Depressions and Holes in the Ground
Arthur Smith, as his wont, raises an interesting point at Not Spaghetti about a stopgap for handling sea level rise
Indeed, BF Chao has long been on the track of the effects of sea level of both impoundment and depletion of water resources on land. The effect is large, and indeed has had measurable consequences on the observed sea level rise.. . . there may be something much simpler we could do that would not require huge energy expenditures in itself: retain more of the naturally precipitated water on continental land. Annual precipitation depth over most land areas of the world is on the order of 1000 mm/year. If we just divert 0.3 to 1% of that rainfall to prevent it from returning to the world's oceans we could stop SLR until whatever storage capacity was involved became full. This could have significant additional benefits. Increasing the world's reserves of fresh water could help alleviate the droughts expected under climate change. Large water reservoirs close in horizontal distance but significantly separated vertically could provide new pumped hydro-electric energy storage that would be the perfect complement to increased solar and wind resource use. Refilling underground aquifers would reverse the salt-water incursion and stop the land subsidence problems that have plagued some parts of the world in recent years. What are the potential total capacities of these systems?
a total of ~10,800 cubic kilometers of water has been impounded on land to date, reducing the magnitude of global sea level (GSL) rise by – 30.0 millimeters, at an average rate of –0.55 millimeters per year during the past half century. This demands a considerably larger contribution to GSL rise from other (natural and anthropogenic) causes than otherwise required.There are some obvious places to put a whole lot of water, for example moving water from the soaked east coast of the US to the west, or simply using it to recharge the Ogallala Aquifer. In Africa, pumping water from the Med to fill the Qattara Depression, and in Asia, refilling the Sea of Aral.
Not that these ideas have gone unmentioned in the past, but, of course the issue is where to get the energy needed to move the water. Eli has half an answer. Solar and wind power, as has also been not unmentioned, suffers from intermittency. To handle this intermittency requires overbuilt capacity, geographically spread out. When there is excess electrical power, that excess can be used to move water into storage reservoirs, both above and below ground. With proper design, some storage schemes (as Arthur points out) can be used for hydro generation of electricity when needed.
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32 comments:
Filling the Qattara Depression with sea water would even generate energy while reducing the sea level temporarily. Downside would be the impact on the nature and the people living there.
There's a freshwater aquifer under Quattara. It'd be a shame to salt it up.
https://info.ngwa.org/GWOL/pdf/911052534.PDF
I keep thinking all this N.American pipeline fu isn't really about petroleum, it's about establishing a right-of-way that can be used later for water, from the melting Arctic and down to Texas.
OT: This ones for Russell. Huffington Post has an article on nuclear winter. Probably doesn't mean a thing. It's just some guy named Tegmark.
http://www.huffingtonpost.com/max-tegmark/climate-change-for-the-im_b_9865898.html
To hold all that water would require damming drainage systems. Dams are a bad idea for lots of environmental reasons.
Contemplation of some of these hairy adaptation schemes only serves to remind us how important mitigation is.
Eli - I'm glad somebody read it - since I've turned off comments on my blog I'm never sure if there's anybody out there paying attention (other than my 84-year-old Dad, who reads everything I write :)
skylanetc - well, damming drainage systems would be one approach - but if you are talking about artificial drainage systems those can be themselves destructive to the environment, that is why there has been so much emphasis on preserving and rebuilding wetlands around the world in recent years. In any case the general point is we are faced with trade-offs, no solution is perfect. Nothing that's been proposed by anybody (even if the Paris accords are exceeded and we go to near 0 CO2 emissions this century) will stop sea-level rise for centuries to come. That's because the CO2 already in the air has produced an energy imbalance that directly heats the oceans and leads to steric sea-level rise. We are due for several meters even if the ice sheets stop melting.
So the question is what feasible cost-effective steps can we take to preserve fragile coastal environments - both built and natural - and islands and coral reef systems etc. that will otherwise disappear. Just looking at the numbers, there seems to be some real potential to move the damage away from the coasts and toward less important internal continental areas - or if we can store it all underground perhaps there need be little environmental cost at all. But there are inevitably losers as well as winners; I suspect a cost-benefit analysis would be prey to all sorts of value judgments that might make it hard to tell what was optimal. It should, I believe, at least be considered. And like all global warming challenges it will require international cooperation as a single nation cannot realistically fix this on its own.
