1. Gaia theory
The word 'Gaia' has over time collected many meanings, several of which are firmly outside the realm of science (e.g., in 'New Age' it is a name for a conscious planet, a generally benevolent 'Mother Earth' figure). In science it is a hotly debated topic, but many scientists accept at least some of its basic concepts. In its simplest form, Gaia theory holds that life and non-life on Earth form a combined system, and have developed in very close connection over Earth's history. Life on Earth has shaped Earth as much as Earth has shaped life, and the actions of living organisms have had the consequence that the planet remained habitable. Therefore we must not just look at the evolution of life on Earth, or just look at geochemical evidence for changes in rocks, but we must look at the combination of traces of living organisms as well as evidence on their non-living environment in order to understand the development over geological time of the Earth System. An example: storage of carbon in the lithosphere instead of in the atmosphere, so that the Earth did not start to overheat when the Sun increased its heat output (overview lecture on climate).
The close links between life and non-life are based on the chemical interaction of life (biosphere) and non-living matter in the atmosphere, hydrosphere and lithosphere: elements move through biogeochemical cycles between these four 'spheres'.
The most familiar biogeochemical cycle is that of the element carbon. Carbon is present in the biosphere in organic matter (in which it is taken up through photosynthesis or chemosynthesis reactions - reaction 1 in overview lecture) as well as in skeletons (precipitation of carbonate - reaction 2, overview lecture). Other elements also move through biogeochemical cycles, many of which have not been studied in detail. The pathways of the elements through the four 'spheres' (the biogeochemical cycles of the elements) are determined by the chemical character of the element. For instance, carbon occurs in the atmosphere as the gaseous compound CO2, whereas phosphorus (P) does not occur in gaseous compounds (at atmospheric temperatures and pressure) and thus is not present in the atmosphere, with the exception of solid dust particles. Nitrogen, however, is abundant in the atmosphere as the fairly inert gas N2 (80% of the atmosphere), as well as in many reactive compounds including ammonia (NH3), and nitrogen oxides (NO, NO2, together also called NOx), as well as in ions that are dissolved in water to form nitrogen-based acid rain (HNO2, HNO3). The nitrogen cycle thus is much more complex than both the simple P-cycle and the already complex C-cycle. The most intensely studied biogeochemical cycles (in addition to that of carbon) are those of nitrogen, phosphorus and sulfur, all of which have been heavily influenced by anthropogenic processes (nitrogen and phosphorus through fertilizers; nitrogen through combustion including cars; sulfur through combustion of coal, including in power plants).
Gaia theory has evolved from studies of possible life on other planets, specifically by a scientist (J. Lovelock) who tried to device ways to test samples from other planets for the occurrence of 'life' - however that might be on other worlds. This led to the ideas that one should test for some form of chemical disequilibrium in the atmosphere of a planet. This consideration of what must be seen as essential for 'life' led to a deeper understanding of the numerous ways in which the biosphere on Earth has influenced the composition of the atmosphere (e.g., oxygen), the oceans and the lithosphere.
A very simplistic way of trying to understand how living organisms can influence the 'habitability' of the planet on which they live is the concept of "Daisyworld" (see reading). The object of looking at 'Daisyworld' is to get a first, basic understanding of how life can influence the habitability of its planet, through long-term climatic stability, without having to get into the enormous complexity of life on Earth. We assume a simple planet, on which the only life forms are daisies, and where climate is determined only by how much heat is reflected back into space from the planet's surface (we do not worry about the atmosphere of this planet). On Earth this reflected heat is about 30% of all heat received by the Sun, so it is a reasonable assumption that the size of this heat flux can influence the climate of a planet. In our simplest Daisyworld, we assume that the daisies come in black and white. White daisies reflect back more heat into space than black ones, and they grow better at higher temperatures than the black ones.
The faint young Sun of Daisyworld is at first too faint to warm it up into the temperature-range where daisies can grow. As it increases its heat output (luminosity), the right temperature for daisies will be reached and they will somehow start to grow on the planet (we assume that all other conditions such as soil, fertilizer etc. are just right for the daisies). Both white and black daisies start to grow, but at the temperatures at the low end of the range for daisies the black daisies do better, out-compete the white ones and grow over a large area of the planet, with the exception of the warmest regions in the tropics. But all those black daisies reflect very little heat back into space, so that the planet heats up. As the temperature rises during the global warming in Daisyworld, the white daisies start to take over from the black ones and expand their growth region - because they grow better at higher temperatures. But the increased area covered with white daisies reflects back more heat into outer space, so that the planet cools down. The black daisies (which grow better at lower temperatures) start to increase their range, which means that less heat is reflected back into outer space, so the planet starts to heat up...... and so one and on. As a result, the planet's climate does not simply reflect the gradual increase in heat influx from the Sun, but is kept stable by the alternating increase in abundance of the two types of daisies. In fact, these alternating abundances have given the climate system of Daisyworld negative feedback (see text on the Earth's thermostat, first lecture). More complex Daisyworlds have been modeled, having many different species with colors between black and white, and varying temperature tolerances. The presence of more species of daisies tends to make the planet's climate more and more stable over long time periods.
