CHAPTER ONE


Introduction

The idea that Earth’s climate could abruptly change in a drastic manner has been around for several decades. Early studies of ice cores showed that very large changes in climate could happen in a matter of a few decades or even years, for example, local to regional temperature changes of a dozen degrees or more, doubling or halving of precipitation rates, and dust concentrations changing by orders of magnitude (Dansgaard et al., 1989; Alley et al., 1993). In the last few decades, scientific research has advanced our understanding of abrupt climate change significantly. Some original fears have been allayed or now seem less ominous, but new ones have sprung up. Fresh reminders occur regularly that thresholds and tipping points exist not only in the climate system, but in other parts of the Earth system (Box 1.1).

What has become clearer recently is that the issue of abrupt change cannot be confined to a geophysical discussion of the climate system alone. The key concerns are not limited to large and abrupt shifts in temperature or rainfall, for example, but also extend to other systems that can exhibit abrupt or threshold-like behavior even in response to a gradually changing climate. The fundamental concerns with abrupt change include those of speed—faster changes leave less time for adaptation, either economically or ecologically—and of magnitude—larger changes require more adaptation and generally have greater impact. This report offers an updated look at the issue of abrupt climate change and its potential impacts, and takes the added step of considering not only abrupt changes to the climate system itself, but also abrupt impacts and tipping points that can be triggered by gradual changes in climate.

This examination of the impacts of abrupt change brings the discussion into the human realm, raising questions such as: Are there potential thresholds in society’s ability to grow sufficient food? Or to obtain sufficient clean water? Are there thresholds in the risk to coastal infrastructure as sea levels rise? The spectrum of possibilities here is very wide, too wide to be fully covered in any single report. In practice, little is known about these and other possible abrupt changes. As such, this report lays out what is currently known about the risks, raises flags to point out potential threats, and proposes improved monitoring and warning schemes to help prepare us for both known and unknown abrupt changes. This report can be viewed as the current frame in an ongoing movie in which we grasp the basic plot, but we are not sure what plot twists lie ahead or even how the various characters are related. As scientific research and monitoring progresses, i.e., as we watch the movie and learn more about the key characters and



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CHAPTER ONE Introduction T he idea that Earth’s climate could abruptly change in a drastic manner has been around for several decades. Early studies of ice cores showed that very large changes in climate could happen in a matter of a few decades or even years, for example, local to regional temperature changes of a dozen degrees or more, doubling or halving of precipitation rates, and dust concentrations changing by orders of mag- nitude (Dansgaard et al., 1989; Alley et al., 1993). In the last few decades, scientific re- search has advanced our understanding of abrupt climate change significantly. Some original fears have been allayed or now seem less ominous, but new ones have sprung up. Fresh reminders occur regularly that thresholds and tipping points exist not only in the climate system, but in other parts of the Earth system (Box 1.1). What has become clearer recently is that the issue of abrupt change cannot be con- fined to a geophysical discussion of the climate system alone. The key concerns are not limited to large and abrupt shifts in temperature or rainfall, for example, but also extend to other systems that can exhibit abrupt or threshold-like behavior even in response to a gradually changing climate. The fundamental concerns with abrupt change include those of speed—faster changes leave less time for adaptation, ei- ther economically or ecologically—and of magnitude—larger changes require more adaptation and generally have greater impact. This report offers an updated look at the issue of abrupt climate change and its potential impacts, and takes the added step of considering not only abrupt changes to the climate system itself, but also abrupt impacts and tipping points that can be triggered by gradual changes in climate. This examination of the impacts of abrupt change brings the discussion into the hu- man realm, raising questions such as: Are there potential thresholds in society’s ability to grow sufficient food? Or to obtain sufficient clean water? Are there thresholds in the risk to coastal infrastructure as sea levels rise? The spectrum of possibilities here is very wide, too wide to be fully covered in any single report. In practice, little is known about these and other possible abrupt changes. As such, this report lays out what is currently known about the risks, raises flags to point out potential threats, and proposes im- proved monitoring and warning schemes to help prepare us for both known and un- known abrupt changes. This report can be viewed as the current frame in an ongoing movie in which we grasp the basic plot, but we are not sure what plot twists lie ahead or even how the various characters are related. As scientific research and monitoring progresses, i.e., as we watch the movie and learn more about the key characters and 19

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abrupt impacts of climate chang E BOX 1.1  EXAMPLES OF RECENT ABRUPT CHANGE IN THE EARTH SYSTEM Stratospheric Ozone Depletion During the early 1970s, concerns arose in the scientific community that inputs of nitrogen oxides (known as “NOx”) from a proposed fleet of supersonic aircraft flying in the stratosphere and of industrially produced halocarbon gases containing chlorine and bromine (CFCs or chlo- rofluorocarbons and chlorofluorobromocarbons) had the potential to deplete the amount of ozone in the stratosphere. Halogen oxide radicals were predicted to form from the degradation of halocarbons in the stratosphere. Intensive study of the stratosphere, extending more than a decade, confirmed the rising concentrations of CFCs and halons in the atmosphere, and of halogen oxide radicals in the stratosphere. International negotiations led to the signing of the Montreal Protocol in 1987, requiring a 50 percent reduction in CFCs and a 100 percent reduction in halon production by 2000 by the developed countries. However, two years prior to the treaty, scientists learned that the column amount of ozone over Antarctica in the austral spring had been declining since the late 1960s, and it had been reduced by almost a factor of two by the mid-1980s (Farman et al., 1985); See Figure A. The continuous record of column ozone abundances measured at Halley Bay, Antarctica, showed Antarctic Column Ozone, October Halley Bay 60 - 90 S 350 300 Ozone total column (DU) 250 200 150 Onset Montr eal P hase Out 100 1960 1970 1980 1990 2000 2010 Year FIGURE A Total column ozone in Antarctica, at the Halley Bay station of the British Antarctic Survey (black) and averaged over the whole polar region of Antarctica (blue, from satellite data). (Adapted from WMO/ Box 1-1 Figure A_R02461.eps UNEP [2010] plus data from the British Antarctic Survey [http://www.antarctica.ac.uk/met/jds/ozone/data/ ZOZ5699.DAT, downloaded 26 April 2013].) 20

