Guest blog by Chris Colose (e-mail: colose-at-wisc.edu)
UPDATE: This is Part 1 of two posts by Chris. Part 2 is here
RealClimate has recently featured a series of posts on the greenhouse effect and troposphere, articulating some of the more important physics of global warming from first principles. It is worthwhile reviewing these elements every so often with different slants just so the broad picture is not lost in the disagreement over details. This post extends on this theme to discuss one of the greatest sources of interest and uncertainty in the physical science of climate change: feedbacks. Feedbacks behave in interesting and often counter-intuitive ways, some of which can only be fully appreciated by mathematical demonstration. The previous posts at RC were criticized for being either too complex or too simple, so this post will feature two parts, with the second part providing some more of the technical details.
Feedbacks are components of the climate system that are constrained by the background climate itself; they don’t cause it to depart from its reference norm on their own, but rather may amplify or dampen some other initial push. These original “pushes” are forcings which are typically radiative in nature (such as adding CO2 to the air) and manifest themselves as a climate change when they are large enough or persistent enough to overcome the large heat capacity of the oceans, and thus change the annual mean radiative energy balance of the Earth. In a broad sense, a feedback means that some fraction of the output is fed back into the input, so the radiative perturbation gets an additional nudge (amplifying the forcing, a positive feedback or damping the forcing, a negative feedback). The major examples such as decline in ice extent in a warmer world, thereby reducing the reflected fraction of incident surface radiation are pretty well known at this point.
Another thought experiment can help to appreciate the implications. When we think about the terrestrial greenhouse effect, it is necessary to distinguish between those gases which condense and then precipitate from the air as solid or liquid rather rapidly (on Earth this substance is water) and those gases which can reside in the atmosphere for a very long time, and whose concentration is not so dependent on the temperature. Once you account for the spectral overlap between the various greenhouse gases and clouds in the sky, it is found that water vapor makes up roughly 50% of the modern greenhouse effect, clouds about 25%, CO2 is 20%, and the remaining gases (primarily methane, ozone, and nitrous oxide) make up the rest (Schmidt et al 2010, in press and a discussion of the paper here). This generally leads to popular claims like ‘water vapor is the most important greenhouse gas’ since it makes up the bulk of the infrared absorption in our atmosphere. This simple picture is incomplete however, since the total water vapor concentration is largely set by temperature and thus the non-condensable, long-lived greenhouse gases (chiefly, CO2) really provide the skeleton by which the greenhouse effect is maintained and what governs its capacity for change. In that sense, water vapor is in large part supported by the other gases and then amplifies their effect significantly.
If you could remove all of the CO2 from our atmosphere, aside from making the planet more efficient at losing its heat to space (thus cooling) you would do a couple of things. First, you’d lose much of the water vapor and cloud greenhouse effect since temperatures would be too cold for them to exist in appreciable amounts. Secondly, you would also get temperatures cold enough to the point where expanding ice cover greatly enhances the surface albedo of the planet and triggers a snowball Earth. This simple picture also holds true as the climate warms today, where it has long been noted that the increase in water vapor content of the upper troposphere should amplify the CO2 signal by a factor of about two.
CO2 concentration itself can act as a feedback to temperature on longer glacial-interglacial timescales in response to changes in ocean dynamics, temperature/salinity, as well as vegetation. It can also be a negative feedback to temperature on still longer timescales, where silicate weathering effects (determined by volcanic output and removal by precipitation) are thought to keep the climate in check over geologic timeframes. Often however, climatologists define some metric CO2 change (such as a doubling) which allows you to ignore the carbon-cycle feedbacks and focus on the ones which alter the radiative balance and temperature further. The principle feedbacks in this category are water vapor responses, surface albedo, cloud, and lapse rate effects.
Feedback behavior
The ultimate constraint on climate change is the Planck radiative feedback, which mandates that a warmer world will radiate more efficiently and therefore provide a cooling effect. For a blackbody, the emission goes like the fourth power of the temperature. So the question of how the other feedbacks behave is really of how they modify the Planck feedback. In order to decide whether a feedback is positive or negative, it is instructive to define a baseline sensitivity value that the climate system would have if no feedbacks operated at all. That is, if we perturb the climate with some forcing, what is the temperature change you would need to have to allow the planet’s energy balance to be satisfied. It can be shown that for every Watt per square meter radiative forcing the climate would warm by about 0.3°C without any other responses. To put this in perspective, it would take about five doublings of CO2 or a 7% increase in the total solar radiation hitting the Earth to produce the magnitude of climate change typical of glacial-to-interglacial transitions. Changes of this sort are well outside the bounds of what is characteristic of proxy records and observations, so this must mean that various feedbacks act to change the temperature much more than 0.3°C for a watt per square meter forcing. In other words, the aggregate effect of feedbacks is to be positive and enhance the so-called climate sensitivity relative to what it would otherwise be. Figure 1 below illustrates this.
