1
Introduction

Evidence that climate is changing—including increasing global temperatures, melting glaciers, rising sea level, increasingly severe weather, and shifting seasons and animal migration patterns—is driving national and international discussions on reducing anthropogenic greenhouse gas emissions, the primary cause of climate change. The principal international framework for greenhouse gas reductions is the United Nations Framework Convention on Climate Change (UNFCCC), which is aimed at “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system” (United Nations, 1992, p. 4). The greenhouse gases covered by the UNFCCC include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulfur hexafluoride (SF6), perfluorocarbons (PFCs), and hydrofluorocarbons (HFCs).1 In 1997, the parties to the UNFCCC approved the Kyoto Protocol, which contains binding emissions targets for developed countries (United Nations, 1998). The United States is not a party to the Kyoto Protocol, but it is considering a variety of proposals for reducing emissions to mitigate adverse effects of climate change, including an international climate treaty.2

For any international agreement to limit greenhouse gas emissions, monitoring and verification of emissions will be essential to assess the effectiveness of emissions reductions and overall compliance with the terms of the treaty and to give nations confidence that their neighbors are also living up to their commitments. As former president Ronald Reagan said: “Trust but verify.” Emissions verification will also be important for correcting errors in reporting.

DOMAIN OF THE REPORT

This report examines methods for estimating anthropogenic greenhouse gas emissions and for observing their changes over time (see committee charge in Box 1.1). The report asks: How accurate is each method for estimating greenhouse gas emissions? How well can emissions reductions required under a climate treaty be monitored? What new measurement

BOX 1.1

Committee Charge

The study will review current methods and propose improved methods for estimating and verifying greenhouse gas emissions at different spatial (e.g., national, regional, global) and temporal (e.g., annual, decadal) scales. The greenhouse gases to be considered are carbon dioxide, chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), nitrous oxide, methane, and perfluorinated hydrocarbons (PFCs). Emissions of soot and sulfur compounds along with precursors of tropospheric ozone may also be considered. The results would be useful for a variety of applications, including carbon trading, setting emissions reduction targets, and monitoring and verifying international treaties on climate change.

1

A separate treaty, the Montreal Protocol on Substances That Deplete the Ozone Layer, covers chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs).

2

International negotiations are intended to culminate in an agreement at a future Conference of the Parties to the UNFCCC, see <http://unfccc.int/2860.php>.



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1 Introduction E vidence that climate is changing—includ - emissions will be essential to assess the effectiveness of ing increasing global temperatures, melting emissions reductions and overall compliance with the glaciers, rising sea level, increasingly severe terms of the treaty and to give nations confidence that weather, and shifting seasons and animal migration their neighbors are also living up to their commitments. patterns—is driving national and international dis- As former president Ronald Reagan said: “Trust but cussions on reducing anthropogenic greenhouse gas verify.” Emissions verification will also be important emissions, the primary cause of climate change. The for correcting errors in reporting. principal international framework for greenhouse gas reductions is the United Nations Framework Conven- DOMAIN OF THE REPORT tion on Climate Change (UNFCCC), which is aimed at “stabilization of greenhouse gas concentrations in This report examines methods for estimating the atmosphere at a level that would prevent dangerous a nthropogenic greenhouse gas emissions and for anthropogenic interference with the climate system” observing their changes over time (see committee (United Nations, 1992, p. 4). The greenhouse gases charge in Box 1.1). The report asks: How accurate is covered by the UNFCCC include carbon dioxide each method for estimating greenhouse gas emissions? (CO2), methane (CH4), nitrous oxide (N2O), sulfur How well can emissions reductions required under a hexafluoride (SF 6), perfluorocarbons (PFCs), and climate treaty be monitored? What new measurement hydrofluorocarbons (HFCs).1 In 1997, the parties to the UNFCCC approved the Kyoto Protocol, which contains binding emissions targets for developed coun- BOX 1.1 Committee Charge tries (United Nations, 1998). The United States is not a party to the Kyoto Protocol, but it is considering a The study will review current methods and propose im- variety of proposals for reducing emissions to mitigate proved methods for estimating and verifying greenhouse gas adverse effects of climate change, including an inter- emissions at different spatial (e.g., national, regional, global) national climate treaty.2 and temporal (e.g., annual, decadal) scales. The greenhouse For any international agreement to limit green- gases to be considered are carbon dioxide, chlorofluorocarbons house gas emissions, monitoring and verification of (CFCs), hydrofluorocarbons (HFCs), nitrous oxide, methane, and perfluorinated hydrocarbons (PFCs). Emissions of soot and sulfur compounds along with precursors of tropospheric ozone may 1A separate treaty, the Montreal Protocol on Substances That also be considered. The results would be useful for a variety of Deplete the Ozone Layer, covers chlorofluorocarbons (CFCs) and applications, including carbon trading, setting emissions reduc- hydrochlorofluorocarbons (HCFCs). tion targets, and monitoring and verifying international treaties 2 International negotiations are intended to culminate in an agree- on climate change. ment at a future Conference of the Parties to the UNFCCC, see . 