The stabilizing effect on the environment by the presence of the vari-colored daisies has some interesting implications for interactions between organisms and environment, and between various type of organisms. The daisies, which directly compete with each other for resources (water, fertilizer, carbon dioxide), are according to the Gaia theory in the long run of life on Daisyworld 'cooperating', in order to keep their planet's climate stable (not consciously, but the net effect is there).
There exist many other examples of co-dependency of organisms which at first sight appear to be in competition with each other. The Calvary Tree on the island Reunion in the Indian Ocean, for example, was declining in numbers because its seeds never germinated, although at least some stands of adult trees did not appear to be under environmental stress. The forest service was worried about the gradually decreasing numbers of Calvary trees, and found out during a tree-census that all individuals were at least about 300 years old (in the 1970s). People then realized that these trees all germinated in the years before the extinction of the Dodo, the large, flightless relative of the pigeon that became extinct in the 17th century. The hypothesis was formed that Dodos had eaten Calvary tree seeds (fossil Dodo remains had been found in areas where the Calvary tree grew). Dodo's had gizzards filled with stones; in birds such as turkeys seeds are worn down by gizzard stones. Calvary tree seeds were then fed to turkeys, and several germinated. Seeds from which the very hard and thick outer layers of the pit were ground down partly also turned germinated. This observation suggested that the stones of Calvary tree fruits became tougher and tougher (so that they could survive the passage through the Dodos), until finally they could no longer germinate without the passage through the bird, and the tree needed the Dodo to reproduce.
Research resulting from the proposal of the Gaia theory has involved unicellular (eukaryotic) algae which photosynthesize and secrete a shell consisting of tiny plates of CaCO3 (the organism is called Emiliania huxleyi; you're not asked to remember that). These organisms live floating in the upper hundred meters or so of the oceans, and are most common at middle latitudes. They grow in enormous numbers in the spring. In winter, there are few of them because there is not enough light during large parts of the day. In addition, the small algae are during the winter storms moved vertically through the water column, so that they are for a large part of the day below the zone where light penetrates into the ocean waters (about 70 m in open ocean). Therefore photosynthesis occurs only to a limited extent during the winter months, and fertilizing materials (called nutrients, mainly nitrogen- and phosphorus-bearing compounds) are not used up by the primary producers. In spring, the organisms receive more and more light from the sun, start photosynthesizing rapidly, using the accumulated nutrients. The spring blooms end when the nutrients run out, and the primary producers are eaten by the small, herbivorous organisms that live floating in the oceans. During the large spring blooms, the unicellular algae secrete large quantities of a gas called dimethylsulfide OR DMS {(CH3)2S} which escapes from the ocean into the atmosphere. In the atmosphere this gas is oxidized into minute particles of sulfate, which serve as cloud condensation nuclei (rain drops form from water vapor around these tiny particles). The algae thus influence weather: the clouds reflect more heat back into space, causing local cooling; they shade the oceans, slowing down photosynthesis of the algae, thus production of DMS.
We do not fully understand the role that these algae play in global climate, and what the function is of the emission of DMS. It has been proposed that the algae (which live dominantly at middle latitudes) benefit because they ensure that they do not overheat, nor suffer under too high a salinity. When the sun shines most, photosynthesis proceeds most rapidly, but sea water also evaporates more rapidly, leading to increased salinity (salt remains behind when water evaporates) and of course increased temperature. The DMS from the algae generates clouds, thus local rain fall, which counteracts the increase in salinity increase as a result of evaporation. The clouds also cool things down and slow down evaporation. The system has its own negative feedback, because the photosynthesis slows down when the clouds from, which slows down the formation of DMS, which slows down cloud formation. The overall effect is that conditions are kept comfortable for the algae.
The Gaia theory is sometimes read as indicating that humans can hardly make a mess of things; that they can count on Gaia ('Mother Earth') to keep things going. This is a very naive view, in that it assumes that 'what is good for Earth, is good for people'. This is, of course, not necessarily true, certainly not on human time scales. Increased amounts of CO2 in the atmosphere, for instance, will be taken up in the deep oceans, dissolve limestone on the bottom of the oceans (reaction 2), and do no harm to Earth as a planet but that will occur only on time scales of millennia. In the mean time, the CO2 is in the atmosphere where it can cause greenhouse warming on time scales of decades to hundreds of years: human time scales. Over Earth's history, CO2 levels have been much higher than today for long periods of time (as we will discuss later in this class), without apparent damage to the biosphere - but that will not be of much help to human civilization if the polar ice caps were to melt and flood small island nations and almost all large cities in the world. The possible damage as a result of greenhouse gas emissions is not to the biosphere as a whole, but dominantly to human structures (economy; buildings close to sea), spread of diseases, and changing food supplies (agricultural changes).