The feedback factor is a value that is proportional to the no-feedback sensitivity value. It relates the fraction of the system output that is fed back into the original input, and takes on a value between 0 and 1 for positive feedbacks and less than 0 for negative feedbacks. It also means that the timescale it takes for Earth to reach a new equilibrium value is longer.
Feedback interactions
When multiple feedbacks operate, they can add together in rather odd ways. For instance, you might think that if you take a feedback that doubles the sensitivity to climate and another that halves it, they would cancel and bring you right back to the no-feedback sensitivity. You might also think that two feedbacks which each amplify the original forcing by 50% would add to double the no-feedback sensitivity. In fact, neither of these is the case, and the behavior emerges because multiple feedbacks interact with each other as well. One can imagine that if water vapor and the ice-albedo feedback are operating, the water vapor boost will mean more ice melt, which will mean further warming, more water vapor, still less ice, and so forth. Note that positive feedbacks do not inherently imply a runaway scenario; it just means that the final temperature change is higher than it would have been without the feedback being there.
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.
Ice-Albedo feedback
The ice-albedo positive feedback arises because sea ice is less dense than its liquid form, it is more reflective, and the extent is highly sensitive to temperature. Water is somewhat unique in this regard since most solids tend to be denser than their liquid form, causing the ice to sink which would forbid an ice-albedo feedback. This means that if the planet warms the ratio of highly reflective ice to relatively high absorbing ocean and land surfaces will be altered. The key in the modern climate is the seasonality between absorption of solar radiation in the summer and the release of energy from the topmost part of the ocean into the lower atmosphere in the cooler months. Temperatures in the Arctic do not become substantially amplified in the summertime as you might expect in large part because a lot of energy is going into melt or evaporation. However, areas of open ocean water develop earlier in the melt season, raising the heat content of the ocean mixed layer and melting more ice. When the melt season is completed, there’s a lot of open water and heat in the mixed layer and large vertical heat transfer from the ocean to overlying air until the sea ice forms, resulting in a seasonal delay of warming from the radiation absorbed in the summer. This surface amplification of Arctic temperatures has emerged primarily in the autumn and winter and should progress into the spring and summer in the future (Serreze et al., 2009).
Lapse Rate Feedback
Why is the lapse rate (the temperature decline with height) important as a feedback? In the tropics, the temperature lapse rate is largely set by convection to stay near a moist adiabatic profile. In principle, this should decline in a warmer world resulting in the upper atmosphere warming more than the surface. This means that the bulk of the atmosphere radiates to space at a temperature warmer than it would have with no lapse rate change, and emission from warmer layers is more efficient than emission from cooler levels. This provides a negative feedback which partially compensates for the water vapor feedback. Interestingly, the two effects act in tango with each other and so the uncertainty in the water vapor+lapse rate feedback is smaller than the uncertainty in the individual terms.
In the context of anthropogenic global warming, all of these complex feedbacks and interactions end up boiling down to the question of how much warming you get from additional CO2 release into the atmosphere. The most recent IPCC AR4 assessment gives a range of about 2 to 4.5ºC at equilibrium. This is the so-called ‘Charney sensitivity’ which takes into account these fast feedbacks discussed above, as well as clouds which provide the greatest uncertainty in narrowing these estimates.
Estimates of this range have been based on not just GCM results, but constraints from observational data (the seasonal cycle, or volcanic eruptions) as well as the past climate record (Knutti and Hegerl (2008) provide a review). One problem is that high values of sensitivity are more difficult to rule out than low values, and some observations that are good for ruling out the low end do not constrain the high end very well. For example, volcanic eruptions display a non-linear relationship with the equilibrium sensitivity, so the peak in the probability distribution shifts only slightly for larger mean values of sensitivity.
Recently, some studies have expanded on this view to also include ‘slow feedbacks’ such as the response of ice sheets and vegetation that are important on hundreds of year timescales (Lunt et al 2010; Pagani et al 2010). These estimates show that the long-term warming should be even more than the Charney estimates, on the order of about 5°C.
In summary, the function of feedbacks is to modify how much you expect the climate to change for a given forcing. Part 2 will describes some of the basic mathematical relationships that are important for discussing feedbacks, as well as elaborate on the water vapor feedback to climate change. Water vapor provides the strongest feedback effect on Earth in terms of enhancing the sensitivity, and is also a key component in understanding the evolution of the terrestrial planets.
