1. Seasonal to Interannual Climate Variability--The USGCRP seeks to obtain the understanding and skills needed to forecast short-term climate fluctuations and to use these predictions in social and economic planning and development in the United States and abroad.
2. Climate Change Over Decades to Centuries--The USGCRP seeks to understand, predict, and assess changes in the climate that will result from the influences of projected changes in population, energy use, land cover, and other natural and human-induced factors; to understand, predict, and assess the consequences of climate change for society and the environment; and to provide the scientific information society needs to address these changes.
3. Changes in Ozone, Ultraviolet Radiation, and Atmospheric Chemistry--The USGCRP seeks to understand and characterize chemical changes in the global atmosphere and their consequences for human well-being.
4. Changes in Land Cover and in Terrestrial and Aquatic Ecosystems- -The USGCRP seeks to understand, predict, and assess the causes, magnitude, and consequences of changes in land cover and in terrestrial and aquatic ecosystems, and to strengthen the scientific basis for sustainable environmental and natural resource practices.

The following research highlights demonstrate some of the ways in which greater understanding of seasonal and interannual climate variability can produce substantial benefits to human society.

Understanding Year-to-Year Climate Fluctuations: Forecasts and Applications The goal of the seasonal to interannual climate variability component of the USGCRP is to obtain the understanding and skills needed to forecast short-term climate fluctuations and to use these predictions in social and economic planning and development in the United States and abroad.

Improved Prediction of El Niño Events and their Regional Impacts

The El Niño Southern Oscillation (ENSO) is a natural phenomenon that causes warming and cooling of large areas of surface water in the tropical eastern and central Pacific Ocean every several years. (Warming phases are termed El Niño events, and cooling phases are termed La Niña or El Viejo events.) These large-scale ocean disturbances also have significant effects on the atmosphere. Research has linked El Niño events to an increased probability of severe weather anomalies, including the failure of the Indian monsoon, droughts in Brazil, Australia, and Southern Africa, and heavy rains and droughts in the United States and in other areas around the world.

A central focus of research in this area has been on developing numerical models that can forecast whether coming seasons will be warmer or cooler, and wetter or drier, than normal. Several of these forecasting systems successfully predicted the onset, though not the full amplitude, of the 1997-98 El Niño event, which is comparable in amplitude to the 1982-83 warming--the previous "event of the century."

Atmospheric model forecasts for North America have captured the general meteorological features of El Niño's effects, providing the forecast information with sufficient lead time that actions could be taken to limit damage. Research is now focusing on making these forecasts even more timely and reliable. Information on extreme rainfall and drought conditions is of particular interest for emergency and water resource managers and others involved in planning endeavors.

An important research challenge is to forecast the differing characteristics among El Niño events that cause differences in regional impacts. All El Niño and La Niña events are not the same, and research has documented that different events have different effects. Understanding these differences and the irregularity of El Niño events will enhance forecast capabilities (see Color Plate 1 below).

Color Plate 1. ENSO Forecast Maps

ENSO Forecast Maps. Each month the NOAA Climate Prediction Center produces seasonal forecasts for temperature and rainfall for the United States out to a year in advance. The forecasts for the fall of 1997 through the spring of 1998 shown in this figure reflect the expected variations in temperature and precipitation that will be caused by the El Niño event of 1997-98. For precipitation, the green shades indicate regions where above normal precipitation is forecast, and the yellow shades indicate regions where below normal precipitation is forecast. The more intense the color, the more likely the odds that such excursions will occur. For temperature, the reds and yellows indicate regions where warmer than normal temperatures are forecast, and the blues indicate regions where colder than normal temperatures are forecast. The forecast for the October-December 1997 period was issued in mid-August 1997; the forecasts for the January-March and March-May 1998 periods were issued in mid-November 1997. Because of these early warnings, planners in different economic and emergency sectors have been able to use the forecasts to improve water management, prepare for potential flooding events, and adjust hydroelectric allocations.

Source:
Ants Leetmaa/NOAA Climate Prediction Center.

The 1997-98 El Niño event is much better documented than any previous El Niño event. An array of observing instruments was placed in the tropical Pacific Ocean as part of the Tropical Ocean- Global Atmosphere (TOGA) program starting in the 1980s. In addition, satellite-derived observations of sea-level elevation and ocean-surface conditions generated by winds were a factor in the 1997 forecasting success and have extended observations to higher latitudes. Satellite measures of water vapor in the upper atmosphere have helped document the event and will improve future forecasts. In addition, new satellite observations of ocean color will document future ENSO variability and help gauge the impacts of warming and cooling events on the marine biosphere.

Changes in precipitation are among the most important features of El Niño events. The USGCRP is merging satellite-based data and ground-based data to produce the best possible depiction of fluctuations in precipitation patterns. The Tropical Rainfall Measuring Mission (TRMM), a joint U.S.-Japan satellite mission, was launched successfully in November 1997, and is expected to provide high- quality satellite observations of precipitation for the entire region between 35°N and 35°S latitudes. TRMM also will provide a basis for estimating how the rainfall associated with El Niño events and other tropical phenomena affects the atmospheric wind patterns on a global basis and thereby generates anomalous conditions in far-removed regions of the world.

