D. Andrew White, 2007
Anyone familiar with botany has, no doubt, noticed recurring patterns in leaf morphology. To use a single but extended example, the palmate leaf form typical of maples also occurs in many plant species, species not very closely related to each other. Palmate leaves are the fingered leaves common, but not universal in the maples. Typically there are 3, 5, 7 or 9 main lobes radiating from the stem. The terminal lobe is typically the longest, and each lobe, as numbered from the apex, is shorter than the preceding lobe. Palmate leaves occur in: grapes (Vitis), Aralia, Cecropia, hemp (Cannabis), gourds (Cucurbita) and even ferns such as Doryopteris pedata and Hemiontis palmata. Elaborating further, the sub-lobes of these leaves are, usually, quite similar in outline to the larger lobes with which they occur. Often indeed even the serrations on these sub-lobes are similar to the large lobes in outline. The self-similarity of leaf lobes are a reflection of the fact that plant growth consists, in large measure, of reiteration of modular units, like an algorithm (White 2005). This modularity reflects the fact that leaves are self-similar because they are not individual modules in and of themselves. Rather, leaves are composites of sub-units. Each lobe develops under the same set of genetic instructions as the other lobes, and each affected by very-similar morphodynamic factors (Halle & Oldeman 1978, Hallé et al 1978).
Because plants are so modular, it has been possible to model their growth via algorithms on a computer. These algorithms are so exacting that they manage to generate leaf and branching architectures that closely correspond to real plants. They can even be used to predict what forms mutations could take (Prusinkiewicz & Remphrey 2000, Mündermann et al 2003).
Leaves, branches and floral elements develop from primordia in the buds, or 'meristems' of green plants. The primordia on a meristem are not randomly arrayed. Rather they are arranged in spirals, and a limited set of spirals at that. These spiral arrangements are called 'phyllotaxies'. What at one time was considered astounding was the fact that the number of overlapping spirals, and the number of major vessels, and also the number of leaves per spiral almost invariably are members of the well known 'Fibonnaci sequence', 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 et cetera. The number 4 being a notable and important exception. In the Fibonacci sequence each successive number is the sum of the preceding pair of numbers. The Fibonacci sequence was discovered, without botany in mind, by Leonardo Fibonacci sometime around 1200 AD. Fibonnaci developed the number system to model the growth rate of animals, he used rabbits as his example, or so the story goes. Oddly enough, the ratio between the higher pairs in the sequence approach the 'Golden Ratio' or 'Phi', an irrational ratio equal to approximately 1 : 1.6182. This particular ratio was, to the ancient Greeks, the ideal ratio on which to proportion buildings for the best aesthetic effect. (The Greeks did not actually know the exact ratio.) Over the years number mystics have made much of the fact that Phi occurs widely in both art and nature. Phi is a ratio implicit in the pentagram, as Pi is implicit in a circle. Like all natural proportions, natural examples of Phi tend to be rather approximate (Livio 2002).
Phi is sometimes called the 'most irrational of irrational numbers'. The continued fractions leading towards Phi converge with their asymptote more slowly than any other continued fraction series.
Phi = 1+ 1/(1+ 1/(1 + 1/(1/ + 1/(1 ... {ad infinitum} )
Physically this means that if each leaf is offset by 360o/Phi of a rotation from the leaf before, it would have the best possible spacing. All other rotation angles bring a leaf closer to leaves lower on the twig. It is well to note that plants do not have exactly 222.469o spacing between leaves. The angle is approached, not duplicated. The fact that the angles are even close to some set angle, is a manifestation of the modular and reiterative nature of plant growth.
Note that the fact that phi ‘is the 'most irrational of irrational numbers’, is about the sum of phi’s role in reality. This fact does give the phi ratio some advantage over others in specific physical processes involving the reiterations of modular units varying in scale. The occurrence of phi in some aspects of plant growth has a physical basis – not a Platonic one. Most of the extravagant and mystical claims about phi appearing in nature are pure hokum.
