Until recently, it was thought that human activity caused only local or, at most, regional environmental change. It is now generally recognized that the effects of human activity on Earth are so extensive that we are actually involved in an unplanned planetary experiment. To recognize and perhaps modify the changes we have initiated, we need to understand how the entire Earth works as a system. The discipline, called Earth system science, seeks to further this understanding by learning how the various components of the system the atmosphere, oceans, land, and biosphere are linked on a global scale and interact to affect life on Earth.
Tools for studying global change
The geologic record
 |
| Trapped air bubbles in glacier picture taken by kevin berne |
Sediments deposited on floodplains or in lakes, bogs, glaciers, or the ocean may be compared to the pages of a history book. Organic material that is often deposited with sediment may be dated by a variety of methods to provide a chronology. In addition, the organic material can tell a story concerning the past climate, life-forms in the area, and environmental changes that have taken place. Ocean sediments are sampled by drilling from a ship or on an ice shelf and extracting a core several cm in diameter that may be a few 100s to over 1000 meters long. On land, sediment may be sampled by drilling, trenching, or from a natural exposure. One of the more interesting uses of the geologic record has been the examination of glacial ice. Glacial ice contains trapped air bubbles that may be analyzed to provide information concerning atmospheric carbon dioxide concentrations when the ice formed. These trapped air bubbles are atmospheric time capsules from the past and have been used to analyze the carbon dioxide content of air as old as 800,000 years. The method of studying the glacial record is to drill the ice and extract an ice core, which can be sampled. Glaciers also contain a record of heavy metals, such as lead, that settle out of the atmosphere, as well as a variety of other chemicals that can be used to study recent Earth history. The geologic record is the primary source of data and evidence for understanding Earth's history and changing environment. Without ice cores and layered sediment, we would know little about long-term change and would not be able to put past change learned from the geologic record in the context of what is changing today.
Monitoring is the regular collection of data for a specific purpose; real-time monitoring refers to collecting these data while a process is actually occurring. For example, we often monitor the flow of water in rivers to evaluate water resources or flood hazard. In a similar way, samples of atmospheric gases can help establish trends or changes in the composition of the atmosphere; measurements of temperature and the composition of the ocean are also used to examine changes within them. Gathering of real-time data is necessary for testing models and for calibrating the extended prehistoric record derived from geologic data. Methods of monitoring vary with the subject being measured. For example, the impacts from mining may be monitored by evaluating remotely sensed data collected by satellite or high-altitude aerial photographs. However, the most reliable data are often derived from ground measurements that establish the validity of the airborne or satellite measurements.
Mathematical Models
Earth's atmosphere and climate change
The Atmosphere
Mathematical models use numerical means to represent real-world phenomena and the linkages and interactions between the processes involved. Mathematical models have been developed to predict the flow of surface water and groundwater, erosion and deposition of sediment in river systems, ocean circulation, and atmospheric circulation. The global change models that have gained the most attention are the climate models. These models predict changes in climate at the global scale. Data used in the calculations are arranged into large cells that represent several degrees of latitude and longitude; typical cells represent an area about the size of Oregon. In addition, there are usually 6 to 20 levels of vertical cells representing the lower atmosphere. Calculations involving equations for major atmospheric processes are then used to make predictions. There is growing confidence that climate models provide believable quantitative estimates of known past and predicted future climate change. The models are based on physical principles (such as conservation of energy and mass) that are used to produce a mathematical representation of Earth's climate system, defined as the system consisting of the atmosphere, hydrosphere, land surface, biosphere, and cryosphere (ice, snow, and frozen ground) which are linked and often interacting with each other in complex ways.
Earth's atmosphere and climate change
To a great extent, the study of global change is the study of changes in the atmosphere and linkages between the atmosphere, lithosophere, hydrosphere, and biosphere. We define climate as the characteristic atmospheric conditions that is, the weather at a particular place or region over time periods from seasons to years to decades. The climate at a particular location may be complex and consist of more than average precipitation and temperature. For example, it may be dependent on infrequent or extreme seasonal patterns, such as rain in the monsoon season in parts of India. Selected processes and changes that produce and maintain the climate system. Global circulation and movement of air masses in the atmosphere produce the major climatic zones. Warm tropical air near the equator rises and moves north and south, descending in the mid-latitudes (sometimes producing deserts). The air then rises again at higher latitudes and, finally, descends at the poles. The lower, active part of the atmosphere, where weather occurs, is the troposphere. Air temperature and concentration of oxygen decrease with altitude in the troposphere. At the top of the troposphere, at an altitude that varies from about 18 km at the tropics to 7 km at the poles, it is very cold, and the temperature remains nearly constant for a few kilometers through the tropopause. The constant temperature with little air movement places a lid on the active lower atmosphere (troposphere). Temperature then increases in the stratosphere only to decline again in the mesosphere. Nearly all (99 percent) of the atmosphere by weight is below an altitude of about 30 km (20 mi).
The Atmosphere
Our atmosphere can be thought of as a complex chemical factory with many little-understood reactions taking place within it. Many of the reactions that take place are strongly influenced by both sunlight and the compounds produced by life. The air we breathe is a mixture of nitrogen (N2)
(78 percent), oxygen (O2) (21 percent), argon (Ar) (0.9 percent), carbon dioxide (CO2) (0.03 percent), and other trace elements (less than 0.07 percent). It also contains compounds, such as methane, ozone, carbon monoxide, oxides of nitrogen and sulfur, hydrogen sulfide, hydrocarbons, and various particulates, many of which are common air pollutants. The most variable part of the atmospheres composition is water vapor (H2O), which can range from approximately 0 percent to 4 percent by volume in the lower atmosphere. Having given this brief introduction to the atmosphere and climate, we will now discuss human-induced global warming in terms of the greenhouse effect the history and process of global temperature change and the potential consequences.