Rod @ 447
For pity’s sake. What passes for discourse these days.
I know that this sort of thing passes for humor in some circles, but it seems to me that given the nature of this forum, refusal to analyze and scrub your own comments for fallacies will only hasten your descent into total trolldom.
See Straw man at Wikipedia.
SecularAnimist (449)
That sounds interesting. I have a question about the 40% figure though. Does that include the change in CO2 production when these techniques are implemented? I would assume there is a different carbon footprint from these techniques and this might also be beneficial (may even be detrimental, I don’t think so but I’m not familiar with this enough to know). The only thing that would likely prevent immediate widespread implementation could be fear of unknown obstacles, particularly by those farmers who have done well with current techniques. They may see it as unproven and not worth the risk until more people have tried it over many years and shown it to be as good as what’s being used now.
435, Jim: Get off of this nonsense. None of you have given the first thought as to what would be required to afforest desert lands, nor any of its climatic effects. It’s a stupid geo-engineering pipe dream, divorced from reality.–Jim
I disagree with you there. We have provided examples of actual afforestation of desert lands (e.g. Eritrea), as well as discussions of watering salt-tolerant varieties in arid coastlines (previous threads, plus the Eritrea example.) Consider the link in post 439 by Pekka Kostamo. It isn’t intrinsically any more of a “pipe dream” than any other of the “stabilization wedges”, such as organic (minimum tillage) gardening. 1.5 million new trees in UAE is a small step, but all the steps are small.
[Response: Where’s the water going to come from and how much will it cost? How are you going to get seedlings established over many millions of hectares without shade and heat protection? Where are you going to get the seeds? What genotypes of what mycorrhizae are you going to infect the seedlings with? How are you going to prevent salt and metal accumulations in the rooting zone from high evaporation rates? What’s going to be the regional effect of a greatly decreased albedo and large increases in water vapor? How will you deal with harvesting for firewood by locals? What unforseen problems with invasive species, including pathogens, are going to arise, and what will be their effect on ecosystems and people? (To list only a few off the top of my head). Get back to me in 40 or 50 years with a progress report.–Jim]
Rod B., you share a widespread misconception, I think. That is that eating locally would mean a less varied diet. It seems intuitive, since you’re “cutting things out,” right? Yet that is not necessarily what happens. Because you don’t have the shrimp or the pineapple, you are motivated to seek out foods that are available–often ones that you may not have tried without that “push.” Moreover, you keep doing this, since what’s available changes with the seasons (assuming you live in a temperate climate.)
See, for example, Barbara Kingsolver’s experience:
It’s an interview well worth reading–for a fun example:
I haven’t read the book yet. I think I will, though; my own experiments with reforming our diet have been decidedly timid so far. Anyway, you can read the interview here:
http://www.loe.org/shows/segments.htm?programID=07-P13-00018&segmentID=4
One last quote:
“. . .this isn’t a story of deprivation. This is a story of gratitude.”
#453–
SM, Jim Bullis specifically rejected identification of his “project” with re-afforestation on normal scales. You can plant trees in places where they are native, or at least appropriately adapted. Probably we’d all of us involved in this conversation agree that that was a good thing.
But trying to kickstart an ecology that’s not adapted to the local conditions (and at the expense, as far as I can tell from the few details that were given, of existing ecosystems in the Southwestern desert and the coastal lowlands of Hudson Bay) is a whole other beast. And not a very intelligent one, IMO.
Kevin McKinney quoted Barbara Kingsolver: “We didn’t for example give up coffee because my husband said, ‘Coffee will get you through times with no food better than food will get you through times with no coffee’.”
There’s a point worth noting there.
It’s one thing to import agricultural products that can really only be grown well in certain locations, like coffee.
It’s quite another thing to grow broccoli in expensively irrigated deserts in California, and then ship it 3000 miles in refrigerated diesel trucks to Pennsylvania, which has an ideal climate for growing broccoli.
Having said that, I hope the Kingsolvers were buying organic, shade-grown, fair-trade coffee (which is what I do, being of the same persuasion as Ms. Kingsolver’s husband when it comes to De Ol’ Debbil Bean).
TNYT, I think it was, maybe elsewhere, had a fine article on reforestation in Burkina Faso, a county in the Sahel region of Africa. Turns out that when a French colony, trees didn’t belong to the landowners, so were all cut down eventually. The farmers there, small holders all, dig pits to collect rainwater rather than having it run off. One farmer put his animal manure in his larger than normal pits. Voila, trees starting growing. Now, on his maybe 43 acress (largely than average anyway), he grows nothing but trees, sell9ing the wood for firewood and making furniture. His neighbors have taken up growing some trees as well, finding this seems to enhane their crop yields.