Climate Variability in North America

A number of studies are underway to improve the representations of physical, chemical, and ecological processes in computer models, thus increasing their ability to predict climate variations. For example, research in the Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project and the Global Ocean- Atmosphere-Land System (GOALS) Pan-American Climate Studies program is seeking to determine the extent to which year-to-year variations in summer precipitation over North America is predictable and how it is linked to forcing influences in other parts of the world, such as ENSO events.

During FY99, research will focus on atmospheric and land-surface processes leading to heavy precipitation in the eastern part of the Mississippi River Basin, the effects of mountain geography on precipitation and hydrology in the northwestern Great Plains, and demonstration projects to show how climate information can be used more effectively to manage water resources in the central United States. Researchers also are developing strategies for assessing the role of the Pacific Ocean, the Gulf of Mexico, and soil-moisture conditions in contributing to the variability of warm-season precipitation in central and western North America.

Using Seasonal Climate Forecasts to Reduce Costs of Impacts

Seasonal climate forecasts can both be used to reduce the impacts of natural disasters and identify potential social and economic opportunities. For example, research is showing that the intensity, frequency, and paths of storms, including hurricanes, are modulated by ENSO events. While it is not possible to prevent destructive storms, improved forecasts can be used to reduce some of the losses associated with these storms.

Better predictions of possible weather extremes could, over time, save the United States billions of dollars in damage costs. Recent estimates of weather-related damages include the 1982-83 El Niño event ($2-3 billion), 1988 Midwest drought ($40 billion), 1993 Midwest floods ($15 billion), 1995 California flooding ($3.3 billion), and 1995-96 Southwest drought ($4 billion).

Advance warning of such events can help water, energy, and transportation managers and agricultural producers to plan ahead and limit some losses. During the 1997-98 El Niño event, national and international mechanisms are being developed for responding to future climate variations. For example, FEMA, NOAA, NASA, USDA, USAID, the Army Corps of Engineers, and the Department of the Interior are cooperating to reduce the impacts of extreme rainfall events and identify opportunities for social and economic benefits. Researchers are working with planners and managers to identify how improved predictions can be used to improve disaster preparedness of Federal, state, and local agencies.

To encourage the development of similar capabilities internationally, NOAA is sponsoring the International Research Institute for climate prediction (IRI). The IRI will include a multinational network of research centers and activities related to use of information from forecasts. Among the Institute's objectives are the following:

1. To develop and issue experimental seasonal to interannual climate predictions based on global and regional modeling of ocean- atmosphere-land interactions.
2. To disseminate forecast guidance to nations and regions that are particularly affected by climate variability associated with El Niño.
3. To tailor global predictions to specific regional conditions and needs.

Figure 1: Multivariate ENSO Index, 1950-Present. The El Niño Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon causing year-to-year global climate variability. The Multivariate ENSO Index (MEI), developed mainly for research purposes, monitors the coupled oceanic-atmospheric character of ENSO events based on the main observed variables over the tropical Pacific Ocean. The MEI can be understood as a weighted average of the main ENSO features contained in the following six variables: sea-level pressure; the east-west and north-south components of the surface wind; sea surface temperature; surface air temperature; and cloudiness of the sky. Positive values of the MEI represent the warm phase of ENSO (i.e., El Niño events), and negative values represent the cold phase of ENSO (i.e., La Niña, or El Viejo, events). The observations used to generate this index are collected and published in the Comprehensive Ocean-Atmosphere Data Set (COADS). For more information, see the NOAA Climate Diagnostics Center Web sites:

http://www.cdc.noaa.gov/ENSO/enso.mei_index.html
http://www.cdc.noaa.gov/~kew/MEI/mei.html
http://www.cdc.noaa.gov/coads/.

Source:
Klaus Wolter and Michael Timlin/NOAA-CIRES Climate Diagnostics Center.

Determining the consequences of global change for the environment and for society requires better estimates of the rate, pattern, and magnitude of future climate change. Management of natural resources, the design and planning of long-term infrastructure, and business decisionmaking are all examples of areas where more complete understanding could reduce the potential adverse consequences of climate change.

The past several years have seen considerable progress in advancing our understanding of global climate processes and the climate changes expected in the decades and centuries ahead. Despite this progress, important uncertainties remain, particularly concerning projections of the detailed regional patterns of climate change. USGCRP research activities are dedicated to reducing these uncertainties.

Predicting Climate Change and Understanding its Implications for Society and the Environment The goal of the climate change component of the USGCRP is to understand, predict, and assess changes in the climate that will result from projected changes in population, energy use, land cover, and other natural and human-induced factors; to understand, predict, and assess the consequences of climate change for society and the environment; and to provide the scientific information society needs to address these changes.

