CHAPTER TWO


Abrupt Changes of Primary Concern

The following section describes potential abrupt climate changes that are of primary concern, 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 considered to be unlikely to occur but have been widely discussed in the literature or media. As such, the Committee did not attempt to create a comprehensive catalog of potential abrupt changes. As described in the Introduction, this section examines both abrupt climate changes in the physical climate system itself and abrupt climate impacts in physical, biological, or human systems that are triggered by a steadily changing climate.

ABRUPT CHANGES IN THE OCEAN

The Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC)—characterized by warm surface waters flowing northward and cold deep waters flowing southward throughout the Atlantic basin—is defined as the zonal integral of the northward mass flux at a particular latitude. The deep limb of this overturning circulation carries waters that are formed via convection in the Nordic and Labrador Seas (Figure 2.1). Collectively, these waters constitute North Atlantic Deep Water, which is exported to the global ocean at depths between about 1000 and 4000 m. The southward-flowing deep limb of the overturning circulation is compensated by an upper limb of northward-flowing surface waters, which head to the Nordic and Labrador Seas to replenish the regions of convection. Together, the upper and lower limbs of the overturning circulation produce a poleward flux of heat that has strong global and regional impacts. The AMOC also plays an important role in the transport of carbon in the Atlantic. Thus, variability in the AMOC’s strength is of much interest, as a diminishment or strengthening would impact the ocean’s effectiveness as a heat and carbon reservoir.

Examinations of paleoclimate temperatures and other variables recorded in both North Atlantic ocean sediments and Greenland ice cores (e.g., Lehman and Keigwin, 1992; Alley et al., 1993; Taylor et al., 1993) have led to suggestions that the AMOC



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CHAPTER TWO Abrupt Changes of Primary Concern T he following section describes potential abrupt climate changes that are of primary concern, 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 considered to be unlikely to occur but have been widely discussed in the literature or media. As such, the Commit- tee did not attempt to create a comprehensive catalog of potential abrupt changes. As described in the Introduction, this section examines both abrupt climate changes in the physical climate system itself and abrupt climate impacts in physical, biological, or human systems that are triggered by a steadily changing climate. Abrupt Changes in the Ocean The Atlantic Meridional Overturning Circulation The Atlantic Meridional Overturning Circulation (AMOC)—characterized by warm surface waters flowing northward and cold deep waters flowing southward through- out the Atlantic basin—is defined as the zonal integral of the northward mass flux at a particular latitude. The deep limb of this overturning circulation carries waters that are formed via convection in the Nordic and Labrador Seas (Figure 2.1). Collectively, these waters constitute North Atlantic Deep Water, which is exported to the global ocean at depths between about 1000 and 4000 m. The southward-flowing deep limb of the overturning circulation is compensated by an upper limb of northward-flowing surface waters, which head to the Nordic and Labrador Seas to replenish the regions of convection. Together, the upper and lower limbs of the overturning circulation pro- duce a poleward flux of heat that has strong global and regional impacts. The AMOC also plays an important role in the transport of carbon in the Atlantic. Thus, variability in the AMOC’s strength is of much interest, as a diminishment or strengthening would impact the ocean’s effectiveness as a heat and carbon reservoir. Examinations of paleoclimate temperatures and other variables recorded in both North Atlantic ocean sediments and Greenland ice cores (e.g., Lehman and Keigwin, 1992; Alley et al., 1993; Taylor et al., 1993) have led to suggestions that the AMOC 39

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abrupt impacts of climate chang E FIGURE 2.1 Schematic of the major warm (red to yellow) and cold (blue to purple) water pathways in the North Atlantic subpolar gyre. Acronyms not in the text: Denmark Strait (DS); Faroe Bank Channel (FBC); East and West Greenland Currents (EGC, WGC); North Atlantic Current (NAC); DSO (Denmark Straits Over- flow); ISO (Iceland-Scotland Overflow). Figure courtesy of H. Furey (WHOI). abruptly changed in the past. Following on this examination, questions have arisen as to the possible likelihood of an abrupt change in the future. The Stability of the Atlantic Meridional Overturning Circulation Climate and Earth system models are used to understand potential changes in the AMOC, including potential feedbacks in the system, although the representation of unresolved physics (such as the parameterization of ocean mixing) could potentially be of concern in long, centennial simulations. Because saltier water is denser and thus more likely to sink, the transport of salt poleward into the North Atlantic provides a potentially destabilizing advective feedback to the AMOC (Stommel, 1961); i.e., a reduction in the strength of the AMOC would lead to less salt being transported into the North Atlantic, and hence a further reduction in the AMOC would ensue. As noted 40