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 VERIFYING GREENHOUSE GAS EMISSIONS methods could be developed within a few years to designed to control them. The scales of interest range independently verify emissions estimates? from national to global and from annual to decades. The focus of this report is on monitoring and veri- Although some of the methods described in this report fication of the emissions themselves (see definitions in have sufficiently high resolution to be used to audit Box 1.2), rather than on implementation of policies individual emissions sources, which may be of inter- BOX 1.2 Definitions of Terms Used in the Report Activity data—Data on the magnitude of a human activity resulting in particular land classification (e.g., forest, peatland) based on ecosystem emissions or removals during a given period of time. Examples include characteristics that affect carbon storage, such as volume of soil carbon data on energy use, metal production, management systems, forest and live and dead above- and belowground biomass, measured from clearing, and fertilizer use. a network of plots. Changes in carbon stock through time (i.e., carbon uptake or release) are measured by differencing two samples from the Annex I countries —The 41 countries included in Annex I (as same plot but separated by 1 to 10 years. amended in 1998) to the UNFCCC, including industrialized countries Inverse model—A model in which observations are used to infer the that were members of the Organisation for Economic Co-operation and Development in 1992 and many countries with economies in transi- values of the parameters characterizing the system under investigation. tion. Under the convention, Annex I countries committed to returning In this report, inverse models are used to infer sources and sinks for individually or jointly to their 1990 levels of greenhouse gas emissions a greenhouse gas from measurements of the atmospheric or oceanic by 2000. By default, the other countries are referred to as non-Annex abundance of that gas. I countries. Monitoring—The observation of emissions or variables correlated Anthropogenic emissions —Emissions of greenhouse gases, with emissions for the purpose of detecting any changes that may precursors of greenhouse gases, and aerosols resulting from human occur over time. activities. Because it is difficult to disentangle anthropogenic and Sector—An emission-producing segment of the economy. The Inter- natural components of emissions and removals from land use, the UNFCCC considers emissions and removals on managed lands as governmental Panel on Climate Change (IPCC) currently specifies four anthropogenic. sectors for greenhouse gas reporting: energy; industrial processes and product use; agriculture, forestry, and other land use; and waste. CO2 equivalent—The amount of carbon dioxide emission that would Sink—Any process, activity, or mechanism that removes a greenhouse cause the same integrated radiative forcing, over a given time horizon, as an emitted amount of a well-mixed greenhouse gas. It is a standard gas, an aerosol, or a precursor of a greenhouse gas or aerosol from the metric for comparing emissions of different greenhouse gases, but atmosphere. Removals of greenhouse gases by a sink are conventionally does not imply exact equivalence of the corresponding climate change shown as negative emissions. responses. The 100-year global warming potential is used to calculate Source—Any process, activity, or mechanism that releases a green- CO2 equivalents. house gas, an aerosol, or a precursor of a greenhouse gas or aerosol Emission factor—The rate of emission per unit of activity, output, or into the atmosphere. Certain activities, such as forestry, can be both a input. For example, a particular fossil-fuel power plant may have a CO2 source and a sink of greenhouse gas emissions. emission factor of 0.765 kg CO2 kWh–1 generated. Survey data—Data from a statistically representative sample. Inventory—An accounting of an item of interest at a specified date. Tracer-transport model—A model used to predict the movement • An emissions inventory accounts for the amount of one or more specific greenhouse gases discharged into the atmosphere from all of greenhouse gases in the atmosphere or dissolved substances in source categories as well as removals by sinks in a certain geographical the oceans. area and within a specified time span, usually a specific year. Under the Verification—An independent examination of monitoring data to help UNFCCC, Annex I countries prepare national inventories of anthropo- genic greenhouse gas emissions and removals for each calendar year. establish whether or not a country’s actual emissions are consistent with • An ecosystem inventory accounts for the carbon stored in a its obligations under a climate treaty. SOURCES: Adapted from IPCC glossaries () and UNFCCC resources ().

