RFF REPORT Measurement and Monitoring of the World’s Forests A Review and Summary of Remote Sensing Technical Capability, 2009–2015 Matthew Fagan and Ruth DeFries DECEMBER 2009 Preface This report is part of a series of studies carried out under an initiative at Resources for the Future (RFF) to consider the economic, technical, and institutional issues associated with the goal of improving global forest measurement and monitoring. Companion studies under this initiative include an RFF discussion paper documenting uncertainties and discrepancies in existing forest measurement (Waggoner 2009) and another RFF report providing perspectives on “how good is good enough” in improving measurement and monitoring (Macauley and coauthors forthcoming). The financial support of the Alfred P. Sloan Foundation is gratefully acknowledged for all work under this initiative. Acknowledgements Special contributions were made by Molly Macauley, Paul Waggoner, Roger Sedjo, Michael Obersteiner, Craig Dobson, Josef Kellndorfer, and Miriam Marlier. Steffen Fritz, Ralph Dubayah, Steven Sesnie, and Bonnie Tice also helped. Table of Contents Executive Summary .................................................................................................................................................. i Scope .......................................................................................................................................................................... i Current and Emerging Satellite Technologies ........................................................................................ ii Remote Sensing and Global Forest Measurement ................................................................................ iii Summary and Conclusions ................................................................................................................................... v Chapter 1. Forests, Their Measurement, and the Need for Global Remote Sensing .................... 1 1.1 Current Needs for Forest Monitoring .................................................................................................. 1 1.2 A Brief History of Global Forest Measurement and Remote Sensing .................................... 4 1.3 Diversity of Forest Types .......................................................................................................................... 5 1.4 For Global Forest Measurement, Why Use Satellite Imagery? ................................................. 6 1.5 Scope and Purpose ...................................................................................................................................... 6 Chapter 2. Remote Sensing and Forest Measurement: An Overview ................................................ 7 2.1 Comparing Satellite and Airborne Remote Sensing ...................................................................... 7 2.2 Passive and Active Remote Sensing: An Overview ..................................................................... 11 2.3 Putting It All Together: Remote Sensing Fusion .......................................................................... 21 2.4 An Overview of Current and Near‐Term Earth‐Observing Technology ............................ 25 Chapter 3. Technical Considerations in Global Forest Monitoring .................................................. 33 3.1 Measuring Forests Globally .................................................................................................................. 33 3.2 Criteria for a Global Forest Monitoring Program ........................................................................ 36 3.3 Developing and Evaluating Reference Forest Maps ................................................................... 36 3.4 Survey Methods for Determining Forest Area and Type ......................................................... 39 3.5 Price and Logistics .................................................................................................................................... 41 Chapter 4. An Overview of Remote Sensing Capabilities for Forest Measurement: Current and Near‐Term Technologies ........................................................................................................................... 43 4.1 Forest Area: Current Measurement Methods ............................................................................... 43 4.2 Forest Structure: Current Measurement Methods ..................................................................... 53 4.3 Estimating Forest Volume and Biomass Using Remote Sensing .......................................... 58 4.4 Estimating Forest Carbon Stocks from Remotely Sensed Data ............................................. 64 Chapter 5. Forests from Space: Unparalleled Measurements, Timely Moment ......................... 69 5.1 Improving Forest Biomass and Carbon Estimates ..................................................................... 69 5.2 In Conclusion: Toward Improved Measures and Monitoring ................................................ 72 Satellite, Sensor, and Remote Sensing Acronyms and Abbreviations ............................................ 74 Acronyms .................................................................................................................................................................. 76 Appendix. Current and Near‐Term Earth‐Observing Satellite Technology: An Overview .... 78 Bibliography ............................................................................................................................................................ 99 FAGAN AND DEFRIES 1 Measurement and Monitoring of the World’s Forests: A Review and Summary of Remote Sensing Technical Capability, 2009–2015 Matthew Fagan and Ruth DeFries * Executive Summary orests are ecosystems of fundamental importance to humanity, yet we know little about the Fglobal status of forests. We can make more current and informative maps of Mars than of the Amazon basin or the Russian boreal forest. This gap in our knowledge arises from several sources, including a historical shortage of tools to observe the entire globe; the lack of consistent global initiatives on forests; balkanized, inconsistent recordkeeping; and the absence of a concerted and systematic effort to inventory and monitor the world’s forests. To understand the planetary carbon budget, it is imperative to generate accurate and reliable estimates of global forest cover and the amount of biomass and carbon harbored by the planet’s forests. Yet widespread uncertainties in forest measurements have hampered efforts to obtain this basic scientific data. Indeed, the most significant weaknesses in estimates of the planetary carbon budget derive from uncertainties about terrestrial ecosystems. Satellite‐based estimates of forest cover and biomass have begun to fill this need. To measure forests worldwide, satellite imagery is a practical necessity. Aerial observations are expensive at present and only cover small areas at a time. Ground measurements are also expensive and are logistically challenging and spatially restricted. Neither aerial nor ground observations are well suited to continuous measurement of the entire global forest. Satellite mapping is necessary to detect deforestation and regrowth in remote tropical forests and to track the northern expansion of boreal forests in a warming world. The greatest strengths of satellite‐based measurements are their unparalleled, unbiased measurements, their monthly to daily frequency, and—above all—their synoptic nature. Satellites provide a general view of the whole Earth that is not possible with any other forest measurement method. Scope We evaluate current and near‐term (2009–2015) technologies for measuring and monitoring global forests. We focus primarily on remote sensing (defined in this report as the analysis of satellite and aerial imagery), because this technology meets the steep logistical challenge of measuring the world’s forests in an accurate, repeatable, and inexpensive manner. We emphasize the observations needed to provide accurate, basic measures of forest attributes for use by forest scientists, decisionmakers, and the public. We also illustrate how the resulting data would address * Matthew Fagan ([email protected]) is a graduate student in the Department of Ecology, Evolution, and Environmental Biology at Columbia University. Ruth DeFries ([email protected]) is the Denning Professor of Sustainable Development in the Department of Ecology, Evolution, and Environmental Biology at Columbia University. FAGAN AND DEFRIES i some of the most compelling questions that persist about our forests. Remote sensing can ultimately help to answer these important questions: • Where are forests being lost and gained? • How are biodiversity and regional climates responding to forest loss and regrowth? • How do forests affect the global carbon budget? • How can forest management be improved by carbon markets and satellite technology? • How will forests respond to climate change? In this report, we evaluate the technical capacity of satellite imagery to measure and monitor global forests. In particular, we examine satellite observations of forest area, volume, biomass, and carbon. These measurements are fundamental to our understanding of the status and trends of forests around the globe and to our grasp of the role that forests play in modulating global climate. We evaluate current capacity and predict future capacity from expected satellite