Strategic Plan for the
Climate Change
Science Program
Review draft, November 2002

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Chapter 6:
Climate Variability and Change

Climate variability and change profoundly influence social and natural environments throughout the world. The consequent impacts on natural resources and industry are large and far-reaching. For example, seasonal to interannual climate fluctuations determine the success of agriculture, the abundance of water resources, and the demand for energy, while long-term climate change may significantly alter landscapes, recreational activities, agricultural productivity, and the services that ecosystems supply. Recent advances in climate science are beginning to provide information for decisionmakers and resource managers to better anticipate and plan for potential impacts of climate variability and change. Further advances in climate sciences will substantially improve our national capabilities to apply science-based information to increase economic efficiency and better protect the environment.

Over the past decade, global change research has indicated that: decreases in Northern Hemisphere sea ice extent exceed what would be expected from natural variability alone; large climate changes can occur within decades or less, yet last for centuries or longer; and the observed global warming during the 20^th century exceeds the natural variability of the past 1,000 years. Moreover, model simulations that incorporate a full suite of natural and anthropogenic forcings have indicated that the observed changes over the past century are likely consistent with a contribution from human activity.

Global change research has also significantly advanced our knowledge of the temporal and spatial patterns of climate variability. Substantial improvements in our ability to monitor the upper tropical Pacific Ocean now provide the world with an "early warning" system that shows the development and evolution of El Niño-Southern Oscillation (ENSO) events as they occur. This improved observational system, together with a greatly improved understanding of the mechanisms that produce ENSO, have led to skillful climate forecasts at lead times of up to a few seasons. This developing capability has given the world an unprecedented opportunity to prepare for, and reduce vulnerabilities to, this major natural climate phenomenon.

Research supported by the US Global Change Research Program (USGCRP) has played a leading role in these scientific advances, which have provided new climate information to help the public and decisionmakers better anticipate and mitigate potential effects of climate variability and change. While progress in this area has been impressive, there still remain many significant unresolved questions about key aspects of the climate system, including some that have enormous societal and environmental implications. For example, we are just now beginning to understand how climate variability and change may influence the local and regional occurrence and severity of extreme events such as hurricanes, floods, droughts, and wildfires. We have identified several major recurrent natural patterns of climate variability other than ENSO, but do not yet know to what extent they are predictable. Our predictive capabilities at local and regional scales show promise in some regions and for some phenomena, but are still quite poor in many instances. We have yet to obtain confident estimates of the likelihood of abrupt global and regional climate transitions, although such events have occurred in the past and, in some climate model simulations, have been projected to occur within this century. Perhaps most fundamentally, we do not yet have a clear understanding of how these natural climate variations may be modified in the future by human-induced changes in the climate, particularly at regional and local scales, and how emerging information about such changes can be used most effectively to evaluate the vulnerability and sustainability of both human and natural systems.

The transformation of knowledge gained from climate research into information that is useful in supporting decisions presents many challenges, as well as significant new opportunities to forge essential relationships between the climate research community and the rapidly expanding base of public and private sector users of climate information. For continued progress over the next decade, research on climate variability and change will focus on answering two overarching questions:

Providing policy and decision-relevant answers to these questions will require new research infrastructure that includes:

In addition, a coordinated research management effort will be essential to ensure a broad-based and collaborative research program spanning academic institutions, government laboratories, and other public and private sector expertise in order to provide sustained basic research into the mechanisms of climate processes and their interactions; and advanced graduate and post-doctoral training for the next generation of climate scientists.

The research effort will require improvements in paleoclimatic information as well as modern observational data systems, because in general the latter have been present for too short a time to extract robust features of climate variability on decadal time scales, or to identify climate variability on centennial to millennial time scales. For example, in the Arctic, few climate stations have records extending back beyond 50 years but those that do indicate that the Arctic warmed by about 1� -- 2�C between 1910 and 1945. Paleoenvironmental data collected from a network of lakes, wetlands, tree-ring sites, ice cores, and marine sources further demonstrate that both the magnitude and spatial extent of 20^th century Arctic warming may be unprecedented over the past 400 years.

As described in the following section, the overarching policy-relevant questions in the areas of climate variability and change can most effectively be addressed by focusing attention on five key science questions and their associated research objectives.

The range in estimates of climate sensitivity accounts for a major part of the range of projections for long-term changes in the climate. Climate sensitivity is a measure of the climate's response to changes in the Earth's radiative balance, (e.g., the change caused by a doubling of the atmospheric concentration of carbon dioxide (CO₂)). Past research has identified important climate feedback processes (e.g., cloud formation, atmospheric convection, and ocean circulation) that amplify or diminish the influence of radiative perturbations. World-class climate models exhibit a large range in the estimates of the strengths of these feedbacks, with the major US models used in recent Intergovernmental Panel on Climate Change (IPCC) assessments lying close to the opposite ends of this range. The uncertainty that this range in climate sensitivity introduces to the overall findings makes US models an ideal setting for investigating sensitivities to feedbacks. In addition, all current climate models fail to accurately simulate certain climate system processes and their associated feedbacks due to anthropogenic forcing.

Among the least well-represented processes are ocean mixing, which to a large degree controls the rate of projected global warming; and atmospheric convection, hydrological, and cloud processes, which strongly influence the magnitude and geographical distributions of global warming. These deficiencies are thought to be related to both limits in understanding the physics of the climate system and insufficient fine-scale treatment of the key processes, together contributing significantly to model uncertainties in projections of climate change. As a result, limitations in model representations of climate feedbacks and climate sensitivity create significant uncertainties in estimating the impacts of future climate change, in consideration of response strategies, and ultimately in formulation of optimal environmental and energy policies.

