Biological Consequences of Global Climate Change
Scientific discussions of climate change often center on numbers: average temperature increases or decreases to be expected, alterations in rainfall amounts, adjustments to the growing season, and the like. It is easy to lose sight of the fact that we are ultimately interested in climate change because it has the potential to alter the livability of regions and, perhaps, the planet itself. This module steps away from those numbers, the questions of “how much” climate change might occur, and takes a look at the question of “what if.” It shows how life forms – plant and animal – might be affected by changes in various aspects of climate. We look at some of the possible biological consequences of the familiar physical aspects of climate change (temperature, rainfall, ultraviolet radiation, etc.). We also explore what we call changes in the “chemical climate,” alterations in the chemical composition of the atmosphere that could be substantial enough to elicit responses in biological organisms.
Uncertainties are plentiful in any discussion of biological consequences of climate change, perhaps even more than in other facets of the climate change debate. But that should not deter us from considering the range of possibilities. It is precisely the abundance of uncertainties that makes this one of the most exciting and crucial areas of global change research. Scientists can be sure that every new finding will receive both intense interest and intense scrutiny. This creates a challenge for the scientists and a dilemma for policy makers, who must chart a course for their countries despite the uncertainties. We each must decide how much of a “climate change insurance policy” we will pay for in the face of many unknowns. I hope that this module will help you to answer that question for yourself.
Public awareness of global climate change is extraordinarily high. From schoolchildren to senior citizens, from farms to factories to Wall Street, the phrase “greenhouse effect” now occupies a prominent place in the vocabulary of everyday life. In a world of so many issues and concerns, how is it that global climate change has been able to capture our attention? Why are you interested in the topic?
This problem strikes home for so many people for one simple reason: global climate change has potential consequences for the Earth’s living systems (biota). All life on Earth, whether plant or animal, is inexorably linked to the surrounding chemical and physical climate. It is obvious to most people that organisms tolerate certain ranges of temperature, have certain requirements for moisture and nutrients, and respond to the chemical composition of the surrounding air or water. The rate, as well as the magnitude, of global environmental changes could potentially challenge the capability of organisms to adapt. We have a vested interest in this topic because we are members of the biological community ourselves, and because we are dependent on other living systems. It’s not surprising that our interest in climate change is intense and that it spans many levels: scientific, economic, social, political, and even emotional.
This module will explore current research concerning the biological consequences of climate change. As with most aspects of climate change, there are few certainties – about exactly how the biota might respond to temperature perturbations, sea-level changes, changes in rainfall patterns, increased ultraviolet radiation, or an altered atmospheric chemical composition. That the range of possible responses is large offers little comfort. In fact, it surely provides even greater motivation to study the topic
The rationale for this module is simple: the forces of global change have the potential to alter physical and chemical climate conditions, which determine where organisms can live. In order to understand how climate change might affect living systems, we must first discuss some basics about the biota and how it is linked to climate. Once we understand these links, we will be able to recognize many of the aspects of global change that could spell trouble for the Earth’s organisms.
The Organization of Living Systems
A staggering array of life forms makes up the biota of the Earth, and we may approach the study of biological interactions with climate and other features of the abiotic (nonbiological) environment on a variety of levels. Ecologists have come up with conceptual groupings of living organisms. At the simplest level, we talk about individual organisms, composed of cells organized into various tissues and organs that carry out the basic physiological processes of life. The characteristics of the individual are determined by chromosomes, thread-like structures in the nucleus of each cell that carry genetic information and consist of DNA (deoxyribonucleic acid) and proteins. A species is a group of individual organisms sharing a common gene pool. Members of the same species will be similar in appearance, behavior, and chemistry. A population consists of all the members of a species in a particular geographical area at one time. For example, all of the kangaroos in Australia make up a population, which can be described by size, density, age structure, and other properties. The populations of different species in an area are referred to as a community. A community of organisms interacting with one another and with their physical environment constitutes an ecosystem. A field, a lake, and a vacant lot are all examples of ecosystems. A biome is a major type of land community of organisms, such as a desert or a tropical rain forest. Finally, the biosphere is the total of all areas on Earth that support life. It encompasses the deep ocean, land, and the part of the atmosphere (up to an altitude of about eight kilometers) where living organisms are found.
In describing these levels, you can see that we have expanded outward from the very close-up view to the broad view of the globe. That is, the “spatial scale” has gone from millimeters to kilometers to continental to global. The times needed to cause significant change at any level can vary from relatively short, in the case of individuals, to centuries and more at the level of biomes and the biosphere (see Figure 1). When we talk about the effects of climate change on the biota, we could be concerned with any of the above levels of organization. Clearly, studies of effects at the ecosystem or biome level will differ greatly from research at the level of individual organisms because of the differences in temporal (time) and spatial scales. The various types and scales of study are interdependent and complementary in the quest to understand the biotic response to change.