I realise that the threads get more off-topic the longer they go on, but they really aren’t the place for our wilder meanderings. Far fewer people are paying attention, and so there isn’t as much informed comment as one might like. It is too late for this thread, but everyone benefits if people stay vaguely on topic. People do come back to these threads many moons after they were active, and if they find them substantive, rather than combative, that reflects well on everyone concerned. Thanks for understanding.
453, Jim: Get back to me in 40 or 50 years with a progress report.
Now aged 63, I don’t expect to be around then. As I have written, I expect the whole energy industry to be much different 20 years from now, and even more so in 40 years. Agriculture will also be much different, with or without greater efforts at reforestation.
As to your questions. The water will come from the oceans and from some of the rivers (as in the areas of China that are being reforested to reverse desertification.) The cost of the water will be less than the cost of the end of civilization as we know it; on an island off the coast of India desalination for agriculture and forestation is provided for by the waste heat from solar power, so the cost of the water is a small addition to the cost of the electricity. In Southern California the water for the avacado plantations comes from the Colorado River, among other places. That might not represent a net gain in CO2 sequestration (the flora of the Colorado River delta were sacrificed, but the delta might support modern salt-tolerant species of all kinds), but it does show that water flows can be redirected hundreds of miles where people are willing to do it. It could profitably be done in the Indus valley as it is being done in western China.
The seeds (and seedlings) will be provided as they were for Eritrea, Senegal and Indonesia; as they are provided for Weyerhauser and Georgia-Pacific and the reforestation of Southern Mexico; and by DeKalb and Pioneer seed companies as they do for American farmers.
I do not now know the solutions to all of your other questions. Is it your assertion that those problems have no solutions?
Success would require continued investment in labor, capital, and ingenuity, but that’s always true of everything.
A note about Australia, which as a long coast line and much arid country within 100 miles of the coast. There is little natural ecology to preserve in Australia because the first immigrants burned the forests and the subsequent immigrants have made additional large-scale changes. Now they export coal and other minerals, but they could easily become one of the world’s largest exporters of wood, should they choose to make the labor and capital investment to do so.
> little natural ecology
That’s a denial talking point.
“Australia is the most megadiverse developed country and supports almost 10 per cent of the biological diversity on earth.”
http://www.environment.gov.au/biodiversity/
“… here in the east — nonhumans have rights, that there are strict limits on how far you can displace a marsupial that’s living in your chimney because after all, they were here first.”
http://www.rifters.com/crawl/?p=1603
http://www.biodiversityhotspots.org/Pages/default.aspx
Dale (448), I’ve nothing against organic farming or its attendant benefits. All for it in fact. I am against the hype and hyperbole that some organic/communal/regional supporters spout. Like more than doubling the yield.
Is your 40 acres in sweet or field corn? If the latter does it go into seed or processed food (the vast great majority of corn utilization). If the latter, how do you get it converted into cereal or corn sugar, e.g? Is your transportation, planting and harvesting all by hand or mule? That would be unique, but admittedly doable. But does that doable remain when you project your successful experience to every farm (as reconstituted) in the country? Don’t mull on it for too long — the answer is no.
And at the societal level don’t forget that organic and small are not synonymous.
SecularAnimist, on the surface the Rodale effort seems atypical of the hype and hyperbole I mentioned. “…regenerative organic agricultural practices — can be the most effective currently available strategy for mitigating CO2 emissions.” Really? The BEST? Better the wind or solar power? electric cars? banning coal? (Or possibly they are hedging behind the skirt, “can.” Later in the fine print they do change it to “substantially mitigate….”) Are regenerative organic farming practices the only way to get carbon into the soil. What does regularly farmed corn and soybeans do with their absorbed carbon?
That being said, I shouldn’t discount it out of hand. Maybe it does better at sequestering CO2 that I would have imagined or understood, and the Rodale effort deserves a further look. Though, maybe admitting my prejudice, 40% of all emitted CO2 is a long, long, very long row to hoe (pun intended).
460, Hank Roberts: “Australia is the most megadiverse developed country and supports almost 10 per cent of the biological diversity on earth.”