Apparently, as experiments indicate, this phyllotaxy is not caused directly by any specific genetic instruction. Rather phyllotaxy is incumbent upon the developmental processes that give rise to primordia. Mathematical models of phyllotaxy have been devised since the late nineteenth century. In the early 1990s the French physicists Stéphane Douady and Yves Couder developed the most concise and descriptive model of this phenomenon. As per usual, the mathematics required to demonstrate this 'simple' process is not very simple itself. Phyllotaxies depend on a few simple principles governing growth. Each successive primordium develops so as to distance itself, as far possible, from the other primordia in its vicinity. Obviously the spacing of the primordia depends intimately upon the timing of growth, the relative size of each primordia, phytohormones and many other factors. Despite all the possible variations in rate and timing of growth, the tendency of buds to space themselves, out automatically results in the generation of the full phyllotaxic order. Even the exceptions to the phyllotaxic norm were explained by the computer models. The most common 'exception' is the decussate pattern. Decussate leaves form as paired opposites, each pair at right angles to the preceding pair. On a meristem two buds develop simultaneously. The most fitting position of the new pair of primordia is to be between, and at right angles to, the next older pair of primordia. As a result there develop four main vascular bundles and four ranks of leaves. This pattern is found in such otherwise dissimilar plants as bromeliads and maples (Jean 1983, Douady & Couder 1992).
The botanists F. Hallé, R.A.A. Oldeman, M. Zimmermann and P.B. Tomlinson have classified the above-ground structures of plants into 23 basic 'Architectural Models', each model representing a fundamental structural algorithm. The algorithms are based on a few simple rules, including: whether the main stems and branches have the same phyllotaxy, and whether or not there is a dominant twig at each forking of a branch, or whether or not the stems fork at all. To some extent the architectural classification of trees is artificial, and reflects the parameters chosen for the classification scheme. Nevertheless, the limited number of models does reflect some real characteristics of plant growth. Intermediates between these models generally do not occur because they are logically impossible. They would not be impossible if plants displayed random shifts between phyllotaxies and branching patterns. They do not. As if to confirm that plant growth is algorithmic, even mutations conform to the architectural models. Architectural mutants do occur. Mutations can affect, ever so slightly, the timing and sequence of bud development. These same mutations can leave other aspects of growth relatively unscathed. Minor tweaks in a plant's developmental programme that vastly alter the overall appearance of the plant. For example the Hyphaene ventricosa palm normally has a single leading stem, as is typical of the palmae. Highly branched mutant forms do occur. Instead of always forming a rosette of leaves around a leading stem, the meristems give rise a rosette of leaves and also a few additional side branches. Viewed from a distance these mutants look for all the world like another species (Good 1974, Hallé & Oldeman 1978, Hallé et al 1978).
So modular are plants that insects can use this modularity to their own advantage. By disturbing the plants' developmental routines via chemical agents, insects can induce the formation of galls. These galls act as protection and food sources for the larvae. Galls require little labour on the part of the larvae because the plants grow the galls for the insects. If one examines galls such as the strobilary galls of the genus Salix, the willows, one shall notice that they are composed of broad stunted leaves on an even more stunted stem between them, the overall effect producing a tight round cone-like structure. These galls house the larvae of a fly called Rhabdophaga strobiloides. Many other examples of insect galls develop from similar disruptions in the development of plant tissues. In most cases these disruptions produce organized structures, not crude masses of tissue (Rose and Linquist 1982).
The existence of modularity, architectures and phyllotaxy has lead to the speculation that the evolution of plants has been to some degree non-gradual, which is to say it can make jumps, that it is 'saltative'. 'Saltational evolution' or 'saltatory evolution' has been championed by the University of California botanist G. Ledyard Stebbins, C.G.G.J. Van Steenis of the Rijksherbarium in the Netherlands, Ronald Good at the University of Hall, and many others. Many examples of plant arrangements cannot be explained as having evolved gradually from one form to another, for the simple reasons that intermediate forms are often logically impossible. Leaves on a twig are either decussate or they are not. Fruits are fused, or they are not. Some of the more subtle difference in floral structure between closely related plants also do not appear to have any possible intermediate forms. And in such cases the differences can best be explained as relatively minor alterations in the developmental sequence. Were once a single seed per fruit was formed, now a mutation makes several seeds per fruit. Where a leaf once had three leaflets, a mutation adds an extra pair. Most of these changes could best be described as re-arrangements of parts, or re-arrangements in timing. Much as one can alter a computer program by changing a single line of code (Good 1974, Hallé & Oldeman 1978, Hallé et al 1978, White 2005).
Douady, Stéphane and Yves Couder. 1992. Phyllotaxis as a physical self-organized growth process. Physical Review Letters. 68: 2098-2101.