Hank Roberts
I keep thinking all this N.American pipeline fu isn't really about petroleum, it's about establishing a right-of-way that can be used later for water, from the melting Arctic and down to Texas.
As I understand it, the USA is eventually looking to get water from the Great Lakes, so that is the area of focus when it comes to pipeline access.
Eli is invited to run the numbers comparing the energy cost of pumping water uphill into Western reservoirs and brightening their surfaces to limit the evaporation of the water that flows downhill into them.
Divert fresh water...yah...because screw Louisiana and their brackish ecosystems anyways. After all whats one more disaster after BP Deep Water Horizon and Hurricane Katrina. Fuck'em if those Cajuns can't take a joke.
I'm sorry for the scatalogical response, but that idea is so desperately foolishly grasping at any straws rather than forthrightly facing reality.
Divert fresh water into aquifers...yah...because screw Louisiana and their brackish ecosystems anyways. After all whats one more disaster after BP Deep Water Horizon and Hurricane Katrina. Fuck'em if those Cajuns can't take a joke.
I'm sorry for the scatological response, but that idea is desperately foolishly grasping at any straws rather than forthrightly facing reality.
Russell, you have heard of and?
forshortened, you have noticed that some places are getting more rain?
> As I understand it, the USA is eventually looking to get water from the Great Lakes
Well of course the US is not going to TELL Canada about plans to extend a long sucking pipe across their country to get at their melting Arctic ice.
But look at it logically. The Great Lakes water is full of nasty chemicals and lampreys and such.
The melting Arctic ice (once the upper century's accumulated lead and other fallout has washed into the ocean) has nothing contaminating it but stardust. Why waste that on raising sea level directly, when we can run it through our depleted aquifers and pump it back out to use in our sewers and irrigation systems en route to the ocean, eh?
Listen for the slurping sound ....
There has long been talk of using water from the Great Lakes for other areas/states. This would be met with much opposition - both from our treaty partner (Canada) and residents of the Great Lakes. On the US side withdrawal of water is governed by the Great Lakes Compact.
Even cites just 20 miles from Lake Michigan can have difficulty getting approval (see Waukesha, Wisconsin). It is governed not by proximity, but by watershed.
It has always seemed a short-term (read, silly or stupid) solution. It wouldn't take long before the Great lakes would then be running at low levels. For many communities around the Great Lakes tourism is their main source of income. Anything that jeopardizes that would be looked at askance.
[I tried to post this yesterday, but RR started to gimme '502' (as did Sou's). Still, better late than never.]
Hank, others have wondered that too...
On dams, I've often wondered why graphs of SLR don't more frequently tip a hat to the work of the likes of Chao et al. There, potentially, be some dragons.
Also, there are some big problems with the construction of dams, in that a lot of the prime impoundment real estate is already flooded, a lot of what's left has crucial ecological value, and a lot of what's already in use is well on its way to silting up.
Dam use in 50-100 years is going to be a very different beast to the status quo that the world currently takes very much for granted...
Leveed lakes though, that might be a Thing. Ecology/space, appropriate water sources, and nasty folk with levee-busting intentions notwithstanding.
Maybe I've been reading the physics section of the IPCC wrong, but my impression was that global warming was characterized by an overall long term shift of atmospheric energy transport away from convection towards radiation, with resultant drying and warming, thanks to the Stefan-Boltzmann law. More importantly was that any increases in evaporation occurring in the lower troposphere would not be able to keep pace with the overall decrease in convection due to warming in the upper troposphere, which saturates out the temperature gradient that drives Archimedean buoyancy. I thought this was more or less encapsulated in the early work on 1-D RCMs. Maybe I read the physics incorrectly.
The obvious corollary is that water management will be critical in global warming risk management and impact mitigation. However, undertaking yet more ecologically disruptive industrial water projects will only compound the effects of global warming on already threatened and vulnerable ecosystems. The trick will be managing water supplies to buffer and reinforce existing ecosystems, and perhaps even provide pathways for ecological adaption.
PS sorry about the double post, I had a page vanish on me.