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Introduction BOX 1.1  CONTINUED that October ozone column amounts started to drift lower in the late 1960s and 1970s. Satellite records and measurements from other stations confirmed that this change was occurring on the continental scale of Antarctica. This was an abrupt change in the timescale of human activities, the scale of the whole polar region, but lack of continuity and rejection of data perceived to be anomalous prevented the detection of the change from space observations. The Montreal Protocol was amended to require complete phase-out of most ozone-deplet- ing CFCs by 1996 in developed countries and by 2010 in rest of the world. In addition, the Protocol was amended or adjusted multiple times to reduce emissions of all ozone-depleting substances. As a result of the Montreal Protocol and its amendments, stratospheric ozone is expected to return to its pre-1980 values as the atmospheric abundances of ozone-depleting substances decline in the coming decades. Column global ozone amounts prevalent in the early1970s are expected to be restored by the mid-21st century, although stratospheric cooling associated with changes in greenhouse gases will alter the trajectory of the restoration. The Antarctic ozone hole is expected to no longer occur towards the late 21st century, and this recovery is not expected to be influenced as much by climate change as the global ozone amounts (WMO/UNEP, 2010). The Antarctic ozone hole represents an abrupt change to the Earth system. Although it is not specifically an abrupt climate change, for the purposes of this report, it is a recent example of the type an unforeseen global threshold event. The Antarctic ozone hole appeared within a few years after a threshold was crossed—when the concentrations of inorganic chlorine exceeded the concentration of nitrogen oxides in the lower altitudes of the polar stratosphere—and it affected a large portion of the globe. Thus, it exemplifies the scope and magnitude of the types of impacts that abrupt changes from human activities can have on the planet. Bark Beetle Outbreaks Bark beetles are a natural part of forested ecosystems, and infestations are a regular force of natural change. In the last two decades, though, the bark beetle infestations that have occurred across large areas of North America have been the largest and most severe in recorded history, killing millions of trees across millions of hectares of forest from Alaska to southern California (Bentz, 2008); see Figure B. Bark beetle outbreak dynamics are complex, and a variety of circum- stances must coincide and thresholds must be surpassed for an outbreak to occur on a large scale. Climate change is thought to have played a significant role in these recent outbreaks by maintaining temperatures above a threshold that would normally lead to cold-induced mortality. In general, elevated temperatures in a warmer climate, particularly when there are consecutive warm years, can speed up reproductive cycles and increase the likelihood of outbreaks (Bentz et al., 2010). Similar to many of the issues described in this report, climate change is only one contributing factor to these types of abrupt climate impacts, with other human actions such as forest history and management also playing a role. There are also feedbacks to the climate sys- tem from these outbreaks, which represent an important mechanism by which climate change may undermine the ability of northern forests to take up and store atmospheric carbon (Kurz et al., 2008). continued 21

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abrupt impacts of climate chang E BOX 1.1  CONTINUED FIGURE B  Photographs of a pine bark beetle and of a beetle-killed forest in the Colorado Rocky Mountains. Source: Top: Photo by Dion Manastyrski; Bottom: Photo from Anthony Barnosky. 22

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Introduction how they interact, it is hoped that scientists and policymakers will learn to anticipate abrupt plot changes and surprises so that societies can be better prepared to handle them. PREVIOUS DEFINITIONS OF ABRUPT CLIMATE CHANGE As recently as the 1980s, the typical view of major climate change was one of slow shifts, paced by the changes in solar energy that accompany predictable variations in Earth’s orbit around the sun over thousands to tens of thousands of years (Hays et al., 1976). While some early studies of rates of climate change, particularly during the last glacial period and the transition from glacial to interglacial climates, found large changes in apparently short periods of time (e.g., Coope et al., 1971), most of the paleoclimate records reaching back tens of thousands of years lacked the temporal resolution to resolve yearly to decadal changes. This situation began to change in the late 1980s as scientists began to examine events such as the climate transition that occurred at the end of the Younger Dryas about 12,000 years ago (e.g., Dansgaard et al., 1989) and the large swings in climate during the glacial period that have come to be termed “Dansgaard-Oescher events” (“D-O events;” named after two of the ice core scientists who first studied these phenomena using ice cores). At first these variations seemed to many to be too large and fast to be climatic changes, and it was only after they were found in several ice cores (e.g., Anklin et al., 1993; Grootes et al., 1993),1 and in many properties (e.g., Alley et al., 1993), including greenhouse gases (e.g., Severinghaus and Brook, 1999) that they became widely accepted as real. This perspective is important, as first definitions of abrupt climate change were tied directly to these D-O events, which themselves are defined by changes in temperature, precipitation rates, dust fallout, and concentrations of certain greenhouse gases. For this reason, previous reviews of abrupt change have tended to focus on the physical climate system, and the potential for abrupt changes and threshold behavior has been expressed primarily in climatic terms (key references listed in Box 1.2). The first systematic review of abrupt climate change was by the National Research Council (Abrupt Climate Change: Inevitable Surprises; NRC, 2002). This study defined abrupt climate change as follows: “Technically, an abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the 1  http://www.gisp2.sr.unh.edu/. 23

