Climatic Stability: Coincidence or Control?
Summary of Major Biological Influences on the Climate
Introduction
The concept of Gaia, which views the Earth as a living, self-regulating entity has raised a few eyebrows and provoked a good deal of discussion over the past 15 years. In this chapter, we explore the extent to which the biosphere controls the global climate system by means of a model representing the climatic evolution of a fictitious planet called Daisyworld, created by James Lovelock, the originator of the Gaia concept.
Climatic Stability: Coincidence or Control?
Although it is difficult to determine the climatic conditions during much of Earth's early history, there are a number of lines of evidence that suggest that the climate was not so different then as compared to now, at least in terms of the average planetary temperature. Various water-deposited sedimentary rocks tell us that the hydrologic cycle was up and running and that the oceans were not entirely frozen at a very early date (around 3.8 Byr), gypsum crystals in ancient sediments indicate that the temperature could not have been too high since it is not stable above about 40°C. Occasional glacial deposits in the Precambrian also argue for a planet that was not too hot. Recently, we have learned that there may have been some important, yet brief climatic excursions (the Snowball Earth Hypothesis), but on the whole, most paleoclimatologists consider that the climate has remained relatively stable over the long time periods. This is somewhat of a puzzle given the history of our Sun, and the implications for Earth's temperature, shown in the graph below.
It is generally believed that the answer to this problem of a faint young sun is that the Earth had a more potent greenhouse in the past and that the strength of that greenhouse has declined over time. This makes sense if you simply think about all the limestone that has accumulated over time; limestone effectively is a sink for CO2. So, if you imagine going backwards in time and burning limestone that formed at different stages in the past, this would release the CO2 back to the atmosphere. This is effectively what is attempted in the figure below.
If this history of atmospheric CO2 is approximately correct, then in combination with the solar history, the planetary temperature could remain more or less stable (ignoring short-term excursions). Is this simply a fortuitous coincidence, or is it a consequence of some control mechanism? Recall that limestone is a biogenic sedimentary rock -- it is a consequence of life.
Gaia
The above question is one of many that is addressed by the Gaia theory or paradigm, as proposed and advocated by James Lovelock. At the core of this idea is the notion that the Earth is essentially a living entity that has a capacity for homeostasis, the self-regulation of temperature and chemical composition and metabolic functions that manifests itself in the form of a stable climate system and an atmospheric composition that is starkly different from that of our neighboring lifeless planets like Venus and Mars. This claim rests on a particular way of defining a living organism, one in which we do not require that the organism has the ability to reproduce. As support for this definition, Lovelock offers up the example of Lombardy Poplars, which are all male and can only be propagated by transplanting cuttings -- surely we would not argue that the Lombardy Poplar is not a living organism. The tree analogy also works in the sense that with most trees, only the outer skin contains living cells that are actively engaged in metabolic processes; on Earth, the same is true.
Some more traditional members of the scientific community have chafed at the notion of Gaia in part because it is sometimes referred to as a theory or hypothesis and perhaps also because of the use of the term Gaia (earth goddess, mother earth), with its spiritual overtones. As a hypothesis, Gaia is not easily tested and so cannot easily be falsified; thus it is considered to be unscientific. Mot scientists are also uncomfortable at the slightest incursion of religion or spirituality into their work. So, it is not surprising that this notion has not been embraced too strongly by the scientific community. The situation is not helped by the fact that its inventor and principal advocate, James Lovelock, is not in the academic world and is a bit eccentric and combative at times.
Nevertheless, there are very few earth scientists who do not accept that notion that life has always had a tremendous impact on the physical and chemical characteristics of the Earth's surface. Every nook and cranny of our planet's surface is covered with organisms -- at least 30 million species by some estimates -- all of whom are exchanging gases with the surface environment, leaving their imprint on the atmosphere and altering the surface of the Earth.
Summary of Major Biological Influences on the Climate
Life is a powerful force on our planet and many biological processes can and do influence the climate on a variety of scales. Some of the major processes are listed below to make the point that life definitely does play a major role in the climate. In addition, since most life forms respond to their environments, these processes contain feedbacks, most of which are negative (i.e., stabilizing).
Plants -- their distribution, density, and color affect the planetary albedo and thus play a role in the amount of solar energy absorbed and reflected from the surface. To the extent that higher temperatures leads to greater evaporation and more moisture in the atmosphere, the albedo-effect of plants can be a weak positive feedback mechanism -- the more the grow and cover the surface, the hotter and moister the climate gets, enabling the plants to colonize previously barren areas, decreasing the albedo and warming the planet.
