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Climate Sensitivity: Feedbacks Anyone?

Posted on 28 August 2011 by James Wight

Climate modeling is notoriously complex, but it all boils down to one key question: How sensitive is the Earth’s climate to perturbations like an increase in the greenhouse effect?

“Climate sensitivity” describes the amount of global warming you get from a specified forcing once all climate feedbacks are taken into account. A forcing is something that changes the Earth’s energy budget: the difference between the amount of energy entering the Earth system and the amount leaving it. If the energy budget is in balance, the Earth’s temperature is stable. A forcing creates an energy imbalance, causing global temperature change until the system gets back in balance. Forcings can be quantified in watts per square metre (W/m2), and causes can include variation in sunlight or the Earth’s orbit, surface reflectivity (“albedo”), and the greenhouse effect. Climate sensitivity is usually expressed in degrees per doubling of atmospheric carbon dioxide (CO2), a forcing of about 4 W/m2.

A recently published paper by Hansen and Sato (2011), Paleoclimate Implications for Human-Made Climate Change, examines evidence from past climates about the various feedbacks that affect climate sensitivity. A feedback is a mechanism that either amplifies (positive feedback) or dampens (negative) the initial effect. Interest is a feedback on a loan. If there were no feedbacks in the Earth’s climate system, physics tells us climate sensitivity would be 1.2°C for a doubling of CO2. In reality, a complex array of interacting positive and negative feedbacks come into play. Climate models include “fast feedbacks” like water vapor, clouds, sea ice, and aerosols (reflective particles that hang in the atmosphere), but exclude longer-term “slow feedbacks” like ice sheets (an icy surface reflects more heat than a dark surface) and greenhouse gases (warming releases gases from the oceans, melting permafrost, etc).

Fast feedbacks

There is a broad consensus that fast-feedback sensitivity is 3°C for doubled CO2. In other words, fast feedbacks multiply the 1.2°C direct warming by two-and-a-half. Model estimates come with large error bars that have proven difficult to reduce as climate models have become more realistic over the decades, because modeling all the positive and negative feedbacks is so complicated. However, the paleoclimate record allows us to circumvent that problem, as past climate changes obviously included all existing feedbacks. Climatologists study past climates with climate proxies like ice cores and sediment cores.

The most accurately known climate changes are the ice age cycles of the last few hundred millennia, recorded by ice cores and ocean sediment cores. During that time the Earth oscillated from brief “interglacial” periods like today, when ice sheets are confined to Antarctica and Greenland; to long “glacial” periods when global temperature plunged by 5°C, ice sheets covered much of Canada and Europe, and sea level fell over 100 metres. The forcings largely driving these climate swings were surface albedo and greenhouse gases – themselves slow feedbacks on tiny orbital forcings sustained over long periods, but for the purpose of finding fast-feedback sensitivity they are considered forcings. The sensitivity Hansen and Sato derive is 3°C, exactly as the models predicted.

A landmark 1979 report by the National Academy of Sciences, whose lead author was Jule Charney, defined climate sensitivity as including the fast feedbacks of water vapor, clouds, and sea ice. This gave us the term “Charney sensitivity”, but its meaning varies: it can encompass all fast feedbacks, or it can exclude feedbacks Charney excluded, such as aerosols. Hansen and Sato make a distinction between the “all-fast-feedback sensitivity” (Sff) and a definition excluding aerosol feedbacks (Sff-a). They argue Sff is more useful as it can be measured with much greater precision from paleoclimate. If aerosols are counted as a forcing then the uncertainty is very large, because we don’t know how aerosols changed in glacial periods. If they are counted as a feedback then the answer is 3.0±0.5°C, consistent with but more precise than the IPCC model value of 3.0+1.4/–0.9°C.

hansen sato 2011

Figure 1: Implied fast-feedback climate sensitivity from ice age transition measurements.  0.75°C per W/m2 corresponds to a fast feedback sensitivity of 3°C for doubled CO2 (Source: Hansen and Sato 2011).

Slow feedbacks

But in the long run, what will be important is the climate sensitivity including fast and slow feedbacks. Slow feedbacks have received far less attention. Current models don’t take them into account, so paleoclimate is the only available tool to estimate them. Again, Hansen and Sato use the glacial-interglacial cycles. Though earlier we counted ice sheets and greenhouse gases as forcings in the cycle, both were actually slow positive feedbacks on tiny energy imbalances, lagging behind the initial temperature increase by centuries. The large magnitude of the glacial-interglacial swings tells us that slow feedbacks are large and positive on millennial timescales.