Global Carbon Cycle

Stabilization of greenhouse gas concentrations in the atmosphere (including the concentration of carbon dioxide), at a level that would prevent dangerous anthropogenic interference with the climate system, is the ultimate objective of the Framework Convention on Climate Change. At the present time, however, emissions of greenhouse gases continue to rise, as do their atmospheric concentrations (with the exception of CFCs and other halocarbons, which are now controlled by the Montreal Protocol). While it is known that slowing or halting the rise in the atmospheric CO₂ concentration would require significant cutbacks in emissions over the next century, it is not yet possible to be precise about the needed cutbacks because uncertainties continue to surround the factors determining the rates and magnitudes of changes in the concentrations of CO₂ and other greenhouse gases in the atmosphere.

USGCRP-supported research is underway to reduce the differences between modeled and measured concentrations of CO₂ in the atmosphere. Investigations are in progress to better determine the sources of CO₂ to the atmosphere and its removal to the ocean and land surface via sinks, both natural and as altered by human activities. Studies are showing that processes that control CO₂ concentrations are variable over seasons, years, and decades, revealing a more dynamic and complex carbon cycle than was previously recognized. Support for long-term measurements of atmospheric CO₂ concentrations and research to understand the natural processes controlling atmospheric CO₂ levels remains a high priority for the USGCRP.

In the late 1980s, scientists established the existence of a net CO₂ sink in Northern Hemisphere terrestrial ecosystems. The effect of increased atmospheric CO₂ on photosynthesis, long known from laboratory studies, was proposed as one factor causing the additional ecological uptake of CO₂. Early research focused on this "CO₂ fertilization" effect. In the mid-1990s, it was recognized that the ability of plants and soils to store carbon through CO₂ fertilization is limited by the availability of nitrogen, deposition of which is also being affected by human activities. This process is difficult to test experimentally, but its implications have been examined in a series of modeling exercises. This modeling work suggests that CO₂ fertilization alone can account for only part of the terrestrial carbon sink. With the new awareness that nutrients may also be influencing carbon cycling, several researchers have shown that nitrogen pollution resulting from human activities could, unintentionally, be creating a substantial terrestrial sink of CO₂.

Predictive Models of Climate Change

Over the past 10 years, largely as a result of USGCRP-sponsored research, atmospheric and oceanic general circulation models have improved significantly. There is strong potential for continuing advances through improved representations of critical climate processes and through finer model resolution for regional-scale predictions.

As the spatial resolution and detail in predictive climate models increases, the need for better descriptions of clouds and ocean processes in these models also increases. Researchers are seeing the early benefits of data gathered by USGCRP-sponsored field research and satellite programs. The rate of progress can be expected to increase with the additional investments that have been made in both surface- and space-based measurement systems.

Climate variations on time scales of years to decades are largely controlled by how the ocean stores and transports heat from the warm regions near the equator to the colder regions at higher latitudes. Weather patterns change in response to changes in sea- surface temperatures, resulting in climate oscillations ranging from El Niño events to smaller but longer-term interdecadal variations.

More accurate simulation of ocean circulation is now possible through very-high-resolution ocean models. One example is the work of scientists at the Los Alamos National Laboratory, in collaboration with university scientists. Using a high-resolution global ocean model, they reproduced the long-term average behavior of the ocean currents that transport heat, as well as the shorter term fluctuations that lead to climate variations (see Color Plate 2 below).

Color Plate 2. High-Resolution Ocean Modeling

High-Resolution Ocean Modeling--Global Image with Mediterranean Salt Eddies (inset). More accurate simulation of ocean circulation is now possible through use of very-high-resolution ocean models. One example is based on the work of scientists at the Los Alamos National Laboratory (LANL), in collaboration with the National Center for Atmospheric Research and the Naval Postgraduate School. Using a high-resolution global ocean model, they have achieved accurate simulations of the long-term behavior of the ocean currents that transport heat from the warm regions near the equator to the colder regions at higher latitudes, as well as the shorter term seasonal fluctuations that affect climate variability.

The global image was generated from a simulation using the Parallel Ocean Program (POP) at LANL, using a Mercator grid with 1/4- degree resolution at the Equator and an average latitudinal resolution of 1/6-degree over the globe. The field displayed is the 7- year average sea-surface height variability (in cm), which highlights the regions of active circulation. The inset figure shows, based on a different simulation using the same model, a snapshot of the ocean's salinity (salt content) at a depth of 1,100 meters in the Atlantic Ocean near the Strait of Gibraltar. Because enhanced evaporation in the Mediterranean Sea creates higher salinity water, very salty water (in red) flows out of the Mediterranean and mixes with the less saline water of the Atlantic Ocean (in blue). Very small features of the turbulent flow can be seen in this simulation of the Atlantic Ocean, which used a Mercator grid with 0.1° resolution at the equator.

Sources:

Global Image
Albert Semtner/Naval Postgraduate School. The modeling work was done by Robert Malone, Richard Smith, John Dukowicz, and Mathew Maltrud (LANL), and Albert Semtner (Naval Postgraduate School).

