CHAPTER ONE

Introduction

Climate information is being used by a vast array of organizations within the public and private sectors, with decisions based on climate information being made every day. Users of climate information include national security planners, infrastructure decision makers, public policy makers, insurance companies, water managers, agricultural managers, and more. Each of these communities has different needs for climate data from numerical simulations, with different time horizons and different tolerances for uncertainty. Many user groups want very highly spatially resolved information about the likely range of climate variability and extreme events such as droughts, floods, or heat waves, while others are looking for data on long-term trends. Some concrete examples of current users of climate information are farmers, city planners, water managers, and insurance companies, and details about their use of climate information are described in Box 1.1.

BOX 1.1 EXAMPLES OF CLIMATE DATA USERS

Climate data are needed by many individuals and companies. Below are several representative examples of individuals and organizations who use climate data, why they need them, how they are used, and what the payoff is.

Farmers

Farmers have always been close to weather and climate, as their economic success depends on the right timing of planting, irrigation, and harvesting and the right choice of crops for the local climate. In their day-to-day decision making about irrigation, farmers depend heavily on short-term weather forecasts, which give them information not only about temperature and precipitation but also about soil moisture levels that are crucial for many crops. One concrete example is corn farming—a $15.1 billion business in the United Statesa—which is very sensitive to drought and low soil moisture. Decisions made on the time scales of weeks to seasons rely on short-term and seasonal forecasts of the soil moisture, which have become invaluable tools to help farmers decide on irrigation needs during drought conditions; it is estimated that by 2015 improved weather forecasts will allow the agriculture sector to save $61 million on irrigation water costs (Centrec Consulting Group, 2007). On time scales of seasons to years, forecasts of El Niño/La Niña conditions help farmers to decide when to plant and harvest their crop, with an estimated economic benefit on the order of $500 million to $950 million per year from the seasonal El Niño/La Niña forecast for the U.S. agricultural sector (Chen et al., 2002). On even longer time scales, the changing climate is shifting growing seasons and regions. Farmers are directly



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CHAPTER ONE Introduction C limate information is being used by a vast array of organizations within the public and private sectors, with decisions based on climate information being made every day. Users of climate information include national security planners, infrastructure decision makers, public policy makers, insurance companies, water man- agers, agricultural managers, and more. Each of these communities has different needs for climate data from numerical simulations, with different time horizons and differ- ent tolerances for uncertainty. Many user groups want very highly spatially resolved information about the likely range of climate variability and extreme events such as droughts, floods, or heat waves, while others are looking for data on long-term trends. Some concrete examples of current users of climate information are farmers, city plan- ners, water managers, and insurance companies, and details about their use of climate information are described in Box 1.1. BOX 1.1  EXAMPLES OF CLIMATE DATA USERS Climate data are needed by many individuals and companies. Below are several representa- tive examples of individuals and organizations who use climate data, why they need them, how they are used, and what the payoff is. Farmers Farmers have always been close to weather and climate, as their economic success depends on the right timing of planting, irrigation, and harvesting and the right choice of crops for the local climate. In their day-to-day decision making about irrigation, farmers depend heavily on short-term weather forecasts, which give them information not only about temperature and precipitation but also about soil moisture levels that are crucial for many crops. One concrete example is corn farming—a $15.1 billion business in the United Statesa—which is very sensitive to drought and low soil moisture. Decisions made on the time scales of weeks to seasons rely on short-term and seasonal forecasts of the soil moisture, which have become invaluable tools to help farmers decide on irrigation needs during drought conditions; it is estimated that by 2015 improved weather forecasts will allow the agriculture sector to save $61 million on irrigation water costs (Centrec Consulting Group, 2007). On time scales of seasons to years, forecasts of El Niño/La Niña conditions help farmers to decide when to plant and harvest their crop, with an estimated economic benefit on the order of $500 million to $950 million per year from the sea- sonal El Niño/La Niña forecast for the U.S. agricultural sector (Chen et al., 2002). On even longer time scales, the changing climate is shifting growing seasons and regions. Farmers are directly 19