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abrupt impacts of climate chang E BOX 1.2  PREVIOUS REPORTS ON ABRUPT CLIMATE CHANGE Key references on the subject of abrupt climate change: • Abrupt Climate Change: Inevitable Surprises (NRC, 2002) • IPCC Fourth Assessment Report: Climate Change 2007 (IPCC, 2007c) • Synthesis and Assessment Product 3.4: Abrupt Climate Change (USCCSP, 2008) • Tipping Elements in the Earth’s Climate System (Lenton et al., 2008) • Climate and Social Stress: Implications for Security Analysis (NRC, 2012a) • 2  013 National Climate Assessment (National Climate Assessment and Development Advi- sory Committee, 2013) climate system itself and faster than the cause. Chaotic processes in the climate sys- tem may allow the cause of such an abrupt climate change to be undetectably small.” This early definition is critically important in two regards. First, it focuses on the climate system itself, a focus that remains widely used today. Second, it raises the possibility of thresholds or tipping points being forced or pushed by an undetectably small change in the cause of the shift. The 2002 report goes on to expand on its definition by plac- ing abrupt climate change into a social context: “To use this definition in a policy setting or public discussion requires some additional context, … because while many scientists measure time on geological scales, most people are concerned with changes and their potential impacts on societal and eco- logical time scales. From this point of view, an abrupt change is one that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it. Abrupt changes in climate are most likely to be significant, from a human perspective, if they persist over years or longer, are larger than typical climate variability, and affect sub-continental or larger regions. Change in any measure of climate or its variability can be abrupt, including change in the intensity, duration, or frequency of extreme events.” This expanded definition raised the issues of persistence, of changes being so large that they stand out above typical variability, and that changes in extremes, not just baselines, were considered to be abrupt climate changes. It also placed climate change into the context of impacts of those changes, and the change being considered abrupt if it exceeds the system’s capacity to adapt. In the subsequent years many papers were published on abrupt climate change, some with definitions more focused on time (e.g., Clark et al., 2002), and others on the relative speed of the causes and reactions. Overpeck and Cole (2006), for example, defined abrupt climate change as “a transition in the climate system whose duration is 24

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Introduction fast relative to the duration of the preceding or subsequent state.” Lenton et al. (2008) formally introduced the concept of tipping point, defining abrupt climate change as: “We offer a formal definition, introducing the term ‘‘tipping element’’ to describe subsystems of the Earth system that are at least subcontinental in scale and can be switched—under certain circumstances—into a qualitatively different state by small perturbations. The tipping point is the corresponding critical point—in forcing and a feature of the system—at which the future state of the system is qualitatively altered.” In 2007, the Intergovernmental Panel on Climate Change Fourth Assessment report defined abrupt climate change as: “forced or unforced climatic change that involves crossing a threshold to a new cli- mate regime (e.g., new mean state or character of variability), often where the transi- tion time to the new regime is short relative to the duration of the regime.” In late 2008 a report of the U.S. Climate Change Science Program (USCCSP) was dedi- cated to the topic of abrupt climate change. The Synthesis and Assessment Product 3.4: Abrupt Climate Change (USCCSP, 2008) defined abrupt climate change as: “A large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems.” This simple definition directly focuses attention on the impacts of change on natural and human systems and is important in that it directly combines the physical climate system with human impacts. As an increasingly interdisciplinary approach was taken to studying abrupt climate change, there was an accompanying evolution in think- ing, expanding from abrupt changes in the physical climate system to include abrupt impacts from climate change. More recently, Climate and Social Stress: Implications for Security Analysis (NRC, 2012b) examined the topic of climate change in the context of national security and briefly addressed the issue of abrupt climate change. They noted that events that did not meet the common criterion of a semi-permanent change in state could still force other systems into a permanent change, and thus qualify as an abrupt change. For example, a mega-drought may be followed by the return of normal precipitation rates, such that no baseline change occurred, but if that drought caused the collapse of a civilization, a permanent, abrupt change occurred in the system impacted by climate. The 2002 NRC study introduced the important issue of gradual climate change caus- ing abrupt responses in human or natural systems, noting “Abrupt impacts therefore have the potential to occur when gradual climatic changes push societies or ecosys- 25