Good, Ronald. 1974. Features of Evolution in the Flowering Plants. Dover Publications, Inc. New York.
Hallé, F. , R.A.A. Oldeman and P.B. Tomlinson. 1978. Tropical Trees and Forests -An architectural analysis. Springer-Verlag. New York.
Jean, Roger V. 1983. Croissance Végétale et Morphogenèse. Masson. Paris.
Livio, Mario. 2002. The Golden ratio - the story of phi, the world's most astonishing number. Broadway Books. New York.
Mündermann, L., MacMurchy, P. Pivovarov, J. and Prusinkiewicz, P. 2003. Modelling lobed leaves. Proceedings of Computer Graphics International CGI. 60-65.
Prusinkiewicz, P. and Remphrey, W.R. 2000. Characterization of architectural tree models using L-systems and Petri nets. In M. Labrecque (Ed.): L’ arbre – The Tree 2000: Papers presented at the 4th International Symposium on the Tree. 177-186.
Rose, A.H. et Lindquist, O.H. 1982. Insectes des feuillus de l'est du Canada. Ministère de l'Environnement Service canadien des foréts. Ottawa. 202-203.
Shinozaki, K.; Yoda, K.; Hozumi, K. and Kira, T. 1964. a quantitative analysis of plant form - the Pipe Model Theory. I. Basic analyses. Japanese Journal of Ecology. 14: 97-105.
Tomlinson, P.B. and Martin Zimmermann. 1978. Tropical Trees as Living Systems. Cambridge University Press. Cambridge. 217-218.
White, D. Andrew. 1992. Relationships between foliar number and cross-sectional areas of sapwood and annual rings in red oak (Quercus rubra) crowns. Canadian Journal of Forest Research. 23: 1245-1251.
White, D. Andrew. 2005. Architectural mutation and leaf form, for the palmate series. Journal of Theoretical Biology. 235 (2): 289-301.
It is perhaps now common knowledge that Earth owes its oxygen rich atmosphere to photosynthetic organisms, cynanobacteria, green algae and plants. About two thirds of the organic matter produced per year is by land plants, and the rest by aquatic plants. However, at least half of the oxygen in Earth's air comes from photosynthetic algae in the oceans, algae such as the diatoms and other planktonic plants. Oceanic algae are apparently more effective in liberating oxygen than land plants.
The atmospheres of Venus and Mars are very similar in composition, despite their vastly different temperatures. Earth has a special air about it.
James Lovelock, an atmospheric scientist at Princeton, proposed the now famous "Gaia Hypothesis" in the early 1970s. James Lovelock was driven to his hypothesis by the astounding observation that the Earth's atmosphere is not in chemical equilibrium. Rather, Earth's atmosphere is in dynamic equilibrium. It is held in an unstable mixture of nitrogen and oxygen by photosynthesis, which in turn is powered by the Sun. Lovelock made the prediction that Mars is either poorly stocked with life, or completely devoid of life. The atmosphere of Mars can be explained by abiotic chemical processes, Earth's atmosphere cannot. Both the Martian and Venusian atmospheres are dominated by carbon dioxide (98% +). If all of the carbon locked up in Earth's limestone, chalk, oil and coal were to be liberated, and to react with oxygen, Earth's atmosphere would then come to resemble that of Venus.
According to most theoretic models, the Sun should not have remained stable in radiant output during its long duration. Yet life has not been exterminated by any solar vagary. Also, the amount of carbon dioxide has never been so much that life was blotted out by a run-away greenhouse effect. Nor has the carbon dioxide level ever fallen so low that all living things were obliterated by an extreme glacial epoch. There are other details of the ecosystem that are likewise controlled by the collective action of algae and plants. These processes keep the Earth's biosphere in dynamic equilibrium.
Earth has rain. Venus is too hot for rain, and Mars is now too cold for rain. Rain, being slightly acidic, erodes surface rock and generates carbonates from the reaction of carbon dioxide and basic elements. These carbonates are washed into ocean sediments, forming limestone and dolomite. The net result being a culling of carbon dioxide from the air. Green plants play a major role also, they remove carbon dioxide, and release dioxygen. Many protozoa, with carbonate shells, store up carbon as they live. The precipitation of a portion of these testae to the ocean floor is another carbon sink. Vast sediments of chalk testify to the quantity of carbon removed from the air by these plankta. Furthermore, not every living thing is recycled in the food chain. Sometimes large plants are covered in water borne mud, hence coal is formed. Some of the fats and oils of organisms becomes trapped in sediments, resulting in the formation of petroleum. All of these processes also remove carbon dioxide from the air. Over the eons these carbon sinks have greatly reduced Earth's store of carbon dioxide gas.