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G BOX 1-1  CONTINUED impacted because many of them have specialized in growing specific crops, which in turn are often highly specialized for the climatic conditions they tolerate (see Figure 1). Longer-term re- gional climate projections of precipitation, temperature, and soil moisture will allow farmers to decide which crops to focus on in the future and to prepare for investments in new technologies needed to successfully grow new crops. Mayors of Large Cities One of the main concerns about climate change is associated with the projected increase in the frequency, duration, and intensity of heat waves. According to the National Weather Service (NWS),“heat is the number one weather-related killer in the U.S.,” b claiming more lives each year than floods, lightning, tornadoes, and hurricanes combined. Heat waves also increase the peak demand for electricity, with the potential for blackouts and the high economic cost associated with them. (Estimates for the August 2003 blackout that affected numerous cities in the United States and Canada ranged from $4 billion to $10 billion [U.S.-Canada Power System Outage Task Force, 2004]). Using a heat index that considers absolute temperature and humidity to assess how hot it really feels, the NWS forecasts extreme heat events several days in advance. This allows city officials to prepare for heat waves by warning the public, instituting energy-saving programs, and designating community cooling centers, reducing some of the negative impacts of heat waves FIGURE 1  U.S. Department of Agriculture plant hardiness zone maps are used extensively by gardeners and growers to determine which plants are most likely to thrive at a location. Maps are based on the average an- nual minimum winter temperature, divided into 10°F zones. The map on the left is based on data from 1974- 1986, and the map on the right is based on data from 1976-2005. The more recent map (right) is generally one half-zone warmer than the previous map (left). SOURCES: http://arborday.org/media/map_change.cfm; http://planthardiness.ars.usda.gov/PHZMWeb/AboutWhatsNew.aspx (both accessed October 11, 2012). 20

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Introduction BOX 1-1  CONTINUED and saving lives. In the longer term, climate projection data allow mayors and other planners to develop adaptation strategies (NPCC, 2010) to help plan for some of the negative impacts of these changes. These adaptation strategies include programs to increase the energy efficiency of buildings, investments in power grid infrastructure, and zoning changes to mandate the planting of street trees in heat-stressed neighborhoods. Improved climate data (Figure 2) can help cities make more informed decisions on long-term infrastructure investments that will help to protect the health and economic interests of their constituents. FIGURE 2  Heat waves are projected to occur more frequently in the future. Map shows the projected fre- quency of extreme heat for later in the century (2080-2099 average). Extreme heat refers to a day so hot that it occurred only once every 20 years in the past, and the projections show that extreme heat will occur every 1-3 years in much of the United States by the end of the century. 21

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G BOX 1-1  CONTINUED Hydropower System Managers The Federal Columbia River Power System generates more than 76,000 gigawatt-hours (GWh) of electricity per year, accounting for about 30 percent of the electricity used by the more than 15 million people in the Pacific Northwest and having an estimated worth of approximately $4 billion per year (BPA, 2010). To continue generating power at this level, river managers like those for the Columbia River power system need to make both short- and long-term decisions regarding how much water to store (compared to natural flow), which requires climate data to predict and adapt to future changes in river flow. The climate data most needed by river power management are temperature, precipitation, and wind, with information preferably at high spatial resolutions of 1-10 km and with daily or higher frequency. Current climate data are only available at much lower resolutions, but even these data have been useful in projecting seasonal changes, such as increased winter runoff but less spring/summer runoff. Managers also use longer-term projections of climate change to make decisions on modifying existing infrastructure and/or acquiring additional infrastructure (for example, Figure 1.2). Managers such as those who monitor the Columbia River desire more reliable and higher-resolution climate data to help with planning and ultimately their ability to continue to supply power to millions of Americans (Figure 3). FIGURE 3  Managers of hydropower systems such as those of the Federal Columbia River Power System re- quire climate information for both short-term operational decisions and long-term infrastructure planning. SOURCE: Steven Pavlov, http://commons.wikimedia.org/wiki/File: Grand_Coulee_Dam_in_the_evening.jpg (accesed June 8, 2012). 22

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Introduction BOX 1-1  CONTINUED Insurance Companies Insurance companies provide insurance to people and businesses against the impacts of natural disasters. Insurance rates for weather- and climate-related disasters such as floods, high winds, droughts, etc., are based on the expected occurrence of those events. To realistically assess the probabilities of weather- and climate-related natural disasters, insurance companies have been using climate data on past weather events for many years to develop specific risk models for different regions and operations (e.g., transportation, farming, and construction). Weather- and climate-related losses have increased rapidly in recent years (Figure 4), with record-breaking FIGURE 4  Annual occurrence of natural disasters in the United States, broken down by origin as of 2010, shows that the past may no longer be a reliable guide to the future. Record-breaking insured losses from weather- and climate-related disasters of over $50 billion were recorded in 2011. SOURCE: Munich RE; http://www.munichre.com/app_pages/www/@res/pdf/media_relations/ press_dossiers/hurricane/2011- half-year-natural-catastrophe-review-usa_en.pdf (accessed September 14, 2012). 23

