For more than three decades, scientists have predicted that a doubling of carbon dioxide in Earth’s atmosphere from preindustrial levels would warm Earth’s surface by an average of between 1.5°C and 4.5°C (about 3°F to 8°F). The latest report from the Intergovernmental Panel on Climate Change (IPCC) confirms this finding, with greater confidence, and furthermore affirms that the primary cause of the observed increase in global-average temperature is anthropogenic greenhouse gas (GHG) emissions (IPCC, 2013b). The IPCC further concludes that, if current emissions trends continue, by the end of the century the planet will experience a warming of up to 5°C (Figure 1.1), sea level will rise by as much as 1 m (Figure 1.2), and the Arctic will be ice free in the summer by midcentury. As part of this change in climate, society will experience an increase in the frequency and severity of heat waves, droughts, and heavy precipitation events (also see NCA, 2014).
To date, scientists have observed a number of manifestations of the changing climate, all of which will likely be amplified in the future (IPCC, 2014b). Moreover, the ability to predict these changes carries considerable uncertainties that suggest that while the adverse effects of climate change may not be as severe as many predictions, it is also quite possible that they may in fact be considerably worse (NRC, 2013a). One very visible example is the reduction in Arctic perennial sea ice cover, which has diminished at a rate of 13 percent per decade (relative to the 1979-2012 mean; see Fetterer et al., 2012; Stroeve et al., 2012b). This reduction in ice cover far exceeded model predictions (Stroeve et al., 2012a) and serves as a stark indication that the challenges we may face with climate change may occur sooner rather than later. Such a circumstance underscores the potential mismatch between the timescales at which detrimental change may occur and the timescales at which meaningful mitigation strategies may be implemented.
Globally, greenhouse gas emissions have been increasing as the growing demand for energy has more than offset what progress there has been from improved efficiency and deployment of new energy sources with lower GHG emissions (Le Quéré et al., 2013). In May 2013 the CO2 concentration measured at the Mauna Loa Observatory in Hawaii briefly exceeded 400 parts per million (ppm) for the first time in the modern era, before the spring bloom in the Northern Hemisphere temporarily drew down CO2 levels (Figure 1.3). Concentrations of CO2 in the atmosphere have been increasing from preindustrial levels of 280 ppm largely as the result of the combustion of
FIGURE 1.1 Temperature increase for various emission scenarios. A temperature rise of up to 5°C is possible by the end of the century if current emission trends continue. CMIP5 multimodel simulated time series from 1950 to 2100 for change in global annual mean surface temperature relative to 1986-2005. Time series of projections and a measure of uncertainty (shading) are shown for two representative concentration pathway (RCP) scenarios, RCP2.6 (blue) and RCP8.5 (red). The RCP scenarios represent a family of hypothetical future scenarios for emission of CO2 and other greenhouse gases. They are labeled according to the peak radiative forcing from all gases up to the year 2100, so that higher-numbered RCP scenarios correspond to climate futures with greater emissions. The full set of scenarios consists of RCP2.6, RCP4.5, RCP6.0, and RCP8.5, and the middle two have been selected for the analysis in this section. The RCP2.6 trajectory involves very aggressive emission mitigation and also requires negative emissions (e.g., carbon dioxide removal) to help meet its target. SOURCE: IPCC, 2013b, Fig. SPM.7.
fossil fuels. Unlike many other air pollutants—such as nitrogen oxides and sulfur oxides, which are removed by natural physical and chemical processes in just hours to days after they are emitted—the GHGs most responsible for causing climate change remain in the atmosphere for decades to centuries.1 In order to stabilize or reduce atmospheric concentrations, and thus avoid the worst impacts of warming, global emissions of GHGs must be reduced by at least an order of magnitude (NRC, 2011a).
1 Excess carbon is absorbed by the land biosphere and ocean over decades and centuries, and it reacts with carbonate and silicate materials over thousands of years; nevertheless, most of the excess carbon emitted today will still be in the atmosphere, land biosphere, or ocean many tens of thousands of years later, until geologic processes can form rocks and deposits that would incorporate this carbon (Archer et al., 2009; Berner et al., 1983).