Evapo-transpiration -- cools surface locally, accounts for up to 20% of atmospheric water vapor and thus contributes to the greenhouse effect in an important way. There is an element of positive feedback here in the sense that a warmer Earth may also be moister, thus there would be more water for the plants to use and evapo-transpiration would increase. But, it is also true that plants tend to be more efficient in their use of water when it is warmer.
Photosynthesis -- consumes atmospheric CO2, stores it, and transfers it to the soil; this rate is sensitive to the carbon content of the atmosphere and thus acts to regulate the greenhouse -- it is a negative feedback mechanism.
Photosynthesis -- produces oxygen, some of which eventually forms ozone high in the atmosphere; this ozone shields harmful ultraviolet rays from the surface and warms the stratosphere, creating a temperature inversion that helps trap water in the lower part of the atmosphere, preventing its escape. At some stage, this was probably a positive feedback mechanism in the sense that as the ozone layer built up, the surface became a more hospitable place and may have encouraged more life forms to evolve, and some of those were likely to have been photosynthesizers. On the other hand, photosynthesis also gave rise to a whole class of organisms that ate plants and consumed oxygen -- a form of negative feedback.
Microbial Respiration in Soil --
1. returns carbon to the atmosphere; this rate is also sensitive to temperature (and thus carbon content of the atmosphere). This process represents a positive feedback mechanism, as long as the carbon in the soil holds out.2. some of the products of this process are organic acids that aid in the chemical weathering of minerals within the soil; the weathering products are transported to the oceans where they contribute to the alkalinity of the oceans, which controls the CO2 dissolved in the oceans, which then affects the CO2 in the atmosphere. Increased alkalinity in the oceans allows for the uptake of more atmospheric CO2, thus cooling the planet, so this is a negative feedback.
3. adds CO2 to the air spaces within the soil, which then combines with water to form carbonic acid that is the main agent of chemical weathering of minerals; this effectively takes CO2 from the atmosphere and since the dissolution reactions are temperature sensitive, this represents one of the major negative feedback mechanisms for the control of the climate on a timescale of about a million years.
Biogenic Methanogenesis -- some kinds of microbes living in marine sediments and wetlands produce methane, a potent greenhouse gas. In marine sediments on the continental shelf, this biogenic methane may be trapped in the form of gas hydrates (tiny cages of ice crystals that trap methane within), which are believed to represent vast reservoirs of carbon on a global scale. These gas hydrates can melt and release their methane if the water temperature rises or the water pressure drops (sea level falls), conceivably leading to rapid, dramatic increases in atmospheric methane and thus global warming. Here, the microbial process is creating conditions that make the climate susceptible to sudden changes rather than exerting a steady control on climate. In terrestrial wetlands (including rice paddies), biogenic methane is released to the atmosphere, contributing to a warmer Earth, which in turn accelerates the processes of biogenic methane production, making this a positive feedback (but not one that is capable of dominating the climate due to the limited amount of methane produced).
Respiration of Oceanic Plankton -- produces gases (DMS) that, upon reaching the atmosphere, condense and serve as nuclei for water condensation in the atmosphere, and thus contribute to the formation of clouds, which control the amount of solar energy reaching the surface. Some people consider that the production of DMS is greater at higher temperatures, making this a negative feedback mechanism, but the data are not entirely clear at this point in time.
Microbial Respiration in Ocean Sediments -- returns carbon to the oceans in the form of CO2; this CO2 then dissolves some calcite in the sediment and releases Ca2+ ions to the water, which increases the alkalinity of the oceans, which in turn decreases the CO2 dissolved in seawater, which then enables the oceans to absorb more CO2 from the atmosphere; this is thought to be a major contributing factor in the reduction in atmospheric CO2 during the glacial periods of the Pleistocene. To the extent that this respiration is enhanced by greater supply of oxygen to the deep sea floor by deep circulation, which is enhanced by ice at the poles, this may constitute a positive feedback (but the microbes are also limited by available food, thus the rate of deposition of organic matter is also important).
Marine Organisms -- construct shells of calcite, which later accumulates on the seafloor to form limestone, representing a form of long-term storage for carbon. Overall, this is probably a negative feedback mechanism in the sense that when sea level is higher (and the planet is presumably warmer), there is a greater surface area for shallow-water reef-forming organisms to live, and they draw more carbon out of seawater, allowing for more atmospheric CO2 to be taken up by the oceans. In addition, if it is warmer and there is greater precipitation on the land surface, there will be greater run off and greater supply of nutrients to the oceans, thus greater productivity of marine life.