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G BOX 1-1  CONTINUED insured losses of more than $50 billion in 2011.c More and more large insurance and reinsurance companies are recognizing that climate change poses an enormous challenge to their busi- ness. Accurately reflecting changed risks and actively and profitably managing climate change impacts, rather than withdrawing from high-risk markets, is a major challenge for the insurance industry. To address it, new kinds of climate data are required, focusing on projections rather than historical observations. High-quality regional climate projections of variables such as sea level, temperature, precipitation, wind, and extreme events will be crucial for the insurance industry to rise to this challenge, so insurers can continue to provide disaster coverage for people and businesses in the United States and use their past experience with risk mitigation (e.g., fire and earthquake building codes) to help prevent losses of lives and property (Mills and Lecomte, 2006). National Security Sector National security planners and decision makers use climate information and forecasts over a broad range of time scales. The February 2010 Quadrennial Defense Review notes that climate change will play a significant role in the future security environment for the United States (Gates, 2010). Concurrently, the U.S. Department of Defense and its military services are developing poli- cies and plans to understand and manage the effects of climate change on military operating environments, missions, and facilities (NRC, 2011c). It has been estimated that $100 billion of naval facilities are at risk from sea-level rise of 3 feet or more (NRC, 2011c) (Figure 5). The national security risks associated with a changing climate have also recently been assessed in a report by the Center for American Progress (Werz and Conley, 2012). The Navy would like to use climate model outputs for information related to increasing Arctic maritime activity, water and resource scarcity, and the impact of sea-level rise on installations (NRC, 2011c). In order to use climate model projections to inform its decisions, the Navy would need high-spatial-resolution regional climate models on decadal time scales, uncertainty quantification of the models, and probability distribution functions in the model output. The Navy is a “good example of a stakeholder that has very specific needs in applications related to its infrastructure and operations, disease, civil instability, migration, water resources, and energy” (NRC, 2011c). The Building Community The built environment (buildings, communications, energy, industrial facilities, transporta- tion, waste, water, and associated natural features) shelters and supports most human activities and constitutes a large portion of the nation’s wealth (Figure 6). It has important roles in reduction of greenhouse gas emissions and in measures to help society adapt economically, environmen- tally, and socially to climate change. The building community includes professionals—including architects, engineers, geologists, landscape architects, and planners—as well as owners, investors, facilities managers, contractors, manufacturers of building materials, health and safety regulators, and stakeholders served or affected by the built environment (nearly everyone). The building community uses climate information, particularly on extremes, to ensure that buildings are safe, functional, and resilient. Historically, the extreme environments used in assess- 24

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Introduction BOX 1-1  CONTINUED FIGURE 5  The amphibious assault ship USS Kearsarge (LHD 3) pulls away from its berth at Naval Station Norfolk. An estimated $100 billion worth of naval facilities are at risk from sea-level rise of 3 feet or more. SOURCE: http://www.navy.mil/view_single.asp?id=125450 (accessed June 6, 2012). ment and design of the built environment have not been based on climate or weather models. Rather, extreme environments have been defined by statistics of historical records, albeit to within observation and sampling errors. With climate and weather changing, historical records no longer are adequate predictors of future extremes. However, advanced modeling capabilities potentially can provide useful predictions of extreme environments. Often decisions about buildings and other infrastructure are made for very long time scales—decades and beyond. When looking at building decisions related to material choices, siting, and building design, there are any number of questions related to climate, including: How heavy are future rains and/or snowfalls likely to be? What range of temperatures is likely? What will average precipitation rates mean for the water table? Will it flood? Adaptation of the built environment to climate change is particularly important because it has significant resource implications. The U.S. Department of Commerce estimates total construction spending in the United States to be more than $820,000 million annually.d 25

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G BOX 1-1  CONTINUED FIGURE 6  Construction of the Sovereign, Atlanta, Georgia. The building community uses climate informa- tion to make decisions about building materials, siting, and building design. These types of infrastructure decisions can have implications for decades. As the climate changes, information from climate models is being used as a guide to future climate conditions. SOURCE: Conor Carey, http://commons.wikimedia.org/ wiki/File:Sovereign-Atlanta.jpg (accessed June 6, 2012). a www.epa.gov/oecaagct/ag101/cropmajor.html (accessed October 11, 2012). b http://www.nws.noaa.gov/os/heat/index.shtml(accessed November 30, 2012). c www.noaa.gov/extreme2011/ (accessed October 11, 2012). d www.census.gov/construction/c30/c30index.html (accessed October 11, 2012). 26

