Friday, February 20, 2009
Mammoth Threat from Methane Melt.
Now, it has been assumed that it was climate change that reshaped the region, but a new perspective has been offered by Sergey Zimov, who is the director of the Northeast Science Station at Cherskii, which is located in the Republic of Sakha (Yakutia), leading to the demise of the large animals there. He turns the argument on its head, and reasons that they were hunted to extinction and it was the loss of the animals that resulted in the ecological transformation there. To test this notion, Zimov's team of researchers are reintroducing animals such as bison, horses, and eventually tigers, among other species in an effort to reconstitute the Pleistocene ecology. If it is successful, the strategy will support another example of the way human activities can impact on the environment. Rather than this being a castigation I do sometimes muse that maybe part of the reason that humans are on earth is to change it, as part of some grand design.
The northern ecosystem originally stretched from France and on across the Bering Strait to Canada and from islands in the Arctic over as far as China. It was big. Over the million year duration of the Pleistocene epoch, the ebb and flow of the ice-sheets ploughed the lands of most of northern Europe and North America, while northern Siberia remained largely untouched. At the same time that the herds of herbivores disappeared in their millions, the grasslands on which they depended also vanished. Holocene vegetation is dominated by moss and shrubs, in contrast to the Pleistocene megafauna, and has insufficient transpiration of moisture to dry-out the soil. When soil is saturated with water, the decomposition of biomass is inhibited and so are the nutrients required to underpin plant growth.
In contrast, when mosses are decomposed, the land becomes overgrown with grasses within a short time of perhaps one to two years. These grasses then dry-out the soil through rapid transpiration and create a steppe-like ecosystem. When herbivorous populations are low, the grass remains uneaten and it builds-up on the surface of the soil, thus shading it and reducing its fertility. Thus mosses and shrubs with relatively low water and nutrient needs become dominant. When the mammoths thrived, in the winter the animals ate the grasses from the previous summer's growth, and throughout the year they kept the soil productive with their dung and also trampled down mosses and shrubs thus preventing them from taking hold.
We now get to the interesting part in relation to climate change, which may be influenced by what happens in northern Siberia, in particular to its permafrost. In total, these frozen soils, with an average carbon-content of around 2.5%, encapsulate around 500 Gt of potentially mobile carbon. This amounts to two and a half times the carbon in all the world's rainforests combined. It is known that the tundra permafrosts are melting, and when the soil does thaw, microbes will begin to convert that long-held soil carbon into carbon dioxide (if there is oxygen present) or to methane under anoxic conditions. Methane would in principle be worse, since it has around 100x the global warming potential of CO2, and could cause a potential feedback mechanism where consequently rising temperatures melts more of the permafrost and releases more GW gases, at least so the argument goes.
Can the restoration of Pleistocene ecology, in which grasses and their root systems stabilise the soil, put the brakes on this process? Indeed, can stabilising the soil prevent the permafrost from melting? Probably not. Millions of herbivores surely also emit large amounts of methane, which must be offset against the gain in albedo of the earth from maintaining these frozen lands, which reflects solar radiation back into space. There are many factors involved in the global warming scenario, all of them interconnected. However, looking toward nature to find solutions to our energy and climate problems is I believe the way to go.
Wednesday, February 18, 2009
Low-Carbon Retrofit for a Quarter of British Homes.
The move is likely to be applauded by energy poverty action groups (you are in energy poverty if you spend 10% or more of your disposable income on energy) and environmental campaigners concerned about anthropogenically induced climate change. Now the latter theory has fallen into a quite acrimonious debate, and the likes of James Hansen are being called alarmist (or worse), since there is credible scientific evidence that shows the whole global warming phenomenon to be shall I say, less clear-cut than we are generally led to believe. I noted the other day that Hansen's old boss has come clean that he isn't totally convinced any longer by the "it's all our fault" rhetoric. Given the huge impact on resources that will be necessary to adopt low-carbon, and carbon-capture strategies whether the issue is right or wrong is no affair of mere semantics.