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Abrupt Changes of Primary Concern by Rahmstorf (1996), the presence of this slow salt-advection feedback is critical to the existence of stable multiple equilibria. Climate and Earth system models have been used to investigate the stability of the AMOC, in particular the number of stable states that the system can exist in, which is an important characteristic to know for fully understanding the climate system. Care- fully designed non-linear modeling experiments using Earth system Models of Inter- mediate Complexity (EMICs; and also the FAMOUS AOGCM; Hawkins et al., 2011) have revealed a model-dependent threshold beyond which an active AMOC cannot be sustained (Rahmstorf et al., 2005; see Figure 2.2). However, analysis of the AMOC in the models that submitted simulations in support of the third phase of the Community Model Intercomparison Project1 (CMIP3; Meehl et al., 2007a) suggested that the CMIP3 models were overly stable (Drijfhout et al., 2011; Hofmann and Rahmstorf, 2009), i.e., that an abrupt change in the AMOC was not likely to be simulated in the models even if it were to be likely in reality. Several studies (de Vries and Weber, 2005; Dijkstra, 2007; Weber et al., 2007; Huisman et al., 2010; Drijfhout et al., 2011; and Hawkins et al., 2011) have suggested that the sign of the net freshwater flux into the Atlantic across its southern boundary via the overturning circulation determines whether or not the AMOC is in a monostable or bistable regime. Observations suggest that the present day ocean resides in a bistable regime, thereby allowing for multiple equilibria and a stable “off” state of the AMOC (Hawkins et al., 2011). By examining the preindustrial control climate of the CMIP3 models, Drijfhout et al. (2011) found that the salt flux was mostly negative (implying a positive freshwater flux), indicating that these models were mostly in a monostable regime. This was not the case in the CMIP5 models where Weaver et al. (2012) found that 40 percent of the models were in a bistable regime throughout their integrations. Although this question of the number of stable states of the system is important for a complete understanding of the climate system, it is important to emphasize that regardless of this stability question, the CMIP5 models also show no evidence of an abrupt collapse for the 21st century. In addition to the main threshold for a complete breakdown of the circulation, other thresholds may exist that involve more-limited changes, such as a cessation or dimin- ishment of Labrador Sea deep water formation (Wood et al., 1999). Rapid melting of the Greenland ice sheet causes increases in freshwater runoff, potentially weakening the AMOC. None of the CMIP5 simulations include an interactive ice sheet compo- nent. However, Jungclaus et al. (2006), with parameterized freshwater melt as high 1  http://www-pcmdi.llnl.gov/ipcc/about_ipcc.php. 41

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abrupt impacts of climate chang E FIGURE 2.2  Schematic diagram illustrating the hysteresis behavior of the equilibrium strength of the AMOC in response to the addition of a North Atlantic surface freshwater perturbation of variable magni- tude. Positive values indicate the sustained addition of freshwater to the surface; negative values indicate the sustained subtraction of freshwater from the surface; the zero value corresponds to the present-day situation. The two upper heavy branches indicate the possibility of multiple states with different convec- tion sites. Transitions between stable equilibria of the AMOC with and without active deepwater forma- tion are indicated by: (a) transition associated with slow advective instability, (b) transition associated with fast convective instability, and (d) initiation of convection and subsequent spin-up of North Atlantic Deep Water (NADW) formation. The S indicates the point beyond which a stable equilibrium with active NADW formation cannot exist. (c) indicates a possible transition between active modes of NADW formation with different location of convection. Note: Hysteresis is defined as “a lag in response exhibited by a body in reacting to changes in forces” (Random House Kernerman Webster’s College Dictionary) and is used in many fields such as engineer- ing, economics, biology, etc. to refer to a system that depends on the current but also past environmental conditions. SOURCE: Rahmstorf, 1999. 42

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Abrupt Changes of Primary Concern as 0.09 Sv, and Hu et al. (2009), using the NCAR Community Climate System Model (CCSM) with year 2000 initial parameterized freshwater melt of 0.01 Sv increasing at a rate of 1 percent/year, 3 percent/year and 7 percent/year, as well as Mikolajewicz et al. (2007) and Driesschaert et al. (2007), using coupled ice-sheet/climate models, found only a slight temporary effect of increased melt water fluxes on the AMOC. The impact of these fluxes on the AMOC was generally small compared to the effect of enhanced poleward atmospheric moisture transport and ocean surface warming; or it was only noticeable in the most extreme scenarios. But this point needs to be further quantified. While many more model simulations were conducted in support of the IPCC AR5 (Collins et al., 2012) under a wide range of forcing scenarios, projections of the behav- ior of the AMOC over the 21st century and beyond have changed little from what was reported in the IPCC AR4 (Meehl et al., 2007b). In the case of the CMIP5 models, Weaver et al. (2012) showed that the behavior of the AMOC was similar over the 21st century under four very different radiative forcing scenarios (RCP 2.6; RCP4.5; RCP 6.0; RCP8.5— these Representative Concentration Pathways [RCPs] are detailed in Moss et al., 2010). All models found a 21st century weakening of the AMOC with a multi-model average of 22 percent for RCP2.6, 26 percent for RCP4.5, 29 percent for RCP6.0 and 40 percent for RCP8.5. While two of the models eventually realized a slow shutdown of the AMOC under RCP8.5 (the scenario with the largest amount of warming), none exhibited an abrupt change of the AMOC. The similarity of the model responses despite the widely varying transports of salt into the North Atlantic across its southern boundary (and hence sign and magnitude of the salt advection feedback) suggests that like the CMIP3 models (Gregory et al., 2005), the reduction of the AMOC in the global warming experiments performed by the CMIP5 models is mainly driven by local changes in surface thermal flux rather than surface freshwater flux. North Atlantic surface warming decreases water density there, thus reducing the rate of sinking. In addition, as noted above, none of the CMIP models incorporated the additional freshwater effects of ice sheet melting. This is an important caveat since asymmetric freshwater forcing is capable of initiating a fast, convective instability that could cause the AMOC to abruptly shut down if it were in a bistable regime and suitably close to its stability threshold. This would explain why abrupt changes of the AMOC appear to be pervasive features of the paleoclimate re- cord when vast reservoirs of freshwater were available in the form of ice and proglacial lakes on land. A question that needs to be further addressed is the extent to which projected changes in Greenland ice sheet melting could affect the amount and location of 43