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abrupt impacts of climate chang E tems across thresholds and lead to profound and potentially irreversible impacts.” The 2002 report also noted that “…the more rapid the forcing, the more likely it is that the resulting change will be abrupt on the time scale of human economies or global ecosystems” and “The major impacts of abrupt climate change are most likely to occur when economic or ecological systems cross important thresholds” (NRC, 2002). The 2012 NRC study embraced this issue more fully and expanded on the concept. The first part of their definition is straightforward: “Abrupt climate change is generally defined as occurring when some part of the climate system passes a threshold or tipping point resulting in a rapid change that produces a new state lasting decades or longer (Alley et al., 2003). In this case “rapid” refers to timelines of a few years to decades.” The second part of their definition echoes the 2002 report in emphasizing the role of abrupt responses to gradually changing forcing (emphasis added): “Abrupt climate change can occur on a regional, continental, hemispheric, or even global basis. Even a gradual forcing of a system with naturally occurring and chaotic vari- ability can cause some part of the system to cross a threshold, triggering an abrupt change. Therefore, it is likely that gradual or monotonic forcings increase the probability of an abrupt change occurring.” DEFINITION OF ABRUPT CLIMATE CHANGE FOR THIS REPORT The committee embraces the broader concept of abrupt climate change described in the 2002 NRC report and the definition from the 2012 Climate and Social Stress report, while expanding the scope of the definition further by considering abrupt climate impacts, as well as abrupt climate changes (Box 1.3). This distinction is critical, and represents a broadening of the focus from just the physical climate system itself to also encompass abrupt changes in the natural and human-built world that may be triggered by gradual changes in the physical climate system. Thus, the commit- tee begins by defining that, for this report, the term “abrupt climate change” as being abrupt changes in the physical climate system, and the related term, “abrupt climate impacts,” as being abrupt impacts resulting from climate change, even if the climate change itself is gradual (but reaches a threshold value that triggers an abrupt impact in a related system) This definition of abrupt climate change also helps to set a time frame for what kinds of phenomena are considered in this report. Environmental changes occurring over timescales exceeding 100 years are not frequently considered in decision-making by the general public, private sector, or the government. For some, projected changes oc- 26

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Introduction BOX 1.3  DEFINITION OF ABRUPT CLIMATE CHANGE FOR THIS REPORT The subject of this report includes both the abrupt changes in the physical climate system (hereafter called “abrupt climate change”) and abrupt impacts in the physical, biological, or hu- man systems triggered by a gradually changing climate (hereafter called “abrupt climate impacts” . These abrupt changes can affect natural or human systems, or both. The primary timescale of concern is years to decades. A key characteristic of these changes is that they can unfold faster than expected, planned for, or budgeted for, forcing a reactive, rather than proactive mode of behavior. These changes can propagate systemically, rapidly affecting multiple interconnected areas of concern. curring over less than 100 years begin to raise questions related to inter-generational equity and can be viewed as a relevant time frame for certain policy settings. Changes occurring over a few decades, i.e., a generation or two, begin to capture the interest of most people because it is a time frame that is considered in many personal decisions and relates to personal memories. Also, at this time scale, changes and impacts can oc- cur faster than the expected, stable lifetime of systems about which society cares. For example, the sizing of a new air conditioning system may not take into consideration the potential that climate change could make the system inadequate and unusable before the end of its useful lifetime (often 30 years or more). The same concept applies to other infrastructure, such as airport runways, subway systems, and rail lines. Thus, even if a change is occurring over several decades, and therefore might not at first glance seem “abrupt,” if that change affects systems that are expected to function for an even longer period of time, the impact can indeed be abrupt when a threshold is crossed. “Abrupt” then, is relative to our “expectations,” which for the most part come from a simple linear extrapolation of recent history, and “expectations” invoke notions of risk and uncertainty. In such cases, it is the cost associated with unfulfilled expecta- tions that motivates discussion of abrupt change. Finally, changes occurring over one to a few years are abrupt, and for most people, would also be alarming if sufficiently large and impactful. In this report, the committee adopts the time frame for “abrupt” climate changes as years to decades. The committee chose to focus their discussions of abrupt climate changes to those relevant to human society, including changes in the physical climate itself, and re- sulting changes to human expectations. Given our reliance on natural systems for ecosystem services, impacts to natural systems are of great concern to society as well. This consideration of unexpected impacts to societies and ecosystems broadens the discussion beyond the physics and chemistry of the climate system to include effects on humans and biota on local, regional, national, and international scales occurring 27

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abrupt impacts of climate chang E over years to decades. This is a broad definition that could easily encompass too many topics to cover in one report, and in this report, the committee has attempted to steer clear of the temptation to craft a laundry list of topics. As the climate science com- munity is in the early stages of examining many potential socioeconomic impacts, the discussion is thus necessarily limited in this report to those impacts for which there is good reason to suspect they are both abrupt and could actually occur. There is a nascent but rapidly growing literature on the theory behind how abrupt transitions occur (Box 1.4). This research is also beginning to tackle the even harder question of how to anticipate abrupt transitions, across many disciplines and systems, a topic that this report returns to in more depth in Chapter 4. BOX 1.4  MECHANISMS OF ABRUPT CHANGE Shocks or sudden events in the environment have often been classified into categories based on duration: (1) large temporary disturbances (e.g., earthquakes, hurricanes, tsunamis); and (2) shifts in long-term behavior (e.g., El Niño events, glacial cycles) (Lenton, 2013). However, both of these categories are really different aspects of the same fundamental phenomenon, a change in the system dynamics from the “normal” behavior. Although much is unknown about the mechanisms that can result in abrupt changes, some examples where there has been progress include positive feedbacks and bifurcations. Positive feedbacks occur when the system’s own dynamics enhance the effect of a perturbation, lead- ing to an instability. If these positive feedbacks are not controlled via damping mechanisms or negative feedbacks, the system can pass through a “tipping point” into a new domain (Scheffer et al., 2012a). Bifurcations occur when changes in a parameter of the system result in qualitatively different behavior (e.g., stable points become unstable, one stable point becomes multiple stable points). The presence of bifurcations can easily result in abrupt changes. For instance, random fluctuations from within a system (stochastic endogenous fluctuations) can cause the system to depart from an equilibrium or quasi-equilibrium state (e.g., fast, weather time-scale phenomena forcing changes on the longer time-scale climate). Rate-dependent shifts can also occur: a rapid change to an input or parameter of the system may cause it to fail to track changes, and thus tip (Ashwin et al., 2012). More generally, however, a key characteristic required for abrupt changes to occur is the property of state dependence (aka nonlinearity or nonseperability), where the dynamics (i.e., behavior) of the system are dependent on the system’s current state, which may also include its history (time-lagged manifolds). Generically, it is not correct to study these systems using linear methods or by examining variables in isolation (Sugihara et. al. 2012). Overall, research on abrupt changes and tipping points is moving from examining simple systems to investigations of highly connected networks (Scheffer et al., 2012a). The literature on downstream consequences of climate change has not arrived at a clear, common framework analytically as is the case for the physical aspects. 28