FIGURE 1.2 Sea level rise for emission scenarios RCP2.6 (blue) and RCP8.5 (red). A sea level rise of up to 1 m is possible by the end of the century if current emission trends continue. SOURCE: IPCC, 2013b, Fig. SPM.9.
To date, little progress has been made toward achieving such a major reduction (IPCC, 2011; NRC, 2010c).
Although many uncertainties remain in our understanding of climate science, it is clear that the planet is already experiencing significant climate change as a result of anthropogenic influences (IPCC, 2013b). To avoid greatly increased risk of damage from climate change, the international community has been called upon to embark on a major program to reduce emissions of carbon dioxide and other greenhouse gases (e.g., Hoffert et al., 1998; IPCC, 2013a, b, 2014a; NRC, 2011b). Because major actions to reduce emissions have been delayed, considerable additional climate change is inevitable (Cao et al., 2011). There is a portfolio of responses and proposed strategies for diminishing climate damage and risk (Figure 1.4). As outlined below in the section “Decarbonizing the Energy System,” implementing an aggressive program of emissions abatement or mitigation presents major challenges to how we live and function as a society. These challenges have to date been a major barrier to the undertaking of substantive steps to reduce greenhouse gas emission, even though doing so is technologically well within our grasp and constitutes the lowest-risk and most efficacious
FIGURE 1.3 Record of the concentration of atmospheric carbon dioxide measured at the summit of Mauna Loa in Hawaii. The carbon dioxide data (red curve), measured as the mole fraction in dry air, on Mauna Loa constitute the longest record of direct measurements of CO2 in the atmosphere; the black curve represents the seasonally corrected data. The collection of this record was begun in 1958 by Charles David Keeling of the Scripps Institution of Oceanography. Today, similar trends are observed in locations all around the planet (see http://www.esrl.noaa.gov/gmd/ccgg/carbontracker/). SOURCE: Scripps CO2 Program.
path toward reducing the threats associated with anthropogenic climate change. Even if an aggressive global mitigation program is undertaken, substantial reductions in greenhouse gas levels would not be realized for several decades, and the halting or reversing of some of the detrimental effects already built into the climate system (e.g., ocean warming, ocean acidification, polar ice melting, sea level rise) would not follow for many decades or even centuries beyond that. Although there is considerable opportunity to limit the future growth of climate change, the world cannot avoid major climate change. As a result adaptation will be required and is indeed already happening (discussed below in “Adapting to Climate Change”). Adaptation will become increasingly costly and disruptive as the magnitude of climate change increases.
FIGURE 1.4 There is a portfolio of responses and proposed strategies for diminishing climate risk and damage at various steps in the causal chain of the human-climate system. Carbon dioxide removal approaches if proven effective could reduce the amount of CO2 in the atmosphere. Albedo modification strategies have been proposed as a method to reduce the amount of warming that results from the accumulation of CO2 in the atmosphere. SOURCE: Adapted from Caldeira et al., 2013.
This slow implementation of mitigation and the challenges of adaptation have led some people to consider whether strategies might exist to reduce the climate impacts of greenhouse gases after they have been emitted to the atmosphere. The committee refers to purposeful actions that are intended to produce a desired change in some aspect of the climate (e.g., global mean or regional temperature) as “climate intervention.” Climate intervention includes actions designed to remove carbon dioxide or other greenhouse gases from the atmosphere or to mask some of the climate effects of these gases by changing Earth’s radiation balance. This report examines approaches that actively increase the amount of short-wavelength radiation that is reflected to space, referred to as “albedo modification.” The terms “climate engineering” and “geoengineering” have been used to refer to highly heterogeneous and poorly defined collections of activities. The committee believes that these overarching terms
do little to advance the discussion of the set of activities under consideration here. Therefore, the committee refers instead to carbon dioxide removal (CDR) and albedo modification strategies independently. These two classes of strategies have very different characteristics (see Box 1.1).
