1
Introduction

1.1
THE FOCI OF THIS REPORT: CLIMATE CHANGES AND IMPLICATIONS FOR A RANGE OF ALTERNATIVE FUTURE WORLDS

The concentrations of several greenhouse gases (including carbon dioxide, methane, nitrous oxide, and other chemicals) have increased markedly since the start of the 20th century due to human activities. The changes in these gases very likely account for most of the globally averaged warming of the past 50 years (Hegerl et al., 2007). Carbon dioxide dominates the anthropogenic radiative “forcing” of Earth’s climate due to manmade greenhouse gases. It has increased by more than 35% since 1750, now reaching the highest levels in at least 800,000 years (Forster et al., 2007; Luthi et al., 2008).

Human society faces important choices in the coming century regarding future emissions and the resulting effects that should be anticipated on Earth’s climate, ecosystems, and people. One way of evaluating these choices is to consider the climate changes and impacts that are projected if human actions were to cause greenhouse gases to increase to particular concentration levels and then stabilize. The focus of this study is on the alternatives for the planet’s future represented by stabilization of greenhouse gases at a broad range of “target” levels, hereafter referred to as stabilization targets. Transient climate changes and impacts experienced for increasing concentrations of greenhouse gases are also considered.

This report does not recommend or justify any particular target. Rather, our goal is to present the best scientific information available regarding the implications of different targets for human and natural systems. The charge to the committee was to evaluate the issue of serious or irreversible impacts of climate changes.

It should be emphasized that choosing among different targets is a policy issue rather than strictly a scientific one, because such choices



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1 Introduction 1.1 THE FOCI OF THIS REPORT: CLIMATE CHANGES AND IMPLICATIONS FOR A RANGE OF ALTERNATIVE FUTURE WORLDS The concentrations of several greenhouse gases (including carbon diox- ide, methane, nitrous oxide, and other chemicals) have increased markedly since the start of the 20th century due to human activities. The changes in these gases very likely account for most of the globally averaged warm- ing of the past 50 years (Hegerl et al., 2007). Carbon dioxide dominates the anthropogenic radiative “forcing” of Earth’s climate due to manmade greenhouse gases. It has increased by more than 35% since 1750, now reaching the highest levels in at least 800,000 years (Forster et al., 2007; Luthi et al., 2008). Human society faces important choices in the coming century regard- ing future emissions and the resulting effects that should be anticipated on Earth’s climate, ecosystems, and people. One way of evaluating these choices is to consider the climate changes and impacts that are projected if human actions were to cause greenhouse gases to increase to particular concentration levels and then stabilize. The focus of this study is on the alternatives for the planet’s future represented by stabilization of greenhouse gases at a broad range of “target” levels, hereafter referred to as stabilization targets. Transient climate changes and impacts experienced for increasing concentrations of greenhouse gases are also considered. This report does not recommend or justify any particular target. Rather, our goal is to present the best scientific information available regarding the implications of different targets for human and natural systems. The charge to the committee was to evaluate the issue of serious or irreversible impacts of climate changes. It should be emphasized that choosing among different targets is a policy issue rather than strictly a scientific one, because such choices 49