Even though there are some abiotic processes that remove carbon dioxide, there are no simple chemical reactions that liberate free oxygen. Earth's atmosphere is about twenty one percent oxygen gas (dioxygen). Dioxygen is too reactive to endure for long in an atmosphere. It is now known, with a great deal of certainty, that photosynthesis is the driving force behind Earth's rich unstable atmosphere.
Science has tended to confirm the existence of the Gaian balance. However, Gaia's explanation has eluded science. It is difficult to comprehend how such a fine balance could have originated, through the independent natural selection of its constituent organisms. Perhaps, as Lovelock suggested, complex ecosystems tend to develop feedback loops that tend to self-stability. Some mathematical models seem to support this hypothesis. The latest climatic models suggest that the latent heat of the oceans keeps the Earth's temperatures within liveable bounds. And that even if the Earth had a much more elliptical orbit, the Earth's oceans and atmosphere would resist extreme seasonal swings. Gaia, according to this hypothesis, may be a fortunate consequence of both thermodynamics and ecodynamics on a grand scale. Others propose that the Gaian balance is evidence of a Creator Deity. Just because this Theistic evolution hypothesis falls outside of the scope of empirical science, at present, does not imply that it can be dismissed.
Some people have argued against the existence of a perfect Gaian balance. For example, there is evidence that there was an extreme glacial epoch just prior the Cambrian. This ice age was so extreme that it is possible that life had a "close call" with total obliteration. The oceanic ice may have become so thick that most photosynthetic bacteria and algae barely survived. However, within ten million years of ice age's end the frond-like Ediacaran creatures were thriving. A few tens of million more and the oceans were teaming with protozoa, plants and animals. Since life obviously did survive, perhaps there was not a total freeze-over. Or perhaps, algae survived near volcanic hot-spots. Even this close call, if it was one, was not an isolated disaster. There been seven or so major extinctions since the Cambrian. Some extinction events, evidence suggests, may have been due to asteroid/comet impacts. The mere existence of such 'close calls' has been used as evidence that the Earth was just plain lucky. On the other hand, the fact that life has survived all of these disasters has been interpreted as evidence for the robust resiliency of the Gaian balance.
The Gaia Hypothesis is not without its critics. As originally framed, the hypothesis was a little too wildly metaphorical - almost metaphysical. It left the impression that earth is demonstrably sentient. In later more detailed versions of the hypothesis Lovelock was less metaphoric. His mechanism for Gaia was in essence merely ‘feedback stabilisation’ - i.e. complex interconnected systems tend to resist change. This toned down version of Gaia was a far cry from claiming that Earth is literally alive.
The word Gaia comes from the Greek name of the Earth Goddress. This was originally intended to be taken metaphorically, not literally. Nevertheless, it is not surprising that the Gaia Hypothesis has become a doctrine of the so-called New Age religion(s). Believers in vital force, Earth energies, telluric forces, ley lines, dowser’s energy lines, earth lights, qi gong and feng shui, have all referenced the Gaia Hypothesis as 'evidence' of their claims. All of these claims go beyond the realm of science into speculative metaphysics. All involve suppositions that do not follow logically from the scientific evidence currently available. No doubt these concepts and wild fancies reflect the psychological appeal of the Earth Mother archetype. One can appreciate the mythopoetic allure of these ideas, but it is unwise to take them too literally.
What ever the true origin of the Gaian Process, it certainly now exists. Green plants are essential to it. We owe our every breath to them.
Since photosynthesis powers the majority of the Earth's biota, one would think that the Plantae Kingdom dominates the world. This is not exactly true.