The term “Earth system sensitivity” is sometimes used for slow-feedback sensitivities, but as with Charney sensitivity, usage varies. Hansen and Sato propose a series of definitions including different combinations of feedbacks:

  • Sff+sur – all fast feedbacks plus surface albedo feedbacks
  • SCO2 – all fast feedbacks plus surface albedo and non-CO2 greenhouse gas feedbacks
  • Sff+sf – all fast feedbacks plus surface albedo and all greenhouse gas feedbacks

Surface albedo feedbacks

Sff+sur is the long-term sensitivity to a specified greenhouse gas forcing. It is useful for cases where greenhouse gases are the initial cause, as with anthropogenic global warming. When using this definition, any greenhouse gas feedbacks must be calculated by separate carbon-cycle models.

The dominant surface albedo feedback is ice sheet area (though there is some contribution from vegetation cover). This positive feedback is not stable over geologic time, because it only works when the planet is cool enough to form ice sheets. Earth was ice-free for most of its history; in such times albedo feedbacks must have been near-zero, and Sff+sur about the same as Sff. The Earth has cooled dramatically in the last 50 million years due to geologic CO2 change. The current ice age began with the formation of the Antarctic ice sheet 35 million years ago. The Northern Hemisphere ice sheets emerged in the last few million years. As the world cooled, orbit-forced climate oscillations became greater and greater, as increasing ice increased the ice albedo feedback. The more ice on the planet, the more sensitive the climate.

The same glacial cycles Hansen and Sato used to estimate Sff also tell us about Sff+sur during that time. About half of the forcing was from ice sheet feedbacks and half from greenhouse gas feedbacks. Since here we’re defining greenhouse gases as a forcing and ice sheets as a feedback, Sff+sur must be about twice Sff, so Sff+sur = 6°C. Albedo feedbacks were approximately linear in those cycles, so the 6°C sensitivity applies to the range of climate states between interglacial and glacial.

But we are pushing the climate not towards glacial conditions, rather the opposite direction, in which the ice albedo feedback shrinks and eventually vanishes. So Sff+sur is less than 6°C for positive forcings relative to today, perhaps only ~4-5°C for climates between now and 3 million years ago. However, for a positive forcing just enough to melt the Antarctic ice sheet, there is a large nonlinear albedo feedback. Hansen and Sato explore this matter further in their 2008 paper Target Atmospheric CO2: Where Should Humanity Aim? The current global temperature and total required forcing (assuming Sff = 3°C) are about halfway between the temperature 35 million years ago and in recent glacial periods. So averaged over the climate range between today and 35 million years ago, Sff+sur works out to be almost 6°C. The ice albedo feedback is still in play.

Greenhouse gas feedbacks

SCO2 is the long-term sensitivity to a specified CO2 forcing. Levels of non-CO2 greenhouse gases are not readily measured by proxies, so in paleoclimate studies it is easier to count them as feedbacks than forcings. These gases include methane (CH4) and nitrous oxide (N2O), and they are thought to be a net positive feedback.

CO2 constituted three-quarters of the greenhouse gas forcing between glacial and interglacial. Since here we’re defining CO2 as a forcing and other greenhouse gases as feedbacks, SCO2 must be about a third higher than Sff+sur, so SCO2 = 8°C within that climate range. With ice sheet feedbacks varying as before, SCO2 to positive forcings from today’s climate must be: ~5-6°C for small forcings, almost 8°C averaged over the period of ice sheet loss, and 4°C on an ice-free planet.

Ultimate sensitivity

Sff+sf is the “ultimate Earth system sensitivity” including all feedbacks. It is relevant to forcings which are completely external to the Earth system. In principle it applies to orbital cycles, though figuring out the calculations is tricky because the regionally varying nature of orbital forcing is not fully understood. Sff+sf cannot be accurately estimated from paleoclimate data, but must be extremely large for negative forcings from the current climate state.

Sff+sf is the only one of Hansen and Sato’s definitions which includes CO2 feedbacks. At the moment the carbon cycle is acting as a negative feedback, as oceans and vegetation are removing some of our CO2 emissions, but as global warming continues, those carbon sinks are expected to fill up and start emitting CO2. Like other greenhouse gases, CO2 was a large positive feedback in the glacial-interglacial cycles. Eventually, excess CO2 is removed from surface reservoirs by a negative weathering feedback, but this takes hundreds of millennia.