However, while I await the conclusions of experts on this, it is clear that curbing carbon fuel use must mitigate global warming if indeed this is the cause of it, but more immediately it will ameliorate the terrible threat of running short of conventional fuels if we do indeed transfer our needs to renewable energy sources. I am skeptical about how quickly we can install millions of wind-turbines etc. but a move to a biomass economy (bioeconomy) insofar as this is possible does seem the way forward, since we can begin growing stuff immediately: it is however the processing of it into fuel and chemical feedstocks on the grand scale, requiring vast new engineering that is the real challenge. However, in an economic downturn of conventional production and commerce, might this not be an opportune time to begin honing such new developments, both to drive the economy and to prepare for an inevitable future with far less oil?
We will need to reskill our population at some stage for practical tasks and now seems as good a time as any to begin doing that. It would also be a good time to look at what exactly the universities are turning-out in terms of graduates, in measure against what skills the country actually does need, rather than a bums-on-seats policy that simply makes the government's unemployment statistics look less bad than they would otherwise. In all likelihood the new universities will return to their useful job as the polytechnics who trained the workforce for industry. When manufacturing industry "fell" in the 1970's/early 1980's, that manufacturing base was "lost" and it seems to me that now is the moment to kick-start a new industry based on biomass/renewables and the more efficient use of energy in buildings and for whatever level of transport provision we need in a revamped, relocalised society.More small scale (e.g. CHP) low carbon heat systems are to be encouraged, probably by financial incentives - tax breaks etc., although since the Sustainable Energy Academy estimates that if home-owners spend £15,000-20,000 on their carbon-improvements, they would recoup that in fuel savings over a period of 10-15 years, or sooner if fuel prices increase, as they almost certainly will, it is the punter who will bear the burden. Entire districts may also be offered community clean energy schemes, or a mass refitting with more efficient energy devices. The Conservatives take the idea of "self-help" a stage further and might award grants of up to £6,500 per household, which would be recovered over up to 25 years from expected average reductions of £160 in annual gas and electricity costs.
I find a similarity here with the self-help "go to university" scheme (also known as loans and top-up fees), where a customer (they are not students any more) ends up owing an average of £15,000 - 20,000 in exchange for that piece of paper whose worth must yet be proved in the rapid skid of a world whose future cannot be guaranteed by past outcomes. We are now in entirely unassailed territory.
Saturday, February 14, 2009
Newcastle Coal Gas.
Miller quotes that one ton of Newcastle coal yields 9,250 cubic feet of gas and 13 cwt of coke. One cwt is a "hundredweight" of which there were 20 to the ton and so the quantity of coke amounts to 65% of the initial charge of coal into the retorts.
There are 35.3 cubic feet (cu. ft.) to the cubic meter (m^3) and so that ton of coal yields 9,250/35.3 = 262.0 m^3 of a gas with a "specific gravity" of 0.410. The latter is referenced to air and so if one m^3 of air has a mass of 1.20 kg, the equivalent volume of coal-gas has a mass of 0.410 x 1.20 = 0.492 kg. Multiplied by 292.0 m^3 this gives a total mass of 128.92 kg.
The Imperial "long ton" amounts to 2240 pounds (lbs) or 1016.05 kg (or 1.01605 tonnes). Thus the mass gas yield is 128.92/1016.05 = 12.7%.
The difference between 65% coke plus 12.7% gas and the initial ton of coal leaves 22.3% which we may attribute to coal tar.
I noted in the previous posting that one m^3 of the gas of the composition quoted there has a mass of 536.69 kg, and if a similar quantity of it were produced by retorting one ton of some coal, this amounts to a similar 262.0 x 0.53669 kg/1016.05 kg = 13.8%. The somewhat (9%) higher mass of the gas in the latter case may be due to the presence of more higher hydrocarbons, which will increase its calorific value slightly, or to carbon dioxide which will reduce it.
"Miller's Elements of Chemistry", Part III. Organic Chemistry", John W. Parker and Son., London, 1855.
Thursday, February 12, 2009
Heating Value of Wood Gas and Coal (Town) Gas.
A composition is: N2, 50.9%; CO, 27.0%; H2, 14.0%; CO2, 4.5%; CH4, 3.0%; O2, 0.6%.
Per cubic metre (m^3) of gas, this amounts to:
270,000/24450 (volume of one mole of an ideal gas at 25 deg C and atmospheric pressure) = 11.04 moles CO, x 28 = 309.12 g.
140,000/24450 = 5.73 moles H2 = 11.46 g.
30,000/24450 = 1.23 moles CH4 = 19.68 g.
