Chapter 1. Understanding Global Change

The Earth System and Global Change

The Earth is a forever-changing planet. Its orbit around the Sun varies, continents drift, mountains are driven upwards and erode, animal and plant species evolve, and terrestrial and marine ecosystems change. Large changes have generally occurred as the result of natural forces beyond human influence or control.

Humans have become powerful agents of environmental change on global, regional, and local scales. With an increasing world population, an expanding global economy, and the development of new technologies, the human impact on the environment will become even more significant in the future.

Research supported through the U.S. Global Change Research Program (USGCRP) is documenting environmental change and leading to a better understanding of its significance. For example:

Central Purposes of the U.S. Global Change Research Program

  • To observe and document changes in the Earth system

  • To understand why these changes are occurring

  • To improve predictions of future global change

  • To analyze the environmental, socio-economic, and health consequences of global change

  • To support state-of-the-science assessments of global environmental change issues

These and other changes in the Earth system are having profound consequences. Scientific research is the means to develop an understanding of the changes, their causes, and their consequences.

Changes in any single component of the Earth's system will affect the entire system. For example, changes in the tropical Pacific Ocean affect weather in North America and other parts of the world. The complexity of the Earth system and the many feedbacks among its components make understanding and predicting climatic and environmental change and all of its ramifications an exceedingly difficult challenge.

Progress Over the Past Decade in Global Change Research

Over the past decade, scientific research has greatly advanced our understanding of global change. The growing understanding that the current and future state of the Earth system is inexorably linked to human activities, and the increasing societal concern about the implications of global environmental change, underscore the need for and importance of these scientific efforts.

Science continues to improve our understanding of global change. Research supported by the USGCRP is providing answers to important questions about the Earth system, how it is changing, and the implications of global environmental change for society. Following are a few key examples from research that has focused on:

Ozone Depletion

In 1974, research scientists hypothesized that industrial emissions of CFCs could cause depletion of the stratospheric ozone layer, which serves to shield life on Earth from harmful levels of ultraviolet (UV) radiation. In 1985, after extensive study, the scientific community released its first international assessment, predicting only a small thinning of the ozone layer with continued emissions of chlorinated substances.

With the discovery of the Antarctic ozone hole in 1985 -- a phenomenon resulting in the springtime destruction of more than 50% of the ozone over Antarctica in a period of several weeks -- it became clear that the potential danger of ozone depletion was significantly greater than shown in earlier scientific studies. The atmospheric science community rapidly embarked on a series of experimental field campaigns. This work, together with satellite and aircraft observations, laboratory studies, and the development of atmospheric chemistry models, clearly identified human emissions of chlorinated and brominated chemicals (e.g., CFCs and halons) as the causes of stratospheric ozone depletion.

A Successful Partnership Between Science and Policymaking

In 1987, in parallel with scientific developments and in response to the 1985 assessment, the nations of the world entered into a landmark global agreement, the Montreal Protocol on Substances that Deplete the Ozone Layer. The Parties to the Montreal Protocol agreed -- on the basis of scientific predictions -- to reduce emissions of the chlorinated and brominated ozone depleting substances (ODSs). Thus was born a new era of cooperation between the science and policy communities.

The Parties to the Montreal Protocol have since periodically requested that the science community provide new assessments of the state of the ozone layer, to be used as the basis for revising the international rules limiting emissions of ODSs. In response, the science community, with U.S. funding increasingly focused through the USGCRP, has engaged in extensive monitoring, and process and modeling studies. These studies have elucidated the causes of the observed global thinning of the ozone layer in increasing detail. At the same time, they have identified many ozone-friendly substitutes for the ODSs.

Based heavily on this research, new assessments were prepared in 1989, 1991, and 1994 by international panels of leading scientists, under the co-chairmanship of USGCRP scientists, and with USGCRP scientists serving prominently as authors and reviewers. These authoritative, internationally recognized documents provided the scientific understanding underlying the subsequent Amendments and Adjustments to the Montreal Protocol, which have proven necessary for the protection of the stratospheric ozone layer.