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Introduction Over the next several decades climate change and its myriad consequences will be further unfolding and likely accelerating (NRC, 2011a). Probable impacts from climate change, including sea-level rise, a seasonally ice-free Arctic, large-scale ecosystem changes, regional droughts, and intense flooding events, will increase demand for climate information. The value of this climate information is large. One of the more prominent places to see this is through the impacts of extreme climate and weather events; extreme climate and weather events are one of the leading causes of eco- nomic and human losses, with total losses between 1980 and 2009 exceeding $700 billion (NCDC, 2010) and damages from more than 14 weather- and climate-related disasters totaling more than $50 billion in 2011 alone.1 Climate change is affecting the occurrence of and impacts from extreme events, such that the past is not necessarily a reliable guide for the future, which further underscores the value of climate informa- tion in the future. An example of the value of climate information on shorter time scales comes from the flooding throughout the Upper Midwest in the spring and summer of 2011. Extensive rainfall in the spring and summer of 2011 led to flooding of the Mississippi and Mis- souri rivers. Prior to that spring, climate predictions showed increased risk of flood- ing throughout much of the Upper Midwest as a result of above-average snowpack melting and precipitation levels (Figure 1.1), allowing government authorities to plan ahead. According to the National Oceanic and Atmospheric Administration (NOAA), these climate predictions allowed the government to coordinate “with local, state and federal agencies before and during the flooding, so that emergency officials could make important decisions to best protect life and limit property damage.”2 Such deci- sions included evacuations and destruction of levees in some locations to allow excess waters to flow into floodways. In looking at longer time scales, climate models can provide information on projected rainfall runoff for the coming decades (Figure 1.2). Some areas of the United States, such as the Southwest, are projected to see decreases in average rainfall, while some areas, like the Northeast, will see increases. Such changes will have major implications for future water supplies, crop yields, and wildfire risks, among other effects. This type of projected information allows counties and states to plan ahead for these condi- tions, including decisions regarding infrastructure. However, the relationship between regional drought and predictable patterns of climate variability is complicated, so users of climate information must understand and deal with considerable predictive uncertainty. 1  http://www.noaa.gov/extreme2011/ (accessed October 11, 2012). 2  http://www.noaa.gov/extreme2011/mississippi_flood.html (accessed October 11, 2012). 27

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G FIGURE 1.1  The spring flood risk outlook from NOAA’s National Weather Service for 2011. Extensive flood- ing of Mississippi and Missouri rivers occurred in 2011. SOURCE: http://www.noaa.gov/extreme2011/mis- sissippi_flood.html (accessed October 11, 2012). WHAT IS A CLIMATE MODEL? Information about the future of the climate system comes from computer models that simulate the climate system. Climate models are mathematical representations of physical, chemical, and biological processes in Earth’s climate system (Figure 1.3). Computer models are a part of everyday life—there are models that forecast weather, simulate how to fly an airplane, predict tides, and aid in drug discovery. Models are used to study processes that are inherently complex, require large amounts of infor- mation, or are impractical to study directly. They are essential tools for understanding the world and allow climate scientists to make projections about the future. 28

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Introduction FIGURE 1.2  Longer-time-scale climate projections can assist in long-term planning. The figure shows pro- jected changes in annual average runoff for 2041-2060 relative to a 1901-1970 baseline by water resource region, based on analyses using emissions that fall between the lower and higher emissions scenarios. Lower average runoff is expected in the Southwest and greater runoff is projected for the Northeast. Col- ors indicate percentage changes in runoff, with hatched areas indicating greater confidence due to strong agreement among model projections. SOURCE: USGCRP, 2009. The many different kinds of climate models are all derived from fundamental physi- cal laws such as Newton’s laws of motion and the chemistry and thermodynamics of gases, liquids, solids, and electromagnetic radiation. These are supplemented by em- pirical relationships determined from observations of complex processes such as ice crystal formation in clouds; turbulent mixing, and waves in both air and water; biologi- cal processes; sea-ice growth; and glacier movement. The main components within a climate model include • atmosphere (simulates winds, temperatures, clouds and precipitation, turbu- lent mixing, transport of heat, water, trace chemicals and aerosols around the globe), • land surface (simulates surface characteristics such as vegetation, snow cover, soil water, rivers, ice sheets, and carbon storage), 29