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 INTRODUCTION est for trading schemes or offset projects, the report focuses on the national emission totals that include these activities. Only public domain data (not classified or commercial data) are considered because confidence in a treaty relies on open data for transparency and scientific scrutiny. Greenhouse Gases This report considers the anthropogenic green- house gases required by the committee charge—CO2, CH4, N2O, CFCs, HFCs, and PFCs—and SF6, but not the optional soot or precursors of tropospheric ozone. The greenhouse gases required by the commit- tee charge, along with SF6, are currently covered by international agreements (CFCs under the Montreal Protocol and the others under the UNFCCC) and were the targets of negotiations at the 2009 United Nations Climate Change Conference (COP 15) in Copenhagen. Thus, there is an immediate practi - cal need to verify emissions of the gases included in this report, which does not extend to the greenhouse agents that were omitted. The short-lived greenhouse agents (soot and other aerosols, aerosol precursors, and precursors to tropospheric ozone) are not covered by international agreements, although many countries FIGURE 1.1  The relative importance of emissions of anthro- pogenic greenhouse gases and soot (black carbon) and other  have a highly developed capability to monitor them to aerosols.  The  bars  show  the  20-  (lower  panel)  and  100-year  support air pollution regulations. A comparable capa- Fig 1.1 FINAL.eps (upper panel) radiative forcing of emissions in 2000. SOURCE:  bility for the greenhouse gases discussed in this report Figure 2.22 from IPCC (2007a), Cambridge University Press. type in graph has been converted to paths does not exist. The focus of international agreements on CO2, CH4, N2O, CFCs, HFCs, PFCs, and SF6 is likely do not entail the same penalties for delay. Because to continue for three reasons. First, these gases are they are removed from the atmosphere in less than a collectively more important greenhouse agents than year, today’s emissions will have a smaller impact on soot, sulfur compounds, and precursors of tropospheric global warming in coming decades when the problem ozone (Figure 1.1). Commonly cited mitigation targets, becomes most acute. Third, the net radiative forcing such as a maximum of 2°C of warming or a maximum from the emission of short-lived gases and aerosols concentration of 450 parts per million (ppm) CO2 depends greatly on the location and timing of emis- equivalent, cannot be achieved without large reduc- sions. The time required for air to mix globally is on the tions in emissions of CO2, CH4, N2O, CFCs, HFCs, order of 2 weeks in the east-west direction and 1 year in PFCs, and SF6. Second, the gases included in the report the north-south direction across the equator, which is are long-lived in the atmosphere (decades to millen- less than the lifetime of short-lived greenhouse agents. nia or more), whereas the omitted gases and soot are For this and other reasons, the greenhouse impact of short-lived (less than a year). Longevity in the atmo- the ozone precursor NOx (nitrogen oxide) can vary by sphere means that delayed mitigation is costly—CO2 a factor of 10, depending on whether it is emitted in emissions today will add to global climate change for northern Europe or in the tropics (Wild et al., 2001; centuries. In contrast, short-lived greenhouse agents see also Table 2.15 of Forster et al., 2007). This makes