The committee recognizes that altering Earth’s albedo is an extreme measure, one that many already dismiss as unwise. However, the fact that the risks associated with climate change may themselves be unmanageable and irreversible through mitiga-
BOX 1.1 WHY THERE ARE TWO SEPARATE REPORTS
This committee was tasked with conducting a technical evaluation of examples of both carbon dioxide removal (CDR) techniques and albedo modification techniques (also known as “solar radiation management” or “sunlight reflection methods,” both going by the initials SRM).a
Some carbon dioxide removal techniques such as reforestation have already been considered in the public policy process as a form of mitigation—the effort to reduce net greenhouse gas emissions resulting from human activity. Linking direct air capture of carbon with carbon sequestration (DACS) has the potential to lead to a net reduction of CO2 from the atmosphere if and when fossil fuel use is significantly reduced. As such, CDR approaches such as reforestation and DACS have more in common with widely discussed climate change mitigation approaches than they do with, for example, stratospheric aerosol injection. Reforestation and bioenergy with carbon capture and sequestration figured prominently in the IPCC Working Group III chapter on Mitigation of Climate Change, where mitigation is defined as “a human intervention to reduce the sources or enhance the sinks of greenhouse gases” (IPCC, 2014b).
In contrast, even the lowest-risk albedo modification approaches entail unknown and potentially large international political and environmental challenges, and therefore more research is required to better understand consequences of a possible implementation. The political ramifications, environmental risks, and research needs associated with albedo modification differ dramatically from those associated with carbon dioxide removal. Table S.1 summarizes the many contrasts in cost, risk, impact, and scale between these two approaches.
Although both share the goal of reducing the climate consequences of high greenhouse gas concentrations, CDR methods have more affinity with solutions aimed at reducing net anthropogenic CO2 emissions (e.g., transitions to near-zero-emission energy systems), whereas albedo modification approaches aim to provide symptomatic relief from only some of the consequences of high greenhouse gas concentrations. The committee sees little benefit in or rationale for closely associating these carbon dioxide removal approaches with only distantly related and highly controversial albedo modification approaches. Therefore, the committee has decided that it can most effectively carry out its charge by producing two separate volumes: one on carbon dioxide removal and another on albedo modification.
a Appendix A describes the charge to the committee for this study and Appendix B lists the committee membership.
tion efforts that are implemented too late makes examination of alternatives such as albedo modification a prudent action at this time, so that the limits and potential can at least be understood and weighed against the alternatives.
The most important human activity contributing to GHG emissions is the burning of fossil fuels (coal, oil, and natural gas) (IPCC, 2013b). Hence stabilizing or reducing atmospheric concentrations of carbon dioxide, and thus the climate, will require performing a massive transformation in the energy and transportation system (NRC, 2010b). Most knowledgeable observers understand that humanity should embark on an aggressive program to reduce emissions, although the scale of this challenge is underappreciated by some but not as daunting as it is made out to be by others.
According to the International Energy Agencyn (IEA), the total electricity consumption worldwide in 2011 was approximately 20,000 TWh (a rate of ~2,300 GW), and the United States accounted for just over 4,000 TWh (a rate of ~460 GW), or about 20%, of that amount (IEA, 2013). To gain some perspective on what will be involved in reducing fossil fuel dependence, a large power plant can produce about 1 GW of electrical power (EIA, 2013b; see also http://www.eia.gov/electricity/annual/), so the above numbers can be thought of as the amount of electricity produced by 2,300 large power plants globally or 460 large power plants for the United States alone. If society is to decarbonize the electricity system, it will be necessary to replace much of that infrastructure with carbon-free energy sources or to modify existing power plants to be carbon free. It took the United States more than five decades to create its existing electrical system infrastructure, and the lifetime for an existing coal-fired power plant is typically several decades (EIA, 2013a; Smil, 2010).
Further, global energy use is conservatively projected to rise between 15 percent and 30 percent by 2035 (from 2011 levels2), adding to the challenge of decarbonizing global energy. In addition to the electric power sector, the transportation, industrial and residential and commercial sectors currently account for the majority of energy use in the United States. As Figure 1.5 shows, energy input into electricity is only about 35 percent of U.S. total energy consumption. Most of the remainder involves the direct combustion of fossil fuels in transportation, heating and cooling of buildings, and industrial processes. In order to decarbonize the entire energy system, all of these
2 2011 total energy consumption = 8,918 Mtoe (million tons oil equivalent; 10, 400TWh); 2035 projections are between 10,390 and 11,750 Mtoe (12,100 and 13,700 TWh); http://www.iea.org/publications/freepublications/publication/KeyWorld2013.pdf; accessed October, 2014.