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G and Intercomparison (PCMDI) at Lawrence Livermore National Laboratory has been instrumental in developing CMIP, including archiving, analysis, and quality control of model output, although CMIP now has broad international institutional support. CMIP has developed into a vital community-based infrastructure in support of climate model diagnosis, validation, intercomparison, documentation, and data access. What Can Climate Models Do Well? Climate models have evolved into remarkably sophisticated tools for addressing a diverse range of scientific and societally relevant issues. Their fidelity can be assessed by comparing them statistically with such observations (Box 1.2) as the mean sea- sonal cycle, seasonal extremes of temperature, rain and snowfall, and other routinely measured quantities around the globe, as well as statistics of the El Niño-Southern Oscillation and other important forms of climate variability and the observed changes of climate over the past century and across previous eras. The evolution of climate models over the past 50 years and the diversity of models used for different purposes and across different space and time scales are discussed in more detail in Chapter 3. Climate models skillfully reproduce important, global- to continental-scale features of the present climate, as assessed in more detail by IPCC (2007c, Chapter 11). For in- stance, over most parts of the globe, the simulated seasonal-mean surface air temper- ature is within 3°C of observations (IPCC, 2007c), compared to an annual cycle that can exceed 50°C in places, and simulated seasonal-mean precipitation has typical errors of 50 percent or less on regional scales of 1,000 km or larger that are well resolved by these models (Pincus et al., 2008). In the oceans, projected seasonal-mean sea-surface temperatures are within 1-2°C of those observed over most of the globe, and major ocean current systems like the Gulf Stream are correctly positioned (IPCC, 2007c). The simulated seasonal patterns of sea-ice extent, snow cover, and cloudiness are also in broad agreement with observations (IPCC, 2007c; Pincus et al., 2008). Swings in Pacific sea-surface temperature, winds, and rainfall associated with El Niño are simulated by a number of climate models with fairly realistic amplitude, location, and period (Achuta- Rao and Sperber, 2006; Neale et al., 2008). Other forms of natural climate variability, such as the year-to-year range of seasonally averaged temperature or rainfall over regions of 1,000 km or larger in size and their spatial patterns of year-to-year variabil- ity, are also simulated reasonably well (Gleckler et al., 2008). Simulation of the statistics of extreme hot and cold spells has also improved (IPCC, 2007c), especially in models using grid spacings of less than 100 km. In many ways climate models have become remarkably accurate tools for simulating observable statistical aspects of the Earth system (see Chapter 3 for more details of historical model improvements). 36

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Introduction Climate models do have well-known limitations for simulating the current climate, stemming from both the coarseness of their grid spacing and the challenge of encap- sulating the complex physical interactions between all parts of the climate system. For instance, the grid of current climate models cannot represent fine-scale details of mountain ranges important for simulating snowpack, rainfall, and glaciation in such regions, details of coastal processes such as oceanic upwelling or tidal currents, or hurricanes and severe thunderstorms. Tropical rainfall and many cloud processes rely on interactions between very small scale air motions and other processes such as condensation or freezing that are also not straightforward to represent in current climate models. Other limitations include a lack of fully coupled land-ice or ocean biogeochemistry models in many simulations, which are areas of active research but which are just starting to be included in climate simulations. Furthermore, credible simulations of some processes, such as the formation of continental ice sheets, would require model runs of tens of thousands of years that are not yet feasible on current computers. The main concern among scientists, decision makers, and the interested public is the extent to which climate projections can be trusted based on model simulations for the next decades, the next century, and beyond. Here the crucial problem is that hu- man greenhouse gas and aerosol emissions are quickly moving the climate outside its natural range over at least the past few million years, so it is doubtful that the past can act as a guide to the future. Furthermore, theory, observations, and climate models all point to strong positive internal feedbacks within the climate system that increase its response to changes in its composition. How can we be sure our best climate models can reliably simulate not only the current climate, but also how human influences (presupposing we know what they will be) will change climate? The best indicators are (i) the ability of models to simulate observed climate change of the past 150 years and especially the more rapid and more comprehensively measured changes of the past 30 years, and (ii) the spread in projections made using different climate models or model versions, both taken in the context of paleoclimate observations and simulations that suggest circumstances that may favor abrupt or rapid changes in climate regime. Comparisons of multiple state-of-the-art models against one another (and observations) advance understanding of the climate sys- tem and help build trust in model projections. Intermodel differences provide a lower bound on the uncertainty of climate projections. They may miss sources of error common to all current models; one might hope that, as climate models become more comprehensive, the likelihood of such errors diminishes as long as the model compo- nents and their interactions are carefully tested against observations. Chapter 10 of 37

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G BOX 1.2  HOW DO CLIMATE MODELS GET EVALUATED? As the IPCC report Climate Change 2007: The Physical Science Basis notes, “there is consider- able confidence that climate models provide credible quantitative estimates of future climate change, particularly at continental scales and above” (IPCC, 2007c). There are three primary reasons for this confidence: (1) As noted above, the fundamentals of a climate model are based on established physical laws, such as laws of conservations of energy, mass, and momentum. (2) Climate model simulations are routinely and extensively assessed by being compared with ob- servations of the atmosphere, ocean, cryosphere, and land surface (Figure 1). (3) Climate models are able to reproduce features of past climates and climate changes (e.g., the warming of the past century, and the mid-Holocene warming of the Northern Hemisphere 6,000 years ago). FIGURE 1  Climate model development and testing involves multiple stages and the contributions of the model development community, the model user/evaluation community, and the data community. SOURCE: Jakob, 2010. 38