If we are going to undertake considering ridiculous engineering "solutions" to global warming how about this:
We tow all the calving ice sheets to the mouths of tropical rivers, like the Amazon and the Congo. We then break the sheets up with explosives and tow the icebergs upstream to where the melt water can be feed into ecosystems and agriculture.
Frankly that makes about as much sense as artificially diverting even 1% of all rainfall into aquifers.
Your physics is weak short one.
Physics DEMANDS that you remove the carbon dioxide you put into the atmosphere, strip the oxygen off of it, reducing it back to carbon where it can be manufactured into carbon products or put back into the ground where you removed it. Physics is laughing at your mitigation plans with soon to be 10 to 12 billion religious nuts running wild.
It's your planet. You screwed it up. Now you have to deal with the consequences. One thing I know, nature is not going to miss you.
And neither will I.
I'm drafting the enviromental impact forms you will have to fill out before I give this any more thought.
I think the point is: doing nothing (about SLR) has a huge environmental and economic impact. Are there alternatives that have less impact? I suspect so. More research is needed. It looks like some has been done, but I think a significantly more in-depth study of this would be very useful. Suppose we, say, flood the entire Great Basin of the US (the part that drains into the Great Salt Lake) and a few other similar places, to prevent the loss of Florida, much of the gulf coast, a lot of prime real estate on the eastern seaboard and some California coastal real estate as well. That's a trade-off we could choose to make. Is the balance of costs and benefits positive? Seems like it could be... But there should certainly be better options than that one too.
Anyway I confess to being totally baffled by foreshortened's logic above - water from land either drains into the oceans, or it doesn't. How would retaining a bit more on land every year "compound the effects of global warming on already threatened and vulnerable ecosystems." ??? A specific example would be nice. The theory quoted doesn't seem to justify that statement at all.
I think the obvious solution Arthur is to continue to make fresh water and ice from sea water and rain water via evaporation, transpiration, respiration and precipitation, to store that fresh water on polar ice caps and mountain tops, and to use that fresh water to continue to force the hydrological cycle through creative planetary agriculture.
In order for that plan to work, solar energy, along with to a lesser extent wind energy and hydraulic damming, needs to be exploited on a planetary scale, and part of that electrical energy produced will have to be applied to the atmospheric carbon dioxide draw down. You can do what you want with the biochemical energy, since your life depends on it. Be creative. You'll have lots of self reproducing human labor. Certainly your gene pool and biodiversity is adequate.
We know all terrestrial ecosystems will become increasingly vulnerable to climate change (recent Nature for example http://www.nature.com/nature/journal/v531/n7593/full/nature16986.html)
Furthermore the only way to capture fresh water into aquifers and storage is by diverting fresh water run off away from already imperiled terrestrial ecosystems (consider for example the impact of just a few percent drying had on Northern Alberta this year).
To any terrestrial ecosystem a sustained year over year diversion of its water, even a few percent, will have dramatic consequences. You would be hard pressed to find any terrestrial ecosystems that are not critically sensitive to moisture.
If you don't believe me try travelling through the Southern Interior of BC, say the Kelowna-Kamloops region, were just a few hundred meters of elevation gain increases moisture enough to transition from semi-arid grass land to Ponderosa forests.
Furthermore the timescale on which you will be altering the continental terrestrial ecosystems will be very much shorter than the time scales on which the coasts will retreat. In terms of ecosystem adaption, you always have to go with the slower change to give the ecosystems half a chance at survival.
In this case the cure, radically altering all continental terrestrial ecosystems even more so than what global warming will do, is worse than the disease, continental coastal retreat.
Arthur,
I'm afraid you have a vastly over simplified understanding of terrestrial hydrological cycles. It is not simply a matter of land-to-sea run off.
Removing water, or altering its course, in any step of the ecological transfers that occur in terrestrial environments has radical and multiplied down stream impacts. Consider, as an extreme example, the fates of the ecosystems downstream of the Colorado River.
Let me point it out another way: at what point in the terrestrial continental hydrological cycle do you want to capture all this fresh water?
Straight from mountain run off? Well guess what in the Rocky Mountains snow packs are already half of what that used to be, and because mountain snow fall accounts for a large part of the 1000mm average, you would have to take something like a 10% haircut off of the dwindling snow pack.
Out of continental water ways? Well now we have less water flowing through out water ways, which reduces water levels which further increases fresh water temperature which will decrease fish productivity. And that is just one impact of thousand!