Based on assumed compliance with the most recent Amendments to the Montreal Protocol, stratospheric chlorine and bromine abundances are expected to peak in the next few years, then slowly decline as international controls on ODSs take effect. All other things being equal, global ozone losses and the Antarctic ozone hole are expected to recover in about 50 years. The ongoing research program has shown that such recovery would have been impossible and that ozone losses would have been so large as to cause very large increases in skin cancer without the controls agreed to in the Montreal Protocol and its Amendments.

Both the ozone layer and the atmospheric abundances of ODSs are monitored globally by USGCRP scientists. Long-term observations have revealed that atmospheric abundances of the ODSs have recently started to decline as their phase-out takes effect. Simultaneously, the abundances of their substitutes have been increasing. These important observations confirm the effectiveness of the international policy response to this scientifically identified global threat.

Seasonal to Interannual Variations in the Climate

The weather fluctuates day-to-day, even hour-to-hour. Weather forecasters have developed enough understanding about atmospheric behavior to be able to make socio-economically useful predictions about the specific conditions of the atmosphere up to almost a week in advance. Climate -- the general patterns of weather in a region averaged over seasons, years, and decades -- also exhibits natural variations and fluctuations. Thus, monthly and seasonally averaged temperature, precipitation, sunshine, cloudiness, and wind vary from year to year and decade to decade.

Scientists are seeking to understand how the climate system worked in the past and how it is working now, including the influence of human activities. Improving this understanding is essential to developing the capacity for society to adapt to change and to achieve maximum benefit under the prevailing climatic conditions. Over the past decade, scientists studying the season-to-season and year-to-year fluctuations of the climate have improved their ability to predict rainfall and temperatures up to a year in advance.

Even though they are still experimental, these seasonal-to-interannual forecasts are now being made with useful skill and are used by agricultural and water resource planners in some parts of the world, particularly in the tropics, to adjust planting schedules, crop selections, and water releases from dams in order to reduce the economic and social impacts of droughts and floods.

The El Niño-Southern Oscillation

The ability to make seasonal-to-interannual forecasts, particularly for tropical and subtropical regions and for the southern and western United States, has come from an improved understanding of the irregular cycling of El Niño/Southern Oscillation (ENSO). The ENSO phenomenon involves the warming and cooling of large areas in the tropical Pacific Ocean, with large associated shifts in atmospheric pressure and rainfall in other regions.

The Tropical Ocean/Global Atmosphere (TOGA) program from 1985 to 1995 untangled these complex interrelationships sufficiently to allow prediction of the ENSO cycling under some conditions. With this understanding, scientists have been able to associate changes in ENSO with important consequences in the United States, including:

Prediction of ENSO events and their impacts has enabled South American farmers to adjust their crops and planting in ways that have greatly alleviated the impacts of drought and enhanced the abilities to have bountiful harvests during periods of good rains. ENSO variations have also been found to influence outbreaks of some diseases.

Expanding the Scope of Research

Research in this area is now focusing on the following needs:

In addition, research focuses on understanding how individuals, industries, and resource sectors respond to climate fluctuations. This understanding, coupled with improved knowledge about the climate system, will make it possible to identify options for reducing vulnerability to extreme climate events such as droughts and flooding, and, in some regions, for taking advantage of opportunities provided by rainfall or temperature changes.

Climate Forcings

Human activities have become so pervasive that they are creating changes in the atmosphere and at the land surface that perturb the Earth's natural fluxes of solar and infrared (heat) radiation. These human-induced changes, often called "enhanced radiative forcings," lead to changes in temperature, precipitation, and other climatic variables.