In 1988 two oceanographers, Sallie W. Chrisholm and Robert J. Olson, discovered the most common genus of cyanobacterium. Prochlorococcus is probably the most important single taxon of photosynthetic organism alive on Earth today. By shear number and ubiquity the bacterial genus dominates the seven seas. This common blue-green alga is a genus of cyanobacterium or bacterial alga. The Prochlorococcus are only 0.5 to 0.7 microns wide, and live mostly within 200 metres of the surface, where there may be 1-3 x 105 cells per millilitre of sea water. They have a mere 1.7 x 106 base pairs in their DNA, which is very brief for an independent organism. This group of bacters accounts for roughly a quarter of all photosynthetic oxygen liberation, and a similar proportion of all biotic carbon dioxide absorption, on Earth. Together with other algae they account for more than half of all oxygen production and half of all carbon dioxide consumption. There are at least 35 species of Prochlorococcus known. Its cells are spread thinly, but widely, through all of the world's oceans. Indirectly the sea is powered by algae such as them. The sea could be considered like one big leaf.
Judging by what is visible to the human eye, the oceans seem top-heavy with carnivorous animals. Seaweeds and other visible plants comprise a minority of what is visible in the sea. In the sea there are several layers of carnivores eating carnivores before one reaches algae-eaters. Hence,the greater bulk of the oceanic flora is invisible to the human eye!
Ellis, Richard. 2001. Aquagenesis - the origin and evolution of life in the sea. Viking Penguin. London.
Hecht, Jeff. 2003. Giant creatures appeared millions of years early. New Scientist. 177: 13.
Hoffman, Paul F. & Schrag, Daniel P. 2000. Snowball Earth. Scientific American. 282(1): 68-75.
Lee, Robert Edward. 1999. Phycology. 3rd Edition. Cambridge University Press. Cambridge.
Lovelock, James E. 1975. Gaia: a new look at life on Earth. Oxford University Press. New York.
Morton, Oliver. 1999. Is the Earth alive? Discover. 20(10):98-102.
Speed Weed, William. 2002. Circles of Life. Discover. 23(11): 42-47.
Szalai, Veronika A. & Brudvig, Gary W. 1998. How plants produce dioxygen. American Scientist. 86(6): 542-551.
Symbiosis is the co-existence of two different species, each of which mutually benefits the other. Lichens are symbiotic combinations of algae and fungi, allowing the fungus to photosynthesise like a plant! Many mammals use bacteria to aid in their digestion of foodstuffs. Mycorrhizal fungus-plant interactions are one of the most interesting examples of symbiosis.
Vascular plants commonly have a symbiotic relationship with fungi. In this relationship the fungal mycelia intertwine with the rootlets of plants. Roots can absorb water and highly soluble minerals. Fungi are generally better at absorbing low mobility ions, such as phosphorous, than are plants. Fungi, however, do not produce sugar and are not themselves photosynthetic. Fungi must feed upon sugars which originate, ultimately, from green plants.
| Phylum | Spores | Examples |
| Archaemycota (Pin-moulds) |
 |
Endogone spp. & zygomycetes pin-mould fungi. |
| Glomeromycota (Glomales) |
 |
Many fungi in the order Glomales. Most widespread clade of mycorrhizae fungi. |
| Ascomycota (Sac Fungi) |
 |
Truffles (Tuber spp.), morels (Morchella spp.) & many other soil ascomycetes. |
| Basidiomycota (Club Fungi) |
 |
Amanitas (Amanita spp.), brittlegills (Russula spp.), boletes (Boletus spp.), & many other toadstools. |
The term mycorrhizae is given to those soil fungi commonly found in association with plant roots. Mycorrhizae gain sugar from the roots of plants and, as if by way of trade, pass disolved minerals to the plants' rootlets. This system is symbiosis at its best. The plant gains enhanced mineral uptake. The fungus gains an energy source.
Endomycorrhizae occur where fungal hyphae penetrate the root cells. Arbuscular endomychorrhizae form branching hyphae inside cells of rootlets. ‘Arbusculars’ are most common in the Glomales order of the glomeromycetes. Ectomycorrhizae have hyphae which encircle rootlets and the spaces between the outer layer of rootlet cells. Often these hyphae form complete sheaths around the tips of rootlets. Ectomycorrhizae are commonly formed by the ascomycetes, basidiomycetes, and some of the zygomycetes. Some plant species have strictly ecto- or endo-mycorrhizae, but not both. However, other plant species can have both forms of mycorrhizae. Certain species can even switch mycorrhizal forms during their life cycle.
Mycorrhizal associations were discovered by Professor A.G. Frank of the Landwirtschaftiche Hochschule in Berlin. A.G. Frank was commissioned by the King of Prussia in 1885 to find a way to grow truffles (Tuber spp.). Truffles are a fungus which produces an edible, and delicious, fruiting body that grows underground. These have been esteemed delicacies in Europe for centuries. Pigs and dogs were, and are, used to find these ‘mushrooms’ by smell.