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abrupt impacts of climate chang E freshwater release into the North Atlantic and hence the subsequent evolution of the AMOC. As noted in Meehl et al. (2007b) it is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century. Delworth et al. (2008) pointed out that for an abrupt transition of the AMOC to occur, the sensitivity of the AMOC to forcing would have to be far greater than that seen in current models. Alternatively, significant ablation of the Greenland ice sheet greatly exceeding even the most ag- gressive of current projections would be required. As noted in the ice sheet section later in this chapter, Greenland ice has about 7.3m equivalent of sea level rise, which, if melted over 1000 years, yields an annual rise rate of 7 mm/yr, about 2 times faster just from Greenland than today’s rate from all sources, and more than 10 times faster than the rate from Greenland over 2000–2011 (Shepherd et al., 2012). Although neither pos- sibility can be excluded entirely, it is unlikely that the AMOC will collapse before the end of the 21st century because of global warming. Observations of the Atlantic Meridional Overturning Circulation Recent observational studies have focused on ascertaining two questions of relevance to the AMOC response to climate change: What is the impact of variable North Atlan- tic Deep Water production on the ocean’s meridional overturning? And, what is the current state of the AMOC and its variability? Studies relevant to both questions are briefly reviewed here (material drawn from Lozier, 2012). Though many modeling studies have demonstrated the impact of deep water forma- tion changes on the overturning circulation, the observational evidence for such a linkage has been hard to come by for two reasons: (1) Deep water formation is difficult to quantify because the time and locale of production are highly variable from winter to winter, and (2) overturning circulation measures require observations that span the basin, which have been limited in space and time. Because of this second difficulty, a measure of the Deep Western Boundary Current (DWBC) transport has traditionally been considered a shortcut to the measure of the AMOC: while the upper limb of the AMOC was considered inextricably linked to the much more energetic wind-driven circulation, the lower limb was considered to be “channeled” through the DWBC. An opportunity to assess the linkage between deep water formation variability and DWBC changes was afforded by the deployment of a moored array east of the Grand Banks (Clarke et al., 1998; Meinen et al., 2000; Schott et al., 2006). In an extensive analy- sis of the time series from these two deployments, Schott et al. (2006) found that the transport rates of Labrador Sea Water (LSW) over these two time periods were remark- ably similar despite the large differences in convective activity in the Labrador Sea 44

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Abrupt Changes of Primary Concern during the two time periods: the earlier time period was marked by strong convective activity, while LSW production was considerably weaker during the latter time period (Lazier et al., 2002). This result raised questions about the responsiveness of the AMOC to changes in deep water production; however, the linkage could not be conclusively ruled out because of increasing indications that the DWBC was not the sole conduit for the passage of deep waters to the lower latitudes (Schott et al., 2006). And in fact, recent observational (Lavender et al., 2000; Fischer and Schott, 2002; Bower et al., 2009) and modeling studies (Gary et al., 2011; Lozier et al., 2010) of subsurface floats have revealed that the DWBC is not the sole, and perhaps not even the dominant, conduit for the transport of the waters within the deep limb of the AMOC. Thus, a measure of the DWBC is no longer considered a sufficient monitor of AMOC changes. For a full accounting of the AMOC and its variability, it is now understood that trans- basin measurements of transport are necessary. Attempts to understand trans-basin AMOC variability over the modern observational record traditionally have had to rely on indirect estimates assessed from hydrography. Bryden et al. (2005) used five repeat surveys at 25°N from 1957 to 2004 to show that the overturning slowed by 30 percent over the period of the surveys, an astounding and unanticipated change over such a relatively short time. However, an assessment of transports at 48°N using five repeat World Ocean Circulation Experiment sections and air-sea heat and freshwater fluxes as input to an inverse box model yielded no significant trend in the meridional overturn- ing at that latitude (Lumpkin et al., 2008), though the time period studied was rela- tively short (1993-2000). In 2004 an observational system was put in place to provide the first continuous measure of the AMOC (Cunningham et al., 2007). The RAPID/MOCHA program (Rapid Climate Change/Meridional Overturning Circulation and Heatflux Array) comprises in- struments deployed along a section at 25°N stretching from the North American con- tinent to the west coast of Africa. After just one year of measurements, the conceptual understanding of overturning variability changed dramatically. As seen in Figure 2.3, the overturning strength changed six-fold from April of 2004 to April of 2005, from a minimum of ~5 Sv to a maximum of ~30 Sv. With the demonstrated intraseasonal vari- ability, synoptic sections were now understood to be inadequate to capture measures of interannual transport variability. The continuation of the time series has revealed a strong seasonality (Rayner et al., 2011) that dominates the record, as well as strong intrannual variability (McCarthy et al., 2012). Unfortunately, the strong intraseasonal variability of the AMOC revealed by the RAPID/MOCHA array seriously constrains our ability to recreate AMOC variability over the modern observational period, since synoptic hydrographic sections are the 45