Given the molar enthalpies of combustion for these gases, we get:
11.04 x -283 (kJ/mol) + 5.73 x -286 + 1.23 x -890 = 3.12 MJ + 1.64 MJ + 1.09 MJ = 5.85 MJ/m^3.
Remaining (incombustible) gases:
509,000/24450 = 20.82 moles N2 = 582.90 g.
45,000/24450 = 1.84 moles CO2 = 80.98 g
6,000/24450 = 0.25 moles O2 = 8.00 g
Total mass of 1 m^3 of gas = 1,012.14 g. Hence the calorific value of wood gas is 5.85 MJ/m^3 or 5.85 x 1000/1012.14 = 5.78 MJ.kg.
A composition is: H2, 51%; CO, 15%; CH4, 21%; C2H4 (ethene), 3%; CO2 + N2 = 10%.
Per m^3 of gas, this amounts to:
510,000/24450 = 20.86 moles H2 = 41.72 g.
150,000/24450 = 6.13 moles CO = 171.78 g.
210,000/24450 = 8.59 moles CH4 = 137.42 g.
30,000/24450 = 1.23 moles C2H4 = 34.44 g.
100,000/24450 = 4.09 moles CO2 + N2 = 151.33 g (assuming a 50:50 mixture).
Making a grand total of 536.69 g.
Again, using the molar enthalpies of combustion we derive:
20.86 x -286 + 6.13 x -283 + 1.23 x -1423 + 8.59 x 890 = 5.97 + 1.73 + 1.92 + 7.65 = 17.27 MJ/m^3. This translates to: 17.27 x 1000/536.69 = 32.18 MJ/kg.
Wednesday, February 11, 2009
Even at a yield of 10 tonnes/hectare of biomass residue, we need 3.33 x 10^9 hectares or 3.33 x 10^7 km^2 of land to produce it on, which at 33 million km^2 is over twice the area of arable land on earth (15 million km^2) and more than one fifth of the total land area of around 150 million km^2 (30% of the total 500 million km^2 of the surface of this blue planet). Clearly to provide all our energy from biomass is a very tall order, and it is obvious that we cannot simply substitute biomass in matching amount for fossil mass, as supplies of oil, gas and coal begin to wane. Since however, we will not need to convert overnight from fossil mass to biomass, and energy conservation will be forced on us by a simple lack of resources, biomass offers the potential to provide a significant proportion of the final energy bill, once we have made efforts to use less energy overall. Certainly it can make a significant contribution to the transitional period from the high energy status quo to a future civilization based on a more efficient use of energy and which furthermore is generated from renewable resources like biomass.
Most biomass is simply burned to provide heat, and this can be done more efficiently in CHP (Combined Heating and Power) systems particularly in small-scale units. However, we need a more adaptable form of energy which is best provided in the form of liquid and gaseous fuels. In the latter aspect, synthesis gas or "syngas" is especially flexible, since not only can it be piped and burned directly, but also converted to methanol, other alcohols including ethanol and synthetic diesel using Fischer-Tropsch catalysis.
The simplest firm of gasification is done by pyrolysis, which usually involves heating biomass, e.g. wood, in a restricted supply (or the absence of) air. Thus, the cellulose, hemicellulose and lignin is decomposed to a mixture of solid (char), liquids (bio-oil) tar and a mixture of gases, mainly CO2, H2, CO and methane. The relative amounts of the different phases can be changed according to the temperature of the pyrolysis, the contact time with the heated zone, the pressure and the amount of oxygen present either in the diluted form of air or in some applications pure oxygen is used, but providing this adds-in its own contribution to the overall energy budget.
In terms of gasification, at temperatures >1000 degrees C, and short contact times of less than a second around 70% or more of the initial charge of biomass is converted to gas. There are gasifiers that work at lower temperatures say 400 degrees C and use more air, but provide a gas with a low thermal content of maybe 6 GJ/tonne which is around one fifth that of coal-gas (27 GJ/tonne) and about a tenth that of natural gas (methane, 55.7 GJ/tonne).
During WWII, cars and tractors were run using on-board wood-gasifiers, to cope with the fuel shortages in Europe, petrol and diesel being reserved for the military. The unit was called Gazogene. Full EROEI analyses are necessary to evaluate such gasification strategies, it is generally assumed that (as in making biochar by pyrolysis) the external heat source will come from biomass too. The beauty of using air/oxygen is that the gasification reaction becomes self-sustaining, i.e. the material effectively "burns" albeit in a controlled manner.
