Written Testimony of

NOAA Climate Monitoring and Diagnostics Laboratory

Before the

Senate Agriculture, Nutrition and Forestry Committee

Subcommittee on Production and Price Competitiveness

Atmospheric Carbon Dioxide - Current Understanding and Plans for Future Research

May 4, 2000

Introduction

Good afternoon. I am Dr. David Hofmann, the Director of NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL) in Boulder, Colorado. Thank you, Chairman Roberts, for inviting me to provide testimony on carbon cycle research and the importance of the terrestrial biosphere (soils, trees, plants, etc.) in taking up excess carbon dioxide. I am honored to be here today. I will briefly review what we know about carbon dioxide uptake by the terrestrial biosphere, which may be important for agricultural management practices, and how carbon cycle research can be improved. The scientific community understands the mechanics of the carbon cycle better now than just 10 years ago. However, there is a great need to better understand the natural variability in carbon dioxide uptake by the terrestrial biosphere.

Carbon cycle research is focused on understanding the science of one element of forced climate change: greenhouse gases. Understanding the sources and sinks (uptake mechanisms) of carbon dioxide and methane, two of the most important greenhouse gases known to be increasing in our atmosphere, is the primary research goal in carbon cycle research.

Greenhouse Gasses

The beginning of the industrial age, a time when the burning of fossil fuels (wood, coal, oil and gas) and conversion of forested land for agricultural use began to increase, initiated an increase in the concentration of carbon dioxide in the atmosphere. One of the most famous environmental data sets showing this trend over the past 42 years is the atmospheric carbon dioxide record that has been measured at Mauna Loa, the CMDL Baseline Observatory in Hawaii (see figure below).

Records such as this provide incontrovertible evidence that greenhouse gases are increasing in the atmosphere. However, the processes which affect the concentration of greenhouse gases in the atmosphere are complicated and a better understanding is required to properly represent them in climate models.

We depend on natural greenhouse gases to make Earth habitable. Without them the planet would have an average temperature of about 5 degrees Fahrenheit, well below freezing. But future increases in greenhouse gases of the magnitude expected during the 21^st century must be considered seriously as these increases could affect the future economy and well-being of the nation. For this reason, enhanced study of the global carbon cycle deserves our highest attention and vigilance.

The Carbon Cycle and the Primary Carbon Reservoirs

The carbon cycle is the process by which the Earth exchanges huge amounts of carbon between three major reservoirs, the land, the oceans and the atmosphere. On land, carbon dioxide is taken up by trees and other plants during photosynthesis, and is sequestered in soils when plants die and undergo incomplete decay. Carbon dioxide is given off during respiration of biological systems. Oceans exchange carbon dioxide with air at the surface, although some of the dissolved carbon dioxide in the ocean water is not returned to the atmosphere. The amount of carbon dioxide that remains in the atmosphere is the difference between these huge carbon dioxide inputs and outputs of the land and oceans. We can now measure the atmospheric content of carbon dioxide on a global average quite accurately. We also know the magnitude of fossil fuel emissions. Deforestation in the tropics is less well known but can be estimated. Putting these numbers together tells us how large the global uptake or "sink" for carbon dioxide must be.

While the net amount of carbon dioxide from natural emissions and uptakes is only a few percent of the total amount of carbon dioxide exchanged each year, it nevertheless represents a large amount of carbon, measured in units of billions of metric tons per year. One metric ton equals 1000 kilograms or 2200 pounds. Each year, the world emits almost 7 billion metric tons of carbon in the form of carbon dioxide from fossil fuel emissions. An additional one to two billion metric tons is added through deforestation in the tropics. Of the roughly 8 billion metric tons total involved, the terrestrial biosphere takes up, on average, about 3 billion metric tons per year, and the oceans about 2 billion metric tons per year, leaving on average about 3 billion metric tons per year in the atmosphere. From year to year, the terrestrial uptake and the amount remaining in the atmosphere vary by more than 1 billion metric tons per year.

The increase in carbon dioxide in the atmosphere appears to generally follow the increase in carbon dioxide from fossil fuel emissions. The proportion of carbon dioxide absorbed in the ocean and land has, since measurements began, been about half of the amount emitted through human activity, on average, indicating that as carbon dioxide increases in the atmosphere, the uptake processes become more efficient. However, the fact that about one half of the new carbon dioxide which enters the atmosphere is not absorbed means that as fossil fuel emissions continue to increase, the atmospheric concentration of carbon dioxide will also continue to increase.