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Introduction HISTORICAL PERSPECTIVE—PREVIOUS REPORTS ON ABRUPT CHANGE Here the committee summarizes several previous reports on the topic of abrupt climate change and their recommendations, with the purpose of placing the present study and its recommendations within the context of this previous work. It is particu- larly instructive to report where progress has been made, and where previous recom- mendations continue to be echoed but not acted upon. 2002 NRC Report on Abrupt Climate Change As mentioned above, the first NRC report to comprehensively address abrupt climate change was entitled Abrupt Climate Change: Inevitable Surprises (NRC, 2002). This study remains one of the most comprehensive investigations of abrupt climate change to date, addressing the evidence, the potential causes, the potential for the current greenhouse-gas-induced warming to trigger abrupt change, and the potential im- pacts, ranging from economic to ecological to hydrological to agricultural. One of their key findings was captured in the title of the report, and is summarized by the follow- ing quotation: “Abrupt climate changes were especially common when the climate system was being forced to change most rapidly. Thus, greenhouse warming and other human al- terations of the Earth system may increase the possibility of large, abrupt, and unwel- come regional or global climatic events. The abrupt changes of the past are not fully explained yet, and climate models typically underestimate the size, speed, and extent of those changes. Hence, future abrupt changes cannot be predicted with confidence, and climate surprises are to be expected.” The report made five recommendations in two general categories: implementation of targeted areas of research to expand observations of the present and the past, and implementation of focused modeling efforts. The first recommendation called for research programs to “collect data to improve understanding of thresholds and non-linearities in geophysical, ecological, and eco- nomic systems.” The report particularly called out for more work on modes of coupled atmosphere-ocean behavior, oceanic deep-water processes, hydrology, and ice. In the intervening decade, progress has been made in some of these areas. Since 2004 the ocean’s meridional overturning circulation has been monitored at 26°N in the North Atlantic (Cunningham et al., 2007). Progress has been made in ice sheet observations from satellites (e.g., Pritchard et al., 2010; Joughin et al., 2010) and in a better under- standing of modes of ocean-atmosphere behavior. Nonetheless, as detailed in this 29

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abrupt impacts of climate chang E report, additional improvements in monitoring for abrupt climate change could be undertaken. For example, the interface between ocean and ice sheet is known to be critical part of ice sheet functioning, yet there are few observations, and no systematic monitoring of the changing conditions at this interface. Also, satellite observations of ice sheets are tenuous as satellites age and funding to replace them, let alone expand their capabilities, is uncertain. The 2002 report also called for economic and ecological research, a comprehensive land-use census, and development of integrated economic and ecological data sets. Again, some improvements have been made in these areas, notably the National Ecological Observing Network (NEON) to monitor key ecosystem variables in the United States. Other areas, such as a comprehensive land-use census, remain largely unaddressed. In its second recommendation, the 2002 report called for new, interdisciplinary model- ing efforts that would bring together the physical climate system with ecosystems and human systems in an effort to better predict the impacts of abrupt climate change on humans and natural systems, and to better understand the potential for abrupt climate change during warm climate regimes. During the last decade, considerable improvements have been made in many aspects of coupled climate models. Although biases remain, simulation quality has improved for the models in the Coupled Model Intercomparison Project 5 (CMIP5) compared to earlier models (e.g., Knutti et al., 2013). In addition, many climate models have transitioned to include Earth system model- ing capabilities (e.g., Hurrell et al., 2013; Collins et al., 2012) in that they incorporate biogeochemical cycles and/or other aspects beyond the standard physical climate model components (atmosphere, ocean, land, sea ice). These new capabilities allow for prognostic simulation of the carbon cycle and the assessment of biogeochemical feedbacks (e.g., Long et al., 2013). In some cases, models now also include the ability to simulate atmospheric chemistry (e.g., Lamarque et al., 2013; Shindell et al., 2013) and large ice sheets (e.g., Lipscomb et al., 2013 ). This has resulted in more complete system interactions within the model and the ability to investigate additional feedbacks and climate-relevant processes. The inclusion of isotopes into some models (e.g., Sturm et al., 2010; Tindall et al., 2010) is also allowing for a more direct comparison with paleo- climate proxies of relevance to past abrupt change, and a more comprehensive evalu- ation of the sources and sinks of the atmospheric water cycle that is critical in assess- ing the risk of future abrupt change and its impacts (e.g., Risi et al., 2010). Efforts are also underway to more directly link human system interactions into Earth system models. This includes the incorporation of new elements such as agricultural crops (Levis et al., 2012) and urban components (Oleson, 2012). It also includes new efforts to link Integrated Assessment Models to Earth System models (e.g., van Vuuren 30