Climate and the Distribution of Living Systems
Living organisms are found everywhere on this planet, even in areas that humans may consider hostile, such as hot springs, the deep sea, and under Antarctic sea ice. However, a given species does not occur everywhere. Species have very specific requirements that govern their distributions. These physical and chemical limits of tolerance, together with availability of food and biological interactions such as predation and competition, determine the global distribution patterns of species. The climate plays a key role in establishing these distribution patterns, insofar as it determines the physical and chemical attributes of the abiotic environment.
Land plants (terrestrial vegetation) offer a good illustration of the connection between climate and the biota. A look at a vegetation map of the world reveals a very close association with the climate zones (tropical, temperate, boreal, etc.; see Figure 2). Both climates and biomes show strong latitudinal zones proceeding from pole to equator. The broad terms forvegetation zones (such as boreal forest, tropical forest, temperate forest) in fact carry climatic connotations. This offers the first clue that climate variables influence plant distribution. Likewise, the fossil record provides evidence of climatic factors acting over millions of years to control species distributions. Modern science can hardly lay claim to the discovery of this basic relationship; it was recognized by Greek scholars between the third and fifth centuries B.C. One emphasis of contemporary research is to go beyond the fundamental climate-distribution correlation in an effort to understand the mechanisms by which climate exerts its control.
What are some of the fundamental physiological and ecological principles that link living systems so strongly to climate factors?
The Importance of Temperature
Temperature is the most important climatic factor that governs the biology of animals and plants. It has a direct effect on the rate of most biological processes (e.g., photosynthesis, respiration, digestion, excretion). If proper internal temperatures are not maintained, these processes cannot proceed normally and an individual will be stressed or possibly die. You may wonder why temperature is so critical. It’s because physiological processes are catalyzed (that is, their rate is determined) by a group of molecules known as enzymes. The structure of these enzymes is dependent on temperature; if temperature is too high, the enzyme molecule is rearranged (“denatured”), and it simply doesn’t work the way it should. If temperature is too low, the enzyme isn t effective at catalyzing physiological processes, and those processes slow down. Thus chemistry, or more specifically biochemistry, is behind the basic temperature-tolerance limits of organisms. Figure 3a illustrates in a general way how an organism’s functioning may respond to environmental parameters such as temperature. Figure 3b shows the specific case of the response of plant photosynthesis (the process whereby plants use sunlight, carbon dioxide, and water to make carbohydrate food) to temperature.
Species differ in their abilities to regulate body temperature. The internal temperature of “poikilothermic” organisms (e.g., insects and reptiles) tends to vary over a much greater range than that of warm-blooded or “homeothermic” organisms (e.g., mammals and birds), which are able to regulate their body temperature by internal physiological mechanisms. For example, mammals maintain a body temperature of approximately 35-39° C and birds 38-42° C. The mechanisms for controlling internal temperature, such as shivering, sweating, and panting, work by altering the organism’s metabolic rate. Because warm-blooded animals are able to regulate their body temperature, they are less affected by external temperature variation. Poikilothermic organisms are able to achieve some degree of temperature regulation by modifying their behavior (e.g., lizards bask in the sun). Many cold-blooded organisms are dark colored, an adaptation that allows them to directly absorb more solar radiation. (Note that this links living organisms to another climate factor, sunshine, which is not the same as temperature.)
Several aspects of temperature may limit a species. In some cases it may be the maximum temperature, in others the minimum, the average, or the frequency with which temperature changes. If environmental temperatures rise, those species living near the upper thermal limits of their existence will be stressed. Although death may not be the immediate response, sublethal effects such as a decrease in the number of offspring may contribute to the eventual decline or demise of the species.
Water and Living Systems
Moisture is also a critical variable limiting the distribution of organisms. Water, which accounts for 85 to 90% of the weight of most living organisms, is essential for proper plant and animal physiology. This is because the chemistry of physiological processes is carried out in the water of living tissues. Water is the equivalent of an organism s transportation system, permitting the transport of biological molecules to and from the sites of biochemical reactions such as photosynthesis and respiration. Nevertheless, organisms differ greatly in their water requirements and their abilities to withstand dryness and flooding. Some lower plants, such as algae and fungi, adjust their activity according to the humidity of the surrounding air. In periods of low humidity, they are able to enter a resting state. Some water-dwelling microorganisms have a similar ability to withstand periods of total dryness.