Or from near ocean deltas? Well doesn't that scream stealing from Peter to pay for Paul. You are going to decimate by water withdrawal the very ecosystems you wanted to protect.
Each of these choices will come with exacerbating impacts to ecosystems reaching much further inland than the coastal margins.
Each of these choices will come with exacerbating impacts to ecosystems reaching much further inland than the coastal margins.
You mean those ecosystems already being stomped into the ground by billions upon billioons of religious nuts, only half of which have IQs less than 100? Right? So, what have you got to lose?
Indeed, BF Chao has long been on the track of the effects of sea level of both impoundment and depletion of water resources on land. The effect is large, and indeed has had measurable consequences on the observed sea level rise.
The excerpt following only covers impoundment contributon.
I did some light trawling on the topic a few years back. A 2012 paper by CHAO and others begins:
Recent studies suggest the increasing contribution of groundwater depletion to global sea-level rise. Groundwater depletion has more than doubled during the last decades, primarily due to increase in water demand, while the increase in water impoundments behind dams has been tapering off since the 1990s. As a result, the contribution of groundwater depletion to sea-level rise is likely to dominate over those of other terrestrial water sources in the coming decades. Yet, no projections into the 21st century are available. Here we present a reconstruction of past groundwater depletion and its contribution to global sea-level variation, as well as 21st century projections based on three combined socio-economic and climate scenarios (SRES) with transient climate forcing from three General Circulation Models (GCMs). We validate and correct estimated groundwater depletion with independent local and regional assessments, and place our results in context of other terrestrial water contributions to sea-level variation. Our results show that the contribution of groundwater depletion to sea-level increased from 0.035 (±0.009) mm yr−1 in 1900 to 0.57 (±0.09) mm yr−1 in 2000, and is projected to increase to 0.82 (±0.13) mm yr−1 by the year 2050. We estimate the net contribution of terrestrial sources to be negative of order −0.15 (±0.09) mm yr−1 over 1970–1990 as a result of dam impoundment. However, we estimate this to become positive of order +0.25 (±0.09) mm yr−1 over 1990–2000 due to increased groundwater depletion and decreased dam building. We project the net terrestrial contribution to increase to +0.87 (±0.14) mm yr−1 by 2050. As a result, the cumulative contribution will become positive by 2015, offsetting dam impoundment (maximum −31 ± 3.1 mm in 2010), and resulting in a total rise of +31 (±11) mm by 2050.
Link to full paper
...suggesting we'll have to mitigate groundwater depletion as well as build damns to make a negative impact to sea level rise in the long-term.
barrymoose
Barry,
Thank you, that certainly throws some cold water on the idea. I'll see your groundwater depletion and raise you a geological CO2 sequestration by continental run off:
http://www.sciencedirect.com/science/article/pii/S0921818198000162
http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2486.2001.00383.x/full
But I can do one better and present as evidence exhibit A, the Rocky Mountains. This limestone mountain chain is ample evidence of the dramatic importance of surface ocean abundance, enriched by continental nutrient run off, on the process of long term geological sequestration of CO2, as organically derived CaCO3 deposits.
Taking the first article's estimate of 0.721 Gt/y C carried by run off into the oceans, and using the overly simple boundary condition of detailed balance in the C chain, we can conclude that continental run off accounts for an order of ~0.7 Gt/y C sequestration into geological stores. Now the idea of diverting even 1% per year of fresh water run off would translate to an approximately 1% reduction in annual geological sequestration from run off, or keeping an additional 0.007 Gt/y C in the atmosphere (water evaporates releasing gases, and O2 reacts with substrates in suspended particulate matter to release CO2). That is quite a few power plants.
Now given the scale of the limestone and sedimentary deposits on the surface of the planet, I would hazard that continental run off constitutes the bulk of geological CO2 sequestration. The only possible counter argument is that surface ocean abundance might be more closely tied to algae than continental nutrient enrichment. There are two flaws with this counter argument: first, ocean ecosystems are rate limited by nutrient availability, which is tied to continental run off; second, algae blooms are small net sources of CO2, because they eventually die off and sink. Upon sinking the blooms decompose and in the process generate CO2 and remove O2 from the ocean. In turn the anaerobic zones then kill off ocean eukaryotes reducing over all abundance, and thus the rate of overall sequestration.