Transient climate response

All the above terms (Sff-a, Sff, Sff+sur, SCO2, Sff+sf) are possible definitions of equilibrium climate sensitivity (ECS). They all refer to the final amount of warming after accounting for climate inertia and feedbacks; the differences between them arise from different decisions about which processes to count as forcings or feedbacks. You may also sometimes hear about transient climate response (TCR), which the IPCC defines as the temperature increase at the time of CO2 doubling if CO2 increases by 1% per year. The IPCC estimates TCR is ~2°C, with a model range of 1-3.5°C.

How slow are slow feedbacks?

In the glacial-interglacial cycles slow feedbacks took millennia, but perhaps that was only because orbital forcing changed very slowly. The peak sea level rise rate occurred at the same time (within measurement error) as the peak forcing, suggesting ice sheets could melt faster if the climate changed faster. Sea level rises of several metres per century were not uncommon in past deglaciations; it is the present stability that is unusual.

Mainstream projections of sea level rise for 2100 are ~1-2 m (substantially higher than the 18-59 cm in the last IPCC report). An upper limit of 2 m by 2100 has been proposed, but this is based on the questionable assumption that glaciers will not move faster than their fastest speed in recent decades. In Antarctica, an amount of ice worth 20-25 m is rooted below sea level and held back by ice shelves. Hansen and Sato argue that satellite data show ice sheet loss occurring exponentially, with a doubling time of perhaps a decade. If this trend continues sea level could rise 5 m within a century; however, exponential ice loss is limited by the temporary negative feedback of regional iceberg cooling.

The response time of ice sheets is shorter than the negative weathering feedback which removes excess CO2. So we can expect the slow feedback response to eventually be realized: several degrees of warming will ultimately lead to tens of metres of sea level rise and double the original warming. Post-2100 sea level rise might seem a long way off, but it will be determined by policy decisions taken in the near future. Once an ice sheet begins to collapse there is no way to stop it sliding into the ocean. We would be subjected to centuries of encroaching shorelines.

What it all means

The exact value of climate sensitivity depends on which feedbacks you include, the climate state you start with, and what timescale you’re interested in. While the Earth has ice sheets the total climate sensitivity to CO2 is up to 8°C: 1.2°C direct warming, 1.8°C from fast feedbacks, 1°C from greenhouse gas feedbacks, and nearly 4°C from ice albedo feedbacks. The slow feedbacks have historically occurred over centuries to millennia, but could become significant this century. Including CO2 itself as a feedback would make climate sensitivity even higher, except for the weathering feedback which operates on a geologic timescale.

As is explained in Target Atmospheric CO2, 4 W/m2 of greenhouse gas forcing sustained long enough would ultimately return the Earth to an ice-free state, raising the global sea level by 75 metres. The preindustrial level of atmospheric CO2 was ~275 ppm, so 4 W/m2 would be the effect of doubling the CO2-equivalent of all greenhouse gases (CO2e) to 550 ppm, or increasing CO2 to ~450 ppm with other greenhouse gases responding as feedbacks. Currently CO2 is at 390 ppm and rising; CO2e levels are at 470 ppm and counting; implying significant feedbacks are already in the pipeline. However, we may still be able to prevent them if we can get the Earth back in energy balance by reducing atmospheric CO2. In practical terms, that means cutting global CO2 emissions to near-zero as soon as possible.

Contrarians often argue the paleoclimate record shows CO2 and climate change are nothing to worry about. What it actually tells us is the climate system is extremely sensitive to perturbations – and we are running out of time to prevent the global warming we started from spiraling out of our control.

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Comments

Comments 1 to 28:

  1. There are different views about what the climate system is. And what can be considered as an external forcing to it depends on the definition of system.

    The terminology that equates "climate sensitivity" with equilibrium response of the climate system to atmospheric CO2 concentration implies such a definition of the climate system that excludes CO2 concentration from variable components of the climate system. If we also treat ice sheets and vegetation as fixed (rather than variable), we can say that we exclude them from the active components of the climate system. We can construct a coupled system of the physical (but not biogeochemical) atmosphere-ocean system (including part of the cryosphere such as seasonal snow cover and sea ice). Then, the sensitivity of this climate system to CO2 concentration as an external forcing, i.e. "fast-feedback sensitivity" in James Wight's text, is meaningful both within science and as a piece of information to be referenced in real-world applications.