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Introduction BOX 1-2  CONTINUED Current climate models are calibrated during their development process to match observa- tions within reasonable uncertainty ranges. However, the warming to date due to greenhouse gas increases has been partially compensated by an uncertain amount of cooling caused by human-induced enhancement of light scattering by aerosols and by their effect on clouds; this compensation has been estimated to be from 20 to 70 percent (with 90 percent confidence) based on a range of observational and model-based studies (IPCC, 2007d). Over the 21st century, global aerosol emissions are expected to not increase further, but greenhouse gas emissions are likely to accelerate for at least the next few decades, so this compensation will become less significant. Because of the uncertain cooling by aerosols the current warming cannot be used to constrain the “climate sensitivity.” Thus, the simulated 21st-century global-average warming varies across the international suite of climate models with a range of approximately 30 percenta as is further discussed in Chapter 4. Models provide quantitative estimates of future climate change, but with significant sources of uncertainty—lack of knowledge, or imperfect knowledge about specific quantities or the behavior of a system. These include the uncertainty in the “forcing” on the climate system from future greenhouse gas and aerosol emissions, as well as natural processes such as volcanic erup- tions and solar variability, used as inputs to climate models; the uncertainty in the climate system response to this forcing; the uncertainty from natural internal variability of the climate system; the uncertainty from incomplete representations of known but complicated and small-scale processes (such as cumulus clouds) and of poorly understood processes (such as ice nucleation in clouds); and the uncertainty from “unknown unknowns” (see Chapter 6 for more information on uncertainty). a More specifically, the interquartile range is 30 percent of the mean, where the interquartile range is a measure of statistical dispersion, and measures the difference between the 75th percentile and the 25th percentile of the data. 39

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G IPCC (2007c) discusses some other strategies that are also used to estimate or bound model uncertainties. Figure 1.7 shows which aspects of climate can be most robustly predicted, separated by phenomenon and time scale, based on such assessments. In general, climate models more robustly predict trends at larger space and time scales, and they predict tempera- ture trends more reliably than precipitation trends. They all project a reduction in sum- mer sea-ice extent, but not as large as that observed in recent years. They robustly pre- dict the contribution to global sea-level rise from heat uptake in the oceans, but most do not include a representation of ice-sheet melt and the disintegration of the tongues of large glaciers that may considerably accelerate sea-level rise over the next century. They agree that the polar regions will become wetter and that the subtropics will become drier, but they do not agree on which regions of the subtropics will experience strong drying. As climate models become more comprehensive and their grid scale becomes finer, they can provide meaningful projections of more parts of the climate response and their possible feedbacks on the overall climate system, but this does not necessar- FIGURE 1.7  Time and space scales of key climate phenomena. Color coding shows relative reliability of climate model simulations of these phenomena (or their statistics in the present climate, for climate variability/extremes). 40

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Introduction ily reduce projection uncertainty about some aspects of climate change. Indeed, global climate sensitivity, defined as the global warming simulated by a climate model in re- sponse to a sustained doubling of atmospheric CO2 concentrations, still shows a similar 30 percent spread3 across leading models as it did 20 years ago. Climate Information Delivery to Users Although a number of aspects of the climate system can be projected with some degree of confidence, this climate information may not be useful for making deci- sions. As climate models have become more ambitious, so have their users. Many users of climate model outputs need to make decisions on how or whether to respond to climate change, in some cases within institutions where the reality or importance of climate change is not universally acknowledged. Users consider the information from the climate models a valuable commodity, but they are not always sure what data are available to them or how to best use them to inform their decisions. The research community, both by limited capacity and by culture, is often hard pressed to respond to the desires of the user community for new types of model output at high time and space resolution. Quantifying uncertainty in climate projections is still a multifaceted research problem, making communication of relevant uncertainties with diverse user groups challenging, especially when these uncertainties are perceived to be discour- agingly large or the climate model output is only part of a modeling chain. WHY THIS STUDY? With many studies and reports showing that there will likely be significant impacts as a result of climate change (IPCC, 2007a,b,c; NRC, 2010a,b,d,e, 2011a), now is an appropriate time to examine the capabilities of the nation’s climate modeling enter- prise to ensure that it is advancing adequately. The modeling community has already developed plans to make continued progress over the next 3-5 years. However, both the climate science and applications communities would enormously benefit from a major advance in improving the usefulness of climate projections, especially on regional space scales and decadal time scales and including trends in extreme events. Is this possible? Is this likely? How can the United States best position itself to advance and better use climate models? What resources and planning will that take? The need has arisen for a forward-looking, comprehensive, strategic assessment of how best to improve the United States’ capabilities to simulate past, present, and future climate on local to global scales and at decadal to centennial time scales. 3  This is a 30 percent interquartile spread; see previous footnote for definition of interquartile. 41