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abrupt impacts of climate chang E FIGURE 2.3  Time series of the meridional overturning transport at 26° N from the RAPID/MOCHA array. The meridional overturning circulation (MOC) is computed from the sum of the Gulf Stream transport through the Florida Straits, directly measured via electromagnetic cables; the Ekman transport, estimated from QuikSCAT winds; and the midocean geostrophic transport, estimated from the moored array instru- ments. Importantly, this time series demonstrates the significant interannual transport variability. SOURCE: Rayner et al., 2011. only past trans-basin measurements. Furthermore, as detailed in a recent review by Cunningham and Marsh (2010), modeling estimates have been unable to help in this regard: there is currently no consensus on the strength of the AMOC in assimilation/ re-analysis products, and ocean general circulation models are in disagreement about the strength and variability of the AMOC. Indeed, an active area of research within the climate modeling community is focused on the cause for such wide ranges of AMOC estimates from state estimates that are drawing from the same observational data- bases (U.S. CLIVAR Project Office, 2011) and in ocean simulations forced with the same 46

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Abrupt Changes of Primary Concern atmospheric conditions (e.g., Danabasoglu et al., 2013). It is important to note that the models run for the IPCC discussed above also have varying AMOC strength and interannual variability, yet they are in agreement on a lack of abrupt change for the 21st century. In lieu of consistent model estimates, proxy measures of the overturning derived from satellite altimetry and Argo float data are appealing, but to date these measures have been limited to latitudes of steep topography on the western boundary (Willis, 2010), and are of limited duration to provide a temporal context of decades. Thus, to date direct AMOC observations are limited to one latitude (26°N), and past measures of change remain elusive. Although the RAPID array is providing unprecedented mea- surements, recent modeling and data analysis studies (Bingham et al., 2007; Baehr et al., 2009; Lozier et al., 2010; Biastoch et al., 2008a; Biastoch et al., 2008b) reveal gyre- specific measures of the AMOC, suggesting that the AMOC variability measured by the RAPID array cannot safely be assumed representative of AMOC variability outside of the North Atlantic subtropical basin. Summary and the Way Forward Although models do not indicate that AMOC is likely to change abruptly in the com- ing decades, it is important to monitor the North Atlantic to confirm the understand- ing of how AMOC responds to a changing climate. Observational studies over the past decade or so reveal a meridional overturning circulation with a tenuous link to the production of deep water masses via local overturning at high latitudes in the North Atlantic. However, the deep ocean remains vastly undersampled, particularly so with respect to measures appropriate for the calculation of AMOC variability. To ascertain with confidence the extent to which deep water production impacts the ocean’s meridional circulation and hence the ocean’s contributions to the global poleward heat flux, continuous measures of trans-basin mass and heat transports are needed. Although such measurements are underway with the RAPID/MOCHA array, the stud- ies cited above have made it increasingly clear that AMOC fluctuations are coherent over only limited meridional distances: break points in coherence occur at key lati- tudes, in particular at the subpolar/subtropical gyre boundary in the North Atlantic. Therefore, a transoceanic line in the subpolar North Atlantic, currently being planned by the international community, that measures the net contributions of the overflow waters from the Nordic Seas as well as those from the Labrador Sea, to the AMOC, would directly test the legitimacy of the decades-long supposition that variability in North Atlantic Deep Water production translates into meridional overturning vari- ability (Figure 2.4). This measurement system would—in conjunction with the RAPID/ 47

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abrupt impacts of climate chang E Figure 2.4 Existing and proposed monitoring locations for the Atlantic Ocean. Source: Adapted from Schiermeier, 2013. 48

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Abrupt Changes of Primary Concern MOCHA array—provide a means to evaluate intergyre connectivity within the North Atlantic and allow for a determination of how and whether deep water mass forma- tion impacts overturning and poleward heat and freshwater transports throughout the North Atlantic. Additionally, such an observing system, by measuring the temporal and spatial variability of the AMOC for approximately a decade, would provide essen- tial ground truth to AMOC model estimates and would also yield insight into whether AMOC changes or other atmospheric/oceanic variability have the dominant impact on interannual sea surface temperature (SST) variability. To make clear assessments of the AMOC’s response to anthropogenic climate change, it is expected that a multi-decadal observing system will be necessary. An observing system serving this purpose would be one where a few critical in situ observations, coupled with satellite observations and the Argo float array, provide a reliable and sustainable measure of the AMOC for decades to come. Ice Sheets and Sea Level Based on both simple physics and observations of the past, there is high confidence in the conclusion that sea level rises in response to warming. Sea-level rise can have large impacts (e.g., Nicholls et al., 2007), such as damage to or loss of infrastructure near coasts, loss of freshwater supplies, and displacement of people whose homes are lost to a rising ocean. Although sea-level rise typically is slow compared to many en- vironmental changes, even this type of gradual sea-level rise may force other systems to cross thresholds and trigger abrupt impacts for natural or human systems unless adaptive measures are taken. For example, rising sea level increases the likelihood that a storm surge will overtop a levee or damage other coastal infrastructure, such as coastal roads, sewage treatment plants, or gas lines—all with potentially large, expen- sive, and immediate consequences (Nordhaus, 2010). (See Box 2.1 for discussion of vulnerabilities of US coastal infrastructure.) A separate but key question is whether sea-level rise itself can be large, rapid and widespread. In this regard, rate of change is assessed relative to the rate of societal adaptation. Available scientific understanding does not answer this question fully, but observations and modeling studies do show that a much faster sea-level rise than that observed recently (~3 mm/yr over recent decades) is possible (Cronin, 2012). Rates peaked more than 10 times faster in Meltwater Pulse 1A during the warming from the most recent ice age, a time with more ice on the planet to contribute to the sea- level rise, but slower forcing than the human-caused rise in CO2 (Figure 2.5 and 2.6). One could term a rise “rapid” if the response or adaptation time is significantly longer than the rise time. For example, a rise rate of 15 mm/yr (within the range of projec- 49