FIGURE 1.5 Flows of energy through the U.S. economy. The light gray bands on the right indicate energy that performs no useful service (i.e., waste). The dark gray bands on the right indicate energy that is used in the residential, commercial, industrial, and transportation sectors. Note that roughly 88 percent of the energy that presently enters the U.S. economy involves combustion of a fuel, which releases carbon dioxide to the atmosphere (1 quad is 1012 BTUs or 293 TWh). SOURCE: Lawrence Livermore National Laboratory, https://flowcharts.llnl.gov/.
applications will also need to be converted to systems that emit little or no carbon dioxide, in many cases by converting them to run on cleaner sources of electricity.
“Decarbonization” of the energy system could be facilitated by adopting the following strategies (IPCC, 2014b; NRC, 2010b):
- Improve the efficiency with which the energy enters and is distributed within the system and increase the efficiency of all technologies that use energy.
- Convert the electricity, residential, commercial, industrial, and transportation systems to sources of energy that release less carbon dioxide to the atmosphere. Examples of such sources could include nuclear energy; systems that capture and “sequester” carbon dioxide from power plants that use coal or natural gas; hydroelectricity, wind and solar power; some systems based on biomass (though not all bioenergy has low net carbon emissions); and geothermal energy.
A recent NRC report (2010b) assesses the feasibility of decarbonizing the energy system as follows:
There are large uncertainties associated with these sorts of projections, but the variation among them illustrates that the United States has many plausible options for configuring its future energy system in a way that helps meet GHG emissions-reduction goals. Note, however, that all cases involve a greater diversity of energy sources than exist today, with a smaller role for freely emitting fossil fuels and a greater role for energy efficiency, renewable energy, fossil fuels with CCS, and nuclear power. The virtual elimination by 2050 of coal without CCS—presently the mainstay of U.S. electric power production—in all the scenarios is perhaps the most dramatic evidence of the magnitude of the changes required. (NRC, 2010b)
Because they produce varying and intermittent power, it is thought that wind and solar cannot currently be the sole replacement for conventional fossil fuel–fired power plants. A reliable and affordable supply of carbon-free electricity will require a broad mix of generation types and energy sequestration approaches. Figure 1.6 shows three examples of potential scenarios for the mix of future generation types.
Although such estimates of future deployment of carbon-free energy sources indicate that it may be possible to achieve a decarbonized energy system, great uncertainties remain regarding the implementation of such scenarios due to factors such as costs, technology evolution, public policies, and barriers to deployment of new technologies (NRC, 2010b). Furthermore, simply accounting for the emissions from existing fossil fuel energy facilities over their remaining lifetime commits the planet to an additional
FIGURE 1.6 Three examples of alternative energy system transformation pathways are presented, where each pathway is consistent with limiting CO2-equivalent (CO2-eq) concentrations to about 480 ppm CO2eq by 2100. The scenarios from the three selected models (Model for Energy Supply Strategy Alternatives and their General Environmental Impact [MESSAGE], Regional Model of Investments and Development [ReMIND], and Global Change Assessment Model [GCAM]) show that there are different strategies for combining renewable and nonrenewable energy sources with increases in energy efficiency to meet the target. The left-hand panels show the energy supply for each scenario by year, which, in absence of new policies to reduce GHG emissions, would continue to be dominated by fossil fuels. Right-hand panels show alternative scenarios that limit GHG concentration to low levels through rapid and pervasive replacement of fossil fuels. Between 60 and 300 EJ of fossil fuels are replaced across the three scenarios over the next two decades (by 2030). By 2050 fossil energy use is 230-670 EJ lower than in non-climate policy baseline scenarios. SOURCE: IPCC, 2014b.