Mediterranean Salt Eddies Image
Robert Malone/Los Alamos National Laboratory. The modeling work was done by Richard Smith and Mathew Maltrud (LANL), and Frank Bryan and Matthew Hecht (National Center for Atmospheric Research), with support from the Department of Energy CHAMMP Program and the National Science Foundation.

Natural climate variability on decadal and longer time scales results from the complex interactions between the climate system components--the atmosphere, oceans, land surface, and sea ice. Theoretical and observational studies are planned to quantify the sources, patterns, and magnitudes of long-term variability, as well as the possible effects of long-term climate change on shorter term seasonal to interannual climate variations.

An important USGCRP-supported advance was a recent 300-year computer simulation conducted to assess the ability of one of the new generation of climate models to reproduce the natural variability of the Earth's climate. This simulation, which used a model developed by scientists from the National Center for Atmospheric Research, universities, and other Federal laboratories, included coupled representations of the atmosphere, oceans, land surface, and sea ice. A control run of the model used fixed current-day CO₂ concentration levels to simulate natural variability.

Several interesting results emerged from this simulation. Until now, even the best climate models, after simulating a few decades, produced results that "drifted off" into unrealistic climates unless fairly large "flux adjustments" were made to adjust for limitations in representing physical processes. For this model, however, the control run reproduced a stable value for the long-term global average temperature without the need for corrections. Moreover, the simulated short-term fluctuations were about the same magnitude as those observed in the real world during the past 100 years. The model also succeeded in reproducing, with a good degree of realism, the observed seasonal and geographical variability of the climate system.

Climate models are judged by how closely their results simulate nature and how well they simulate past climate. These results, which are available to all interested climate researchers for further analysis, are a strong indication that significant advances are being made in climate modeling.

Climate Change and Deep Ocean Circulation

New attention is being given to the couplings among components of the Earth system. For example, climate changes in the Arctic polar region could affect ocean salinity by changing the amount of freshwater runoff. This salinity plays an important role in determining the intensity of the deep ocean circulation that brings substantial amounts of heat to the North Atlantic Ocean. Warming temperatures in polar regions therefore could trigger reductions in the poleward transport of heat by the Atlantic Ocean, with significant effects on the climates of Europe and North America (see Figures 2 and 3). New ocean model simulations are able to represent the Arctic region with impressive realism.

Figure 2: Ocean Thermohaline Circulation. The global ocean thermohaline circulation, which involves the joint effects of temperature (thermodynamics) and salinity (haline dynamics), is sometimes referred to as the ocean's "conveyor belt." It important because it is responsible for a large portion of the heat transport from the tropics to higher latitudes in the present climate. For example, the Gulf Stream in the Atlantic Ocean forms part of the global thermohaline circulation, transporting warmer waters northward, thereby contributing to western Europe's relatively mild climate for its latitude.

The conveyor is a simplified diagrammatic representation of a major globe-encompassing ocean circulation system. Salty water near the surface of the Atlantic Ocean is carried northward (largely by the Gulf Stream) to the vicinity of Iceland. There, during the winter months, the heat this waters carries is extracted by the cold, westerly winds that flow across the Atlantic from North America. The air is warmed, greatly ameliorating the winter conditions downwind in northern Europe. The water is cooled by the heat extraction (hence made denser) and sinks to the abyss, forming the lower limb of the Atlantic's conveyor. The amount of water transported by the Atlantic's conveyor averages about 16 million cubic meters per second (i.e., comparable to the world's total rainfall, or 100 times the amount of water transported by the Amazon River).

The water carried by the Atlantic's lower limb passes around the southern tip of Africa, where it joins a powerful circum-Antarctic current. This current is also fed by new deep water generated beneath the ice shelves surrounding the Antarctic continent. The mixture created in this way feeds northward flows into the deep Indian and Pacific Oceans. This water eventually upwells to the surface. As shown in the figure, one branch of the return flow to the Atlantic passes through the Indonesian Straits, across the Indian Ocean and around the tip of Africa into the South Atlantic.

Sources:
W.J. Schmitz, Jr.,  Reviews of Geophysics, May 1995; W. Broecker/Lamont Doherty Earth Observatory, with illustration courtesy of Lawrence Berkeley National Laboratory.

Figure 3: Impact of Climate Effects Caused by Increased CO₂ on Ocean Circulation. New attention is being given to the couplings among components of the Earth system. For example, climate changes in the Arctic polar region could affect ocean salinity by changing the amount of freshwater runoff. This salinity plays an important role in determining the intensity of the deep ocean thermohaline circulation that brings substantial amounts of heat to the North Atlantic Ocean. Warming temperatures in polar regions therefore could trigger reductions in the poleward transport of heat by the Atlantic Ocean, with significant effects on the climates of Europe and North America.