Greenhouse Gases

The most important enhancements of radiative forcings are a result of the emissions of carbon dioxide (CO2), methane (CH4), and other greenhouse gases, which have the ability to increase the natural atmospheric trapping of infrared radiation. Emissions from combustion of coal, oil, and natural gas and from deforestation and land cultivation have increased the natural CO2concentration by almost 30% over the past 200 years. The atmospheric CO2concentration is currently rising at about 0.5% per year due to annual emissions of about 7 GtC (gigatons of carbon -- 1 GtC equals 1 billion metric tons) as carbon dioxide per year from fossil-fuel combustion and land-clearing activities.

Aerosol Effects on Solar Radiation

It has long been recognized that small particles in the atmosphere (called aerosols) can reflect some solar radiation back to space, thus exerting a cooling influence. However, a relatively new finding is that regional increases in short-lived aerosols resulting from human activities are sufficient to alter the Earth's radiation balance. Emissions of sulfur dioxide from coal combustion, and of other gases from biomass and other burning, cause the atmosphere over and downwind of major industrial regions and regions of tropical deforestation to reflect back to space some of the incoming solar radiation. This exerts a regionally distributed cooling influence.

In many industrialized nations, including the United States, the particle loading from fuel burning is being controlled in order to reduce human health impacts, acid rain, and visibility problems. However, as aerosol concentrations are reduced, the global warming they have masked would become apparent.

See Figure 1

Climate Change Over Decades to Centuries

Predicting the future influences of human activities on the climate requires an understanding of how natural forces affect the climate. Gaining sufficient understanding to predict future changes requires consideration of the many variables and processes that describe and control how the Earth system behaves, including how human activities may be influencing natural components. Various approaches are used to develop and test prediction capabilities. Confirming that different approaches yield similar predictions can add to confidence in the results.

Paleoclimate Studies

Paleoclimate studies -- studies of past variations in the climate -- indicate that future warming projected by current climate model simulations would lead to global average temperatures during the next century that have not occurred on the Earth in millions of years. Furthermore, the rate of global warming projected for the next century would be more rapid than any natural climatic change that has occurred during the past 10,000 years.

Paleoclimate studies also indicate that past climate has been quite sensitive to relatively modest changes in the factors governing the climate -- for example, changes in atmospheric composition, changes in solar radiation related to the Earth's orbit around the Sun, and the locations and elevations of mountains. In addition, paleoclimate evidence indicates that climate changes of the magnitude forecast for the next century have caused major shifts in the geographical distribution of forests and other vegetation types in the past, implying significant ecological disruption as a consequence of future climate changes.

Predictive Models

Developing quantitative predictions of how these complex changes might occur in the future requires the use of computer-based climate models that incorporate as much as possible of our theoretical understanding about how the Earth system works. Exploratory global climate models were first developed in the 1960s and started to be used to study human influences on the climate in the 1970s.

Over the past decade, predictive climate models have improved as scientific understanding has advanced and computers have become more powerful. The early climate models (referred to as general circulation models, or GCMs) were only able to calculate the climate change from a large and fixed change (e.g., a doubling) in the atmospheric concentration of CO2. Because they include the capacity of the oceans to act as a heat sink and thereby slow the rate of temperature increase, current models are now able to simulate more realistically the climatic effects from the gradually increasing atmospheric CO2 concentration. Capabilities are also being developed to treat other human influences on atmospheric composition, including sulfate aerosols, other greenhouse gases, and changes in land cover.

A climate model that incorporates many of these new capabilities is now available for research and application studies. Developed and maintained at the National Center for Atmospheric Research in Boulder, Colorado, this is the world's first comprehensive and interactive Climate System Model available for widespread use by the scientific community. In FY98, continued enhancements of this model will facilitate a wide range of global change sensitivity studies and predictions by a broad array of USGCRP researchers and scientists throughout the world.

Comparing and Evaluating Models

Because computer models only approximate the real world, it is important to analyze and test their results carefully. This became especially clear when it was recognized that the several different models used by researchers in the United States and around the world were giving different estimates for the most basic model result: How much global warming would result from a doubling of the CO2 concentration. The model estimates range from about 1.5-4.5°C (about 3-8°F). The reasons for the differences relate primarily to the limited understanding and representation of cloud-radiation feedbacks. Research on these feedbacks is a major focus of the USGCRP effort.