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 VERIFYING GREENHOUSE GAS EMISSIONS it difficult to design a practical international agreement based on measurements of human activities (i.e., data to monitor them as greenhouse agents. such as cement or coal-fired electricity production) Of the long-lived greenhouse gases included under and corresponding emission factors (see definitions the UNFCCC, CO2 is responsible for 77 percent of in Box 1.2). Because future international agreements the greenhouse forcing on a 100-year time horizon are likely to build on this foundation, the committee (Figure 1.2). For this reason, the report devotes con- evaluates UNFCCC inventory methods and extensions siderably more space to CO2 than to the other gases. of them that would improve their comprehensiveness Note, however, that if we include the short-lived gases and accuracy and increase the rigor of self-reporting. and soot and adopt a 20-year horizon, the contribution The committee also discusses the capacity building of CO2 falls to less than 40 percent. necessary to procure regular inventories from develop- ing countries. Remote Sensing of Land Use. Greenhouse gas emissions and sequestration from land use are difficult to estimate because they have the same chemical signature as much larger background sources and sinks in the natural bio- sphere and because they are thinly spread over an enor- mous area. The dominant sources of land-use emissions are from forestry (primarily tropical deforestation and forest degradation) and agriculture. Land-use emis- sions in parts of the temperate zone are negative (i.e., net removals by sinks) due to net forest regrowth and other processes (Pacala et al., 2007). Because deforesta- tion is the second largest source of anthropogenic CO2 (the first is fossil-fuel combustion) and because forest conservation and planting are likely to be important mitigation activities in the future, this report devotes considerable attention to methods for monitoring forest cover and structure by satellites. A comparable FIGURE 1.2  Global anthropogenic greenhouse gas emissions  understanding of N2O and CH4 emissions from crop- in  2004.  The  F-gases  include  HFCs,  PFCs,  and  SF6.  The  area  lands and grasslands does not exist, both because of the in the pie diagram shows 2004 emissions of the gases covered  by the UNFCCC, weighted by their 100-year radiative forcing.  diversity of agricultural practices and because we lack SOURCE: Figure 1.1b from IPCC (2007b), Cambridge University  the technology to measure the dominant emissions Press. sources remotely. For these reasons, it remains difficult to provide a useful check on self-reporting of emissions from agriculture, except in specific instances. Methods of Monitoring This report evaluates three categories of monitor- Atmospheric Methods. A global network of surface ing approaches: national inventories, satellite measure- monitoring stations, aircraft, balloons, and satellites ments of land use, and atmospheric methods. routinely measures greenhouse gas abundances in the atmosphere and oceans. Models of the atmosphere National Inventories. Under the UNFCCC, Annex and/or oceans are used to estimate greenhouse gas I (developed) countries are required to report annual emissions from the abundance data, a method known as anthropogenic emissions and removals of greenhouse tracer-transport inversion. An emissions source located gases. Developing countries also report national inven- between two monitoring stations will cause the concen- tories, but less frequently and in far less detail than tration of the gas to be higher at the downwind station d eveloped countries. The emissions estimates are than the upwind station. How much higher depends on