The figure shows North Atlantic Circulation Intensity on atmospheric concentration pathways that lead to stabilization at 2xCO₂ and 4xCO₂, using the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) coupled ocean/atmosphere climate model. ("2xCO₂" refers to an atmospheric CO₂ concentration of twice the preindustrial level; "4xCO₂" refers to an atmospheric CO₂ concentration of four times the preindustrial level.) GFDL climate model simulations project that the global thermohaline circulation will decrease in intensity as greenhouse gas warming occurs, due to enhanced precipitation and runoff from the continents in high latitudes. In the 4xCO₂ experiment, the thermohaline circulation essentially disappears in the GFDL model. In the 2xCO₂ experiment, the thermohaline circulation initially weakens to less than half its original intensity, but eventually recovers to its initial strength after several centuries. Further experiments have shown that the rate of CO₂ build-up has an important effect on the evolution of the thermohaline circulation: The faster the build-up of CO₂, the greater the eventual reduction in the thermohaline circulation and the longer the delay in its recovery (see caption for Figure 2 above).

For additional information on GFDL's analysis of the climate change impact of a concentration pathway leading up to a quadrupling of atmospheric CO₂, see the GFDL Web space:

http://www.gfdl.gov/~tk/climate_dynamics/climate_impact_webpage.html

Source:
Jerry Mahlman/NOAA Geophysical Fluid Dynamics Laboratory.

Past Climate Changes

Current research is seeking to understand how natural forcing mechanisms and internal climate dynamics affect climate change on the time scales of seasons to centuries. Information about past climates allows researchers to reconstruct conditions beyond the short time span of modern instrumental records. This "paleoclimate" record of past variations in the climate also is vital for developing and validating models and hence for reducing uncertainties in predictions of future climate.

Until recently, the climate of the past 10,000 years was thought to be relatively stable, with none of the abrupt variations that characterized the cold climates of the previous Ice Age. New terrestrial, marine, and ice core data, however, document significant climate swings during historical times. Research attention is being focused on changes such as the Little Ice Age (approximately 1400- 1850 AD) when the annual temperatures of the Northern Hemisphere were about 0.5-1°C cooler than today--enough to choke ports with ice and freeze rivers in North America and Europe.

Based on ice cores recovered from Greenland, the paleo-atmospheric circulation record of the Little Ice Age shows the most abrupt onset (1400-1430 AD) of any climate fluctuation since the last full Ice Age. In addition, a comparison of annually resolved ice cores from Greenland and Antarctica demonstrates the near-synchronous onset of increased marine storminess in the North Atlantic and South Pacific at the beginning of the Little Ice Age. These preliminary studies point to a joint ocean-atmosphere process underlying these historical climate cycles.

Understanding the Implications of Climate Change

A large and growing proportion of the world's population lives in coastal areas. USGCRP-supported work on new ocean models, together with more accurate measurements of the Earth's shape from a future satellite mission, will provide improved estimates of the potential rise in sea level due to global warming. Research suggests that rising sea level will flood some coastal wetlands and communities, and amplify the impacts of storm surges, in which sea levels rise because of severe storm winds. Improved models of vegetation also will provide estimates of shifts in vegetation, soil moisture, and runoff, allowing more complete studies of the consequences of long-term climate change.

USGCRP-supported work on integrated assessment models is improving the ability to predict the impacts of climate change on society. A particular challenge is modeling the impacts of global climate change at regional scales. Another challenge is modeling the impacts on various natural resources, on sectors of the economy, and on public health.

Progress in understanding global atmospheric chemistry will help policymakers protect human health, preserve the cleansing and shielding qualities of the atmosphere, and ensure that new chemical compounds released into the atmosphere do not lead to adverse consequences from changes in atmospheric composition. The following research areas represent important advances.

Understanding Atmospheric Chemistry and its Links to Human Well-Being The goal of the atmospheric chemistry component of the USGCRP is to understand and characterize the chemical changes in the global atmosphere and their consequences for human well-being.

Unusually Low March Total Ozone Observed in Arctic

Unusually low values of total ozone were observed over the Arctic in the spring of 1997. During an approximately 2-week period in March 1997, the satellite-based Total Ozone Mapping Spectrometer (TOMS) instruments found that the ozone levels over a large region centered over the North Pole were as much as 40 percent below the normal amount for this time of year. These levels, which were confirmed by ground-based measurements, are significantly lower than levels observed in most recent years.

The low ozone amounts were closely correlated with the position of the wintertime polar vortex, a cold and relatively isolated region of air in the lower stratosphere. The Northern Hemisphere polar vortex in 1997, which was the most persistent on record, allowed the ozone- depleting chemistry to persist well into the spring, significantly later than in a typical northern winter, resulting in large ozone losses. This ozone-depleting chemistry involving CFC-derived chlorine and other manufactured chemicals is the same as that which produces the Antarctic ozone hole each year.

The lowest ozone values observed in the Arctic are still much higher than the corresponding values observed in the Antarctic and the area of ozone depletion is much smaller. This reflects meteorological differences between the poles, which lead to persistently colder conditions with concomitantly greater chlorine activation and ozone loss in the Antarctic. However, the losses demonstrate that, for appropriate meteorological conditions (which can vary significantly from one year to the next), appreciable chemically induced ozone depletion resulting from human activities can also occur in the Arctic. The Arctic ozone losses are clearly worsening and are becoming comparable to those observed over Antarctica in the mid-1980s (see Figure 4, and Color Plate 3 below).