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Introduction et al., 2012; Schneider, 1997 Goodess et al., 2003; Bouwman et al., 2006; Warren et al., 2008; Sokolov et al., 2005). Enhancements in model resolution are also enabling the simulation of high-impact weather events of societal relevance (such as tropical cy- clones) within climate models (e.g., Jung et al., 2012; Zhao et al., 2009; Bacmeister et al., 2013; Manganello et al., 2012). However, computational resources, while increasing, still remain an obstacle for climate-scale high-resolution simulations. Additionally, model parameterizations and processes need to be reconsidered for simulations at these scales, a task that remains an active research area. The third recommendation of the 2002 report called for more and better observa- tional data on how our planet and climate system have behaved in the past, with a focus on the high temporal-resolution paleoclimate records required to assess abrupt climate changes. The past decade has witnessed a number of advances in this area, no- tably terrestrial records from temperate latitudes from cave deposits (e.g., Wang et al., 2008, and references therein), more and better resolved records from ocean sediments (NRC, 2011b), and expanded reconstructions of regional scale hydrological data—in- cluding mega-droughts—and changes to the monsoons from records including tree rings and pollen (e.g., Cook et al., 2010c). However, although scientists clearly have an improved understanding of past abrupt climate changes today compared with a de- cade ago, in many cases the data still remain too sparse spatially to test mechanisms of change using models. Multi-proxy data sets, in which a number of aspects of the climatic and environmental systems are simultaneously reconstructed, remain sparse as well. The fourth recommendation of the 2002 report focused on improving incorporation of low-probability but high-impact events into societal thinking about climate change. The tendency is to assume a simple distribution of outcomes, and focus on the most probable ones. This approach underestimates the likelihood of extreme events, even ones that would have high impact. If one views risk as the product of likelihood and consequence, then highly consequential, “extreme” events, even if they are unlikely, may pose an equal risk to common events that are not as consequential. The damages resulting from recent extreme weather events (e.g., Hurricane Katrina, Superstorm Sandy, etc.) suggest that there is still a need to better plan for low-probability, high- consequence events, regardless of whether or not their cause is statistically rooted in observed climate trends. That most model predictions for future climate change include more frequent extreme events only heightens the need to take this recom- mendation seriously. The fifth and final recommendation of the 2002 report dealt with “no regrets” mea- sures and their application to the potential for abrupt climate change. The report called for taking low-cost steps such as slowing climate change, improving climate 31

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abrupt impacts of climate chang E forecasts, slowing biodiversity loss, and developing new technologies to increase the adaptability and resiliency of markets, ecological systems, and infrastructure. While there has been some progress in this regard over the past decade, progress has been slow, and remains inadequate to match the scope and scale of the problem. The sci- entific community has worked to improve climate models, for example, but little has been done to limit greenhouse gas emissions. In fact, the rate of greenhouse gas addi- tion to the atmosphere continues to increase, with many policies in place to accelerate rising greenhouse gases (IMF, 2013). It is sobering to consider that about one-fifth of all fossil fuels ever burned were burned since the 2002 report was released.2 If indeed, as the 2002 report states, “… greenhouse warming and other human alterations of the Earth system may increase the possibility of large, abrupt, and unwelcome regional or global climatic events” then the danger that existed in 2002 is even higher now, a , decade later. 2007 IPCC Fourth Assessment Report The next major report on climate change following the 2002 report was the 2007 Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC, 2007c).3 The AR4 did not specifically call out abrupt climate change and address it separately, but abrupt climate change was discussed in both the physical science context and in the context of mitigation and adaptation. The Working Group I report, the Physical Science Basis (IPCC, 2007b), acknowledged that our understanding of abrupt climate change was notably incomplete and that this limited the ability to model abrupt change, stating that “Mechanisms of onset and evolution of past abrupt climate change and associated climate thresholds are not well understood. This limits confidence in the ability of climate models to simulate realistic abrupt change.” How- ever, the Working Group I report did specifically address the issue of a shutdown of the formation of North Atlantic Deep Water and concluded from modeling studies that although it was very likely (>90 percent chance) that the deep water formation would slow in the coming century, it was very unlikely (<10 percent chance) that this process would undergo a large abrupt transition, at least in the coming decades to century. This was an important advancement in the understanding of the potential threats of 2  Sum of global emissions from 1751 through 2009 inclusive is 355,676 million metric tons of carbon; sum of global emissions from 2002 through 2009 inclusive is 64,788 million metric tons of carbon (Boden et al., 2011). Total carbon emissions for 2002-2009 compared to the total 1751-2009 is thus greater than 18%. 3  The Working Group I report of the Fifth Assessment Report (AR5) of the IPCC was released after this report had been submitted for peer-review. The Committee drew their conclusions from the broader sci- entific literature, which is also the basis for IPCC AR5. Although this report only references the IPCC AR5 in a few instances, the broader conclusions of this report are consistent with the IPCC AR5. 32