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abrupt impacts of climate chang E 3. Interaction of pressures induced directly by climate change with non-climatic anthropogenic factors, such as habitat fragmentation, overharvesting, or eutro- phication, that magnify the extinction risk for a given species—for example, the checkerspot butterfly subspecies Euphydryas editha bayensis became extinct in the San Francisco Bay area as housing developments destroyed most of their habitat, followed by a few years of locally unfavorable climate conditions in their last ref- uge at Jasper Ridge, California (McLaughlin et al., 2002). 4. Climate-induced change in biotic interactions, such as loss of mutualist partner species, increases in disease or pest incidence, phenological mismatches, or tro- phic cascades through food webs after decline of a keystone species. Such effects can be intertwined with the intersection of extinction pressures noted in mecha- nism 3 above. In fact, the disappearance of checkerspot butterflies from Jasper Ridge was because unusual precipitation events altered the timing of overlap of the butterfly larvae and their host plants (McLaughlin et al., 2002). BOX 2.4  MASS EXTINCTIONS Mass extinctions are generally defined as times when more than 75 percent of the known species of animals with fossilizable hard parts (shells, scales, bones, teeth, and so on) become extinct in a geologically short period of time (Barnosky et al., 2011; Harnik et al., 2012; Raup and Sepkoski, 1982). Several authors suggest that the extinction crisis is already so severe, even without climate change included as a driver, that a mass extinction of species is plausible within decades to centuries. This possible extinction event is commonly called the “Sixth Mass Extinc- tion,” because biodiversity crashes of similar magnitude have happened previously only five times in the 550 million years that multi- cellular life has been abundant on Earth: near the end of the Ordovician (~443 million years ago), Devonian (~359 million years ago), Permian (251 mil- lion years ago), Triassic (~200 million years ago), and Cretaceous (~66 million years ago) Periods. Only one of the past “Big Five” mass extinctions (the dinosaur extinction event at the end of the Cretaceous) is thought to have occurred as rapidly as would be the case if currently observed extinctions rates were to continue at their present high rate (Alvarez et al., 1980; Barnosky et al., 2011; Robertson et al., 2004; Schulte et al., 2010), but the minimal span of time over which past mass extinctions actually took place is impossible to determine, because geological dating typically has error bars of tens of thousands to hundreds of thousands of years. After each mass extinction, it took hundreds of thousands to millions of years for biodiversity to build back up to pre-crash levels. 116

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Abrupt Changes of Primary Concern These dangers of extinction from climate change are well documented for mammals, birds, reptiles, amphibians (Foden et al., 2013; Pimm, 2009; Sinervo et al., 2010), and cor- als (Hoegh-Guldberg, 1999; Mumby et al., 2007; Pandolfi et al., 2011; Ricke et al., 2013). Theoretical considerations and some empirical data also indicate that continued climate change at its present pace would be detrimental to many species of marine clams and snails, fish, tropical ectotherms, and some species of plants (examples and citations below). For such species, continuing the present trajectory of climate change would very likely result in extinction of most, if not all, of their populations by the end of the 21st century. The likelihood of extinction from climate change is low for species that have short generation times, produce prodigious numbers of offspring, and have very large geographic ranges. However, even for such species, the interac- tion of climate change with habitat fragmentation may cause the extirpation of many populations. Even local extinctions of keystone species may have major ecological and economic impacts. The interaction of climate change with habitat fragmentation has high potential for causing extinctions of many populations and species within decades (before the year 2100 if not sooner). The paleontological record and historical observations of species indicate that in the past species have survived climate change by their constituent populations moving to a climatically suitable area, or, if they cannot move, by evolving adaptations to the new climate. The present condition of habitat fragmentation limits both responses under today’s shifting climatic regime. More than 43 percent of Earth’s currently ice-free lands have been changed into farms, rangelands, cities, factories, and roads (Barnosky et al., 2012; Foley et al., 2011; Vitousek et al., 1986, 1997), and in the oceans many continental-shelf areas have been transformed by bottom trawling (Halpern et al., 2008; Jackson, 2008; Hoekstra et al., 2010). This extent of habitat de- struction and fragmentation means that even if individuals of a species can move fast enough to cope with ongoing climate change, they will have difficulty dispersing into suitable areas because adequate dispersal corridors no longer exist. If individuals are confined to climatically unsuitable areas, the likelihood of population decline is en- hanced, resulting in high likelihood of extinction if population size falls below critical values, from processes such as random fluctuations in population size (Maurer, 1999) or Allee effects (Stephens et al., 1999). These considerations make it very likely that at least some populations and species would likely go extinct, and even more will likely drop below viable numbers of indi- viduals within the next few decades simply because they could not disperse across fragmented landscapes fast enough to keep pace with movement of their required climate zones. Concerted efforts of human-mediated translocation of species could help mitigate this, but the practice is still regarded as controversial and experimental 117