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G In recognition of this need, the Committee on a National Strategy for Advancing Cli- mate Modeling was tasked by NOAA, NASA, DOE, NSF, and the intelligence community to produce a high-level assessment, providing a strategic framework to guide progress in the nation’s climate modeling enterprise over the next 10-20 years (see Appendix A for the full statement of task). STRUCTURE OF THIS REPORT In response to the statement of task for this committee (Appendix A), this study has built upon recent efforts to engage and coordinate the national and international climate modeling community, recent NRC and interagency reports that have made recommendations about both U.S. climate modeling and its role in the broader and more diverse climate research and applications communities, and recent actions and progress by federal agencies and other domestic groups. Ultimately, the report at- tempts to provide a coherent set of recommendations understandable to nonexperts (Box 1.3 includes the definition of a number of key terms), and to set out a compre- hensive, unified, and achievable vision for climate modeling for the next decade and beyond that can form the basis of a national strategy that advances climate models, climate observations,4 and user needs. To obtain advice from a broad spectrum of climate modelers, researchers using climate model output, and the diverse and growing community of users of climate model outputs and projections, the committee convened a 50-person community workshop to engage with leaders from the modeling and user communities. During day-long open sessions at four other meetings, the committee heard from other stake- holder groups, both nongovernmental and from various levels of government, that are trying to use climate projections for long-term planning (Appendix B has more detail on the information-gathering process). The presentations and discussions encom- passed global and regional models, downscaling, computing and data, user needs and education, the role of the private sector, and cultivating a coordinated national model- ing and user community that spans many goals and applications. 4  One cannot consider advancing climate modeling without attention to the supporting climate obser- vations, both space-based and in situ, needed to initialize, force, and validate climate models, as well as for monitoring climate variability and change. The United States currently does not have a coordinated climate observing system, or a strategy that could lead to a coherent system, across both in situ and remotely sensed observations. As noted in the report Improving the Effectiveness of U.S. Climate Modeling (NRC, 2001b): “the lack of a suitable sustained observing system for climate limits progress in climate modeling.” This statement still rings true today, and therefore this report only discusses observations at a high level. 42

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Introduction The committee was charged with examining “decadal to centennial” time scales (Appendix A) but decided to extend the report to also touch on shorter time scales, including intraseasonal to interannual (ISI) time scales, even though ISI climate predic- tion has recently been assessed by another NRC report (NRC, 2010c). Four motivations for this were the following: (1) Seasonal to interannual prediction is a valuable test of climate models, because phenomena that evolve on this time scale like El Niño have been well observed for more than 25 years, during which they have gone through enough cycles to allow the seasonal prediction skill of climate models to be tested and compared. (2) Decadal prediction of climate is a natural extension of interannual climate prediction, because it also requires a detailed initial knowledge of the ocean state. (3) For many users, simulation of climate variability about the long-term trends we project is also very important; ISI simulations observationally test aspects of the skill of climate models in predicting this variability. (4) Currently, ISI climate prediction is a nexus between U.S. operational weather and climate forecasting at short time scales (e.g., as performed at NCEP) and research-oriented climate modeling at long time scales. Hence, it may be a fruitful arena to explore closer interactions between the operational and research modeling communities. This report is structured in three sections. In addition to the introduction in this chap- ter, this first section reviews the history of previous reports as context for this report (Chapter 2). Building on that background material, the second section of the report examines a number of the issues that are currently facing the U.S. climate modeling community. These issues include climate model hierarchy, resolution, and complex- ity (Chapter 3); scientific frontiers in climate modeling (Chapter 4); integrated climate observations (Chapter 5); characterization, quantification, and communication of uncertainty (Chapter 6); the climate model development workforce (Chapter 7); the relationship of U.S. climate modeling efforts with international efforts (Chapter 8); and operational climate prediction systems (Chapter 9). This final section of the report examines several key issues in the U.S. climate model- ing enterprise where the committee presents its primary recommendations and an overarching national strategy for advancing climate modeling in the United States over the next two decades. These issues include the challenges and opportunities related to computational infrastructure (Chapter 10), unified climate modeling (Chap- ter 11), interfacing with the trained climate model user and educational communities (Chapter 12), and optimizing U.S. institutional arrangements (Chapter 13). A number of specific recommendations are presented throughout the text. These recommenda- tions are synthesized into an overarching strategy in the final chapter of the report (Chapter 14). 43

