Biogeochemistry, Climate, and Interactions with Other Factors
Societal options for addressing links between climate and biogeochemical cycles must often be informed by connections to a broader context of global environmental changes. For example, both climate change and nitrogen deposition can reduce biodiversity in water- and land-based ecosystems. The greatest combined risks are expected to occur where critical loads are exceeded.4,5 A critical load is defined as the input rate of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge.5 Although biodiversity is often shown to decline when nitrogen deposition is high due to fossil fuel combustion and agricultural emissions,5,6 the compounding effects of multiple stressors are difficult to predict. Warming and changes in water availability have been shown to interact with nitrogen in additive or synergistic ways to exacerbate biodiversity loss.7 Unfortunately, very few multi-factorial studies have been done to address this gap.
Human induced acceleration of the nitrogen and phosphorus cycles already causes widespread freshwater and marine eutrophication,8,9,10 a problem that is expected to worsen under a warming climate.9,11,12 Without efforts to reduce future climate change and to slow the acceleration of biogeochemical cycles, existing climate changes will combine with increasing inputs of nitrogen and phosphorus into freshwater and estuarine ecosystems. This combination of changes is projected to have substantial negative effects on water quality, human health, inland and coastal fisheries, and greenhouse gas emissions.13,9
Similar concerns – and opportunities for the simultaneous reduction of multiple environmental problems (known as “co-benefits”) – exist in the realms of air pollution, human health, and food security. For example, methane, volatile organic compounds, and nitrogen oxide emissions all contribute to the formation of tropospheric ozone, which is a greenhouse gas and has negative consequences for human health and crop and forest productivity.14,15,16 Rates of ozone formation are accelerated by higher temperatures, creating a reinforcing cycle between rising temperatures and continued human alteration of the nitrogen and carbon cycles.17 Rising temperatures also work against some of the benefits of air pollution control.16 Some changes will trade gains in one arena for declines in others. For example, lowered NOx, NHx, and SOx emissions remove cooling agents from the atmosphere, but improve air quality.18,19 Recent analyses suggest that targeting reductions in compounds like methane and black carbon aerosols that have both climate and air-pollution consequences can achieve significant improvements in not only the rate of climate change, but also in human health.19 Finally, reductions in excess nitrogen and phosphorus from agricultural and industrial activities can potentially reduce the rate and impacts of climate change, while simultaneously addressing concerns in biodiversity, water quality, food security, and human health.20
Estimating the U.S. Carbon Sink
Any natural or engineered process that temporarily or permanently removes and stores carbon dioxide (CO2) from the atmosphere is considered a carbon “sink.” Temporary (10 to 100 years) CO2 sinks at the global scale include absorption by plants as they photosynthesize, as well as CO2 dissolution into the ocean. Forest biomass and soils in North America offer large temporary carbon sinks in the global carbon budget; however, the spatial distribution, longevity, and mechanisms controlling these sinks are less certain.2 Understanding these processes is critical for predicting how ecosystem carbon sinks will change in the future, and potentially for managing the carbon sink as a mitigation strategy for climate change.
Both inventory (measurement) and modeling techniques have been used to estimate land-based carbon sinks at a range of scales in both time and space. For inventory methods, carbon stocks are measured at a location at two points in time, and the amount of carbon stored or lost can be estimated over the intervening time period. This method is widely used to estimate the amount of carbon stored in forests in the United States over timescales of years to decades. Terrestrial biosphere models estimate carbon sinks by modeling a suite of processes that control carbon cycling dynamics, such as photosynthesis (CO2 uptake by plants) and respiration (CO2 release by plants, animals, and microorganisms in soil and water). Field-based data and/or remotely sensed data are used as inputs and also to validate these models. Estimates of the land-based carbon sink can vary depending on the data inputs and how different processes are modeled.21 Atmospheric inverse models use information about atmospheric CO2 concentrations and atmospheric transport (like air currents) to estimate the terrestrial carbon sink.22,23 This approach can provide detailed information about carbon sinks over time. However, because atmospheric CO2 is well-mixed and monitoring sites are widely dispersed, these models estimate fluxes over large areas and it is difficult to identify processes responsible for the sink from these data.21 Recent estimates using atmospheric inverse models show that global land and ocean carbon sinks are stable or even increasing globally.24
The U.S. Environmental Protection Agency (EPA) conducts an annual inventory of U.S. greenhouse gas emissions and sinks as part of the nation’s commitments under the Framework Convention on Climate Change. Estimates are based on inventory studies and models validated with field-based data (such as the CENTURY model) in accordance with the Intergovernmental Panel on Climate Change (IPCC) best practices.25 An additional comprehensive assessment, The First State of the Carbon Cycle Report (SOCCR), provides estimates for carbon sources and sinks in the U.S. and North America around 2003.2 This assessment also utilized inventory and field-based terrestrial biosphere models, and incorporated additional land sinks not explicitly included in EPA assessments.
Data from these assessments suggest that the U.S. carbon sink has been variable over the last two decades, but still absorbs and stores a small fraction of CO2 emissions. The forest sink comprises the largest fraction of the total land sink in the U.S., annually absorbing 7% to 24% (with a best estimate of 16%) of fossil fuel CO2 emissions during the last two decades. Because the U.S. Forest Service has conducted detailed forest carbon inventory studies, the uncertainty surrounding the estimate for the forest sink is lower than for most other components (see Pacala et al. 2007, Table 23). The role of lakes, reservoirs, and rivers in the carbon budget, in particular, has been difficult to quantify and is rarely included in national budgets.26 The IPCC guidelines for estimating greenhouse gas sources or sinks from lakes, reservoirs, or rivers are included in the “wetlands” category, but only for lands converted to wetlands. These ecosystems are not included in the EPA’s estimates of the total land sink. Rivers and reservoirs were estimated to be a sink in the State of the Carbon Cycle analysis,3 but recent studies suggest that inland waters may actually be an important source of CO2 to the atmosphere.27 It is important to note that these two methods use different datasets, different models, and different methodologies to estimate land-based carbon sinks in the United States. In particular, we note that the EPA Inventory, consistent with IPCC Guidelines for national inventories, includes only carbon sinks designated as human-caused, while the SOCCR analysis does not make this distinction.
| Land Area | C sink (Tg C/y) (95% CI) | Method |
|---|---|---|
| Forest | -256 (± 50%) | inventory, modeled |
| Wood products | -57 (± 50%) | inventory |
| Woody encroachment | -120 (± >100%) | inventory |
| Agricultural soils | -8 (± 50%) | modeled |
| Wetlands | -23 (± >100%) | inventory |
| Rivers and reservoirs | -25 (± 100%) | inventory |
| Net Land Sink | -489 (± 50%) | inventory |
Table 15.1: Carbon (C) sinks and uncertainty estimated by Pacala et al. for the first State of the Carbon Cycle Report.3 Forests take up the highest percentage of carbon of all land-based carbon sinks. Due to a number of factors, there are high degrees of uncertainty in carbon sink estimates.