300 billion tons of CO2 (Davis and Socolow, 2014).3 With whatever portfolio of technologies the transition is achieved, eliminating the carbon dioxide emissions from the global energy and transportation systems will pose an enormous technical, economic, and social challenge that will likely take decades of concerted effort to achieve.
The likely impacts of climate change have been described at length in reports of the IPCC (IPCC, 2013b; NRC, 2010a). Impacts likely to be experienced in the territories of the United States have been described in the U.S. National Climate Assessment (NCA, 2014) and the Arctic Assessment (ACIA, 2004; NRC, 2010a). These and similar studies conclude that, although it will be difficult and expensive, with a deliberate effort industrialized societies and economies can adapt to the climate change that may occur over the remainder of this century. There is much to do to build the capacity to adapt in the United States (NRC, 2010a, 2012a). The outlook is more pessimistic for the less industrialized societies and economies of the world, and grimmer still for many natural terrestrial, aquatic, and oceanic ecosystems (IPCC, 2013b).
The past 10,000 years have been a period of relative climatic stability that has allowed human civilization to flourish, agrarian sedentary communities to replace a nomadic lifestyle, and cities to emerge on mostly stable shorelines. This has been true despite notable exceptions, such as the Little Ice Age and episodes of volcanic-influenced weather that resulted in famine and widespread travail (Parker, 2013; Wood, 2014). What swings there have been in the global climate system have occurred within a relatively narrow range compared to those in the longer paleoclimate record. History suggests that some ancient civilizations have not adapted well to past climate changes. For example, it is believed that natural climate excursions, along with other factors, contributed to the end of the Anasazi and Mayan civilizations in the southwestern United States and Central America (Diamond, 2011; Tainter, 1988).
Globally, communities are already experiencing changing conditions directly linked to climate change—including rising seas that threaten low-lying island nations, loss of glaciers and sea ice and melting permafrost that expose Arctic communities to increased shoreline erosion, and consecutive record years of heat and drought stress (IPCC, 2013a,b, 2014a; NCA, 2014).
3 Units of mass adopted in this report follow the convention of the IPCC and are generally those which have come into common usage; GtCO2 = gigatonnes of carbon dioxide, where 3.67 GtCO2 = 1 GtC.
As described above, the challenge of decarbonizing the energy system is indeed daunting, and adapting to climate change is also likely to present substantial challenges. For example, much of the current infrastructure essential for commerce of coastal cities such as New York, Boston, Miami, Long Beach, Manhattan, New Orleans, Los Angeles, San Diego, and parts of San Francisco today could end up below sea level as the ocean continues to rise and, thus, could be submerged in the absence of protective dikes or other adaptive measures (NRC, 2012b; Strauss et al., 2012, 2013; Tebaldi et al., 2012). With sufficient planning, the possibility of moving infrastructure to higher ground is a cost-effective mitigation strategy for many localities, but there is little history of abandoning commercial use of coastal land in anticipation of sea level rise and there are many social and societal factors involved in potentially relocating communities (NRC, 2010a). Anticipatory adaptation is made more difficult because disruption to human lives and property typically does not occur gradually (see, for example, NRC, 2013a) but rather as a result of major weather events, such as hurricanes and other large storms, that cause billions of dollars in damage.
Food production is also sensitive to climate change. Although the relationship is complex—some regions will experience longer growing seasons while others will suffer from more heat stress—global yields of wheat, barley, and maize have decreased with increasing global-average temperature (Lobell and Field, 2007). There are numerous adaptation strategies that are available to cope with various climate changes—including changes to temperatures, precipitation, and ambient CO2 concentrations—but all require substantial effort and investment (see Table 3.3 in NRC, 2010a). But even with adaptation, climate change can still cause long-term loss (for example, long-term loss of land due to sea level rise).