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 INTRODUCTION both the strength of the source and the pattern of air OVERVIEW OF GREENHOUSE GAS flow, including wind speed, direction, and turbulence. EMISSIONS Thus, to produce emissions estimates from abundance data, one needs an atmospheric model to reconstruct Relative Contribution to Climate Change the three-dimensional pattern of air and water flow and A greenhouse gas’s instantaneous tendency to mixing around the globe. For this reason, the report change the climate is measured by its radiative forc- devotes considerable space to uncertainties in atmo- ing, which multiplies the increased abundance of the spheric transport models. gas caused by anthropogenic emissions and the gas’s The report also evaluates extensions of the atmo- potency as a greenhouse agent. Of the four groups spheric sampling network that could significantly of gases considered in this report, CO2 has the larg- improve our ability to estimate national emissions est radiative forcing (1.66 W m–2 for emissions up to and emissions trends. These include measurements of the end of 2005), followed by CH4 (0.48 W m–2), the concentrations that would fill spatial gaps in the cur- HCFCs and CFCs (collectively 0.32 W m–2), N2O rent sampling grid—for example, samples taken near (0.16 W m–2), and the HFCs, PFCs, and SF6 (collec- large sources such as power plants and municipalities tively 0.02 W m–2; see Forster et al., 2007). that were avoided when the current sampling network was established. Longevity in the Atmosphere Uncertainty The longevity of a greenhouse gas in the atmo- sphere is important because it determines the number This report evaluates uncertainties in annual of years that today’s emissions will affect climate. emissions estimates derived from the three monitor- Short-lived gases, such as the precursors to tropo- ing methods described above. In some cases, standard spheric ozone, are rapidly cleared from the atmosphere; statistical methods can be used to evaluate the uncer- thus, the perturbation caused by emissions appears tainties, but in others, standard methods cannot be to adjust rapidly to a change in emissions. However, applied because our underlying scientific understanding short-lived chemically reactive gases are coupled with is too incomplete or our measurement capabilities are the longer-lived greenhouse gases and thus produce insufficient. In such cases, we rely on other methods, long-lived perturbations to radiative forcing that take including expert judgment, that are specified in tables decades to reach a steady state (Wild et al., 2001). For of uncertainty estimates. Uncertainties are categorized even small levels of anthropogenic emissions, the atmo- in five bins—0-10 percent, 10-25 percent, 25-50 per- spheric abundances of very long-lived gases, such as the cent, 50-100 percent, and >100 percent (for the last PFCs, will continue to rise in proportion to emissions category, it is unclear whether the activity is a source and remain well below the steady-state value at which or a sink)—to facilitate cross-comparison between annual emissions are balanced by annual removals. To estimates from different methods. An uncertainty of eventually halt climate changes caused by greenhouse 10 percent means that measurements are accurate to gases, their abundances in the atmosphere must be within 10 percent of the true value. Unless indicated stabilized. otherwise, uncertainties are reported for two standard The lifetime of CO2 in the atmosphere cannot be deviations of the mean (2σ or 95 percent confidence ascribed a single value because the carbon cycle consists interval). of a series of interacting reservoirs, each with a differ- Uncertainties in decadal changes can be computed ent time scale (see Figure 1.3). For example, although from the values for annual emissions using standard land ecosystems or the oceans take up approximately time-series methods, including simple regression, for one-sixth of the CO2 in the atmosphere every year, they calculating the uncertainty of regression slopes. A rea- also return almost the same amount (IPCC, 2007a). sonable expectation is that uncertainties in the decadal Thus, the lifetime of a pulse increase in the atmospheric change of emissions will be lower than the annual abundance of CO2 is set not by the short stay of an uncertainty.

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 VERIFYING GREENHOUSE GAS EMISSIONS FIGURE 1.3  The global carbon cycle and changes in the sizes of CO2 reservoirs over the last two decades. All values are billions  of metric tons of carbon. Arrows show annual fluxes. Values for the 1990s are in blue; those for 2000-2008 are in red. SOURCE: Le  Quéré (2009), International Geosphere-Biosphere Programme/Global Carbon Project. Data from Le Quéré et al. (2009). individual molecule in the atmosphere, but by the small Methane is short-lived in the atmosphere relative imbalance that the pulse creates between the uptake to CO2. A molecule emitted into the atmosphere is and removal rates. For example, CO2 molecules spend oxidized to CO2 in an average of about 8 years, but thousands of years in the oceans once they have been chemical feedbacks extend this time scale to 12 years transported into the abyss. Consequently, dissolved (Prather, 1994). This means that the current abundance CO2 in the deep oceans reflects the atmospheric abun- of methane is derived from the last several decades of dance before the industrial revolution, rather than the emissions. Nitrous oxide has an average residence time increased abundance caused by fossil-fuel burning in of 114 years in the atmosphere before it is photochemi- the last 200 years. The upshot is that a fraction of the cally decomposed in the stratosphere. The average fossil-fuel CO2 emitted is taken up rapidly by the upper atmospheric lifetimes of the other gases considered in ocean and biosphere, but the remainder of the perturba- this report range from 45 to 1,700 years for CFCs, 1 tion acts like a very long-lived gas, requiring thousands to 270 years for HFCs, 3,200 years for SF6, and tens of of years to decay away (Denman et al., 2007). thousands of years for PFCs (Forster et al., 2007).

