Dynamics of energy flow and the transport of matter in hydrological and biogeochemical cycles have evolved over millions of years into complex, finely tuned systems. As humans began to inhabit parts of Earth, initially the environmental impacts of their activities were very localized and had little influence on global processes. In the recent past, human populations have grown tremendously, spreading out over land areas. Research now indicates that the influence of human activity on energy flow and biogeochemical cycling can lead to significant changes in global climate and primary productivity.
This includes changing amounts of elements stored and released to various Earth realms (for example, addition of carbon dioxide to the atmosphere by the burning of fossil fuels), altering pathways of nutrient and energy flow, depletion and destruction of soils, disruption and removal of habitats (for example, the conversion of vegetated land to bare land or agriculture), and decreased diversity in plant and animal communities.
The direct use of food, fuel, fiber and timber by humans (and their domestic livestock) amounts to about 3% of global NPP. However, when indirect usage of NPP is considered, the total of global terrestrial NPP coopted by humans and domestic animals ranges from roughly 40-58%. The lower value of 40% represents direct use plus all the productivity of lands devoted entirely to human use, and the energy that human activities (such as land clearing) consume. The maximum 58% estimate further includes productive capacity lost as a result of converting open land to cities, forests to pastures, or environmental degradation such as overuse or erosion.
This indirect appropriation of NPP by humans is substantially disproportionate to direct use. The more NPP humans coopt, the less available for natural ecosystems. The extended impact of such inefficient and lopsided consumption on the health of the biosphere is currently unknown, however observations of increasing habitat disruption and declining biological diversity signal that human activities can affect long-established processes of energy flow and biogeochemical cycling.
In agricultural ecosystems, humans actively monitor and modify biogeochemical cycles. Although the basic principles of the flow of energy and materials are also present in agricultural ecosystems, the source and paths of these cycles differ. For instance, in many managed systems, there is energy input in the form of fossil fuels to power machinery and produce fertilizers, herbicides and pesticides. In order to sustain yields, fertilizers need to be added to replace nutrients depleted through harvests. Whatever is physically removed from the system must be manually replaced.
Agricultural ecosystems often require intensive inputs of water and other resources that natural systems do not require. Natural, diverse ecosystems have adapted over a long time to physical conditions of the native environment. In contrast, many agricultural systems are relatively new, human imposed systems focused on large area cultivation of a single crop (monoculture) and are more susceptible to disease or insect attack than "natural" systems that have developed resistance through natural selection. For this reason, many managed agricultural crops require input of pesticides and herbicides to resist pests and pathogens. Soils between plant rows in monoculture crops are typically left bare, which decreases their ability to conserve water and increases erosion.
Humans now highly depend on managed ecosystems and agriculture for survival. Although these systems are typically more productive than their native counterparts, the energy and material requirements to sustain the crops are also much greater. Efforts are ongoing to find new ways to maximize food production while minimizing the inputs required, including the use of naturally adapted vegetation (e.g., drought tolerant species) and natural predators to control pests (bioremediation).
One of the most obvious examples of environmental modification by humans is deforestation. Though deforestation is not new to humankind, demands for timber, forest products, and cleared land for agriculture, pastures and settlement expansion have grown along with increasing world population. This has resulted in exploitation of some of the most biologically diverse ecosystems on Earth, particularly in tropical rainforests. Sustainable use of forests by humans has become more difficult as demands have risen and technology for timber harvesting has advanced. It is currently possible to reduce a large forested area to fallen timber in a matter of days.
The short-term economics of exploitation have outpaced the ability or incentive to manage forests for long-term sustainability. Thus, deforestation continues in response to demand for forest products (whether timber, paper or packaging) and cleared land for settlement expansion or agriculture, even as the remaining tree stocks dwindle and are replaced by biologically impoverished plantations or secondary regrowth. Given sufficient time, forest regrowth will regenerate into biologically rich ecosystems, but the time required cannot match the rapid rates of additional deforestation. Research has documented the logic of sustainable resource use, but the balance between short- vs. long-term economic gain is a delicate one, subject to the pressures of special interest groups and the characteristic shortsightedness of many humans.
In addition to reducing biological diversity, deforestation has multiple effects on the global carbon cycle and associated climate mechanisms. Regrowing forests are CO2 sinks, while mature forests approach a balance between photosynthesis and respiration, neither sinks nor sources. Deforestation is also typically associated with biomass burning, particularly in less developed nations. Biomass burning (due largely to deforestation and grassland burning) is done to release nutrients to the soil and make them available for agriculture, however many of these nutrients end up quickly washed away in increased runoff. Biomass burning also releases trace gases and particulates into the atmosphere, which affect atmospheric absorption and transmission of solar energy.
Soils denuded of forests are subject to increased erosion and runoff. Irreplaceable soil and nutrients get washed away and eventually end up in aquatic and marine ecosystems. These excess nutrients increase water turbidity and give rise to intense algal blooms which deplete oxygen in the water. This ultimately leads to conditions that cannot sustain diverse wildlife and increases the rate of eutrophication of the water body. Deforestation also changes local and sometimes regional climate patterns by altering surface albedos, wind speeds, and latent heat cooling. These effects can accumulate regionally and alter global circulation patterns.
Deforestation practiced in a sustainable manner (i.e., at a pace that permits equivalent reforestation) and employing cutting patterns less disruptive than clearcuts could moderate detrimental effects on energy flow, biogeochemical cycling, climate and biological diversity. Sustainability is a crucial criteria which requires careful monitoring and possibly regulation for the long-term benefit of humanity and healthy functioning of the biosphere.
The impact of humans on the consumption of primary production and other energy resources varies throughout the world. Differences between nations are due in part to various agricultural and landcover conversion practices, and cultural lifestyles (including fertility and population growth rates). More developed nations employ monoculture, which requires intensive application of fertilizers, herbicides and pesticides, and involves nonlocal distribution along with competition for the international market. In contrast, less developed nations grow food on smaller, more intensively managed plots with crop mixtures and less use of applied chemicals, for largely local consumption.
Populations in less developed nations tend to live in smaller homes, with larger family units, less environmental control (e.g., air conditioning), and less fuel-powered mobility (e.g., more bicycles than automobiles). During the 1990s, electricity consumed by the average person in China was only 6% of that used by the average American (780 vs. 12,711 kilowatt hours per year respectively).
In more developed countries, the per capita consumption of water, food (including meat), and wood products is much greater. Per capita consumption of paper and packaging materials in China is highly disproportionate to that of the United States (20 vs. 320 kilograms per year). Despite lower population densities, countries with high standards of living (as measured by consumption statistics) generally have a greater impact on the biosphere through greater consumption and inefficient resource use, however in less developed nations some types of pollution and disease are relatively more uncontrolled. Increased awareness of all these impacts may inspire technological advances enabling much more efficient resource utilization by all nations. For example, household appliances like refrigerators and air conditioners have become much more energy efficient in recent years, and new toilets use much less water.
In following chapters, the use of technologically advanced satellite observation systems to monitor various aspects of resource use on a global basis (e.g., deforestation, primary production) is described. In the end, the ability of this technology to help resolve human problems, including human impacts on the biosphere, will only be effective to the extent that the derived knowledge is disseminated and applied, by not only resource managers, planners, engineers, and scientists, but consumers as well.