The differences were also an important incentive for the establishment of a series of intercomparison studies, particularly the Atmospheric Model Intercomparison Project. In the future, more powerful predictive models will contain the finer spatial and temporal resolution and the improved representations of climate processes that are needed to represent more completely the many aspects of the hydrologic cycle, including clouds and upper tropospheric water vapor, and processes governing the coupling of the land, ocean, and sea ice with the atmosphere.

Detection and Attribution of Climate Change

Scientists have been searching for several decades for evidence that climate change is occurring and that it is a result of human activities. The Intergovernmental Panel on Climate Change (IPCC) was created in 1989 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess the state of knowledge of climate change. More than 2,000 scientists from around the world were involved in the IPCC Second Assessment Report, released in December 1995. The Second Assessment Report came to the important conclusion that "the balance of evidence suggests that there is a discernible human influence on global climate."

Detection of Climate Change

For some time there has been clear evidence that detectable global warming is occurring:

A number of additional records also point to long-term global warming:

The 17-year satellite record of temperature in the lower atmosphere does not directly show a warming trend. However, analyses that take account of several additional factors do suggest that greenhouse gases are exerting a warming influence. Issues relating to the satellite data set are currently being addressed by USGCRP-supported research.

Attribution of the Causes of Climate Change

Distinguishing the clear signal of human activities from the "noise" of natural variability has been a difficult challenge for two reasons. First, both natural and human influences can cause climate to change. Second, greenhouse gases exert a warming influence while sulfate aerosols exert a cooling influence.

New statistical tests indicate that much of the warming that has been observed can indeed be attributed to human activities. This is because both the geographical pattern  across the Earth's surface and the vertical pattern  into the atmosphere of temperature change are what is expected from changes induced by human influences:

These patterns are unlike those expected from naturally varying forcing factors, such as solar radiation and volcanic eruptions.

The amount of warming since the 19th century is also in accord with the range of model estimates of the surface warming that is expected from the increases in greenhouse gases and aerosols that have occurred. Inclusion of a more complete set of human influences, including the cooling influences of aerosols and stratospheric ozone depletion, has been important in reconciling observed and predicted changes and in clarifying that it is essential to consider all important factors that can influence the climate, rather than drawing conclusions or making comparisons based on the estimated change from any single factor (like greenhouse gases).

Climatic Extremes

From model predictions about climate change, inferences can be drawn that there will be:

Analyses of the climate record are seeking to determine if these inferences are in accord with recent observations.

It is inappropriate to attribute any particular extreme weather event to climate change, but the trends in their occurrence can be examined. While detailed worldwide information is not available to explore this question, analyses of data for the United States during the 20th century show a consistency between inferences drawn from model predictions and observed responses. Such modeled and observed changes include, for example, an increase in intense rainfall events.

Terrestrial and Aquatic Ecosystem Feedbacks and Effects

Changes in the atmospheric composition and climate will have a significant influence on terrestrial and aquatic ecosystems. Because these ecosystems also have a major influence back on the atmosphere and climate, understanding their role in potential positive and/or negative feedbacks is critical. Over the past 10 years, significant advances have been made in understanding terrestrial and aquatic processes and their effects on the climate system.

The feedbacks and effects that the land and aquatic environments exert on climate are of two types:

  1. Biogeochemical:  Changes in the distribution and circulation of chemicals such as carbon- and nitrogen-containing compounds between the atmosphere, oceans, biosphere, and soils, resulting in changes in the atmospheric composition of greenhouse gases

  2. Physical:  Changes in land-surface properties such as albedo (the ability to reflect light) and roughness (the ability to alter wind speed and direction), resulting in changes in the Earth's energy balance.