The self-limiting nature of this sequestration process is very interesting. Too much run off yields algae blooms which then limit ocean abundance. Too little and there is insufficient nutrient replacement. But just the right amount and you get equilibrium sequestration rates.
Kinda smacks of James Lovelock's Gaia hypothesis doesn't it?
To fully appreciate the conveyor belt of run off sequestration try this mental exercise then next time you are hiking in forested hills:
When you reach a high clearing and can look over at a nearby forested hill with a drainage splitting its side, imagine that you have a camera that has record the last millennia of forestation of the hillside. If you play the video in fast forward what you will see would look like a conveyor belt of trees moving inward towards the drainage and downhill. This is due to the process of soil subsidence towards the drainage. As the tree tops meet the edge of the drainage they collapse inwards towards the drainage, with new trees continually growing on the margins of the hill side. Overtime the material of the soil and the trees gets washed down the drainage and carried into the rest of the continental waterways. Eventually reaching the lakes, rivers, estuaries, deltas, and finally the ocean.
The nutrients from these former plant and soil material then feeds continental shelf ecosystems, whose decomposing animals and wastes deposit over time to become sedimentary sequestration of geological CO2. It is a poetic process. Stretching over hundreds of thousands, and millions of years.
forshortened - thanks for the greater detail and more specific examples. I think our disagreement may be because we are envisioning some very different scenarios here. First off, climate change is not expected to (or so far seen to) lead to increased drying everywhere. IPCC AR4 WG1 stated "Long-term trends from 1900 to 2005 have been observed
in precipitation amount over many large regions.
Significantly increased precipitation has been observed
in eastern parts of North and South America, northern
Europe and northern and central Asia. Drying has been
observed in the Sahel, the Mediterranean, southern
Africa and parts of southern Asia. Precipitation is
highly variable spatially and temporally, and data are
limited in some regions. Long-term trends have not
been observed for the other large regions assessed". The story in AR5 was a little more nuanced but also clear that precipitation has increased in some areas (the concern is now more with the increase in extremes - very heavy precipitation events).
Since I live in the US East coast perhaps it's been clear to me that we often have more water than we know what to do with. Our town has had several over-1-foot-in-24-hours rainfall events in recent years, and a couple of huge snowfalls too. The story is certainly different elsewhere. Most of those huge water inputs just wash right out to sea - despite the fact that we indeed have some depleting aquifers below us, even some where groundwater extraction has had to cease due to saltwater incursion. Our built environment encourages the water to leave, rather than to stay. And that makes sea level rise worse when I believe we could just change a few practices to help.
Obviously something like this would have to be well-planned, and as I've repeatedly mentioned, balance the costs and benefits. I expect the best place to divert water would be closer to the mouths of rivers than the sources. Perhaps a mechanism could be developed to capture rainfall over the ocean (while it's still fresh and not salt water) and bring it back to land, so it wouldn't have any negative impact on land water resources at all. Since precipitation varies greatly from year to year it might be sensible to plan considerably larger diversions (water storage) in years with a lot of rainfall, particularly when there is risk of major flooding, and then not divert much or any in dry years. A lot of details would need to be worked out. But I don't see your objections as being anything more than cost issues that would need to be accounted for, not substantive roadblocks. In principle this should be possible, and may be worth it.
Arthur,
I think we are getting closer to being in agreement. My impression of IPCC, as well as my own 30 years of local observations, is that overall average precipitation will continue to decrease as convective energy transport decreases, as well the spatial-temporal-volume predictability will decrease. So that single events are larger and less predictable, but overall in the long term and over large geographies precipitation will delivery less. Kind of like holding a lid tightly on a boiling pot.
Water management will be the key to our future, but from what I have seen mega projects will only exacerbate the impacts of global warming. Rather we need global scale micro-management of our water. We need to ensure larger volumes of water are moving more slowly through our ecosystems. In short humanity needs to become beaver ecologists.