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 INTRODUCTION Emission Sources BOX 1.3 Measurement Units for Greenhouse Gases in the Atmosphere CO2 emissions to the atmosphere are caused pri- marily by fossil-fuel burning (~74 percent in 2004; The concentration of a gas, which is defined as the number IPCC, 2007b) and tropical deforestation (~22 percent), of molecules per volume, will vary with altitude and weather although recent work suggests that the contribution systems as the density of the air changes, even if there are no from deforestation has decreased to as little as 12 percent sources or sinks. When a parcel of air rises and expands at lower of CO2 emissions in 2008 (van der Werf et al., 2009a). pressure, the concentrations of all species decrease by the same Other contributors include industrial processes such factor. What is conserved is the mole fraction, the relative abun- as cement production. The primary sources of anthro- dance of each. When water evaporates or condenses, which adds or removes an extra gaseous component, the mole fraction of all pogenic methane are energy production, ruminant other components will decrease or increase, respectively, by the animals, rice agriculture, landfills, and biomass burning same proportion. Thus, the property that reflects additions and (Denman et al., 2007). Natural sources of methane are removals of a trace component is its mole fraction in dry air, which dominated by wetlands and are approximately one-half changes only when there are sources or sinks. The dry air mole the size of anthropogenic sources. Although under- fraction of CO2 is expressed as parts per million. A mole fraction of standing of anthropogenic N2O sources is incomplete, 385 ppm means that, on average, in every 1 million molecules of dry air there are 385 CO2 molecules. The mole fraction of methane agriculture is likely the largest source because of the is typically expressed in parts per billion, and that of the HFCs, oxidation of nitrogen fertilizer and reduction of nitrite PFCs, and CFCs in parts per trillion. (see Table 7.7 in IPCC, 2007a). Natural sources are dominated by soils under natural vegetation and by microbial transformations of nitrogen compounds in the oceans and are thought to be roughly comparable from these emissions (4-5 Gt C yr–1) accumulates in size to anthropogenic sources (Boumans et al., 2002; in the atmosphere and the rest is taken up by carbon Nevison et al., 2004; Hirsch et al., 2006). The presence reservoirs in the oceans and on land (see discussions of CFCs, HFCs, SF6, and PFCs in the atmosphere is below and Figure 1.3). Because 1 ppm of CO2 in the due almost entirely to human manufacture for a wide atmosphere equals 2.12 Gt C, the atmospheric growth range of industrial applications in the latter half of the rate currently averages ~2 ppm per year. twentieth century (IPCC, 2007a). The abundance of methane in the atmosphere today is much higher than in the millennium before the Atmospheric Concentrations, Emissions, and industrial era (1,774 parts per billion [ppb] in 2005 ver- Trends sus approximately 700 ppb; see IPCC, 2007a). For the 1970s and 1980s, the growth rate of methane was about Atmospheric abundances of greenhouse gases are 1 percent per year; the rate slowed dramatically in the best quantified by dry air mole fractions—the number 1990s and dropped to nearly zero from 2000, but began of molecules of the gas in a set volume divided by the to grow again in 2007 (Rigby et al., 2008b).3 Several total number of molecules of dry air in the same vol- reasons for this anomalous growth pattern have been ume (see Box 1.3). The mole fraction of CO2 in the proposed, but no clear explanation is available (Dlu- atmosphere is currently 387 ppm (Figure 1.4), which gokencky et al., 2001). The literature on emissions of is more than 100 ppm higher than in the pre-industrial methane is summarized in Table 7.6 of IPCC (2007a). period. Annual anthropogenic emissions of CO2 are Estimates for anthropogenic sources range from 264 to between 9 billion and 10 billion metric tons of carbon 428 Tg CH4 yr–1 (1 Tg CH4 equals one million metric (Gt C yr–1), increased at 1-2 percent per year over the tons of methane) and for natural sources from 145 to last three decades of the twentieth century, and 3.4 260 Tg CH4 yr–1, although total emissions are more percent per year from 2000 to 2008, and are projected tightly constrained (493 to 667 Tg CH4 yr–1). to decline in 2009 by almost 3 percent due to the weak economy (Canadell et al., 2007; Le Quéré et al., 2009). 3 S ee also .