A.G. Frank failed to develop a method of truffle cultivation. Truffles were not added to the list of agriculturally grown mushrooms. A.G. Frank did, however, discover that truffles are symbiotic with tree roots (Heinrich 1997) .
There are thousands of mycorrhizal fungi species. Some are specifically associated with particular plant hosts. This is why certain mushrooms are only found near specific tree species. Others are generalists. Mycorrhizae may be ascomycetes, basidiomycetes, glomeromycetes or zygomycetes. Some of these fungi are common mushrooms and toadstools. Mycorrhizal pin-moulds are especially widespread. Nature has apparently invented mycorrhizae several times.
Plants with mycorrhizal associates occur in the mosses, conifers and angiosperms. Some mycorrhizae fungi even pass nutrients to both mosses and trees! So established is the phenomenon of mycorrhizae, that some plants exploit the fungi involved. Mycoheterotrophic parasites include young orchids, pinesaps, Cryptothallus liverworts and a host of other plant species. These parasites do not make fair exchanges of carbohydrates for mineral nutrients.
Study after study has shown that mycorrhizae really do enhance the health of mosses, herbs and trees. Mycorrhizae are just one example of the incredible relationships between living things.
Glomeromycota were, until recently, classified in the phylum Zygomycota. However, genetic comparisons in the 1990s suggested that the glomales are best considered a taxon unto themselves. The members of the Order Glomales are mostly, if not entirely, asexual. The glomales can produce branched sporangia, each sporangium with very few spores. Glomales are arbuscular endo-mychorrhizal fungi, their hyphae penetrate the cell walls of rootlets. The genus Glomus is one widespread example of an endo-mycorrhizal symbiont. Most of common glomale fungi are obligate mycorrhizals. A few of the glomeromycetes (e.g. Geosiphon spp) live symbiotically with cyanobacteria instead of roots.
Glomales are a very widespread clade of mycorrhizal fungi. Many plants have glomales as their main root associates. Fossil evidence suggests that the mycorrhizal association between glomale-like fungi and plants is very ancient. Probably both land-plants and fungi have had an association since times premordial.
As if to add superfluous complexity, there seems to be yet another layer of symbiosis in the glomales. The glomales often have little bladder-like extensions on their mycelia. In these chambers there are colonies of endobacteria. Some of the genes of the Candidatus Glomeribacter gigasorarum seems to be involved in mineral uptake. So quite possibly the ‘glomeribacters’ are symbiotic.
Mycorrhizal fungi in the genus Endogone are still often classed as zygomycetes. These fungi form ectomycorrhizal sheaths around rootlets. Endogones are not imperfect fungi. The sporocarps of the endogone fungi are small puffy masses under forest duff. Mice often feed on these fruiting bodies.
Confusingly, genetic studies show that the endogones are more closely related to some of the chytrid moulds than to other pin-moulds. However, unlike the chytridiomycetes they do not have flagellated gametes. Presently endogones are not considered to be true chytrids. Probably chytrids are simply those archaemycetes that have retained flagellated gametes. Those archaemycetes which are non-aquatic seem to have lost the need for flagellated gametes. Chytrids may be considered the more fully aquatic members of the Archaemycota.
Daniel, W.D., Helms, J.A. and Baker, F.S. 1979. Principles of Silviculture. McGraw-Hill Book Company. New York. 215-221.
Grierson, Donald and Covey, Simon N. 1988. Plant Molecular Biology. Second Edition. Blackie. Glasgow.
Heinrich, Bernd. 1997. The Trees in My Forest. Cliff Street Books. New York. 180-187.
Bonfante, P. 2003. Plants, Mychorrhizal Fungi and Endobacteria: a Dialog Among Cells and Genomes. Biol. Bull. 204: 215-220.
Schüßler A, Schwarzott D, and Walker C. 2001. A new fungal phylum, the Glomeromycota: phylogeny and evolution. Mycological Research 105(12): 1413-1421.
It is ‘normal’ for small planets to have atmospheres of carbon dioxide (CO2), nitrogen (N2), water vapour (H2O) and argon (Ar). Venus’ atmosphere of carbon dioxide and nitrogen is probably much like Earth’s ancient state. Venus is too hot for water molecules, they disassociate in its infernal air. Ancient Earth certainly had both water and rain showers. Nitrogen, carbon dioxide and water vapour were probably dominant in its original atmosphere.