    In the glacial cycles, however, atmospheric CO2 concentration varied, apparently in response to temperature (as well as theoretically certainly forcing to it), and it responded apparently faster than the ice sheet did. So if we include the ice sheet as a variable component of the climate system, it is awkward to treat CO2 concentration as if an external forcing and to discuss the sensitivity.

    It seems interesting and meaningful as an exercise internal to climate system science to discuss the equilibrium response of the climate system to the change of one of the internal component of the system hypothetically held constant for an indefinitely long time. I doubt that it can be evaluated by simply comparing the values of alleged forcing and alleged response in the paleoclimatic reconstructions. I think that more careful discussion is needed both in theoretical considerations and in interpretations of observational data. Even then, I do not think that such scientific exercise is directly very useful to real-world application. Just insights gained by such exercise will be useful.
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  2. An implication of my previous comment: While the new paper by Hansen and Sato contains good points internal to science, I do not recommend following it as if it were a standard of how the climate system (including slow component) is sensitive to forced alteration of the atmospheric CO2 concentration.
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  3. Kooita Masuda,

    I'm not sure you're understanding the distinction between a forcing and a feedback well, so I in turn am unsure how to evaluate your logic.

    A feedback is something that responds to a forcing.

    A forcing is initiated outside of the climate system.

    That CO2 can be a forcing and a feedback, under different circumstances, is in no way inconsistent. What matters in the distinction is not the mechanism, but how the mechanism is initiated. Clearly, human burning of long sequestered and naturally unreachable fossil fuels is a forcing, as is (in the broadest sense) the addition of massive amounts of CO2 to the atmosphere through weathering or extremely active volcanic activity.

    Conversely, CO2 changes that are themselves a response to temperature changes are clearly a feedback, and not a forcing.

    On your own recommendation, I do not see that your logic follows. Hansen and Sato's comparisons to the current climate system, and warnings of the implications that past climate changes imply, are to me all valid points worth serious consideration.

    You have not in anyway established a valid basis behind your contention that you "do not recommend following it as if it were a standard of how the climate system (including slow component) is sensitive to forced alteration of the atmospheric CO2 concentration."

    I think the paper stands well as it is, and needs to be taken very, very seriously by anyone who doesn't want to simply deny any unsettling conclusions.
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  4. For the last 5,000 years (or more) the Arctic sea ice has been a constant ice sheet over most of the Arctic Ocean. We can see by the current sea surface temperature in the Arctic that this sheet is melting and the temperature is rising rapidly, as Hansen and Sato predict. We do not need to wait for Greenland and the Antarctic to melt for this effect. The Arctic sea ice melt is already decreasing the albedo of the Earth (in addition to the decrease in snow cover in summer). In the fall of 2010 the increase in Arctic temperatures from this effect raised the average temperature of the Earth and contributed to the record temperature for that year. As the Arctic continues to melt this effect will grow.
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  5. This was an excellent post, thanks for the summary of the Hansen & Sato paper.

    I have thought about this issue of forcings vs. feedbacks for quite some time, and especially in the case of CO2, it seems the distinction as the whether it is a forcing or a "hyper" positive feedback in the case of anthropogenic release of CO2 seems not to really matter. One could argue, for example, that the rise of human civilization was a result of this particular interglacial, and as such, our release of extra CO2, is simple a biogenic effect on climate not unlike the relationship between plankton and DMS, but simply a positive rather than negative feedback. Homo sapiens, during this particular interglacial, as a biological entity on earth, were primed for civilization to arise. Had the interglacial never occurred, with the subsequent rise in farming and agriculture, it is doubtful that we would have gotten to the point of being able to advance our civilization enough to have the ability to release large amounts of carbon into the atmosphere. In this sense, with the climate as a system on the edge of chaos, some little change from one interglacial to another (like a species being ready for the advance of civilization) is simply a outlier, or black-swan event that takes this particular interglacial in an entirely different direction. In a very real sense then, the Milankovitch cycle leading to this interglacial, was the trigger that brought about the initial conditions, which through initial positive feedbacks of CO2, ice cover reduction, etc. lead to a "hyper" positive feedback situation which seems to be entirely changing the nature of this particular interglacial.
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  6. With regard to the glacial to interglacial sensitivity, one can’t equate the positive feedback effect of melting ice from that of leaving maximum ice to that of minimum ice where the climate is now. There just isn’t much ice left, and what is left would be very hard to melt, as most of is located at high latitudes around the poles which are mostly dark 6 months out the year with way below freezing temperatures. A lot of the ice is thousands of feet above sea level too where the air is significantly colder too. Unless you wait a few 10s of millions of years for plate tectonics to move Antarctica and Greenland to lower latitudes (if they are even moving in that direction), no significant amount of ice is going to melt from just a 1 C rise in temperature. Furthermore, the high sensitivity from glacial to interglacial is largely driven by the change in the orbit relative to the Sun, which changes the angle of the incident energy into the system quite dramatically. This combined with positive feedback effect of melting surface ice is enough to overcome the strong net negative feedback and cause the 5-6 C rise. But we are very nearing the end of this interglacial period, so if anything the orbit has already flipped back in the direction of glaciation and cooling.
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    Response:

    [DB] "There just isn’t much ice left, and what is left would be very hard to melt, as most of is located at high latitudes around the poles which are mostly dark 6 months out the year with way below freezing temperatures."

    Tell that to the Arctic Sea Ice, which has lost more than 50% of its thickness in the past decade alone, or to the GIS which continues to lose mass at an accelerating rate.

    "But we are very nearing the end of this interglacial period, so if anything the orbit has already flipped back in the direction of glaciation and cooling."

    Unsupported assertion (prove it).  By all means, show us where in the past where there has been a CO2 excursion such as mankind has introduced into the carbon cycle in the past 150 years (at a rate 10 times higher than occurred during the PETM).

    Furthermore, evidence is already accumulating that we not only have already skipped the next glacial cycle, but will soon have managed to prevent the next 5 glacial cycles.

    So much for the "cooling" direction.

  7. "Model estimates come with large error bars that have proven difficult to reduce as climate models have become more realistic over the decades, because modeling all the positive and negative feedbacks is so complicated. However, the paleoclimate record allows us to circumvent that problem, as past climate changes obviously included all existing feedbacks."

    I really doesn't, as I've illustrated in comment #6.
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    Response:

    [DB] "I really doesn't, as I've illustrated in comment #6."

    Not hardly.

  8. DB,

    "Tell that to the Arctic Sea Ice, which has lost more than 50% of its thickness in the past decade alone, or to the GIS which continues to lose mass at an accelerating rate."

    OK, calculate the increase in absorbed solar energy from the melting Artic ice that has occurred and explain how the 'feedback' caused this much melting occur, and then show how this effect is proportional to that which occurs when the planet leaves maximum ice. You should also explain why the 'feedback' hasn't caused any melting in Antarctica.
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    Response:

    [DB] It is rather simplistic thinking to compare the Arctic, a maritime region largely comprised of ocean, with the Antarctic, a glaciated plateau, and expect them to react similarly to this early phase of AGW.  And you wrong in implying that Antarctica is losing ice (hint: both the WAIS and the EAIS are losing mass).

    So for starters, try reading these: Ice isn't melting and What causes Arctic amplification?

    An attitude of actually trying to learn about things and asking questions instead of speaking negatively about subjects that it is apparent you lack full understanding about will get better results.  But that would imply that you are here to learn, which is something that you have already stated that you are not here to do...

  9. RW1#8: "calculate the increase in absorbed solar energy"

    You might consider a different strategy on this question this time around. Rather than your calculation-based approach -- which is, in all reality, just a model with its own simplifying assumptions -- take a look at what has actually happened.

    Perovich et al 2008

    There was an extraordinarily large amount of ice bottom melting in the Beaufort Sea region in the summer of 2007. Solar radiation absorbed in the upper ocean provided more than adequate heat for this melting. An increase in the open water fraction resulted in a 500% positive anomaly in solar heat input to the upper ocean, triggering an ice–albedo feedback and contributing to the accelerating ice retreat. The melting in the Beaufort Sea has elements of a classic ice–albedo feedback signature: more open water leads to more solar heat absorbed, which results in more melting and more open water. The positive ice–albedo feedback can accelerate the observed reduction in Arctic sea ice.

    This is what is happening: No models needed. The feedback is there; a baseline sensitivity already exists. Have a good long listen to Denning's video: Arguing about a few decimals of sensitivity almost seems silly at this point.
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  10. DB,

    "It is rather simplistic thinking to compare the Arctic, a maritime region largely comprised of ocean, with the Antarctic, a glaciated plateau, and expect them to react similarly to this early phase of AGW."