In addition to using biomass taken from fields, there is the option to use the technology to convert land-fill waste into useful fuel, as well as directly gasifying wet-biomass including algae which saves energy in drying the material prior to use as is normally necessary. In terms of converting algae to fuel, it may prove more efficacious to gasify the total mass directly rather than choosing a high-oil yielding variety, extracting the oil from it and then transesterifying that into biodiesel. The syngas could be used directly as a fuel or converted instead to synthetic diesel using FT rather than biodiesel. NB the calorific value of biodiesel is around 36 GJ/tonne compared with syn-diesel at 44 GJ/tonne, which is the same as for normal diesel.
The focus on biomass is of course that it is renewable, ideally carbon-neutral (on the grounds that the carbon content of the plant was taken from the air originally through photosynthesis), and is hence a better bet than fossil fuels which are being exhausted continually from their finite reserve and which contribute CO2 to the atmosphere.
I shall post more on this subject as I think more about it, but these are just some initial impressions.
The figure of 500 EJ in 2005 from: http://en.wikipedia.org/wiki/World_energy_resources_and_consumption
There is another link at: www.sfpa.sk/dok/PDFI/MZeman.pdf (on slide number 6) that mentions 10,878 Mtoe which x 42GJ/t = 4.6 x 1020 J for 2005, and is thus also in the same ball-park.
Friday, February 06, 2009
Yorkshire Carbon Capture Project gets Go-Ahead.
Mr O'Brien is quoted as saying: “It is essential to replace older polluting power stations that are reaching the end of their lives with new stations that operate more efficiently."
What is clear is a shift of emphasis by the government, who while being aware of the anthropogenic carbon/greenhouse effect/global warming/climate change theories, are also well informed that the U.K. has an energy supply problem, to put it mildly. Hence avoiding people freezing in the winter and keeping the lights on in general here, is of more immediate concern than GW, the connection of which to wholesale climate change is a matter of model and debate. Thus, coal appears as a useful indigenous fuel, especially as we are running out of our other indigenous fuels - gas and oil, from North Sea fields that are likely to be almost dry in 6 years time.
How much accessible coal we have depends on how it is accounted, but there is enough for some decades in all likelihood, and much longer if underground coal gasification (UCG) is implemented on a grand scale. Uzbekistan has been running a UCG plant since the 1940's. Indeed, the National Coal Board ran an experimental UCG plant in Derbyshire in the 1950's but rejected the technology on the grounds it was too expensive compared to cheap solid coal that we were still producing to the tune of around 150 million tonnes/year.
The Hatfield plant is not a standard coal-fired power plant, which simply burns finely powdered coal, but rather uses coal that has been gasified, and it is the gas that is burned. It is termed a Combined Cycle plant and it was approved along with two more, one at Pembroke in South Wales (2 GW) and another at Kings Lynn in Norfolk (1 GW). Powerfuel, the company which bought the Hatfield colliery in 2006 plans to construct the "world's first large scale integrated gasification combined cycle (IGCC), near zero carbon emissions powers station with carbon capture capability." The grand total of 4 GW worth of electricity from the three new power stations is enough to power 4 million homes.
Their intention is to do this in two stages: first an 800 MW combined cycle gas turbine (CCGT) plant will be fabricated which burns coal that has been gasified into "syngas" (a mixture of CO + H2), and then this will be "upgraded" to an IGCC facility of 900 MW capacity running on coal from the Hatfield colliery. Richard Budge, CEO of Powerfuel, stressed the advantages of having a power producing facility in the north of England that runs on a secure fuel source based in the U.K. - i.e. coal. The first stage at Hatfield is expected to cost around £900 million and the second around £1 billion.
The initial CCGT station will employ gas turbines provided by General Electric, which are proven in their purpose, and it is planned that its construction will start in 2009 and finish in 2012, by when a connection to the national grid will have been implemented. The stage-two gasification technology is licensed from Shell and will capture 90% of the carbon emissions so that the fuel will be essentially hydrogen.
That carbon then has to be "removed for sequestration", i.e. put somewhere safe for hundreds to thousands of years, usually in rock-formations. Where, I wonder? Or, perhaps feed it to algae and turn it into liquid fuels? In that latter case, the carbon would end up in the atmosphere when the fuel was burned but it might be argued that there would be a trade-off against the oil that would otherwise be used as a fuel instead.