Color Plate 3. Arctic Ozone Image Time Series, 1971-1997

Arctic Ozone Image Time Series, 1971-1997. Unusually low values of total ozone were observed over the Arctic in the spring of 1997. During an approximately 2-week period in March 1997, the satellite-based Total Ozone Mapping Spectrometer (TOMS) instruments found that the ozone levels over a large region centered over the North Pole were as much as 40 percent below the normal amount for this time of year. These levels, which were confirmed by ground-based measurements, are significantly lower than levels observed in most recent years. The ozone losses demonstrate that, for appropriate meteorological conditions (which can vary significantly from one year to the next), appreciable chemically induced ozone depletion resulting from human activities can occur in the Arctic as well as the Antarctic. The Arctic ozone losses are clearly worsening and are becoming comparable to those observed over Antarctica in the mid-1980s.

The figure shows March monthly average total ozone polar stereographic images for 1971 and 1972 (Nimbus-4 satellite, BUV instrument); 1979, 1980, 1990, and 1993 (Nimbus-7 satellite, TOMS instrument); 1996 (NOAA-9 satellite, SBUV/2 instrument); and 1997 (Earth Probe satellite, TOMS instrument). High ozone values are yellow-red in color, while low total ozone values are blue-purple in color. Note the distinct differences in ozone between the earlier years and later years in the Arctic region.

Source:
Paul Newman/NASA Goddard Space Flight Center.

Figure 4: Average Total Ozone over the Arctic. The time sequence of total ozone values, in Dobson units, over the Arctic (63°-90°N) for March shows a large decline in average ozone values during the 1990s. The 1971 and 1972 satellite data are from the BUV instrument aboard the Nimbus-4 satellite; the 1979 through 1993 data are from the Total Ozone Mapping Spectrometer (TOMS) instrument aboard Nimbus-7; the 1994 data are from the Meteor-3 TOMS; the 1996 data from from the NOAA-9 SBUV/2; and the 1997 data are from Earth Probe TOMS.

Source:
Paul Newman/NASA Goddard Space Flight Center. The figure is taken from P.A. Newman, J.F. Gleason, R.D. McPeters, and R.S. Stolarski, "Anomalously low ozone over the Arctic,"  Geophysical Research Letters, Vol. 24, No. 22, pp. 2689-2692, November 15, 1997.

Additional Source of Upper Tropospheric Hydroxyl

Direct observations made for the first time from high-flying research aircraft suggest that the abundance of hydroxyl (OH) in the upper troposphere is appreciably different from the predicted abundance. Clarifying the reasons for this may lead to a major advance in our understanding of atmospheric chemistry. Knowledge of hydroxyl abundance is important because of the pivotal role OH plays in removing many trace gases from the atmosphere. Hydroxyl is known as the "scavenger" of the atmosphere because of the way it cleanses the troposphere of many chemicals produced naturally or by human activities.

Until recently, the production of OH was thought to come mainly from photochemical reactions involving ozone and water vapor. To explain the difference between expected and observed OH abundance, laboratory measurements and computer modeling both suggest that the observed greater OH concentrations could result from the breakup by ultraviolet light of other hydrogen-containing precursors, such as acetone or simple peroxides.

Improvements in our understanding of OH production should lead to corresponding improvements in our ability to simulate the distribution of OH with computer models of atmospheric chemistry. This, in turn, should lead to increased confidence in climate-related model predictions.

Slowing Observed Growth of Stratospheric Halogen Amounts

Observations by the Upper Atmosphere Research Satellite of hydrogen fluoride (HF) levels near the top of the stratosphere have provided conclusive evidence for a reduction in the growth rate of the concentration of HF. At this altitude, because there are no natural sources of stratospheric fluorine, HF is a very good indicator of the total amount of industrially produced halogens (especially fluorine and chlorine) in the atmosphere.

A time series of the HF measurements in the stratosphere shows a distinct flattening of the growth curve. These measurements are consistent with the reduced growth rates of the major fluorine- containing chemicals--notably several of the more abundant CFCs-- observed previously near the Earth's surface, with a time lag of several years needed for the gases to reach the stratosphere.

In concert with previously reported findings of decreasing halogen levels in the troposphere, these results prove that regulation of halogen-containing molecules under the Montreal Protocol on Substances that Deplete the Ozone Layer is affecting stratospheric composition. The results further strengthen the evidence that the phaseout of CFCs will significantly reduce amounts of stratospheric chlorine.

Biomass Burning and Global Tropospheric Ozone

Several areas of research show that biomass burning, which is concentrated in the tropics, is having a global effect on the distribution of ground-level ozone and radiative forcing in the troposphere.