They also grow rice and cotton, grains and soybeans, and they suffer desertification caused by horses, cattle, rabbits and camels. And the first immigrants burned a major fraction of the forests that used to be there. They mine uranium, iron, coal and other stuff. I doubt that they want to use solar power to pump and desalinate large amounts of water to grow more eucalyptus, walnuts, almonds, dates, mangroves and miscanthus, but they surely could with little negative impact on the marsupials. Certainly nothing comparable to the damage done by burning the native forests and planting cotton.
Kevin McKinney, your conflating a “still satisfactory diet” with “keeping the diverse diet” — not the same thing. Explain bananas (but remember saying we don’t need bananas is not the same as maintaining the diet.)
I appreciate your anecdotal examples. But they ain’t a societal solution.
Rod, again you post your unchecked assumptions. You could read a bit and ask questions that didn’t just allow you to keep arguments going. Try it.
It took 30 seconds, posting your questions into the search box, to find:
“more than 80 percent of the new farmland created in the tropics between 1980 and 2000 came from felling forests …. the first study to map and quantify what types of land have been replaced by the immense area of new farmland developed across the tropical forest belt during the 1980s and 1990s….”
http://www.physorg.com/news202722944.html
http://www.physorg.com/tags/proceedings+of+the+national+academy+of+sciences/
“The estimate is that we are now losing about 1 percent of our topsoil every year to erosion, most of this caused by agriculture…. . A geomorphologist who studies how landscapes form, Montgomery describes modern agricultural practices as “soil mining” ….”
http://www.seattlepi.com/local/348200_dirt22.html
Sec is right. You could have figured this out for yourself.
You could check it for yourself if you did a little reading.
Spare us another long digression about your doubts and nitpicking.
Here is a note on part of Australia:
http://bravenewclimate.com/2010/10/18/who-crippled-the-murray-darling-basin/#more-3359
It’s tangential to the idea of “af”-forestation with desalinated sea water, but it shows some of the environmental impact of farming. They’d probably benefit from widespread replanting of native drought-tolerant flowers, grasses, legumes, shrubs and trees; considering the damage done by the dairy farms and cotton farms. For that, the water is already there. But it’s up to them. The same advice could probably be given to farmers and ranchers in large areas of Colorado, Nebraska, etc.
@392 Jim’s response:
Thanks for your comments, but you overstate what we have accomplished. Hopefully, in time your comments will reflect our reality. Livin’ on the edge…
[Response: Doing the hard work with little fanfare in a tough environment (and I know enough about Detroit to know how tough what you’re doing must be), is already an accomplishment, and deserves to be publicized widely.–Jim]
@ 400 Jim Bullis: Jim, there are metrics that trump all others. If I wish to play professional football, but weigh 135 lbs. sopping wet and fully clothed, my size is my Liebig Minimum. With large-scale ag there are a number of them, but the simplest is this: it depletes soil and requires constant addition of fossil fuel-based fertilizers and phosphorus. Both are finite. Both impose limits that have nothing to do with any of the many other issues we might disagree on. Those limits mean that large-scale ag is unsustainable, period. Finite resources are finite resources. And, no, technology doesn’t save us. Since 1950, and all the improvements in efficiency, population has a somewhat more than doubled, but consumption has gone up 11 times. Jeavon’s Paradox.
@415 Secular Animist: Familiar. Considering the audience. Far out claims to the uninitiated, thus not going there. Yet. That sort of production takes planning and some years to get right. But, yes, very doable. However, we don’t plan the future on the long tail. Or shouldn’t.
@ 418 Ray: We aren’t discussing subsistance farming, we are talking about regenerative farming building highly productive soil, using and creating all space you have or wish to use and maximizing yield while minimizing work and consumption. The beauty of Mollison and Holmgren’s book is the drawing together of ancient wisdom and knowledge into a modern context. and putting a lot of into in one place. People are growing very large amounts of food on very small areas of land.
@421 JB: The quoted study is curiously contradictory to 100 years of agricultural experience in North America and Brazil; not such small players, I might add.
no, it doesn’t. Regenerative agriculture wasn’t being studied until recently. It generally performs as well and somewhat better than chem aided ag over the long run, as the Rodale study shows. Find and watch Mollison’s Global Gardener series of videos to see what can happen in areas such as Rwanda.
@423 Rod B: I think the numbers cited in that one post are off somewhere, but it depends on where they were starting from. Many African farmers have been encouraged/tricked/forced to go to chem ag, which destroys soils. A destroyed soil changed over to regenerative ag would easily perform as stated over a soil-depleted chem ag farming method.
Apropos diversity — this is where the world is going, if we’re lucky:
http://www.conservationmagazine.org/2010/06/the-new-normal/
This is one reason modeling the biological side of climate change is difficult; we aren’t keeping baselines, everything is changing.