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abrupt impacts of climate chang E (McLachlan et al., 2007; Ricciardi and Simberloff, 2009; Hoegh-Guldberg et al., 2008; Sax et al., 2009; Schwartz et al., 2012). Vulnerabilities of Species to Extinction The demonstrable vulnerabilities of populations and species to extinction by climate change fall into three categories. 1. Those whose physiological tolerances to various climatic parameters will be ex- ceeded by climate change throughout their geographic range. 2. Those that will have their growth, development, reproduction, or survival detri- mentally impacted by climate change or consequent changes in biotic interac- tions, resulting in population decline. 3. Those that are effectively trapped by habitat fragmentation in areas where climate changes detrimentally, even though suitable climatic habitat may exist for them elsewhere in the world. Examples of species in Category 1 are: polar bears, which require sea ice in order to thrive, as their primary hunting strategy to maintain adequate fat reserves is waiting for seals to emerge from openings in the ice (Derocher et al., 2004); mountain spe- cies such as pikas (Grayson, 2005; Beever et al., 2011), which cannot survive sustained temperatures above ~27°C (80°F); endemic Hawaiian silverswords, which are restricted to cool temperatures at high altitudes and die from moisture stress (Krushelnycky et al., 2013); and some coral species, which are known to die at ocean temperatures that are only 0.5-1°C above the maxima experienced prior to 1998 (Hoegh-Guldberg, 1999; Mumby et al., 2007; Pandolfi et al., 2011). In Category 2 are many marine species whose growth and development are affected by calcium and aragonite concentrations in ocean water, which vary with increasing acidification caused by adding CO2 to the atmosphere. Already exhibiting detrimental effects are the oyster Crassostrea gigas (Barton et al., 2012; Gazeau et al., 2011) in the US Pacific Northwest, where warmer, more acidic waters cause the oyster eggs to die after a few days of apparently normal development. Experimental work, where organ- isms are reared in waters simulating ocean chemistry expected by the year 2100, also reveals fatal or potentially detrimental effects on other species, including the oysters Crassostrea virginica (Miller et al., 2009) and Pinctada fucata (Liu et al., 2012), inland silverside fish Menidia beryllina (Baumann et al., 2011), Atlantic cod Gadus morhua (Frommel et al., 2011), sea bass Atractoscion nobilis (Checkley et al., 2009), orange clown fish Amphiprion percula (Munday et al., 2009), and damsel fish (Pomacentrus 118

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Abrupt Changes of Primary Concern amboinensis) (Ferrari et al., 2012). This effect is predicted to be particularly severe for coral-forming species (Ricke et al., 2013). Similarly, many plant populations are stressed by climate change. For example, earlier snowmelt in the Rocky Mountains exposes plants to increased frost damage, (e.g., Inouye, 2008), and declining summer fog causes stress to coastal redwoods (Johnstone and Dawson, 2010). Climate change also causes indirect impacts on plants via outbreaks of pests such as pine bark (Kurz et al., 2008) and spruce bark beetles (National Climate Assessment and Development Advisory Committee, 2013; Bentz et al., 2010). In Category 3 are many species that will probably experience lethal effects in large parts, but not all, of their geographic ranges. For example, warmer river temperatures could reduce habitat for trout in the Rocky Mountain West up to 50 percent, and locally up to 70 percent, by 2100 (Kinsella et al., 2008). Survival for such species will depend on whether or not viable population sizes will remain in areas where climate does not change unsuitably, and on the potential of surviving individuals to disperse from climatically unsuitable areas into regions with favorable climate. Of particular concern are species now much reduced in numbers of individuals and restricted to protected habitat islands, such as national parks, that are surrounded by human- dominated landscapes where survival of the affected species is not possible without changing societal norms (Early and Sax, 2011). Plausible vulnerabilities are potentially more severe than the demonstrable vulner- abilities. Of primary concern are probabilities of novel and disappearing combinations of climatic parameters (Williams and Jackson, 2007). Novel climates are those that are created by combinations of temperature, precipitation, seasonality, weather extremes, etc., that exist nowhere on Earth today. Disappearing climates are combinations of cli- mate parameters that will no longer be found anywhere on the planet. Modeling stud- ies suggest that by the year 2100, between 12 percent and 39 percent of the planet will have developed novel climates, and current climates will have disappeared from 10 percent to 48 percent of Earth’s surface (Williams et al., 2007). These changes will be most prominent in what are today’s most important reservoirs of biodiversity (includ- ing the Amazon, discussed in more detail in the “Abrupt Changes in Ecosystems” sec- tion above) and if they result in loss of critical aspects of species’ ecological niches, a large number of extinctions would result. Other circumstances that have high plausi- bility of accelerating extinctions include climatically induced loss of keystone species, collateral loss of species not necessarily affected by climate directly but dependent on species removed by climate change (for example, the myriad species dependent on coral-building species, see below), and phenology mismatches (disruption of the links between a species’ yearly cycle and the seasons) (Dawson et al., 2011; NRC, 2011a). 119