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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G BOX 1.3  DEFINITION OF KEY TERMS Boundary conditions: External data input into climate models that define conditions that are fixed relative to the dynamic elements of those models. In the case of Earth system models, the boundary conditions define the orbit of Earth, the land/ocean cover, the height of the moun- tains, drainage basins and paths of rivers, and the radiation from the sun, among other things. See also Forcings. Climate models and Earth system models: Climate models are computer codes that encapsulate the physical laws governing the motions and cycles of energy and water in the atmosphere, ocean, and land surface, including sea ice and snow. Earth system models are climate models that additionally incorporate representations of the chemical and biological processes that con- trol the cycling of human-produced and natural aerosols, as well as biogeochemical substances including carbon, nitrogen, and sulfur. Some Earth system models also represent ice sheets and climate-induced changes in the distribution of different types of vegetation. Climate predictions and projections: Climate predictions are model simulations that are started from our best estimate at the state of the climate system at a particular time. Climate projec- tions, on the other hand, are simulations started from a statistically representative initial state. Both predictions and projections are made using estimates of future values of the forcings. The goal of projection is to look at the statistics of the simulated climate and how they change; the goal of prediction is to forecast the evolution of the actual climate state, including variations in El Niño or the Atlantic meridional overturning circulation. Common modeling framework: A group of programs that provides a high-performance, flexible software infrastructure, which enables climate models to run on very large parallel computers and that supports coupling diverse, modular climate model components. Data assimilation: The process of making best use of observational data to provide an estimate of the state of the system that is compatible with a given model and that is better than could be obtained using just the data or the model alone. Forcings: External data input into climate models that drive climate variations and change (e.g., greenhouse gas concentrations, volcanic aerosols, and solar irradiance variations). Model fidelity: The measure of agreement between the statistical distributions of a climate vari- able or group of variables as simulated by a model compared with observations (e.g., the seasonal and geographical root-mean-square difference between simulated and observed rainfall over 1980-2010). Model forecast skill: The typical accuracy of a forecast, e.g., as measured by the agreement be- tween realistically initialized model predictions of some variable (e.g., winter-mean surface air temperature over Kansas based on model predictions from 6 months before) and their corresponding verifications over some period. The relation of fidelity to skill is similar to that between prediction and projection. In particular, model fidelity (correctly predicting the statisti- cal distribution of this quantity) need not imply model skill (skillfully predicting warm winters when they are observed).a 44

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Introduction BOX 1-3  CONTINUED Multimodel ensemble: A set of simulations from several different models, forced by the same ex- ternal forcing. Considerable evidence suggests that an average over simulations from different models produces a better match to observational climatological distributions than similar-sized averages of simulations from any single model. Perturbed physics experiments: Multiple simulations from the same models using a plausible range of parameters or representations of physical processes. These simulations make it possible to analyze the sensitivity of simulation results to some of the choices made in model development. Operational climate prediction: Distinct from climate model research and development, opera- tional climate prediction is a regularly scheduled, user-driven, product-oriented process that conforms to a specified schedule of generation and delivery of products and that depends on dedicated computing and information delivery resources with failsafe contingency plans. Parameterization: The process of representing the effects of processes other than resolved-scale fluid motion (e.g., cumulus cloud dynamics and microphysics; land-surface or sea-ice modeling; or transfer of heat, salt, and nutrients in unresolved oceanic eddies) in a climate model by using the resolved fields whose time evolution is predicted by the model. Reanalysis: The process of reassimilating historical observations of atmospheric and oceanic quantities such as temperature, pressure, wind, humidity, current, and salinity using fixed state- of-the-art models and data assimilation techniques to produce long time series of global fields. Regional climate models: Climate models that are restricted to a portion of the globe so as to reduce the computational cost and thereby increase the spatial resolution, and which use the output of a coarser-resolution global climate model at their boundaries. Such models are often used in “downscaling,” the process of representing global climate model output at the relatively small spatial scales that are more relevant to decision makers. Regional climate models some- times include greater scientific complexity that can inform particular applications and decision makers. Seamless prediction: Viewing weather and climate prediction as problems that share common processes and dynamics and that can be addressed using modeling approaches that span a broad range of time scales and spatial resolutions. Tuning: The process of adjusting the values of parameters used in climate models to achieve the best fit to observations in a dependent control data set. The values are adjusted only within the range of observational uncertainty of those parameters. Uncertainty: Lack of knowledge or imperfect knowledge about specific quantities or the behavior of a system. Unified modeling across time scales: The ultimate realization of seamless prediction whereby a sin- gle climate model is used to predict the weather, seasonal climate, and decadal climate change. a There is some evidence that model fidelity and prediction skill are related (see DelSole and Shukla, 2010). 45

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