The rate of increase in fossil fuel emissions has been very steady, although the increase each year of atmospheric carbon dioxide is not. In some years the planet is able to take up almost all of the human-related carbon dioxide, in others almost none. This indicates that the carbon sinks must be highly variable. Recent measurements suggest that high surface temperatures may enhance emissions of methane from wetlands and may possibly increase carbon dioxide emissions as well. Ocean upwelling, which brings carbon dioxide-rich waters to the surface, changes during ocean warmings associated with the familiar "El Niño." This changes the amount of carbon dioxide taken up by the oceans. Thus, the ability of the planet to take up excess carbon dioxide is intimately related to climate and cannot be considered in isolation.

Measurements of carbon dioxide sinks are being made in an attempt to characterize the sinks so they can be incorporated into predictive climate models. Most of the measurements have been made on land because it is more accessible for measurement than the ocean and it appears to be more variable than the ocean sink, suggesting a more complicated system which will probably require more study to reach an acceptable understanding.

What is Known About the Terrestrial Carbon Sink?

Until recently, estimates of terrestrial carbon sinks have been derived from land use surveys and global mass balance calculations. These techniques have given estimates of the terrestrial carbon sink in the 1-2 billion metric tons of carbon per year range. However, in the past decade, atmospheric measurements of carbon dioxide on a global scale have matured to the point where estimates of the terrestrial carbon dioxide sink can be made independent of these inventories and mass balance calculations. These measurements suggest that the terrestrial biosphere is considerably more important in carbon dioxide uptake than previously believed.

Atmospheric measurements of carbon dioxide cover wide-ranging distance scales. For example, the amount of carbon dioxide coming out of or going in to a local region can be measured to characterize different ecosystems (forest, tundra, cropland, etc.). On a larger scale, carbon dioxide can be measured on coastlines in clean maritime air that has not been altered by terrestrial sources. NOAA operates four Baseline Observatories (Barrow, Alaska; Mauna Loa, Hawaii; American Samoa; and the South Pole) where all the greenhouse gases have been monitored on a continuous basis for up to 42 years. In addition, NOAA operates a cooperative air sampling network consisting of about 45 global sites plus measurements from ships at sea (see map). The air samples are sent to CMDL for analysis. This network has recently been augmented with tall (1300-2000 feet) communications towers and automatic air samplers on

small aircraft.

Comparing the amount of carbon dioxide coming on shore in the western U.S. with that leaving the eastern U.S. under the prevailing westerly winds (see graph below) clearly shows that since 1990, the averaged concentration of carbon dioxide from sites in the northern Atlantic are less than those from the northern Pacific. If we assume there is no escape of air enriched with CO2 from the continental U.S. into Canada, the Caribbean or Mexico, nor entry of low-CO2 air from these regions into the U.S., then this suggests that the North American continent has, for at least the past ten years, been on average absorbing carbon dioxide as air passes across it.

While this analysis is over-simplified and the data are still too sparse to quantify the process adequately, these and other data have been used in conjunction with atmospheric transport models to show that for the 1988-1992 period the global terrestrial biosphere took up about 2.2 billion metric tons of carbon per year with the great majority (1.6 billion metric tons of carbon per year, with an uncertainty of 0.5 billion metric tons of carbon per year) taken up by the North American continent (Fan et al., Science, October 16, 1998). This is a very important result as it indicates that the North American continent is a much larger carbon dioxide sink than previously believed. Yet, prior and subsequent direct measurements of carbon content of the land and recent modeling of land surface carbon exchange do not indicate a large North American carbon sink. This illustrates the need for continued investment in research designed to resolve the scientific uncertainties surrounding terrestrial carbon sinks.

There is additional, independent evidence that the entire terrestrial sink, most of which is in the Northern Hemisphere, is large. Carbon dioxide molecules are not all the same. About one in a hundred molecules will have a heavy carbon isotope (¹³C). It is known that the terrestrial process of photosynthesis preferably takes up regular carbon dioxide thus enriching the amount of heavy carbon dioxide in air that has been in terrestrial biological systems. In contrast, the ocean carbon dioxide exchange process does not discriminate against heavy carbon dioxide.

Since 1990, measurements of the concentration of both regular and "heavy" carbon dioxide and a global transport model have been used by CMDL to "fingerprint" the history of the measured carbon dioxide. (see graph below). These measurements now suggest that from 1991-1997, the terrestrial biosphere was the dominant global sink of carbon dioxide with an average value of 3.1 billion metric tons per year with an uncertainty of about 1.2 billion metric tons per year. The oceans were estimated to have taken up about 2.0 billion metric tons of carbon per year with an uncertainty of 0.4 billion metric tons. The average increase in atmospheric carbon dioxide over this period was 2.8 billion metric tons per year. However, the numbers vary from year to year. For example, in 1998 atmospheric carbon dioxide increased by 6.4 billion metric tons, the largest increase in any year since global measurements began about 1980. The analysis showed that the terrestrial biosphere took up about half of the normal amount, about 1.5 billion metric tons, while the oceans took up only about 1/5 of the normal amount, about 0.4 billion metric tons, possibly a result of the major El Niño in 1998. Thus, there appears to be substantial year-to-year variability in the major carbon dioxide sinks.