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50 CLIMATE STABILIZATION TARGETS involve questions of values, e.g., regarding how much risk to people or to nature might be considered too much. Some climate changes could be beneficial for some people or regions, while being damaging to others. For example, with global warming, fewer people may die in cold waves while more people die in heat waves; similarly, crops may be more productive in parts of Canada while less productive in the United States, raising issues of international food trade and transfer (see Section 5.1). Treatment of such effects as cancelling one another would generally be a value judgment and is not deemed to be appropriate here. Due to these considerations, we do not comprehensively cover possible benefits of climate change that could accrue to some people or regions. The study does not seek to evaluate the financial costs or feasibility of achieving any given stabilization target, nor to identify possible mitigation strategies to attain the targets. The primary challenge for this study is to quantify insofar as possible the expected outcomes of different stabilization targets using analyses and information drawn from the scientific literature. Data available from pub- licly available archives were used in analyses carried out for this study, including, e.g., the CMIP3 climate model intercomparison project, the new representative concentration pathway (RCP) scenarios, etc. Some analyses and runs were also carried out, using published models and methods. The report covers emissions, concentrations, changes in the physical climate (such as temperature, rainfall, sea level, etc.) and their time scales, as well as associated impacts (such as food production, flooding, ecosystem dam- age, etc). In evaluating impacts, we seek to identify a baseline that includes expectations regarding adaptation to climate change where appropriate, but we also identify instances where adaptation is possible but where its feasibil- ity, likelihood, or effectiveness is presently unknown. The report represents a brief summary of a vast scientific literature and seeks to be illustrative and representative rather than comprehensive. Warming is the frame of reference for evaluation of impacts used in this report for both conceptual and practical reasons. Many key future cli- mate impacts are dependent upon the amount of global warming. Indeed, available data and modeling suggest that the magnitudes of several key impacts can be evaluated with relatively good accuracy for given amounts of global warming. Much of the available literature and analysis of climate impacts can be tied to specific warming levels but not readily linked to CO2-equivalent concentrations (due for example to lack of specification of aerosol forcing between studies). Indicated warming levels are related here to the corresponding best estimates and uncertainties in CO2-equivalent concentrations as well as to cumulative carbon emissions.

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INTRODUCTION 51 The study presents best estimates for climate change and its impacts at varying warming levels and associated stabilization targets, and it also briefly describes the level of understanding of the processes involved (e.g., physical, biological, etc.) in such changes. Presentation of best estimates provides a clear view of the best current understanding that may otherwise be poorly communicated, and this is one aim of this study. However, we also balance descriptions of best estimates with their corresponding uncertainties, along with appropriate coverage of issues of uncertain risks. Climate changes and their impacts are discussed on a global basis, and specific regional ex- amples within America and American territories are also presented for the purpose of illustration. We summarize as appropriate the following factors: (1) the extent to which multiple studies are available, and the robustness of findings across work by a range of authors, (2) the scientific confidence in understanding of the underlying processes, and (3) studies that already attribute a contribution of observed changes to anthropogenic effects where available (see Section 1.2). Where attribution is already possible for current levels of climate change, confidence in further future changes is generally strengthened. Many climate changes or impacts currently are understood only in a qualitative manner, and thus are not quantifiable as a function of stabiliza- tion target. The report assesses and identifies these unquantified factors to the extent practical based upon the available literature. It should be empha- sized that these should not be considered negligible; indeed some of these could be very important, or even dominant, in evaluating future risks due to anthropogenic climate change. Many studies involve the use of “pattern scaling” whereby it is assumed that the spatial pattern of future climate change computed for one level of perturbation (i.e., radiative forcing) may be scaled to derive values for an- other test case such as one with stronger forcing and larger perturbations. A variety of studies have shown that such methods generally simulate the results of atmosphere-ocean general circulation models rather well (see Sec- tion 4.2), although results are generally less robust for precipitation than for warming (see Section 4.3), and near regions where strong feedbacks such as sea ice retreat take place. We employ pattern scaling for many of the estimates in this report. Earth’s history shows that climate changes can occur on time scales ranging from decades to centuries to millennia. All of these time scales are considered here. As there is abundant evidence that climate is already changing in part due to human activities, a focus of the report is on the next few decades to century, where the climate changes under increased