Biogeochemical Cycling

A particular challenge has been to identify where all of the CO2 being emitted as a result of human activities is going. Natural biological and physical processes, such as photosynthesis and oceanic uptake, are able to limit the annual increase in the atmospheric loading of CO2 to just under half the annual emissions of CO2. Together, the land and ocean components of the climate system must be taking up the half of the CO2 that does not remain in the atmosphere.

Research over the past decade has helped to clarify the means by which this carbon sequestration on land occurs. Much of the excess carbon from human activities that is being removed from the atmosphere appears to be going into terrestrial systems. Studies suggest that land-surface processes are removing about 2 billion metric tons more of carbon from the atmosphere each year than they release. This net carbon uptake by land ecosystems is related to several processes, including the regrowth of previously deforested areas and to enhanced storage of carbon by plants due to the elevated CO2 concentration. Previously deforested areas, particularly in the northeastern United States, are showing substantial forest regrowth. For example, areas in New England that were 80% farms 100 years ago are now 80% forest-covered.

The role of ocean ecosystems in "pumping" carbon to the deep ocean as dead animals and plants sink to the ocean floor is also important, and would be especially so if the ocean circulation is slowed by global warming. Data collected in a series of ship cruises indicate that about 2 billion metric tons more carbon is being taken up by the oceans than is being released by them through natural processes.

Critical questions remain:

Physical Ecosystem Properties

The characteristics of land and ocean surfaces also affect the climate directly. Plants are particularly important in controlling the amount of evaporation and runoff at the surface, thereby influencing the hydrologic cycle. Moreover, by enhancing or reducing evaporation, soils and surface vegetation have an influence on the near-surface temperature.

Shifts in precipitation, whether brought about by seasonal-to-interannual climate fluctuations, volcanic eruptions, or long-term climate change, will cause shifts in temperature and soil moisture. Depending on their magnitude and timing, persistent changes in precipitation are predicted to cause shifts in ecosystems as well as feedbacks affecting climate.

Interactions Among Feedback Mechanisms

Biological, chemical and physical feedback mechanisms can also interact. For example, climate changes can induce changes in ecosystem structure and function which can alter carbon uptake, which in turn can alter the future climate. New global vegetation models are being developed that can be coupled to climate models to simulate these influences.

In cold land regions where very large amounts of carbon are currently trapped in frozen soils, climate change will warm the soils which can lead to release of methane, a strong greenhouse gas that could amplify climatic warming. Another possibility is that freshening of the ocean as a result of increased precipitation can slow ocean overturning and the oceanic uptake of carbon, thus creating further climate change. Climatic warming can enhance the release of ozone precursors from the vegetation.

As the atmospheric CO2 concentration increases, the capacity for plants to take up CO2 via photosynthesis may increase. This "CO2 fertilization" effect may act as a negative feedback, thereby reducing the rate of climate change. Conversely, decreases in precipitation in some regions can turn forests to grasslands, thereby leading to release of carbon to the atmosphere.

These and other feedbacks emphasize the need for full treatment of surface processes in climate models.

Land Cover and Land Use

Human activity has altered all but a few of the Earth's landscapes. These changes alter vegetation, change the capacities of landscapes to cleanse water and the air, affect how animals, plants, and ecosystems can migrate, and alter biological diversity.

The current pattern of global land cover most often reflects past and present land use. Since different land uses and vegetation types (forests, farmlands, grasslands, urban developments, and so forth) have very different capacities to absorb and store carbon, monitoring land cover and land use are critical.

The larger patterns of land cover are observable and can be monitored from space. From historical archives, including the last 20 years of satellite data, we are building a quantitative assessment of landscape and land-use change. More subtle types of change -- for example, those which take place through the intensification of current uses -- require additional in situ (on site) information.