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Introduction abrupt climate change, and an example of a threat that has been categorized as less likely due to improved understanding of the process. The AR4 Working Group 2 (WG2) report, Impacts, Adaptation and Vulnerability (IPCC, 2007a), addresses abrupt climate change throughout the report, and summarizes the impacts of extreme events and key vulnerabilities including topics such as coastal inundation, food supply disruption, and drought. The AR4 WG2 report repeatedly calls for more research to be done on the impacts of abrupt change, particularly a collapse of the North Atlantic Deep Water formation (which was not considered likely) and a relatively rapid sea level rise of many meters due to rapid (century-scale) loss of ice from Greenland and/or West Antarctica, noting that without a better scientific under- standing of the potential impacts, it was impossible to carry out impact assessments. That report also notes that there has been “little advance” on the topic of “proximity to thresholds and tipping points.” The AR4 Working Group 3 report on Mitigation of Climate Change (Metz et al., 2007) mentions abrupt climate change, but does not consider the topic in detail. It acknowl- edges that abrupt climate changes are not well incorporated into conventional deci- sion-making analysis, which tends to enable substantial vulnerability to high-impact, low-probability events. This potentially increases the damages from any such events that could occur—and perhaps even the probability of such events—through lack of mitigation and adaptation. Similarly, abrupt climate change can challenge assump- tions made in economic cost-benefit analyses, for example the cost of a lost species versus the savings realized in not acting to save that species. 2008 USCCSP Synthesis and Assessment Product 3.4: Abrupt Climate Change The next major report to address abrupt climate change was the 2008 United States Climate Change Strategic Plan Synthesis and Assessment Product 3.4, Abrupt Climate Change report (USCCSP, 2008). This report (also known as SAP 3.4) was focused solely on abrupt climate change, but took a different approach from the 2002 NRC report by focusing on four key areas of interest: 1. Rapid Changes in Glaciers and Ice Sheets and their Impacts on Sea Level; 2. Hydrological Variability and Change; 3. Potential for Abrupt Change in the Atlantic Meridional Overturning Circulation (AMOC); and 4. Potential for Abrupt Changes in Atmospheric Methane. 33

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abrupt impacts of climate chang E As stated in their introduction, “This SAP picks up where the NRC report and the IPCC AR4 leave off, updating the state and strength of existing knowledge, both from the paleoclimate and historical records, as well as from model predictions for future change.” Their findings are woven into the present report, but are too extensive to repeat in this Introduction. A few key findings are discussed briefly, however. “Although no ice-sheet model is currently capable of capturing the glacier speedups in Antarctica or Greenland that have been observed over the last decade, including these processes in models will very likely show that IPCC AR4 projected sea level rises for the end of the 21st century are too low.” This finding re-states the caveat expressed in the AR4 concerning the lack of understanding about glacial dynamics, particularly fast-flowing, large glaciers such as parts of Greenland and West Antarctica. As detailed in the present report, the scientific community has not yet formed a consensus regard- ing the rate with which large glaciers can shed ice, and thus uncertainty remains about the speed and eventual magnitude of sea level rise, both over this coming century, and beyond. The SAP 3.4 raised two questions concerning tipping points in droughts. The first is the model predicted expansion of aridity into the U.S. Southwest accompanying the general warming of the ocean and atmosphere. As they state, “If the model results are correct, then this drying may have already begun, but currently cannot be definitively identified amidst the considerable natural variability of hydroclimate in Southwestern North America.” This remains a key area of concern, and one that is addressed in this report. The SAP 3.4 also raised the issue of monitoring for tipping point behavior in the hydrological cycle (Chapter 4 of that report), including the potential for mega- droughts in a world warmed by greenhouse gases. Physical understanding suggests that mega-droughts are more likely to be triggered by interior reorganization of the ocean-atmosphere system rather than by overall warming of Earth’s surface, although overall warming can cause interior reorganization and thus can be responsible in- directly. The SAP 3.4 report states that it is unclear whether current climate models are capable of predicting the onset of mega-droughts: “… systematic biases within current coupled atmosphere-ocean models raise concerns as to whether they cor- rectly represent the response of the tropical climate system to radiative forcing and whether greenhouse forcing will actually induce El Niño/Southern Oscillation-like patterns of tropical SST change that will create impacts on global hydroclimate…” . Research done since SAP 3.4 suggests that the drying from human-caused climate change (radiatively forced reduction of the net surface water flux, i.e., the precipitation minus evapotranspiration) appears to be comparable to the drying induced by the impacts of La Nina over the Southwestern North America since 1979 (Seager and Naik, 2012). In the future, drying forced by the addition of anthropogenic greenhouse gases 34