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52 CLIMATE STABILIZATION TARGETS human forcing of the climate system represent a transient climate change, linked to transient climate response (or TCR, see Section 3.2), and reflects a changing climate in which relatively fast-responding variables such as water vapor and cloudiness change but slower processes such as ocean warming and ice melt may lag behind. This implies that only a portion of the climate change is “realized” during a period of increasing greenhouse gas concen- trations. If greenhouse gases are stabilized following a period of changing radiative forcing, the climate system response within a few centuries can be expected to reflect the equilibrium climate sensitivity (see Section 3.2). On very long time scales of multi-millennia, changes in factors such as ice sheets and vegetation can lead to different and generally stronger climate changes under stabilization (see Chapter 6). The report considers a range of warming levels, which can be related to stabilization targets ranging from 350 to near 2,000 ppmv of total green- house gas forcing expressed as equivalent carbon dioxide concentration (see Box 1.1 for definition). We note that 1,000 ppmv is a level that a number of studies suggest may be reached by 2100 for a “business as usual” scenario with little or no mitigation of emissions. Even higher concentrations have precedent in the Earth’s paleo history millions of years ago (see Chapter 6), and illustrate how the Earth may be changed if, for example, feedbacks from the Earth system trigger large releases of carbon from peat. This report is organized as follows: The next part of this introductory chapter summarizes the attribution of currently observed climate changes and impacts, as a starting point for discussion of the future. Chapter 2 describes the relationship between greenhouse gas emissions and concentrations for a range of gases and cases. Aerosols and other forc- ings are also discussed. Chapter 3 presents the current understanding of how the climate changes due to greenhouse gas concentrations on a range of time scales including BOX 1.1 CARBON DIOXIDE-EQUIVALENT (CO22-EQ) CONCENTRATIONS Greenhouse gases differ in their warming influence (radiative forcing) on the global climate sys- tem due to their different radiative properties and lifetimes in the atmosphere. These warming influences may be expressed through a common metric based on the radiative forcing of CO2. • CO2-equivalent concentration is the concentration of CO2 that would cause the same amount of radiative forcing as a given mixture of CO2 and other forcing components.

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INTRODUCTION 53 decades, centuries, and millennia. It discusses best estimates and uncertain- ties, including those relating to feedbacks (such as carbon cycle feedbacks whereby more carbon is emitted from the biosphere in a warming planet). Chapters 4 through 6 comprise the core of this report. They present climate changes and impacts responses as a function of warming, that is, corresponding to stabilization targets of 350 to 2,000 ppmv CO2-equivalent, considering time frames up to 2100, as well as very long time frames several thousand years in the future. 1.2 ATTRIBUTION We offer here a brief summary of detection and attribution results as they provide part of the foundation of the discussion of future projected changes in the physical climate system and the impacts on natural and hu- man systems that could ensue from them. This summary is not intended to be comprehensive of all detection and attribution studies to date, but will start from the IPCC Earth Assessment Report (AR4), updating it by those more recent results that are specifically relevant to this report. Formal detection and attribution of an anthropogenic influence over the physical climate system is based on analysis of spatial and temporal patterns in observations of climate parameters and on comparison of their statisti- cal characteristics with those of the same patterns as simulated by climate models. Because models can be integrated by applying the known external forcings in designed experiments (natural only, anthropogenic only, natural and anthropogenic jointly1) or in unforced mode (i.e., a control simula- tion), the behavior of the system subjected to different forcings as well as in control mode can be characterized, and the observed behavior of the real climate system can be compared to test consistency with a naturally vary- ing process or with a process subjected to externally (especially manmade) forcings, to a given degree of statistical confidence. The progress of formal detection and attribution (D&A) is thus linked inextricably to the accumula- tion over time and space of quality observations that allows computation of robust statistics of the parameters under study and comparison to the ability of climate models to reliably simulate those parameters’ natural and forced behavior. Importantly, formal D&A compares spatial and/or tempo- ral patterns of change in observations and model simulations, not simply magnitudes of changes, seeking to test the consistency of the process-driven behaviors between models and observations. These behaviors are defined 1Each of these examples can be split into more complex designs with single natural/anthro- pogenic forcings administered in isolation or jointly.