Another promising independent technique for identifying carbon dioxide sources and sinks is related to measurement of the amount of oxygen in the atmosphere. Recently it has become possible to measure the oxygen to nitrogen ratio with enough accuracy to detect the global decrease in oxygen related to fossil fuel combustion. By comparing the rate of oxygen loss with the rate of carbon dioxide gain in the atmosphere, one can determine the influence of the terrestrial biosphere since oxygen is produced during photosynthesis. Initial results indicate the magnitude of the terrestrial biosphere and ocean sinks to be in general agreement with the "heavy" carbon dioxide work.

All the evidence from atmospheric measurements indicates that there is substantial variability in terrestrial carbon dioxide uptake, which is not well understood. Temperature changes can affect soil respiration, for example. Estimates indicate that about two-thirds of the global terrestrial carbon is in soils and accurate inventories and variability of this component are not available. Future effects of climate change on carbon sequestration must be considered. If we are to have any hope of prudent management with the goal of influencing terrestrial carbon dioxide uptake, we must understand the cause of the variability in carbon dioxide uptake.

The U.S. Carbon Cycle Science Plan

Because of the importance of understanding the global carbon cycle, in 1999, the U.S. Global Change Research Program (USGCRP), at the request of its sponsoring agencies (USDA, DOE, USGS, NASA, NOAA and NSF), and with the help of a consortium of U.S. scientists who are leaders in carbon cycle science, produced a report entitled "A U.S. Carbon Cycle Science Plan." It presents a strategy for a research program to deliver credible predictions of future atmospheric carbon dioxide levels, given realistic emission and climate scenarios. The research program will address two very easily stated scientific questions:

· What has happened to carbon dioxide already emitted by human activities?

· What will be the future atmospheric carbon dioxide concentration from past and future emissions?

If we had the answers to these two questions, climate models would be able to make more accurate predictions.

The basic strategy of the plan is twofold:

· To develop a small number of new feasible, cost-effective, and compelling research initiatives to improve understanding of the three carbon reservoirs: land, ocean and atmosphere, and their mutual interactions.

· To strengthen the relevant research agendas of the agencies involved in the research through better cooperation, focus, conceptual and strategic framework, and articulation of goals.

The scientific goals of the plan include the following:

· Determine the magnitude and variability of Northern Hemisphere terrestrial carbon dioxide uptake.

· Determine the spatial distribution, variability and sensitivity to climate change of oceanic carbon dioxide uptake.

· Determine the impact of historical and current land use change in the Northern Hemisphere and in the tropics.

· Evaluate potential management strategies for enhancing carbon sequestration.

· Incorporate appropriate carbon cycle - climate feedbacks into global climate models.

Some of the specific measurement enhancements deemed necessary include:

· An expanded carbon dioxide exchange (flux) measurement network to characterize uptake for various biomes;

· An expanded airborne carbon dioxide monitoring network utilizing general aviation aircraft at 50 distributed North American sites to characterize uptake on a regional (multi-state) scale;

· An expanded global carbon dioxide surface monitoring network which would increase the present number of stations by a factor of three;

· An expanded terrestrial carbon land use inventory, including vegetation cover, above-and-below ground carbon and rates of change, including satellite observations and analysis of current soil carbon inventories and new measurements of eroded carbon and other effects of land use on soil carbon;

· Regional observational experiments including coordinated airborne, ship, terrestrial and satellite measurements with model development and testing;

· Long-term vegetation, soil, and carbon dioxide and methane flux measurements for major biomes to evaluate natural disturbance and management effects on carbon fluxes; and

· Global ocean measurements of ocean-atmospheric carbon dioxide exchange and atmosphere-ocean-biology interactions.

In addition, ocean process studies and the development and application of models for analysis of data, synthesis, prediction and policy would be enhanced. This would include Earth System models that predict carbon dioxide and climate interactively.

Conclusion

Progress on the understanding of the global carbon cycle and how it has responded to human presence on the planet has been remarkable in the past 10 years. We now believe that the North American continent presents a major sink for carbon dioxide emissions. We also know that this sink is highly variable but we do not know why. The U.S. now has a plan that includes the study of this important sink, and in particular, its regional nature. We also have a community of scientists who are ready to execute that plan.

Thank you, Mr. Chairman, for your interest in this matter. I would be happy to address any questions you and your Committee may have.