First, aircraft observations over the tropical Pacific Ocean (thought to be one of the cleanest areas in the atmosphere) have demonstrated that effluents from biomass burning can be transported over thousands of kilometers. Such effluents are known to be a source of ozone in the troposphere, because ozone is photochemically produced from precursor molecules released during biomass burning of forests and grasslands. Recent analyses of satellite data over South America and New Guinea provide strong evidence that tropospheric ozone levels have been increasing in areas affected by biomass burning.

In addition, computer model simulations show that upper tropospheric ozone from biomass burning can contribute significantly to radiative forcing over large areas of the tropics, and that the resulting effects on forcing and then on climate may be significant even when averaged over the entire globe. Identification of this tropical warming effect should improve the understanding of the spatial patterns of temperature change caused by human activities.

Cooling Role of Aerosols

Atmospheric aerosol particles like dust and soot cool the Earth by scattering sunlight back to space and by altering the properties of clouds. Several international projects have measured the impact of both natural and anthropogenically derived aerosols on the radiative balance of the Earth. Direct measurements of the physical, chemical, and optical properties of aerosol particles have enhanced confidence in estimates of their effects on the global radiative balance, and thus their calculated effects on climate.

These studies have shown that there are major aerosol components (such as sea salt and organic compounds) that are not yet included in aerosol-climate calculations. Airborne measurements over Brazil, however, have shown that the radiative cooling due to smoke from biomass fires may be smaller than previously estimated. These refinements to understanding of the link between aerosols and climate should lead to improved estimates of the climate change to be expected from future emissions of greenhouse gases.

Issues such as tropical deforestation and the collapse of major marine fisheries have attracted international attention in recent years. In the United States, changes in terrestrial and marine ecosystems, including the loss of productive agricultural land, the severe reductions in salmon populations in the Pacific Northwest, and concerns about the sustainability of important ecosystems have underscored the importance of documenting and understanding ecosystem changes.

Increasing concentrations of atmospheric carbon dioxide and other chemicals and changes in climate will add to the challenges of maintaining the benefits of healthy and productive terrestrial and aquatic ecosystems. To provide society with the information needed to manage and preserve these ecosystems, researchers must document and understand the human and natural factors that influence land cover and ecological systems. This task is particularly complex because it involves wildlife, forests, croplands, rangelands, wetlands, lakes and rivers, settlements, transportation arteries, ecosystem fragmentation, migration of species, and such disturbances as pests, diseases, and fires.

Global Ecosystems and Sustainability The goal of the land cover and ecosystems component of the USGCRP is to understand, predict, and assess the causes, magnitude, and consequences of changes in land cover and in terrestrial and aquatic ecosystems, and to strengthen the scientific basis for sustainable environmental and natural resource practices.

Documenting Changes in Land Cover and Ecosystems

At the global scale, many data collection and analysis activities rely on satellites. Key examples include the use of satellite data to produce global and continental land-cover maps (see Color Plates 4 and 5 below), and the analysis and cross-comparison of ocean color data from several international missions to document the extent and quantity of marine phytoplankton.

Color Plate 4. Global Topography

Global Topography. This global elevation image is based on data that provide a significant increase in detail over previously available elevation data. These data can provide information on slope, landforms, water drainage patterns, and mountain effects for users in many diverse fields of study, including studies involving global circulation models, water resources, geology and geophysics, ecology, soil science, botany, and glaciers. Topographic data are also critical to computer-based procedures used to correct and animate remotely sensed satellite and other global data.

The data set was developed by the U.S. Geological Survey in collaboration with other Federal agencies and international Earth science organizations, including NASA, UNEP, USAID, the Instituto Nacional de Estadistica Geografica e Informatica of Mexico, the Scientific Committee on Antarctic Research, the New Zealand Manaaki Whenua Landcare Agency, and the Geographical Survey Institute of Japan. The data are available via the Internet at http://edcwww.cr.usgs.gov/landdaac/gtopo30/.

Source:
Susan Greenlee/USGS.

Color Plate 5. Global Land Cover

Global Land Cover. At the global scale, many data collection and analysis activities rely on satellites. Key examples include the use of satellite data to produce global and continental land-cover maps. The U.S. Geological Survey (USGS), the University of Nebraska-Lincoln, and the European Union's Joint Research Centre generated this global synthesis of information on land cover at a spatial resolution of 1 km (about 0.6 miles). These data, already in wide use, are of great utility in a range of environmental research and modeling applications.

The global land cover characteristics database was developed on a continent-by-continent basis using data obtained by the Advanced Very High-Resolution Radiometer (AVHRR), a NASA-developed instrument that flies aboard a NOAA satellite, during a period spanning April 1992 through March 1993. Each continental data set contains unique elements based on the geographical aspects of the specific continent. In addition, a core set of derived thematic maps, produced through the aggregation of seasonal land-cover regions, is included in each continental data set. The continental data sets are combined to make six global data sets, each representing a different landscape based on a particular classification legend.