    Sorry, I mean Antarctic sea ice extent.
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    Response:

    [DB]

    Try Is Antarctica gaining or losing ice? for a discussion on Antarctic sea ice changes and the causes of the same.

    BTW, Antarctic Sea Ice now (when it should be at maximum) is currently at a low point:

    AASI

    [Source]

    Let us get back to the topic of this thread, Climate Sensitivity: Feedbacks Anyone?.

  11. Hansen and Sato's paper was excellent and Hansen writes in a very straight forward way with no need to impress. He uses technical terms only when they are appropriate and this makes his papers very readable.

    While the sceptics go crazy about the models, they tend to forget that the predictions of AGW are based more on the temperature response to delta CO2 in the past. To disprove climate change, you have to show why this is no longer valid in today's world. The only difference is the speed of the change which the models can be very helpful in deducing.

    In short, the physics says what the physics says.
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  12. michael sweet@4:
    Your point about Arctic Ice is not correct.

    Study of north shores of Greenland

    Paleo study of Greenland North Beach


    Holland also had a very extensive study on Arctic Ice, but it seems the url to the paper is no longer viable.

    There are also paleo studies of bow head whales that dispute that the Arctic Ice has been stable for the past 7,000 years.

    In fact, it has flucuated dramatically.
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  13. 12, Camburn,

    Your own link belies your statement, and you provide no other supporting links.

    Michael Sweet said (correctly):
    For the last 5,000 years (or more) the Arctic sea ice has been a constant ice sheet over most of the Arctic Ocean.
    Your link said:
    ”The climate in the northern regions has never been milder since the last Ice Age than it was about 6000-7000 years ago. We still don’t know whether the Arctic Ocean was completely ice free, but there was more open water in the area north of Greenland than there is today,” says Astrid Lyså, a geologist and researcher at the Geological Survey of Norway (NGU).


    See the difference? 5,000 years versus 6,000-7,000?

    Coincidentally, temperatures have been dropping for the last 7,000 years, which marked the Holocene Climactic Optimum, when temperatures were approximately roughly equal to what they are today. So evidence that the Arctic had less ice 7,000 years ago is further evidence of current warming, what that warming will do, and the fact that warming above and beyond that will have unheard of effects.

    It's all consistent... all except your personal interpretation of it, that is.
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  14. Excellent article. Contrarians often comfort themselves with by suggesting that climate sensitivity is about 1C per doubling and that you can only get to higher sensitivities through "hypothesised positive feedbacks". They don't appreciate the physical reality of the feedbacks, and that they are not only real, present and active today, but also absolutely necessary to explain past climate change. You cannot explain climate with negative net feedbacks. Ice-albedo feedback seems a very appropriate topic as we head below 5 million sq km IJIS extent in Arctic for only the 4th time, which is the 4th time in 5 years. Late summer Arctic sea ice volume is also desperately low, and with futher reduction, extent loss is certain and so will be a further increase in the albedo feedback.
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  15. Ice-albedo feedback due to sea ice and snow cover is (in principle) included in fast-feedback sensitivity or "Charney sensitivity". Ice-albedo feedback due to continental ice sheets and albedo feedback due to vegetation are considered as slow feedbacks.
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  16. Ice-albedo feedbacks should be included in fast-feedback sensitivity as the absorption / reflection of solar radiation is immediate. There is some indication that the high latitude albedo effects are more localized than the ice-albedo feedbacks associated with continental ice, as witnessed by local temperature changes.

    I agree with Sphaerica (a rarity) about the 5000 vs 7000 year distinction about the Arctic sea ice. Although the sea ice has been present for 5000 years, it has been anything but constant. (DB, I think you are confusing sea ice extent with sea ice anamoly in your Antarctic graphic).

    Vegetative albedo feedbacks are slow and highly variable by comparison, and when combined with the absorption feedback results in a net negative.
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    Response:

    [DB] You may want to check the link...both are down.  Just sayin'.

    SH Sea Ice

  17. 16, Eric the Red,
    Although the sea ice has been present for 5000 years, it has been anything but constant.
    I do not agree with this statement. Do you have any citations to support this assertion? Can you quantify the variability that you conjecture has occurred?
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    Moderator Response:

    [DB] EtR must be acknowledging the unprecedented loss of Arctic Sea Ice ongoing in today's time (unprecedented over the past 5,000 years)...