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Introduction to the atmosphere is expected to increase along with earlier melting and reduced storage of mountain snow packs, although whether changes of climate variability would intensify or mitigate such drying remains uncertain (Seager and Vecchi, 2010). In addition, as increasing anthropogenic forcing shifts the surface temperature distri- bution (Trenberth et al., 2007; Meehl et al., 2007b), extreme warm temperatures and soil moisture loss would increase. Thus, the “climate dice” mainly controlled by random , climate variability, would become more “loaded” with the risk of mega-drought even if a particular drought is simply the result of natural climate variability (Hansen et al., 2012, 2013a). The SAP 3.4 also addressed the potential for tipping points in the North Atlantic Deep Water formation; and in the release of methane, a potent greenhouse gas, to the atmosphere. As with the IPCC AR4 report, the SAP 3.4 report concluded that deep- water formation was not likely to “tip,” although it is likely to decrease, with impacts on precipitation patterns that could be tipped on regional scales. The potential for catastrophic methane release, from decomposition of terrestrial carbon stocks in permafrost, or methane ice in clathrates, was considered small. However, the potential for gradually increasing methane and CO2 release from thawing permafrost was con- sidered important, and would accelerate the loading of these greenhouse gases into the atmosphere over many decades to centuries. The report recommended that the United States should “Prioritize the monitoring of atmospheric methane abundance and its isotopic composition with spatial density sufficient to allow detection of any change in net emissions from northern and tropical wetland regions.” Such a prioriti- zation has not occurred; in fact, the primary monitoring network for greenhouse gases globally, the NOAA network,4 has faced funding cuts of over 30 percent in the past several years. 2012 NRC Report on Climate and Social Stress The NRC report Climate and Social Stress: Implications for Security Analysis (2012b) is the most recent report to address abrupt climate change. It dedicates a section to a general discussion of abrupt climate change, with an additional section allocated to the topic of extreme events. The report focuses on the coming decade, and as such they conclude that there is little expectation in the scientific community for an abrupt change on that timescale. It makes several recommendations including enhanced monitoring, such as enhanced drought metrics to assess if a region is entering a new mega-drought. These include social factors as well, for example: 4  http://www.esrl.noaa.gov/gmd/ccgg/flask.html. 35

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abrupt impacts of climate chang E “changes in the social, economic, and political factors that affect the size of the ex- posed populations, their susceptibility to harm, the ability of the populations to cope, and the ability of their governments to respond. Where potentially affected areas are important producers of key global commodities such as food grains, it would also be important to assess the effects of climate-induced supply reductions on global mar- kets and vulnerable populations.” The NRC 2012 report also called for enhanced monitoring of such factors, and noted that society is, in general, rather blind to what is at risk to abrupt climate change, for example, having only limited understanding about the risks posed by sea level rise to coastal infrastructure, toxic materials in landfills, or drinking water aquifers. THIS REPORT Looking back across these previous reports, it can be seen that while a great deal of progress on the topic of abrupt climate change has been made, there is still a long way to go to achieve an understanding of these issues with enough fidelity to be able to anticipate their occurrence. This report takes on that challenge, as per the committee’s statement of task, given in Box 1.5. Organization of the Report The committee recognized that discussions of abrupt climate changes and impacts of abrupt climate changes may have different audiences. As such, the report is orga- nized so that one can seek information on the processes as well as information on the impacts. Chapter 2 gives examples of abrupt climate changes, specifically those examples that the committee believes are worthy of highlighting either because they are currently believed to be the most likely and the most impactful, because they are predicted to potentially cause severe impacts but with uncertain likelihood, or because they are now considered to be unlikely to occur but have been widely discussed in the litera- ture or media. This section includes processes such as the changing chemistry of the oceans and the melting of ice sheets leading to sea level rise. Many of these processes have been discussed in the recent reports (Box 1.2), and the committee provides an updated discussion building on those previous reports. Chapter 3 discusses abrupt climate impacts from the perspective of how they affect humans, building on many of the same processes discussed in Chapter 2. Examples include abrupt changes in food availability, water availability, and ecosystem services. 36

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Introduction BOX 1.5  STATEMENT OF TASK This study will address the likelihood of various physical components of the Earth system to undergo major and rapid changes (i.e., abrupt climate change) and, as time allows, examine some of the most important potential associated impacts and risks. This study will explore how to monitor climate change for warnings of abrupt changes and emerging impacts. The study will summarize the current state of scientific understanding on questions such as: 1.  What is known about the likelihood and timing of abrupt changes in the climate sys- tem over decadal timescales? Are any of the phenomena considered by the committee currently embodied in computational climate models? The committee could consider relevant physical and biological phenomena such as: • large, abrupt changes in ocean circulation and regional climate; • reduced ice in the Arctic Ocean and permafrost regions; • large-scale clathrate release; • changes in ice sheets; • large, rapid global sea-level rise; • growing frequency and length of heat waves and droughts; • effects on biological systems of permafrost/ground thawing (carbon cycle effects); •  phase changes such as cloud formation processes; and •  hanges in weather patterns, such as changes in snowpack, increased frequency and c magnitude of heavy rainfall events and floods, or changes in monsoon patterns and modes of interannual or decadal variability. 2.  the abrupt climate changes and resulting impacts identified by the committee, what For are the prospects for developing an early warning system and at what lead time scales? What can be monitored to provide such warnings? What monitoring capabilities are already in place? The committee will consider monitoring capabilities that include both direct observations and the use of models in conjunction with observations. 3.  What are the gaps in our scientific understanding and current monitoring capabilities? What are the highest priority needs for future research directions and monitoring capa- bilities to fill those gaps? Chapter 4 examines the way forward in terms of both research on abrupt changes and their impacts, and monitoring to detect and potentially predict abrupt changes. This chapter examines priorities and capabilities for addressing research knowledge gaps. It also addresses the question of what to monitor to observe that an abrupt change is coming, and how to identify tipping points in various systems. 37

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