This dependence on water links the biota to several climate-related variables, such as relative humidity, rainfall amount, and the distribution of rainfall through the year. Figure 4 shows how plant growth is related to a very simple climate variable, total yearly precipitation. The frequency of extreme events such as droughts and floods is also critical. Reproduction is especially vulnerable to such extremes, with the survival of eggs and seeds often tied to the presence or absence of moisture. For example, amphibians such as toads and salamanders lay eggs in the fringes of ponds, placing them at risk in periods of drought. Larger animals can sometimes move (migrate) to minimize the impacts of extreme events, but plants and smaller animals don t have this option. The timing and/or duration of rainfall are other critical climate features for some organisms.
All organisms are sensitive to the chemical composition of the medium they live in, whether that medium is air or water. Organisms require certain chemicals, oxygen being the most obvious example for all but the anaerobic microorganisms. In the case of plants, carbon dioxide (CO2) is required.
Beyond these essential needs, other chemicals can only be tolerated at certain levels. For water-dwelling (aquatic) species, the pH (acidity) of the water is a critical variable. Acid rain, currently a problem in some regions, could alter the pH of lakes or coastal areas enough to affect the health and survival of aquatic plants and animals. Pollution of streams or coastal waters by toxic organic chemicals and metals such as mercury and lead also has biological consequences. Water pollution can present serious local problems, as in Mexico City, where water has been rendered unfit for human consumption. While these are serious environmental problems, we shall not be discussing them further because the focus of the module is on problems that are global in their scope.
Likewise, organisms that depend on air have limited tolerance for some atmospheric gases (Table 1). Modern-day air pollutants like sulfur dioxide (SO2) and ozone (O3) affect plants and animals by interfering with basic physiological processes such as photosynthesis and respiration. These gases are the by-products of industrial processes that have increased as the human standard of living has advanced. Some toxic gases such as phosgene have been developed deliberately as chemical weapons because of their rapid and serious disruptions to human physiology. In this module, we will discuss one aspect of air composition – the occurrence of gaseous oxidants – which may be becoming a problem of global dimensions.
Particulate matter Increased respiratory illness and mortality; visibility impairment; materials damage Sulfur dioxide* Increased respiratory illness and mortality; plant and forest damage; materials damage Carbon Monoxide Asphyxiation; mortality in cigarette smokers; impaired functioning of heart patients Ozone Increased respiratory symptoms and illness; plant and forest damage; materials damage Nitrogen oxides* Increased respiratory illness; leads to ozone formation; materials damage Lead Neurological malfunctioning; learning disabilities; increased blood pressure
A few other climate factors also affect living systems. While the activities of humans do not affect the amount of solar radiation striking the outer atmosphere of the Earth, our activities can affect cloudiness and thus the amount of sunlight that reaches the surface of the Earth. Solar radiation is important for many organisms, most obviously plants, which require light for photosynthesis. Visible light in the range of 400-700 nanometers in wavelength, called “photosynthetically active radiation”, or PAR, is captured by chlorophyll and other pigment molecules in plant cells and eventually is converted to chemical energy and stored in the molecular products of photosynthesis. (The chemistry of photosynthesis is explained on pages 9-10). Plants can get too much of a good thing, however. Thin-barked trees such as aspen and birch can suffer from sunscald. Sunshine also plays a role in the distribution of some animals. One ant species requires a habitat with at least 40 days per year of full sunshine. Some reptiles, known as heliotherms, depend on solar radiation to regulate body temperature. While many organisms require visible light, they can only tolerate certain levels of other components of solar radiation. Ultraviolet light of some wavelengths is actually harmful to many plants and animals, as we shall discuss later in this module.
Atmospheric and oceanic circulation are other features of climate that have a bearing on living systems. Wind seems to be important as a means of transport for some insects such as moths, dragonflies, and locusts. The location of the major prevailing winds often determines the distribution of migratory birds, which may use the winds quite deliberately to assist their travels. This strategy occasionally backfires, as winds unexpectedly carry the birds farther than their food reserves allow. Wind is an important mechanism for seed dispersal of some plants, which have evolved seed casings that capitalize on the free ride. (Dandelions and maple trees are familiar examples.) But one species’ gain can be another species’ loss: windy conditions can be deadly to young insects and harmful to coastal-dwelling plants and animals. Oceanic currents can also be either a help or a hindrance to organisms. Phytoplankton and zooplankton, including the immature stages of some bottom-dwelling marine animals, are often carried long distances by oceanic currents. This can be a mechanism to extend the geographical range of a species. But individual organisms will die if they are transported to regions outside their climatic tolerance levels.