  18. “Speaking of Arctic Sea Ice volume...

    Combining ice thickness with sea ice area gives the total sea ice volume. At present, researcher­s cannot measure volume directly, so they estimate the volume with computer models. The University of Washington­'s Pan-Arctic Ice Ocean Modeling and Assimilati­on System (PIOMAS) model combines data on sea ice concentrat­ion with models of ocean and atmospheri­c conditions to estimate total ice volume. Sea ice volume normally changes with the seasons, but monthly estimates through July 2011 show that the volume for each month has tracked well below the 1979 to 2010 average, and below the volume for 2007, which saw the record low ice extent. PIOMAS projects that this year's minimum volume in September will very likely finish below 2007 and could even reach a record low volume.

    Source: Arctic sea ice at the crossroads­, NSIDC, Aug 16, 2011

    http://nsi­dc.org/arc­ticseaicen­ews/”
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  19. #15, as ice loss accelerates (as has been observed in the arctic), the albedo feedback increases. It is not a static value. Additionally, large losses are spreading into the summer months of high insolation, further accelerating the feedback when open water is exposed to strong sun. When ice loss decelerates, probably when the ice is largely gone altogether, the ice-albedo feedback value will reduce of course.

    #18 Badgersouth: To add to your comment, the low PIOMAS volume to beat is 2010, which was significantly lower than 2007 (4428 cubic km compared to 2007's 6458 cubic km). The volume as of the 31st July, the last updated value, was 6494 cubic km, meaning that 2011 probably passed 2007's minimum according to PIOMAS on the 1st August. 2011's 31st July volume is ~500 cubic km below 2010. Source.
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  20. No. My contention was that Arctic sea ice has not been constant throughout the previous 5000 years. See the following:

    http://bprc.osu.edu/geo/publications/mckay_etal_CJES_08.pdf

    That is not to say that todays extent is not as low or lower than that claimed to have existed 7000 years ago.
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  21. Despite the agendas evident, the climate obviously responds to forcings, of which that due to the CO2 bolus mankind has injected into the atmosphere over the past 40 years is paramount.

    As the planet's climate changes towards a new equilibria with the new CO2 levels, the floating ice cap that is the Arctic Sea Ice has been responding as if the proverbial "canary in the coal mine".

    Since 2001 alone, median ice thicknesses in the Central Arctic Basin, including the ice at the North Pole, have dwindled from approximately 2.0 meters thickness to about 0.9 meters thickness currently (from the Polarstern press release, also well-discussed here).

    For a good visual of what that looks like, here's a graphical depiction of ice melt and regrowth from 7 mass-balance buoys monitoring sea ice conditions in the Arctic over a recent 2-year period:



    [Source]

    So don't be taken in by our resident merchants of doubt and the hand-waving "There's no problem! The climates changed before!" routine that they do.

    The Northern Hemisphere's refrigeration system is failing in response to the forcing initiated by mankind. Despite the obvious dissembling of some.
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  22. To go right back to Sphaerica's post at #3: I think you may have misinterpreted what Kooiti Masuda was saying. If I've interpreted posts 1 & 2 correctly, the argument isn't that CO2 can't act as either a forcing or a feedback, but that we should really consider it as both. This is particularly concerning if we look at the Sff+sf sensitivity parameter - while a value isn't given above, it seems apparent that it's higher than any of the others.

    While anthropogenic CO2 may currently be the primary forcing, the paleo record seems clear that CO2 acts as a very strong positive feedback on millennial timescales. The implication, of course, is that if we don't get our emissions under control, and soon, we might see some very large positive feedbacks coming out of the natural carbon cycle.

