imbalances, let alone net positive feedback of 300%. Water vapor and clouds are also not a homogeneous distribution - they are by far most dynamic component of the entire atmosphere. The notion that the feedbacks of water vapor and clouds are both strong net positive cannot be reconciled with physical process and feedbacks that so tightly constrain planet's energy balance as a whole.
    Response:

    [DB] Fixed text.

  6. RW1 given an incoming heat flux, it is the emission of radiation that mainly constrains temperature. Why on earth the Stefan-Boltzman law, the most important negative feedback, is so often forgotten?
  7. Riccardo, I assume you're talking about the so-called 'Planck' response of about 3.3 W/m^2 per 1 degree of warming?
  8. RW1 wrote: "The global temperature anomaly barely moves by more than a couple tenths of a degree per year..." Which, is a massive amount of heat. You dismiss the idea of positive feedbacks (which BTW are estimated at a 200% increase on the forcings - 300% would be the combined total) because the average annual temperature anomaly is so 'constrained', but that is a completely groundless argument. I could just as easily claim that the massive average annual temperature anomaly shows how large the positive feedbacks must be. In reality, there is no direct connection between the two and thus the argument is meaningless. We know feedbacks are positive because every past and current warming forcing which we can estimate with any degree of accuracy has been enhanced by positive feedback effects. Frankly, it is ridiculous to claim that feedback effects will not be positive given that measurements of forcing and feedbacks from every El Nino, every Summer, and indeed every sunrise prove otherwise. The sun comes up, temperatures rise, dew evaporates, atmospheric humidity increases, that water vapor causes further warming... positive feedback. Happens all the time and not remotely controversial. The idea that this would suddenly just 'stop' happening if the warming comes from increasing atmospheric CO2 levels is without any justification at all.
  9. RW1 the so-called Plank sensitivity or response is derived from the Stefan-Bolzman law.
  10. CBDunkerson, Are you aware that the system is almost always in a state of energy imbalance to some degree? Have you noticed that even when the system as whole warms to a significant degree like during an El Nino event, it always seems to revert to its pre-equilibrium state fairly quickly afterward? This is hardly consistent with net positive feedback acting on perturbations. Also, the temperature rising as the Sun comes up each day is not the result of net positive feedback acting on the increased incident energy. You seem to be confusing a causative positive response to an increase in 'forcing' with positive feedback in response to the causative change. They are two different things. No one disputes that increased energy from GHGs or the Sun will cause warming.
  11. Riccardo (RE: 185), "the so-called Plank sensitivity or response is derived from the Stefan-Bolzman law." OK, explain to me how it's derived? Why the 'Planck sensitivity' 3.3 W/m^2 per 1 degree of warming?
    Response:

    [DB] "OK, explain to me how it's derived?"

    Try here:

    http://scienceofdoom.com/2010/10/24/planck-stefan-boltzmann-kirchhoff-and-lte/

  12. RW1 try here, but let's not go offtopic here and choose a more appropiate place to discuss sensitivity.
  13. Riccardo (RE: 187), What I'm asking is why specifically is it 3.3 W/m^2 per 1 degree of warming? Why not 0.33 W/m^2 or 33 W/m^2 (some other number), for example?
  14. 188 RW1 The earth radiates energy as E=epsilon*sigma*T^4, where epsilon is the emissivity (0.61) and sigma is the Stefan-Boltzman constant. If you increase the average earth surface temperature (287K) by one degrees you'll increase E by 3.3 W/m^2
  15. RW1 - "What I'm asking is why specifically is it 3.3 W/m^2 per 1 degree of warming? Why not 0.33 W/m^2 or 33 W/m^2 (some other number), for example?" I'm more than a bit appalled by this question. You look at the physics, you run the numbers, and you get a result. The only reason the number would be different is if the physics were somehow different, RW1. But reality is a harsh critic.
  16. RW1, You have now demonstrated extreme ignorance on climate science (a complete failure to understand Stefan-Boltzmann, the Planck response as a feedback, and the way that we know that this response is 3.3 W/m2 per degree K). At this point, you must recognize the need to study and learn a whole lot more before you "authoritatively" comment any further on things that you clearly do not grasp.
  17. RW1 wrote: "Have you noticed that even when the system as whole warms to a significant degree like during an El Nino event, it always seems to revert to its pre-equilibrium state fairly quickly afterward? This is hardly consistent with net positive feedback acting on perturbations." So... because the daily cycle of temperature fluxes due to sunrise and sunset returns to its 'equilibrium' state in short order this is evidence against Summer temperatures being warmer than Winter temperatures? That is essentially the 'logic' you are presenting... 'if A shows steady fluctuations around a flat baseline then B cannot possibly show long term increases'. "Also, the temperature rising as the Sun comes up each day is not the result of net positive feedback acting on the increased incident energy." In part, yes... it is. Most of the temperature increase is due to the increased radiation. However, water vapor feedback also plays a part... as anyone who has lived in a very humid area would be able to tell you. A climate 'forcing' is just a change from a baseline. Most often we choose baselines from on yearly averages. However, it is perfectly valid to look at the incoming solar radiation over the course of a day with the daily average as the baseline. In such case, sunrise would represent a massive warming forcing. We can then look at feedback responses to this forcing... specifically, that warming forcing from sunlight will cause water to evaporate. Water vapor is a greenhouse gas. Ergo, this will create a positive feedback. Further, it is a positive feedback that we have known about and studied for centuries. We use the term 'relative humidity' because the amount of water vapor the atmosphere can hold increases as the temperature does... so the relative humidity is the percentage of the possible maximum water vapor content of the atmosphere for the current temperature. If the temperature goes up the amount of water vapor the atmosphere can hold increases as well. A long understood and thoroughly measured phenomenon. Now, if the temperature of the entire planet goes up... why wouldn't the same feedback apply? If the planet's atmosphere is warmer it will hold more water vapor. That water vapor will produce more greenhouse warming. You argue that 'the notion' of positive feedback from water vapor cannot be reconciled with 'tightly constrained' temperature variations in various other cycles (e.g. ENSO, seasons, day/night, 11 years solar cycle, et cetera). I am saying that there is no logical way to conclude that water vapor will not be a positive feedback, because it is always a positive feedback to warming.
  18. lanC (RE: #189), "The earth radiates energy as E=epsilon*sigma*T^4, where epsilon is the emissivity (0.61) and sigma is the Stefan-Boltzman constant. If you increase the average earth surface temperature (287K) by one degrees you'll increase E by 3.3 W/m^2" Correct. Now, why specifically is the emissivity 0.61?
  19. CBDunkerson (RE: #192), "You argue that 'the notion' of positive feedback from water vapor cannot be reconciled with 'tightly constrained' temperature variations in various other cycles (e.g. ENSO, seasons, day/night, 11 years solar cycle, et cetera)." No, I'm arguing that positive water vapor feedback in conjunction with positive cloud feedback cannot be reconciled with how the planet's energy balance is so tightly - but dynamically, maintained.
    Response:

    [DB] "I'm arguing that positive water vapor feedback in conjunction with positive cloud feedback cannot be reconciled with how the planet's energy balance is so tightly - but dynamically, maintained."

    The casual reader will note that, in the absence of doing the physics and showing your work, your statement is devoid of substance and amounts to empty assertion without merit.

  20. DB, Is net negative feedback required for basic stability and maintenance of the current energy balance or not?
    Response:

    [DB] Your question presumes that there is a current energy balance at the TOA; the best understanding of the science indicates otherwise. 

    Energy in ≠ energy out (energy in exceeds energy out).

    You also do not define what physical process you refer to that is providing the "net negative feedback required for basic stability and maintenance".  Energy balance is the sum of physical processes, not hypothetical ones.

  21. DB, "Your question presumes that there is a current energy balance at the TOA; the best understanding of the science indicates otherwise." No, my question presumes the opposite - that there is virtually always an energy imbalance of some degree, especially over the more shorter time scales that the dynamic physical processes of water vapor and clouds operate. The imbalance is either energy in > energy out or energy in < energy out. You are aware that the system is almost never in perfect equilibrium at any given time or over any given time period, right? "You also do not define what physical process you refer to that is providing the "net negative feedback required for basic stability and maintenance". Energy balance is the sum of physical processes, not hypothetical ones." Yes, the planet's energy balance is the net result of all the physical processes (and feedbacks) in the system. Do you agree the net feedback that operates on all the physical processes in the system - whatever they actually are, is negative?
    Response:

    [DB] With all due respect, the last time we discussed this several threads ago, you first need to learn more about the physics of climate science before trying to bend them to your will.  To that end I also recommend Raymond Pierrehumbert's textbook "Principles of Planetary Climate".

    In any event, given the nature of your questions here, it is evident that you have not yet pursued that option.  That is your choice.  But that leaves you on your pre-existent orbit of asking the same questions repeatedly in the hopes that one day you might find an answer more to your liking.  All in all a non-effective path to greater understanding and a waste of other's time.  And indicative of your earlier statement that you were "not here to learn anything"

  22. RW1, you seem rather desperate to demonstrate a net negative feedback for the whole climate system, not just clouds, yet how do you explain palaeoclimatic variations with a net negative feedback? Leprechauns? Positive feedback need not equal runaway feedback.
  23. DB, I already have Ray's book and have read a lot of it. I guess you're not interested in answering my questions?
    Response:

    [DB] If full understanding of the text was not achieved then perhaps you should place those questions over at RC where Ray has a presence.

  24. RW1, Net positive feedback does not lead to runaway warming (Venus result) anymore than net negative feedback leads to Snowball Earth. So, the answer to your question is no. If you do not understand why, you did not understand what you read.
  25. Bibliovermis, I know net positive feedback does not lead to runaway warming. "So, the answer to your question is no." Which question?

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