 "Green Light for 900MW carbon capture coal plant in Yorkshire." http://newenergyfocus.com/do/ecco.py/view_item?listid=1&listcatid=32&listitemid=2223§ion=Carbon
 "First 'clean coal' power station gets go-ahead," by Robin Pagnamenta: http://business.timesonline.co.uk/tol/business/industry_sectors/natural_resources/article5670940.ece
Thursday, February 05, 2009
Thinking Positive - Carbon Capture.
The two concepts might be combined however, at least assuming there is enough time, and that the EROEI stacks-up. For the moment, however, let's think positive and assume that it does. Proposals for geoengineering always make me uneasy, including the idea of "seeding the ocean". Principally, my disquiet stems from a feeling that all aspects of nature are interconnected and by messing about with one thing, an unforeseen calamity might ensue elsewhere - the butterfly effect, to use a well-known phrase.
However, if phytoplankton could be caused to bloom, say in the Southern Ocean, 1 Gt (billion tonnes) of carbon could be captured annually. It is claimed that regenerative agriculture might sequester around 3 Gt of carbon each year (although there is some dispute about this), and that by 2050, biochar production could account for another 1 Gt of carbon annually. In principle - and this is where the link comes in - the carbon in the soil can stay there and improve its quality, but if the other kinds of captured carbon could be harvested, it might provide a useful potential source of biomass/fuel. Growing algae on a local level - a "village pond" you might call it - could provide energy to replace fossil fuels for local communities, without impacting on arable land.
Since we emit 7 Gt/year of carbon from fossil fuels, the sum comes out something like (in Gt): 7 - 3 - 1 -1 = 2 Gt left to worry about. A cut in fossil fuel use by 50% through biomass curbs that to 1 Gt. Photosynthesis already absorbs around 3 Gt of carbon/year into oceanic phytoplankton and land-based plants, and if localised algal production cuts emissions from oil by another 1 Gt (assuming that we need 1 Gt less since we have that from algal biomass), the combined scheme is carbon negative by -3 Gt/year.
Hence in 40 years this would have cut 120 Gt of carbon from the atmosphere, which would reduce the concentration of CO2 by around 50 - 60 ppm.
Now this is an extrapolation of sums I have seen done and on paper it looks pretty rosy, implying that we can eke-out our oil, gas, coal and nuclear and at the same time bring down the carbon-content of the atmosphere to pre-industrial levels by the end of this century.
What is rarely mentioned let alone costed-in is the lead-in time, the energy costs, the EROEI, the materials, the engineering and so on... that's when it begins to look less rosy.
For example, while I like the idea of biochar, the stated goal by the International Biochar Initiative (IBI) is that we could have 1 Gt/year of carbon being drawn from the atmosphere by 2050. O.K. let's assume that's 40 years time and that there is currently (in Gt terms) about zero biochar being produced currently. Even if we assume a linear growth in the technology, that "wedge" (if you draw a straight line on a piece of graph paper from 0 - 1 Gt on the y-axis up to 40 years on the x-axis) that only accounts for 20 Gt of carbon, or a reduction of about 10 ppm, which is neither here nor there, and the biomass production and processing would be simply colossal when viewed en mass.
That said, if that level is achieved by, and sustained beyond 2050, 1/7th of all carbon (14%) captured per year is significant, and could be a higher proportion if fossil fuel burning has by then been significantly curbed, either deliberately or because we have less of them available. The main benefit of biochar is likely to be in terms of improving soil quality, if it is employed as a soil-amending agent, and thereby reduces demand on water and nutrients like N and P to grow crops. The latter is likely to be particularly significant in parts of the world where the soil is poor, e.g. Africa and Asia. In the U.K. soil tends to be very rich - too rich sometimes - but even here, the incorporation of biochar into the soil would attenuate problems from run-off waters that contain too much phosphate and nitrate.
Regenerative agriculture is somewhat contested in terms of its carbon capture potential, and there is little evidence that we can "seed" the oceans in a practical fashion, or recover the plankton on any significant scale. Indeed, if massive amounts of phytoplankton were to grow through seeding, the emissions of sulphur compounds (H2S, dimethylsulphide etc.) which are oxidised to particulate "sulphate" matter in the troposphere, would have the effect of further seeding cloud formation. This might help to cool the planet through reflecting more sunlight back into space, which sounds good in GW-terms, but it would surely affect rainfall and how the earth-systems distribute water around the planet.