Shifts in mean temperature, temperature variability, and precipitation patterns are already causing stress on a diversity of ecosystems (NRC, 2013a). Species’ range shifts have already become evident (Chen et al., 2011; Parmesan, 2006; Parmesan and Yohe, 2003; Poloczanska et al., 2013; Root et al., 2003; Staudinger et al., 2012) and are expected to accelerate with increasing rates of climate change, as are changes in the timing of species migrations (Gill et al., 2013) and other important plant and animal life-cycle events. The world’s surface ocean has already experienced a 30 percent rise in acidity since the industrial revolution, and as that acidity continues to rise, there could potentially be major consequences to marine life and to the economic activities that depend on a stable marine ecosystem (NRC, 2013b). These impacts, combined with increasing numbers of exotic species introductions and demands on ecosystems to provide goods and services to support human needs, mean that extinction rates are increasing (Pimm, 2009; Staudinger et al., 2012). With continued climate change,
species will be increasingly forced to adapt to changing environmental conditions and/or migrate to new locations, or face increasing extinction pressures.
There are many climate adaptation and resilience efforts ongoing within the United States, often at the state or local levels (Boston Climate Preparedness Task Force, 2013; Miami-Dade County, 2010; PlaNYC, 2013; Stein et al., 2014; USGS, 2013; http://www.cakex.org/). Although this is a rapidly evolving field, there is still a great deal of research to be done in the field of climate adaptation and there may be insufficient capacity for adaptation (NRC, 2010a). Overall, both humans and ecosystems face substantial challenges in adapting to the varied impacts of climate change over the coming century.
As discussed above, industrialized and industrializing societies have not collectively reduced the rate of growth of GHG emissions, let alone the absolute amount of emissions, and thus the world will experience significant and growing impacts from climate change even if rapid decarbonization of energy systems begins. Given the challenges associated with reducing GHG emissions and adapting to the impacts of climate change, some people have begun exploring whether there are climate intervention approaches that might provide additional mechanisms for facing the challenges of climate change.
In this volume, the committee considers strategies to remove GHGs (largely CO2) from the atmosphere and provide reliable sequestration for it in perpetuity, which are termed CDR. Chapter 2 introduces several CDR approaches and Chapter 3 discusses each approach in more depth. While nature already performs “CDR” by removing the equivalent of more than half of our emissions from the atmosphere each year, all strategies considered for increasing CDR are inherently incremental and, as with most mitigation activities, require many parties to cooperate in order to have a global impact. With the exception of trying to increase uptake of carbon dioxide by fertilizing the ocean, most strategies for CDR, such as directly scrubbing carbon dioxide from the atmosphere, are local in scale. CDR technologies for removing carbon dioxide directly from the atmosphere at scale are unlikely to be energetically or financially advantageous over using carbon capture and sequestration technologies to remove carbon dioxide from stack gases associated with combusting fossil fuels or biomass (see discussion in Chapter 3 below). Thus, CDR may be more likely to be deployed to offset emissions from diffuse sources of carbon emissions (e.g., transport and agricultural activities). CDR is also likely to compete directly with other methods of reducing
or mitigating carbon dioxide emissions. On the margin the environmental value of removing a ton of carbon dioxide from the atmosphere is the same as that of avoiding the emission of a ton of carbon dioxide.4Chapter 4 discusses some of the social and economic considerations surrounding CDR approaches. The balance between CDR and other mitigation methods is likely to be determined by the relative costs of the various technologies at the local and regional levels, together with government policies that limit or attach a price to GHG emissions. As a society, we need to better understand the potential cost and performance of CDR strategies for the same reason that we need to better understand the cost and performance of emission mitigation strategies—they may be important parts of a portfolio of options to stabilize and reduce atmospheric concentrations of carbon dioxide (see discussion in Chapter 5).
The companion volume to this report, Climate Intervention: Reflecting Sunlight to Cool Earth, considers strategies to increase the fraction of incoming solar radiation that is directly reflected back to space (increase the albedo) and related approaches that modify Earth’s radiative balance. The introductory material for both reports is the same (Chapter 1 both reports). The concluding chapter of this volume (Chapter 5 below) summarizes the discussions in this volume; the concluding chapter of the companion volume summarizes both the discussions in that volume, as well as providing an overview of both volumes.
4 As discussed in Chapter 2, the removal of one ton of CO2 from the atmosphere will lead to a reduction less than one ton in the CO2 burden in the atmosphere due to a “rebound” effect where CO2 outgasses from the ocean.