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abrupt impacts of climate chang E Likelihood of Abrupt Changes It presently is not possible to place exact probabilities on the added contribution of climate change to extinction, but the observations noted above indicate substan- tial risk that impacts from climate change could, within just a few decades, drop the populations in many species below sustainable levels, which in turn would commit the species to extinction. Thus, even though such species might not totally disappear as a result of climate change within the next two or three decades, climate impacts emplaced during that time would seal the species’ fate of extinction over the slightly longer term. On the other hand, the risks of abrupt extinction (within 30 to 80 years) are high for many species that live within two kinds of highly biodiverse ecoystems— tropical and subtropical rainforests such as the Amazon, and coral reefs. Although rainforests presently cover only about 2 percent of Earth’s land, they harbor about half of the planet’s terrestrial species,18 and the tropics as a whole contain about two-thirds of all terrestrial animal and plant species (Pimm, 2001). It is these areas that are among those expected to experience the greatest relative difference between 20th century and late 21st century climates, including a large proportion of “disappearing” and “novel” climates (Williams et al., 2007). Coral reefs, which plausibly as a result of climate change could disappear entirely by 2100 and almost certainly will be reduced much in areal extent within the next few decades (Hoegh-Guldberg, 1999; Mumby et al., 2007; Pandolfi et al., 2011; Ricke et al., 2013), are essentially the “rainforests of the sea” (Knowlton and Jackson, 2008) in terms of biodiversity. Coral reefs support 800 hard coral species, over 4,000 fish species, over 25 percent of the world’s fish biodiversity, and between 9-12 percent of the world’s total fisheries.19 Species in high-elevation and high-latitude regions may also be especially vulnerable to extinction as their cur- rent climate zones disappear. It is possible to gain some qualitative insights from natural experiments afforded by the fossil record to bound the worst-case scenarios. A 4oC increase in mean global temperature, which is plausible by the year 2100,20 would make mean global tem- perature similar to what it was 14 to 15 million years ago (Barnosky et al., 2003). Then, areas that are now at the top of the Continental Divide in Idaho and Montana were occupied by large tortoises that could not withstand freezing temperatures in winters 18  http://www.nature.org/ourinitiatives/urgentissues/rainforests/rainforests-facts.xml. 19  http://coralreef.noaa.gov/aboutcorals/values/biodiversity/. 20  The IPCC AR5 RCP8.5 scenario suggests that exceeding 4.0°C of warming is “about as likely as not and the AR4 suggests warming of 4.0°C by 2100 (relative to 1980-1999) as the ‘best estimate’ for the A1F1 scenario (IPCC, 2007c; NRC, 2011a). Society may be closer to this trajectory than to the IPCC AR4 A2 scenario, or the AR5 RCP4.5 or 6.0 scenarios. Davis et al., 2013 note that “actual annual emissions have exceeded A2 projections for more than a decade,” citing Houghton, 2008 and Boden et al., 2011. 120

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Abrupt Changes of Primary Concern (Barnosky et al., 2007). At the same time, what are now arid lands in Idaho and Oregon supported forests of warm-temperate trees like those in the mahogany family, pres- ently characteristic of central and South America, and in the deserts of Nevada, forests were composed of trees that now are native to the southeastern United States and eastern Asia, for instance, maple, alder, ash, yellowwood, birch, beech, and poplars (Graham, 1999). Given current emissions trajectories, there is a chance that the tem- perature increase by 2100 could be near 6oC.21 The last time Earth exhibited a global mean temperature that high, what are now sagebrush grasslands in the southwestern Wyoming and Utah were covered by subtropical, closed canopy forests interspersed with open woodlands (Townsend et al., 2010), reminiscent of subtropical areas in Cen- tral America today. While different continental configurations, elevations, and atmospheric circulation patterns now prevail on Earth, precluding a return to those exact past conditions, the underlying message is that warming of 4o-7o will result in a biotically very different world. At best, changes of such magnitude would trigger dramatic re-organization of ecosystems across the globe that would play out over the next few centuries; at worst, extinction rates would elevate considerably for the many species adapted to pre-global warming conditions, via mechanisms described above (inability to disperse or evolve fast enough to keep pace with the extremely rapid rate of climate change, and disruption of ecological interactions within communities as species respond individualistically). In the oceans, some insights can be gained by tracking how pH values and relative change in pH values correlated with the most severe past mass extinction event, the end-Permian extinction. At current emissions trends, average pH of the oceans would drop from about 8.1 (current levels) to at least 7.9 in about 100 years (NRC, 2011a).22 A similar change occurred over the 200,000 years leading up to the end-Permian mass extinction, which resulted in loss of an estimated ~90 percent or more of known spe- cies (Chen and Benton, 2012; Knoll et al., 2007). The actual extinction event may have been considerably less than 200,000 years in duration, but the vagaries of geological dating preclude defining a tighter time span. While there may well have been mul- tiple stressors that contributed to end-Permian extinctions, hitting critical thresholds of equatorial warming and acidification are now thought to be major contributors 21 The IPCC AR4 scenario A1F1 also yields a 66% chance of warming as much as 6.4°C (IPCC, 2007c; NRC, 2011a) and the AR5 scenario a similar chance of warming 5.8°C (IPCC, 2013). 22  This estimate holds for both the IPCC AR4 A2 Scenario, in which CO concentrations rise to approxi- 2 mately 850 ppm in 2100, or for the the A1F1 Scenario with CO2 concentrations of around 940 ppm in 2100 (IPCC, 2001), and for the RCP8.5 scenario (IPCC, 2013). NRC (2011a) notes that at 830 ppm, tropical ocean pH would be expected to drop .3 pH units; the A1F1 Scenario would result in a larger decrease in pH. 121