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 VERIFYING GREENHOUSE GAS EMISSIONS FIGURE 1.4  Monthly mean CO2 concentration at 3,400 m altitude on Mauna Loa, Hawaii. The red curve shows the trend of in- dustrial emissions of CO2 from fossil-fuel combustion and cement production. The annual cycle is driven by the imbalance between  seasonal photosynthesis and respiration on the continents. Plants take up CO2 only during the growing season, but plants and animals  release it through plant metabolism and the decay of dead organic matter more evenly throughout the year. The long-term increase  in atmospheric CO2 is caused by fossil-fuel combustion and land-use change. SOURCE: Courtesy of Ralph Keeling, Scripps Institution  of Oceanography. Data are from the Scripps CO2 program. Separating Anthropogenic and Natural The abundance of N2O rose from 270 ppb in 1750 Components of CO2 to 319 ppb in 2005 (IPCC, 2007a). N2O is now increas- ing in concentration at an average rate of approximately Anthropogenic emissions are the emissions of a gas 0.25 percent per year. Total emissions (13.9 to 18.9 Tg resulting from human activities. Although this defini- N yr–1) are constrained by the predicted atmospheric tion is easy to apply to fossil-fuel burning, where all lifetime and observed growth rate (Prather et al., 2001), emissions are anthropogenic, it becomes problematic with the growth rate indicating the level of anthropo- for forestry, cropland management, and other land-use genic emissions (~6 Tg N yr–1). Inventory estimates sources and sinks, where it is difficult to distinguish of N2O emissions have considerable uncertainties emissions and removals due to human influence (e.g., (Prather et al., 2009) as illustrated by the large range management practices) from those due to natural fac- in the size of the total global source reported by the tors. For example, climate change and fertilization of Intergovernmental Panel on Climate Change (IPCC): plants by anthropogenic CO2 and nitrogen deposition 8.5-27.7 Tg N yr–1 (IPCC, 2007a, Table 7.7). probably affect plant growth rates all over the world, CFCs are covered by the Montreal Protocol on but our understanding of these effects is incomplete Substances That Deplete the Ozone Layer, and they and will likely remain so for the foreseeable future. To are either stabilized or decreasing in concentration. address the problem, the IPCC and UNFCCC have However, HFC, SF6, and PFC abundances are cur- adopted a convention of treating all emissions and rently increasing (Prinn et al., 2005; Velders et al., 2005; removals (sinks) on land that is managed, as anthro- IPCC, 2007a).