For example to slow the impacts of large unpredictable storm run off, which quickly, very quickly, destroys drainage systems (I've had first hand experience with this in Kananaskis in 2013) we will need to terrace on a small scale the upper reaches of our drainage systems. The benefits of this are many fold, it creates micro-habits that allow the development of niche species that support ecological adaption to climate change, it slows the movement of water without diverting it, it filters the water, while at the same time adding productive nutrients to the water, it cools the water, both by increasing the volume retained and raising the water table (i.e. heat capacity effects) and by encouraging the growth of shading plant cover, which in turn will support fish and amphibian stocks which are adversely impacted by heating waterways.
In cities we will have to move away from the current storm water sewer system for managing run off which greatly exacerbates downstream flooding from cities, by dumping very large amounts of water into waterways in a very short time. Instead we will need to find ways to reserve this water in local ecosystems, so that the impacts of storm events can be diffused over space and time.
We need to very quickly transition to moving very slowly, if a significant portion of our civilization is to remain intact in any sort of meaningful manner.
foreshortened - terracing and otherwise slowing down water flow sound like great solutions, that's the sort of thing that will really make a difference. Raising the inland water table is, basically, storing additional water underground - so that is the kind of thing I was arguing for. Getting the water underground requires some kind of impoundment over porous regions - in our area they do make some attempt by just basically building huge holes in the ground near each block of houses, which does seem to help in recharging groundwater and preventing flooding (it provides a place for all the water in the street to drain to even during the worst events). Local governments will have a lot of influence on this, and I hope we can find ways to encourage it. It may not be enough, and there may be a need for "Army Corps of Engineers"-style mega-projects as well, I don't really know. But I'm definitely all for local efforts on this sort of thing, and finding a way to communicate to people that it's a good thing to do...
On your IPCC argument - I was reading some more from the AR5 SPM and I'm not getting that impression at all. Through the end of this century they are expecting precipitation in some areas to increase, and others to decrease, and overall definitely an increase in extremes. See Section E.2 "Water Cycle" and especially the figures there. Also your argument on physics grounds is wrong, to my understanding. Radiative greenhouse forcing drives all atmospheric circulation - and as we warm up that effect increases, driving all other heat flows along with it. Convection, evapo-transpiration etc. should all increase along with the increase in radiative heat flow. The increased greenhouse effect drives an increase in the height of the effective radiating layer of the atmosphere (and an increase in the overall height of the troposphere) because the temperature of that radiating layer is governed by the total energy flow that is limited to close to the steady state input from the Sun - i.e. 255 K. That means there is a greater difference between surface and top of the troposphere, driving more vertical heat flows of all sorts. Greater heating of the polar regions would decrease horizontal heat flows as the equatorial-polar temperature difference declines, and there are of course many other regional effects as the modeling reported by the IPCC indicates. But a necessary decrease in evaporation or precipitation doesn't seem like a conclusion I've ever heard of before, or supported by the basic physics here.
For the physics you just about argued my point with "total energy flow that is limited to close to the steady state input from the Sun".
If the total energy flow into the troposphere is in, rough, equilibrium with the total energy flow out, and we rightly assume the planet is not absorbing a significant amount more of solar input (which is after all the argument of a certain camp of denialist), then increasing the amount of energy transported by radiation, vis-a-vi the approximate T^4 dependence of the Stefan-Boltzmann law, will by definition decrease the energy transported by other mechanisms.*
I think what is missing is that it is not the forcing alone that drives convection, but rather the vertical temperature gradients. So it comes down to the question of how much will forcing even out vertical temperature gradients?
Which again with your statement "increased greenhouse effect drives an increase in the height of the effective radiating layer of the atmosphere (and an increase in the overall height of the troposphere)" highlights a decreasing temperature gradient (delta kelvin over meters). If you heat an system evenly it won't circulate, conversely if you cool a system evenly it won't precipitate. Its kind of a basic principle of undergraduate thermal physics that only a temperature gradient can do or extract work.
The critical clue is in the altitude dependence of the sensitivity to CO2. In the lower troposphere, that supports a humid atmosphere, you have water vapor saturating the absorption spectrum. Changes in CO2 concentrations just won't directly push a humid sea level environment that hard; both because H2O is such a great absorber, and has such a large specific heat capacity, that "smooths out" rates of change.** But at high altitudes, where the atmosphere is effectively dry, the sensitivity to CO2 concentrations is much more pronounced.