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54 CLIMATE STABILIZATION TARGETS by the direction and geographical shape of the change in the observed and simulated variables, not by the absolute values of trends and spatial anomalies. The best established and robust results in detection and attri- bution of anthropogenic influences are those that have been documented for temperature increases at global and continental scales, at the surface, in the troposphere, and in the oceans. When moving from temperature to other parameters and from global and continental scales to smaller regional scales, the confidence and robustness of the results diminish but are still high for many other measures of physical changes (e.g., sea level pressure, temperature extremes, zonally averaged precipitation, atmospheric moisture and surface specific humidity). The evaluation of the IPCC has also included detection and attribution of changes that have not been analyzed through the statistical machinery of formal D&A but are either closely related to changes that have been so analyzed, or have been proven consistent in a qualitative way with experiments including anthropogenic forcings and not consistent with model simulations forced by natural inputs alone. In all cases the scientific reasoning and understanding of the mechanisms linking anthropogenic forcings to these changes buttress the evidence from obser- vational records and models. In Table 1.1 we list the physical parameters of the climate system whose changes have been detected and attributed. We list first those D&A results that have been documented in IPCC AR4, along with a measure of confidence assigned, followed by a list compiled from more recent peer- reviewed literature. Formal D&A, as just described, relies on model simulations, by which a treatment (human factors included) versus control experiment (only natural factors included) is set up for D&A of climate variables, but D&A of im- pacts requires an additional modeling step, by which the behavior of the climate system is translated into effects on the natural of human systems under study. The additional modeling adds limitations and uncertainties, but, where it is possible using current understanding, D&A of impacts is methodologically similar to D&A of physical parameters’ change, and it is only the degree of significance or uncertainty in the results that will have to account for the compounding of modeling steps and the approximation errors thus added. For many types of impacts, however, a direct modeling, with the ability to include the confounding factors that are often required in addition to the climate changes, is not possible, and D&A has to follow a multi-step pathway:

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INTRODUCTION 55 TABLE 1.1 Physical Parameters of the Climate System Whose Changes Have Been Detected and Attributed to Greenhouse Gas Increases and Other Human Factors by IPCC AR4 and More Recent Peer-reviewed Literature Phenomena Affected by Anthropogenic Contribution Degree of Confidence Reference Global surface air temperature (SAT) increase in the past half Very Likely IPCC AR4 WG1 Ch9 century (IPCC, 2007a) Larger than observed, in the absence of volcanoes and aerosols, Likely IPCC AR4 WG1 Ch9 Global Surface Air Temperature (SAT) increase in the past half (IPCC, 2007a) century. Warming of continental-scale SAT since middle of 20th century Likely IPCC AR4 WG1 Ch9 (IPCC, 2007a) Temperature extremes over land in the Northern Hemisphere Likely IPCC AR4 WG1 Ch9 and Australia (IPCC, 2007a) Tropopause height increases in latter half of 20th century Likely (with contribution IPCC AR4 WG1 Ch9 of stratospheric ozone (IPCC, 2007a) decrease) Tropospheric warming in latter half of 20th century Likely IPCC AR4 WG1 Ch9 (IPCC, 2007a) Simultaneous tropospheric warming and stratospheric cooling Very likely (because IPCC AR4 WG1 Ch9 in latter half of 20th century of their happening in (IPCC, 2007a) conjunction) Warming of the upper layer of the oceans Likely IPCC AR4 WG1 Ch9 (IPCC, 2007a) Sea level rise in latter half of 20th century Very Likely IPCC AR4 WG1 Ch9 (IPCC, 2007a) Sea level pressure trends in latter half of the 20th century Likely IPCC AR4 WG1 Ch9 (spatial patterns of increases and decreases in both (IPCC, 2007a) hemispheresa) Reduction of NH (Arctic) sea ice extent and global glacier retreat Likely IPCC AR4 WG1 Ch9 in latter half of 20th century (IPCC, 2007a) More Likely Than Notb Increases in intense tropical cyclone activity IPCC AR4 WG1 Ch9 (IPCC, 2007a) More Likely Than Notb Increases in heavy rainfall on global land areas during latter half IPCC AR4 WG1 Ch9 of 20th century (IPCC, 2007a) More Likely Than Notb Increased risk of drought in latter half of 20th century IPCC AR4 WG1 Ch9 (IPCC, 2007a) Zonal mean precipitation changes (increases in the Northern NA Zhang et al., 2007c Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics Water vapor and surface specific humidity increases NA Santer et al., 2007; Willett et al., 2007. continued