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 INTRODUCTION pogenic.4 Any changes in emissions and removals 2007a; Le Quéré et al., 2009). Moreover, because of from these lands are thus considered anthropogenic, their comparatively high accuracy, estimates of the oce- regardless of whether natural factors contributed to anic sink provide a valuable constraint on estimates of those changes. the magnitude of land sinks at regional and global scales In addition to the definitional ambiguities, moni- (because the land sink equals the fossil-fuel source toring of anthropogenic CO 2 emissions is greatly minus ocean uptake minus the atmospheric increase). complicated by the natural cycling of CO2 through Fluctuations of natural CO2 sources and sinks the terrestrial biosphere and oceans (Figure 1.3). The create a difficult signal-to-noise problem for efforts to terrestrial biosphere takes up approximately 120 Gt C estimate anthropogenic emissions with atmospheric yr–1 through photosynthesis and releases almost all of it m easurements. Seasonally fluctuating background back to the atmosphere through respiration by plants, sources and sinks that contribute to the CO2 signal may animals, and microbes (IPCC, 2007a). Photosynthesis be of the same order as the emission reductions that occurs only during daylight hours in the growing sea- might be required under a treaty. The signal-to-noise son, whereas respiration occurs at all times, albeit at a problem is further exacerbated by the fact that annual reduced rate in some seasons (i.e., winter outside the fossil-fuel and deforestation emissions represent only tropics). This diurnal and seasonal imbalance can be about 1 percent of the CO2 in the atmosphere (IPCC, quite large; the CO2 sources and sinks that they cre- 2007a). This means that anthropogenic emissions will ate are often larger than fossil-fuel fluxes in the same change the average CO2 abundance by only a small location, except in cities or close to power plants where amount as air moves across a country over a period of fossil-fuel emissions are concentrated. Moreover, if we hours to a few days. Thus, an effective way to uniquely ignore tropical deforestation, terrestrial ecosystems identify many large emissions sources is to measure the represent a net sink that averaged 2.7 ± 1.0 Gt C yr–1 perturbation in air close to the source, before mixing over 2000-2008 (Le Quéré et al., 2009). The cause dilutes the added CO2. The plume of increased con- of this sink is not completely understood, although a centration above a major point source can be of order substantial fraction is due to forest regrowth and other 1-10 percent above the background concentration (see land-use changes in the temperate zone (CCSP, 2007, Chapter 4). Chapters 2 and 3) and the remaining fraction may be Because of the signal-to-noise problem, the natural caused by CO2 fertilization (Friedlingstein et al., 2006). carbon cycle would have to be monitored as part of any The size of the net terrestrial flux can change from year effort to monitor anthropogenic emissions. Monitor- to year by as much as 5 Gt C (Baker et al., 2006a), in ing the carbon cycle would also constrain estimates of part from anthropogenic fires in tropical forests associ- “ leakage,” in which reduced emissions in one region ated with El Niño events (Randerson et al., 2005; van or sector lead to increased emissions in another (i.e., der Werf et al., 2009b), but is usually within a range soil carbon releases from land newly cultivated for of ±1 Gt C yr–1. biofuels; see Searchinger et al., 2008; Tilman et al., The oceans are also a sink for carbon averaging 2.3 2009). Further, the effectiveness of any climate treaty ± 0.5 Gt C yr–1 from 2000 to 2008 (Le Quéré et al., is based on the stabilization of greenhouse gas abun- 2009). By measuring the changing chemical properties dances, whether from anthropogenic or natural sources. (e.g., pH, pCO2) of the surface ocean from research Current models indicate that climate change feeds vessels and commercial ships of opportunity, the annual back on natural ecosystems and the ocean to produce sink assignable to an ocean basin can be estimated to new sources or reduce sinks of greenhouse gases, with a precision of about ±10 percent (e.g., Watson et al., most of the feedbacks amplifying climate change. For 2009). These measurements show that variations in the example, warming might cause arctic tundra to emit oceanic sink are too small to explain the multi-gigaton large quantities of CO2 and CH4, causing further cli- fluctuations in the atmospheric increase of CO2 (IPCC mate change, even more releases of CO2 and CH4, and so on in a positive feedback loop (Walter et al., 2006; Zimov et al., 2006; IPCC, 2007a; Schuur et al., 2009). 4 This rule is not applied consistently, however, because individual countries may define what constitutes managed lands for their These effects need to be detected early to ensure that national inventories.

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0 VERIFYING GREENHOUSE GAS EMISSIONS any agreement to limit anthropogenic emissions has A. The primary gases in these sectors are CO2, CH4, the desired outcome. N2O, and HFCs. Chapter 3 describes remote sensing measurements of land use that could provide inde- pendent estimates of deforestation and some types of ORGANIZATION OF THE REPORT agricultural production. It also describes observations This report examines methods used to estimate and research needed to improve UNFCCC inventories greenhouse gas emissions and identifies enhancements of emissions from agriculture, forestry, and other land or new techniques that could be used to significantly use. Chapter 4 examines atmospheric-based estimates improve emissions estimates over the next few years. of greenhouse gas emissions, which could provide Chapter 2 describes the national greenhouse gas inven- independent checks on emissions from fossil-fuel use tories reported under the UNFCCC and their limita- and industrial processes. Additional information on tions. The chapter focuses on the accuracy of estimates sources of atmospheric and oceanic data, methods for for the gases and activities in the sectors responsible estimating atmospheric signals, and technologies for for most of the emissions: energy and agriculture, for- measuring emissions from large local sources appears estry, and other land use. The primary gases in these in Appendixes B, C, and D, respectively. Biographical two sectors are CO2, CH4, and N2O. Corresponding sketches of committee members appear in Appendix estimates for gases and activities in the industrial pro- E, and a list of acronyms and abbreviations is given in cesses and waste sectors are summarized in Appendix Appendix F.