A real world example of this is the BC coast: a sea level community like Tofino will maintain a moderate 6-12C day and night, year round, but just a few 10s of kilometers away, nearing 3000m daily extremes can go from -20C overnight to +20C daytime near reflective surfaces (e.g. light colored granite and snow fields). What does CO2 do here? Well it is a long wave radiation blanket, so those overnight lows will steadily warm, and those daytime highs will increase as well. So in this, admittedly shoddy, real world example increasing CO2 will increase the average temperature at 3000m from 0C, say, to ~2C, thus decreasing the average temperature gradient from 10K/3000m to 8K/3000m.
ahhh you say, but we still have cold air to fall through the warm lower air, it is just starting higher up. And that is true, but think about the consequences of that fact. The lower troposphere is losing its temperature gradients, which means more of the convection will originate and end in a higher part of the troposphere, taking its water cycle with it. That is a very unfortunate feedback because for the lower troposphere to shed its solar input it must again heat some more to make up for the energy not shed through convection.
Really they had this, at least analytically, worked out by the early 70s, and possibly as far back the 50s. But I think a lot of those results were wrapped up in military research.
*clearly assuming convection transports energy to the Stratosphere, e.g. latent heat of condensation in cumulonimbus formations
** For any thermal system undergoing flows of energy the inverse heat capacity sets the characteristic time scale for the rate of change of temperature. This flows from the relationship: DT/Dt = DU/Dt X (dU/dT)^-1 = (C_v)^-1 DU/dt
Taking lower case d as the partial derivative, lower case t as time, U as energy, and T as temperature.
Due to the enhanced greenhouse effect we will have more energy available at the surface. Part of that goes into warming the air (temperature increases), part of that goes into more evaporation. What goes up, must come down, thus on global average precipitation will also increase by about 1 to 2 % per degree Celsius temperature increase.
Local changes can be very different from the global changes due to changes in the circulation and convection. And not only the precipitation will increase, but also the evaporation. Thus what happens to soil moisture and the ecosystems and agriculture that depends on it is another difficult question.
foreshortened - sorry for the delay in responding. Victor above is right, but it's also interesting to me to see where you go wrong in your claim - when you said "increasing the amount of energy transported by radiation, vis-a-vi the approximate T^4 dependence of the Stefan-Boltzmann law, will by definition decrease the energy transported by other mechanisms." this indicates a fundamental misunderstanding of what's actually happening with the greenhouse effect. The process is considerably subtler than just "increasing the amount of energy transported by radiation".
In a very rough manner, it helps to think about the *net* flow of radiation, rather than the separated upwards and downwards flows (which do both increase according to the Planck law - Stefan Boltzmann for black body averages - as temperatures warm). You can think of the energy somewhat like a fluid flowing through the Earth system. Starting at the surface, the energy can escape through several mechanisms - radiation, evaporation of surface water adding water's latent heat energy to the local air masses, convection to move those warm air masses upwards and cooler ones down, and also to a very limited degree, conduction through the atmosphere. You can imagine if for some reason the gases of the atmosphere suddenly greatly increased in viscosity and it became difficult to move air masses around, then the heat flow associated with convection would greatly decrease, and more energy would go into other modes. Well, that's very close to what happens to the radiation component when you add infrared-absorbing gases. The more CO2 and water vapor in the atmosphere, the shorter the distance radiated energy can move before it gets absorbed. The greenhouse effect results in a DECREASE in the net flow of energy by radiation (also described as an increase in the "back-radiation" from atmosphere to ground), and thus, as temperatures inevitably rise due to the resulting energy imbalance, an increase in energy flow by other modes.
Further on you refer to the decline in lapse rate expected - however this is a second-order effect, a negative feedback that comes about only AFTER the surface has started warming. It also is significant only in areas with high relative humidity - particularly the tropics, where the moist adiabatic lapse rate dominates. Averaged over the whole of the planet the lapse rate reduction is not particularly large. And in any case it is caused by a greater quantity of water vapor in the air, so it is coupled with the positive feedback associated with the greenhouse properties of water vapor - the net effect is definitely to increase, not decrease, the even imbalance, i.e. to reduce even further the net flow of energy via radiation. As Victor says, I think it is a fairly robust conclusion that, averaged over the whole planet, evaporation and precipitation will increase under global warming. But local changes may be quite different.
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