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abrupt impacts of climate chang E (Hönisch et al., 2012; Payne and Clapham, 2012; Sun et al., 2012). The end-Permian extinction started from a different continental configuration and global climate, so an exact reproduction is not to be expected, but the potential for a very large number of extinctions in the next few decades, as a result of elevated CO2 levels that warm the atmosphere and oceans and acidify ocean waters, is analogous (Hönisch et al., 2012; Payne and Clapham, 2012; Sun et al., 2012). More recently in geological time, the climatic warming at the last glacial-interglacial transition was coincident with the extinction of 72 percent of the large-bodied mammals in North America, and 83 percent of the large-bodied mammals in South America—in total, 76 genera including more than 125 species for the two continents (Barnosky and Lindsey, 2010; Brook and Barnosky, 2012; Koch and Barnosky, 2006). Many of these extinctions occur within and just following the Younger Dryas, and generally they are attributed to an interaction between climatic warming and human impacts (Barnosky et al., 2004; Brook and Barnosky, 2012; Koch and Barnosky, 2006). The magnitude of climatic warming, about 5oC, was about the same as currently-living species are expected to experience within this century, although the end-Pleistocene rate of warming was much slower. Also similar to today, the end-Pleistocene extinc- tion event played out on a landscape where human population sizes began to grow rapidly, and when people began to exert extinction pressures on other large animals (Barnosky, 2008; Brook and Barnosky, 2012; Koch and Barnosky, 2006). The main dif- ferences today, with respect to extinction potentials, are that anthropogenic climate change is much more rapid and moving global climate outside the bounds living species evolved in, and the global human population, and the pressures people place on other species, are orders of magnitude higher than was the case at the last glacial- interglacial transition (Barnosky et al., 2012). Summary and the Way Forward The current state of scientific knowledge is that there is a plausible risk for climate change to accelerate already-elevated extinction rates, which would result in loss of many more species over the next few decades than would be the case in the absence of climate change. Many of the extinction impacts in the next few decades could be cryptic, that is, reducing populations to below-viable levels, destining the species to extinction even though extinction does not take place until later in the 21st or follow- ing century. The losses would have high potential for changing the function of existing ecosystems and degrading ecosystem services (see Chapter 3). The risk of widespread extinctions over the next three to eight decades is high in at least two critically im- portant ecosystems where much of the world’s biodiversity is concentrated, tropical/ 122

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Abrupt Changes of Primary Concern sub-tropical areas, especially rainforests and coral reefs. The risk of climate-triggered extinctions of species adapted to high, cool elevations and high-latitude conditions also is high. There are several questions that are still at a nascent stage of discovery: • Exactly which species in which ecosystems are most at risk? • Which species extinctions would precipitate inordinately large ecological cas- cades that would lead to further extinctions? • What is the impact of climate-induced changes in seasonal timing and species interactions on extinction rates? Likewise, much remains to be learned about whether loss of biodiversity in all cases means loss of ecosystem services (see Chapter 3, section on Ecosystem Services), and what loss of diversity through extinctions would actually cost humanity. What can be monitored to see abrupt changes coming? Evaluating trends of species decline/persistence in uniform ways, such as using techniques developed and in place by the International Union for the Conservation of Nature (IUCN), is especially impor- tant for non-charismatic species that may be essential in controlling ecosystem func- tion, and for marine invertebrates. In general, it would be useful to monitor species composition, abundance, phenotype, genetic diversity, nutrient cycling, etc. in uniform ways in many different ecosystems, especially those thought to have little impact by humans or otherwise set aside as protected areas (national parks, remote regions, etc.) (Barnosky et al., 2012). Currently, monitoring is taking place in a variety of contexts, and some data-sharing and uniformity of data sharing is emerging with efforts such as the National Ecological Observatory Network (NEON).23 But in general different things are being monitored in different ecosystems, and there is little coordination among different groups. Overall, a more uniform, worldwide system of ecosystem/species monitoring is needed (e.g., Pereira et al., 2013). Additionally, a longer time perspective is needed to develop ways to separate the ecological “noise” from the significant ecological signals that would presage biodiversity collapse. This requires comparing changes observed over de- cades and centuries to long-term ecological baselines of change interpreted from rel- evant prehistoric records—much as the climate community has done with comparing recent changes with prehistoric proxy data (Barnosky et al., 2012; Hadly and Barnosky, 2009). Further research is also needed in several key areas: 23  http://www.neoninc.org/. 123

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abrupt impacts of climate chang E 1. Developing metrics to set short-term changes observed over decades or cen- turies in the context of long-term (several hundreds to thousands of years or more) variation in specific ecosystems 2. Better monitoring and modeling of population parameters that would predict extinction risk in a wide variety of species 3. Better understanding of what species are most imperiled by climate change— of those IUCN species in the vulnerable categories, for example, which would be substantially affected by climate change and which would be more resilient 4. Better understanding of which species are true keystones, and which of those are actually at risk from climate 5. Better understanding of how particular life history traits of species predict vulnerability 6. Better predictive models of spatial and demographic responses of species to changes in specific climate parameters 7. Better understanding of the role of species interactions in affecting resilience to climate change 8. Better understanding of the costs—in ecosystem services, economics, and aesthetic/emotional value—of losing species through extinction With improved understanding of these issues, society can make more informed deci- sions about potential intervention actions (Figure 2.21). 124

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Abrupt Changes of Primary Concern FIGURE 2.21 Improved understanding of adaptive capacity, sensitivity, and exposure to climate change can allow for more informed policy decisions. Potential actions are shown as a function of these variables. Source: Dawson et al., 2011. 125

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