What I can see is that production of biochar and algae on a local level, as part of a programme of lower-energy living could offer some benefits. There is also (for once) the advantage that there are a lot of people on the planet. Hence if a community of 2000 people could catch and sequester 200 tonnes of biochar per year (100 kg/person), 7 billion of us in total could sequester almost 0.8 Gt/year (close to the IBI projection of 1 Gt/year by 2050). However, it is the curbing of energy use that really counts. Back to the village algae-pond. As a total area, we would need around 3200 km2 of ponds to fuel Britain (more of which could be turned to other purposes than personalised transport through relocalisation), that suggests that each village pond would need to be:
3200 km2 x 100 ha/km2/60 million x 2000 = 10.7 hectares for each 2000 person community. It's big but it doesn't sound impossible when broken down like this. The real problem is how to process the algae either by extraction of its oil/transesterification or bulk thermal gasification. It might be simpler to just grow the algae (and other biomass), dry it out and burn it as a source of thermal energy.
All of the above is going to take an awful lot of engineering, hence energy and time, but let's not depressed about the details, and look at those "happy sums" again.
Monday, February 02, 2009
It's a Gas.
In any case it is a good portent. We do need to focus on renewable sources, but the engineering involved will be massive. As I have alluded before, the jury is still out regarding anthropogenic climate change, but either way, fossil fuels are in limited supply and so the same action - of using less of them - satisfies both agenda.
Sir Richard Budge ("King Coal" as he is dubbed deservedly) is a man to be admired. He has opened a formerly closed (Thanks Maggie!) coal mine in Yorkshire - a scene of dispute of the worst industrial strife in British history; ignoring the Tonypandy riots in South Wales - and adjacently, has planned to implement a combined-cycle power station, which generally gets 56% of the thermal energy recovered rather than the 36% that is dictated by the thermodynamic Carnot Cycle limits, and is also "clean". This, at any rate is his intention.
His company, Powerfuel, has requested planning-permission to build a 900 MW plant, with low-emissions, i.e. "clean-coal", fuelled from the Hatfield colliery, which he unsealed in 2009, funded by Russian investors. I use the world "unsealed" deliberately because Margaret (Maggie) Thatcher's government inaugurated the sealing of the mines with concrete as a demonstration of force against the trade unions who, although I am a socialist, were asking for trouble... sadly it is the population of this country in general that paid the price for both the union militants and Thatcher's unabating worship of monetarism - a kind of academically discredited economic policy that has brought the world to the pivot of bankruptcy.
As I note, it is a combined cycle (IGCC) - that's Integrated, Gasification, Combined Cycle - plant, which first gasifies the coal dust and converts it to synthesis gas (a mixture of H2 and CO) which is burned at high efficiency. The consequent CO2 is separated out and can then be "sequestered" in some way - possibly it could be fed to algae and the resulting yield used in a thermal gasification process to provide more "syngas".
However, not that it matters much, as I have already made the point, Dr John Theon, who was apparently James Hansen's boss - the main climate change/anthropogenic CO2/global warming protagonist - has come clean that he isn't convinced that this theory is right. I keep an open mind on this - I am not a specialist in atmospheric chemistry, but I am a well qualified physical scientist and I can understand many original papers when I read them and do the sums - but Theon alleges that data has been "cherry picked" to fit the whole picture, and that the "models" which is all they are, a mathematical "fit" inside a computer may give particular results according to a given algorithm; i.e. other models will give different predictions. Theon says, "They have resisted making their work transparent so that it can be replicated independently by other scientists. This is clearly contrary to how science should be done. Thus there is no rational justification for using climate model forecasts to determine public policy."
Now, the latter is an extremely significant point. We are projected to invest billions of whatever currency prevails in carbon capture and sequestration, and yet it is not absolutely clear whether letting that element in the form of its most highly oxidised state - CO2 - into the atmosphere will fry the planet or not. But, burning less carbon in the form of fossil fuels is paramount, because we are going to run out of cheap and readily availabile amounts of them sooner not later. Either line of argument (resource considerations or global warming) takes us to the same conclusion.