Mapping and Characterizing Change in the Global Landscape

The USGCRP has supported several current activities to produce and make available global land-cover maps at different spatial resolutions. These maps provide important foundations for efforts to improve land use. The success of these projects in interpreting land use from satellite data has provided critical information. Products derived from the Advanced Very High-Resolution Radiometer (AVHRR) at 8- and 1-km spatial resolution are already being made available.

The Landsat Pathfinder Humid Tropical Forest project has provided the major foundation of data on tropical and subtropical land-cover conversion. Projects sponsored by U.S. agencies with the goal of measuring the rate of tropical forest loss in South America and Southeast Asia are being coordinated with projects sponsored by Brazilian and European colleagues focused on the Brazilian Amazon and Africa. This coordination includes data exchanges and methods intercomparisons.

Other important regional efforts include the following:

Land-Use Change and Habitat Requirements

A major area of emphasis within USGCRP land-management agencies is to combine knowledge of species habitat requirements with measurements of actual land cover, derived from satellite remote sensing, aircraft, and field surveys, to assess the likelihood that sufficient habitat will remain for broad assemblages of species with similar requirements. This analysis program draws on the results of basic biological research, land-use and planning information, and simple modeling to provide direct guidance to managers and policymakers faced with difficult trade-offs over the uses of land.

Climate Impacts on Marine Ecosystems

Climate change can influence marine ecosystems in many ways. Because humans derive about 20% of their food protein from fish and other ocean and freshwater products, understanding the potential for climate change to alter marine ecosystems is important.

Fisheries' yields depend on many effects of climate on the oceans. These effects include, for example, atmospheric and oceanic temperatures and temperature changes, precipitation, runoff, salinity, primary production, and ice dynamics. Due to this complexity of effects, predicting the responses to global change of marine animal populations -- both the resource fisheries and the prey upon which they feed -- is a difficult challenge for research.

Global Warming and Ocean Circulation Dynamics

Future global warming will not be uniform over the Earth's surface. Large-scale ocean currents, such as the Gulf Stream, are driven by heat, by freshwater runoff cycles, and by winds.

Changes in atmospheric circulation caused by changes in the Earth's heat budget may eventually cause changes in these major ocean currents. Scientists know that during past major climate changes, the locations of major ocean currents were significantly different from their present positions. Future greenhouse-induced changes might produce similar shifts in ocean currents, displacing species accordingly. If currents shift, entirely different ecosystems may result.

Some of the most productive of all the world's oceans and the locations for many of the world's most economically valuable fisheries are regions of ocean upwelling (displacement of warm surface water from along a shore by colder water brought up from the subsurface), such as those off the Pacific coasts of North and South America, off the Atlantic coasts of Canada and Africa, and off the coasts of Somalia and the Arabian Peninsula. A climate-driven change in worldwide ocean circulation, which may occur as freshwater increases in the polar regions from enhanced rainfall, would greatly affect marine fish populations and the people who depend upon them for food and their livelihoods. For example, the Georges Bank region off the northeastern United States has the potential for major disruption if ocean flows are changed.

Marine Ecosystem Productivity

Changes in ocean temperature, salinity, and other physical properties can cause a shift in the distribution of populations of primary producers and plankton. These shifts would be expected to cascade throughout the food web, ultimately altering population stability in economically important fish species. The U.S. sardine fishery, and that of the achoveta off the coast of Peru, are excellent examples.

Sardine catches in the United States peaked at more than 700,000 tons in 1936, but drastically declined to a sustained collapse in the 1950s, 1960s, and 1970s. Similar changes occurred elsewhere in the Pacific. Then large increases in catches started in the late 1970s. The nearly simultaneous and cyclic rise and fall of sardine catches in these regions may be linked to global- and decadal-scale alterations in climate patterns.

U.S. researchers are studying how the global El Niño/Southern Oscillation climate phenomenon is responsible for changes in the large-scale distribution of marine life and how this interacts with the human harvesting of fisheries resources. For example, the anchovy fishery off the west coast of South America was severely impacted by the onset of an El Niño event, which wiped out the stocks weakened by overharvesting.

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