Earth has rain and lifeforms, which together tend to scrub carbon dioxide from the air. Over the eons, apparently, earth has irregularly, and slowly, been losing its atmospheric carbon dioxide quota. Nevertheless, Earth’s small carbon dioxide level, along with water vapour, plays a crucial role for the biosphere. It is largely due to the greenhouse gasses that Earth's average surface temperature is above 0oC. Carbon dioxide is a greenhouse gas. It adsorbs higher frequency light waves, and re-emits the energy as infrared light. Infrared light does not easily escape into outer space. Consequently, it recycles thermal energy in the atmosphere which would otherwise radiate off into space, i.e. the temperature goes up. Greenhouse gasses, in effect, make the Earth darker on the infrared side of the spectrum. Least one think that this process is thermodynamically impossible, there is a concrete analogy. In full sunlight, a dark pavement can warm above freezing, even if the air temperature is sub-zero.
Now I have said that carbon dioxide has been falling through the ages. This has not been true in the past century and a half. Since the 1850s the total concentration of carbon dioxide in the atmosphere has risen from 0.0280 to 0.0355 percent by volume. The gas concentration can be calculated from bubbles dissolved in ice cores, as well as from old measurements made by chemists. During the same time period the average temperature of the lower atmosphere has risen by 0.6 Co. A less than one degree shift may seem small, but this is a global average. In reality, it has meant a rise of several degrees in the temperate and polar regions. It has also meant noticeably shorter and warmer winters.
Carbon dioxide levels have increased, temperatures have increased. It is not exactly rocket science to hypothesise that a surge in a 'greenhouse gas' could cause atmospheric temperature to rise. In fact, there is little reason to doubt that our current bout of global warming is due to the release of carbon dioxide from the burning of fossil fuels.
Earth has experienced fairly rapid climate swings before. Climate may change because of: solar variation, the precession of the equinox, axial wobble, asteroid impacts and periods of increased volcanism. The current rate of global temperature change is similar to the shifts which occurred at the beginning and end of each ice age. However, the temperature rise seems to be a bit more rapid than that which occurred after the last ice age. This climatic shift may have adverse affects on wildlife.
Global warming is expected to be a mixed blessing and curse. Global warming may increase crop yields overall. Almost certainly precipitation will increase globally. Nevertheless, certain regions could experience increased drought. Glaciers are already melting at an unprecedented rate. Global warming would also cause the sealevel to rise. Rising sealevel would be very bad news for lowlands such as the Maldives. Increased equatorial temperature could increase the frequency of hurricanes. Rising carbon dioxide levels would certainly increase the oceans' acidity. Of greatest concern is the probability that some ecosystems may not be able to respond quickly enough. Climate change may occur faster than some species can adapt. Some species may face extirpation or extinction if the climate shift is greater than they can tolerate. This stress is especially true today when human activity has fragmented and over-exploited too many ecosystems. In short, unstable temperatures could be very bad for wild ecosystems.
Glacial periods, in the last few millions years, have tended to dominate over warm interglacials. Typically glacial advances have lasted for over 100,000 years, and interglacials for 10,000 to 20,000 years. Most of these warm spells have lasted circa 10,000 years. (Although, the length of these warm spells is uncertain.) The ice age cycles seem to be controlled by cyclical variations in tilt of the Earth, and precession of the equinoxes. If this is true, then in the present era we should be near the end of the current interglacial. (i.e. it has been 11,500 years, so far.) It is possible that anthropogenic global warming is older than was hitherto believed. There have been speculations that humans have actually been causing global warming via CO2 increases for 7000 years! This could be true, because serious and extensive deforestations have been occurring since the agricultural revolution. The CO2 increases could have been due to human instigated reductions in total vegetation mass. Indeed, ice core analyses suggest that the current interglacial has had anomalously high CO2 levels. It is posible that even before the industrial revolution humans have been causing global warming indirectly.
Kunzig, Robert. 2005. Turning Point. Discover. 26 (1): 26-28.
McGregor, Glenn R.and Nieuwolt, Simon. 1998. Tropical Climatology, Second Edition. John Wiley & Sons. Chichester.
Nilsson, Annika. 1992. Greenhouse Earth. John Wiley & Sons. Chichester.
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