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56 CLIMATE STABILIZATION TARGETS TABLE 1.1 Continued Higher temperatures/early snowmelt/early run-off in U.S. NA Maurer et al., 2007; Southwest Barnett et al., 2008; Bonfils et al., 2008, Hidalgo et al., 2009; Pierce et al., 2008 Arctic and Antarctic temperature increases; Arctic seaice NA Gillett et al., decrease; Arctic precipitation increase 2008a; Min et al., 2008. Increased atmospheric winter (JFM and JAS) storminess in high NA Wang et al., 2009 latitudes SSTs warming in cyclogenesis regions of Atlantic and Pacific NA Gillett et al., 2008b oceans Increased ocean salinity in Atlantic Ocean NA Stott et al., 2008 NOTE: The degree of confidence is noted when available from the expert judgment by the IPCC AR4 authors. More recent results (indicated by the light blue background) have appeared in the peer-review literature, but their degree of confidence has not been assessed and thus does not appear in our table. aSee Figure 9.16 of IPCC AR4 WG1 (Chapter 9). bConsistent with theoretical expectations of the effects of anthropogenic forcings but have not been detected and/or attributed, due to modeling uncertainties or lack of skill, and poor quality/limited extent of the data record. 1. a change in the system impacted is first associated with climatic factors, and, separately, 2. the climatic factors are shown to be attributable to anthropogenically caused changes. In order to link the two steps formally, a measure of significant positive spatial correlation between changes in the impacted system and changes in climate was primarily used in the IPCC WG2 report to summarize many studies of impacts all over the globe. This correlation-based argument was also supported by the existence of several studies that performed formal D&A through modeling of specific systems or domains, and, importantly, by scientific understanding of the reasons why the impacts are consistent with warming of the climate system. In summary, most of the attribution of impacts that has taken place thus far has relied on the documented attribution of the warming patterns of the physical system coupled to the scientific understanding of the effects of such warming on natural and human systems, together with significantly

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INTRODUCTION 57 TABLE 1.2 List of Observed Impacts Attributed to Global Warming by IPCC AR4 WG2 with Results from Two Studies Using Formal D&A Methodology Driving Change in Physical Climate Impacts Reference Changing snow/ice/frozen ground Increase in number and extension of glacial lakes, increase IPCC AR4 WG2 in avalanches and instability of permafrost, and changes in (IPCC, 2007b) polar ecosystems Warmer temperatures Increase in runoff and anticipated snow melt in spring; IPCC AR4 WG2 increase in temperatures in lakes and rivers (IPCC, 2007b) Warmer temperatures Earlier timing of spring event (foliage, migrations, and IPCC AR4 WG2 egg-laying) and range shifts (poleward and upward) of (IPCC, 2007b) terrestrial biological systems Rising ocean water temperatures/ Shifts in the ranges of algae/plankton/fish abundance in IPCC AR4 WG2 changes in ice cover, salinity, high-latitude oceans and high-latitude/high-elevation lakes (IPCC, 2007b) oxygen levels, and circulation Increase in atmospheric Ocean acidification (no effect on ocean life documented IPCC AR4 WG2 concentration of CO2 yet (IPCC, 2007b) Summer temperature warming in Increase in area burned by forest fires in Canada Gillett et al., Canada 2004 Increase in temperature Increase in growing season length Christidis et al., 2007 NOTE: Confidence levels are not available because the IPCC AR4 WG2 did not supply them. positive spatial correlation between said impacts and regionally differenti- ated warming. This reasoning is at the basis of the list of impacts attributed to global warming by IPCC AR4 WG2 in its Summary for Policy Makers, which we list, together with results from two studies that applied formal D&A meth- odology,2 in Table 1.2. 2Of these two studies, the first is reviewed by WG,2 but its results are not explicitly listed in its Summary for Policy Makers, SPM; the second appeared later than the release of the report.

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