Land Animals -- production of methane contributes to the greenhouse effect, but in a fairly small way. This is likely to be a very weak positive feedback mechanism.
From the above, you can see that the microbes and plants are the key players in exerting control over the global climate. The supposedly more advanced, sophisticated organisms such as ourselves, appear to be far less important. In fact, in terms of Gaia, there would be good reason to think of humans as a kind of cancer. Will Gaia rid herself of this cancer?
Weak Gaia vs. Strong Gaia
Some people refer to the general appreciation of the importance of life in shaping the Earth as weak Gaia in contrast to strong Gaia which extends and strengthens this influence to the point of nearly complete control. As an example, consider the importance of the plate tectonic cycle to the global climate system -- it is responsible for the subduction and metamorphism that returning carbon dioxide to the atmosphere that would otherwise remain tied up in limestones. Without this process, our atmospheric carbon dioxide would long ago have slipped to levels too low for photosynthesis and the planet would have been extremely cold. Strong Gaia would therefore have to argue that organisms were instrumental in initiating plate tectonics. Here is how such an argument goes: subduction first began when organisms caused the buildup of a large carbonate platform/reef system at the edge of a continent and the weight of this sediment depressed the crust to the point where its lower part was transformed to a denser set of minerals, pulling the crust down with a greater force and eventually breaking free from adjacent crust and beginning its descent into the mantle. Not many geologists find this idea too reasonable, since it is hard to believe that some form of subduction did not occur on the early Earth, long before carbonate-producing organisms evolved. At present, subduction is an integral part of the convection that goes on within the mantle and this convection was almost certainly more active earlier in Earth's history. So, strong Gaia asks us to accept a very shaky argument that cannot be easily falsified or tested.
Perhaps one of the most important aspects of the strong version of Gaia is that it includes the notion of purpose -- that all of the biological influences on the global climate are done purposefully, to create a stable, comfortable physical environment, an environment that is favorable for organisms. In other words, the homeostasis that is represented by climatic stability is intentional. This last statement falls into the category of teleology, which is the idea that there is a purpose and goal to the operations of the natural world as a whole. Teleology is definitely not a popular idea in science.
A Systems View of Gaia
But, what about the possibility that the biosphere and the other spheres of the whole Earth system represent a complexly connected system that is dominated by negative feedback mechanisms that naturally produce a steady state? This amounts to homeostasis in a way, but it could be entirely unintentional, in which case there would be no purpose behind this steady state. In this view, organisms are responsible for many of the important feedback mechanisms and it may be that evolution has occurred in such a way that most life forms do well in the range of conditions represented by the natural steady state of the system. This notion is called co-evolution by some people -- the idea that the climate system and the biosphere have evolved together and the natural result is that the two reside in a steady state system, one in which negative feedbacks are widespread.
In this systems view of Gaia, the question of weak Gaia vs. strong Gaia is simply a matter of how strong and stable the biological negative feedback mechanisms are. One question to consider is whether the biospheric negative feedbacks are more effective than the negative feedbacks associated with the other spheres, which are operate independently from the biosphere. For instance, recall the temperature-dependent process of silicate weathering (higher temperatures increase rates of chemical weathering, thus consuming more atmospheric CO2, leading to cooling) , or the temperature dependence of cloud cover (higher temperatures lead to greater evaporation and greater global cloud cover, thus cooling the planet). It is clear that the biosphere has not cornered the market in negative feedbacks, and many would argue that these purely physio-chemical processes are every bit as potent as the biospheric processes in regulating the climate.
Another question to consider has to do with positive feedback mechanisms associated with the biosphere -- do they exist, and if so, how can they be incorporated into the notion of Gaia? One example is the temperature-dependence of respiration by soil microorganisms -- as it gets warmer, they respire faster and thus release more CO2 to the atmosphere, enhancing the warming. How does this positive feedback fit with the Gaia idea? It is certainly not designed to produce stability. But, if positive feedbacks such as this do have other limiting factors, then they simply add to the responsiveness of the overall system, and could therefore be considered to be useful to the overall operation of the Gaian system as it reacts to random external forcings.
Daisyworld Model
Click here to explore a model that illustrates how a biological negative feedback mechanism can lead to stability of a planet's climate.