    However, I also think RW1 at #6 has a point (although I disagree with much of that post) - the state of the planet now is very different to the glacial maxima, so the degree of feedback (especially albedo feedback, but probably also GHG feedback from thawing permafrost) available will be different. Has anyone had a go at quantifying that difference? Is it enough to significantly change the climate sensitivity?
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  23. I mean that, very unfortunately, it is difficult to quantitatively define the "Earth system sensitivity" to CO2 concentration, since CO2 concentration is an essential internal component of the system and not an external forcing. It is similar to this case: We cannot meaningfully discuss (fast-feedback) sensitivity of the climate system to specific humidity (=water vapor concentration).
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  24. Hello, I am not a trained scientist, so I hope you guys can bear with me. I came upon a yahoo article, "New NASA Data Blow Gaping Hole In Global Warming Alarmism." The article referenced a study published in Remote Sensing in July of this year. The title of the study was "On the Misdiagnosis of Surface Temperature Feedbacks from Variations in Earth’s Radiant Energy Balance". Here is the link to the PDF of the study. I would be curious to see some of your feedback on this study. Is there any validity to the arguments made? Look forward to your comments.
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    Moderator Response: It is debunked in this Skeptical Science post.
  25. I can not think how I missed this important article. What it points out is that for the first time, anthropogenic CO2 emissions have triggered fast and slow feedbacks, that “slow” feedbacks are occurring faster than expected and are not appropriately included in models aiming to forecast climate change. All very true!

    The article notes that …”In Antarctica, an amount of ice worth 20-25 m is rooted below sea level and held back by ice shelves.” If this refers to the WAIS, its thawing could raise sea level by 5-7 m rather than 20-25 m.

    In regard to the albedo slow feedback, I would note that the greatest loss of sea ice area occurs in summer enabling exposed ocean water to increase absorption of solar energy but that in winter ice mass continues to be eroded by warmer water insulated by the ice from the very much colder atmosphere above it. It’s a lose-lose situation.

    Lastly, I note that the article refers to CO2 as a slow feedback – which of course it is – but does not mention the elephant in the room, the far more potent, much larger and very dangerous release of CH4 embedded in permafrost which has now begun thawing. That is a slow feedback which I would argue is about to accelerate. Or is James Wright’s reference to CO2 a metaphor for CH4?
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  26. It was recommended I post here from another article. I have a question regarding climate sensitivity (or feedback) where there has been a fall in temperature rather than a rise.

    As stated in the above article, positive feedback would indicate that the climate warms in response to a warming - presumably in a declining loop otherwise it would be runaway warming.

    Conversely positive feedback would indicate that the climate cools in response to a cooling - again also in a declining loop.

    If these positive feedbacks acted quickly in the short-term (1-2 years) would we not see wild swings in the climate from hot to cold? For instance the warm year of 1998 would have led to an even warmer 1999. Likewise the cool 2009 would have led to even colder 2010.

    I can see positive feedbacks working on longer timescales as evidenced by the transition over a few hundreds of years into and out of ice ages. But from a layman's viewpoint it looks like, if anything, negative feedbacks prevail over the 1-2 year timescale as warm years are followed by cool ones and vice versa.

    I presume I must be missing something!
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  27. Matthew L - Quite correct, a 'positive' feedback amplifies the climate response to forcing changes whether positive or negative.

    Note that the 'loop' for feedbacks is a diminishing one (gain <1), meaning that there is no "runaway" affect in either warming or cooling directions - just a rather larger movement than one would expect with a forcing change an no feedback.

    Also keep in mind that feedbacks have time constants to their forcing response - water vapor responds in a matter of days, vegetation in years, ice in years/decades, and ocean temperature/CO2 solubility (to name a few) in centuries. Short term (weather) changes, including the yearly cycle, simply don't last long enough for the longer term feedbacks to take effect - they get averaged out.
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  28. Now that we're on a (hopefully topical) thread, Matthew L, I'll take a crack at answering. I'm not one of the professionals participating here, so I may well have some major or minor details incorrect.

    As you state, correctly, positive feedbacks will amplify both warming and cooling forcings in the climate. This is what allows the otherwise weak Milankovitch cycles, for example, to cause climate to shift between glacial and interglacial periods during the current Ice Age.

    At the time scales you are thinking of, 1-2 years (to say nothing of shorter time scales), the noise in the system (seasonal & diurnal variability, large-scale energy-shifting oscillations such as ENSO, and the like) will tend to either dampen or amplify the forcings & feedbacks in play.

    So, for example, in terms of surface temps 1998 was aberrantly warm compared to the rest of the 1990s (despite their being, at the time, the warmest decade on record) because of the very strong El Nino, which made the warming seem much stronger. Likewise the recent La Nina phase has dampened the warming of the 2000s and early 2010s.

    In addition, multiple forcings are at play, some warming (e.g. greenhouse gases, shrinking ice albedo) and some cooling (e.g. solar activity until very recently, aerosols) so contradictory forcings will have contradictory positive feedbacks.

    Hopefully that helps!
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