This effort is part of NASA's Earth Observing System Pathfinder Program. Funding for the project was provided by the USGS, NASA, EPA, NOAA, the USDA Forest Service, and the United Nations Environment Programme. The database has been adopted by the International Geosphere-Biosphere Programme Data and Information System (IGBP-DIS) office to fill its requirement for a global 1-km land-cover data set. These data are available on the Internet at http://edcwww.cr.usgs.gov/landdaac/.

Source:
Thomas Loveland/USGS.

Also during FY99, the Landsat-7 mission will provide global data with even finer spatial resolution. Within the United States, several agencies are collaborating to produce the first national fine-scale map of vegetation distribution using Landsat data. These data will be exceedingly valuable for understanding the current distribution of land-cover types across the United States. The documentation of current conditions also will serve as a baseline for evaluating future changes.

Better understanding of land cover, ocean color, and their changes will enable better calculations of sources and sinks of carbon dioxide. This will contribute both to improved climate predictions on national and global scales and to more reliable calculations of sources and sinks of greenhouse gases on national levels.

Ecosystem Dynamics and Responses of Ecosystems to Climate Change and Increases in Atmospheric Carbon Dioxide

The USGCRP supports field research and observational studies that investigate both the dynamics of ecosystems and the interactions of ecosystems with the atmosphere. For example, as a partner in a Brazilian-led effort to understand the consequences of rapid land-use change in the Amazon Basin, the United States is supporting investigations into basic terrestrial and aquatic ecosystem processes that control the movement of carbon and nutrients within the Amazon Basin and back and forth to the atmosphere.

As a second example, field studies of the dynamics of fish and plankton communities are examining the patterns and causes of variability in marine biological populations. This research seeks to quantify the relationship between this variability and both climate variations and direct human impacts.

Several USGCRP agencies are cooperating to support experiments that expose ecosystems to increased concentrations of atmospheric CO₂. The ecosystem-level responses revealed by these experiments can be used in models that predict future changes in ecosystems caused by rising CO₂ concentrations. Mechanistic models are beginning to use the experimental information for predictions of ecosystem response to CO₂ and associated climate variables.

This combination of experimental, observational, and theoretical work is leading to better scientific understanding of the processes that control ecosystem change. Such understanding will lead to an improved ability to assess the impacts of increasing atmospheric CO₂ concentrations and climate variability on local to regional to global scales. It will also produce an improved ability to understand the combined effects of climate and air pollution on natural ecosystem dynamics.

The Role of Ecosystems in the Global Carbon Budget

A key uncertainty about the global carbon cycle has been the role of the terrestrial biosphere. On the one hand, deforestation and the loss of soil carbon through plowing of the soil and other agricultural practices transfer carbon from the terrestrial biosphere to the atmosphere. On the other hand, forests have regrown in some areas, for example in the northeastern United States, and rising CO₂ concentrations appear in controlled experiments to stimulate plant growth and carbon sequestration.

Summing up the overall effects of these processes on atmospheric CO₂ thus requires consideration of diverse influences. Although we cannot measure all of the effects of these processes individually, the collective evidence indicates that the sum over the globe of natural processes affecting the terrestrial biosphere have caused it to act as a net sink for atmospheric CO₂ during the 1980s and 1990s, although the magnitude of the sink has varied considerably from year to year. In contrast, human influences have caused tropical forests to be a net carbon source to the atmosphere due to deforestation, although variation in cutting rates, regrowth of cleared land, and interannual climate variations have caused variations in the magnitude of the source. Due to forest regrowth, forests outside the tropics have generally been a net carbon sink that has varied in size from year to year, depending mainly on climatic conditions. In some years, the terrestrial carbon sinks outside the tropics are collectively large enough to allow the terrestrial biosphere as a whole to act as a net sink in the global carbon budget, while in other years this may not be so. Major questions remain about the size, location, and magnitude of each of these influences, and, most importantly, about whether or not the terrestrial biosphere will continue to act as a significant sink throughout the next century.

The USGCRP is supporting a series of observational studies, such as AmeriFlux, to quantify better the movements of carbon between terrestrial ecosystems and the atmosphere over time. These studies are coordinated with other, similar programs internationally. These programs also are being augmented with measurements of land- cover change to help evaluate changes in the total stock of carbon stored in terrestrial vegetation.

Sustainable Land and Resource Management in a Changing Environment

The agencies represented in the USGCRP have accumulated a great deal of information through studies involving the sustainable management of the Nation's natural resources. For example, case studies and natural resource assessments in such regions as the Columbia River Basin and the Sierra Nevada are providing the land management agencies and their regional partners with important insights into the current stresses on these landscapes. These investigations also are suggesting potential strategies for the long- term management of these systems to ensure their sustainable provision of goods and services.

Over the last several years, the USGCRP agencies have initiated several case studies, both in the United States and abroad, of the relationships between ecosystem changes and the human activities related to global change. In 1997, USGCRP-supported research projects on land-cover and land-use change were initiated that specifically involve collaborations among ecologists, social scientists, economists, and remote-sensing experts on issues of importance to global change. While this research is currently in an early stage, it will provide important information on the links between human activities, including economic decisions, and landscape change, and on the social and economic consequences of this change.

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