High priority research will focus on several sub-questions:

This research will require the undertaking of coordinated observation, process, and modeling programs by teams of scientists with diverse interests and focused common goals. One mechanism for focusing the research will be through Climate Process Teams (CPTs). CPTs will enable the research community to work together to rapidly identify, focus attention on, characterize, and ultimately reduce uncertainties in climate model projections. For problems that are generic to all climate models, the teams of climate process researchers, observing system specialists, and modelers will work in partnership with designated modeling centers (see also Chapter 4, section on Applied Climate Modeling).

Simulations of past climate events as documented in observed or paleoclimatic data, and for which estimates of climate forcings have been obtained, are an effective and practical means for assessing the scientific credibility of climate models. Such simulations also enable detailed investigations of naturally recurring modes of climate variability. Past research has identified a few modes, or patterns, of variability, which have a disproportionately large influence on global and regional climates. These include ENSO, the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), the Pacific Decadal Oscillation (PDO) and the monsoon systems. As the global observation network becomes more complete and long-lived, further exploration of Southern Hemisphere modes will provide a clearer perspective of global climate variability.

Our knowledge of the mechanisms and processes that produce and maintain these natural climate modes is limited, and thus model simulations and projections inadequately represent their influences. This increases the uncertainties in climate projections and in estimates of the limits of climate predictability. In addition, while the models simulate reasonably well statistics of observed global average characteristics of climate variability and the global average structure of climate trends, important details of seasonal and regional-scale variability are poorly simulated. Indeed, the predictability of regional climate and of coupled climate system behavior is just beginning to be studied, although such issues are fundamental to addressing many of the "If..., then..." questions posed by decisionmakers. This research poses major science challenges because of the less-advanced states of coupled and regional climate models relative to models of the global atmosphere.

High priority research will seek answers to the following subsidiary questions:

Essential needs include the development of, and support for, long-term, sustained climate modeling and observing systems; retrospective data including new high-resolution paleoclimate datasets; field observations and process studies, and current operational data necessary for this research; and focused research efforts (e.g., CPTs) to improve climate prediction and projection models. Instrumental sea level observations, geodetic reference frame measurements, ice sheet and glacier volume estimates, as well as advanced modeling are required to further refine sea level change projections. Other research needs include data sets from ensembles of extended model simulations, and an updated, consistent reanalysis product suitable for climate studies, ideally including all of the 20^th century.

Research to address Questions 1 and 2 will provide essential support to the United States and international decisionmakers and resource managers and will assist climate assessment efforts by increasing understanding of critical processes required to evaluate and improve major climate models (see Chapter 4, and Chapter 11).

Paleoclimatic data have revealed that abrupt regional-to-global climate changes have occurred often in the past, and some models suggest the possibility for abrupt changes during the 21^st century. We have learned a great deal about the structure and geographic extent of past abrupt climate changes, but much remains unknown about their causes and probabilities, leading to subsidiary questions such as:

Improved paleoclimatic information will be essential for analyzing past abrupt climate change. This research will also require the development and implementation of expanded observing and monitoring systems, particularly for key regions or phenomena that may be especially vulnerable or contribute most strongly to abrupt climate change, such as the tropical oceans, the Arctic and Antarctic regions, and the thermohaline circulation of the ocean. Moreover, significant research into how to numerically model the full three-dimensional circulation of the ocean will be required in order to accurately project the time scales and impacts of abrupt changes in thermohaline circulation.

Past research has revealed strong relationships between major modes of climate variability and extreme events; for example, between ENSO and severe flooding in otherwise dry regions, and between the AO/NAO and extreme temperature anomalies in high latitude regions. Limited progress has been made in developing methods to downscale information provided by climate models to spatial and temporal scales relevant to those of extreme weather and climate events, including droughts, floods, heat waves, wildfires, hurricanes, and storm surges. Scientific understanding is currently inadequate to answer subsidiary questions such as:

This research requires high-resolution observations in key regions and sectors to evaluate regional projections; and improved capabilities to model climate variations and change on regional and local scales through finer global model resolution, nested model approaches, and/or other downscaling techniques. This research also requires extensive hydrological data sets and more sophisticated coupled physical climate-land surface-hydrology models (see Chapter 7).

Research in this area focuses on climate information needs for integrated assessment and risks management. Ongoing assessment activities have focused on particular end users, such as water managers, to determine how scientists can accelerate development of products that are more useful to decisionmakers, and thereby improve the value of climate information that can be provided to address a broad range of social, economic, and environmental issues. Outstanding questions include:

The scientific underpinnings for this research are the observational, diagnostic, and modeling expertise required to develop new product lines at regional levels, link global to regional climate variability and change, and infuse advances in science and technology into new climate information products. Increasing understanding of regional climate variability under current conditions is vitally important in developing downscaling methods for future climate scenarios derived from climate change model simulations, interpreting how the regional climate changes are likely to produce societal and environmental impacts, and thereby clarifying options for adaptation or mitigation strategies.

Major research needs will be to improve capabilities to describe, interpret, and predict climate variability and change and their potential consequences at regional scales, much of which is contingent on improving understanding of the range of fundamental scientific issues associated with global climate change that are outlined in this strategy. Regional "test beds" or "enterprises" will be required to develop and evaluate the effectiveness and potential use of climate information at regional scales. Such test beds will enable more effective, sustained interactions between the climate research community and the rapidly expanding base of users of climate information, particularly on regional to local scales.

Owing to the complex and coupled nature of the climate system it is critically important for the Climate Variability and Change research community to work cooperatively with other Climate Change Research Initiative and USGCRP research elements and other programs. Chief among these are the Water Cycle (Chapter 7), Carbon Cycle (Chapter 9), and Atmospheric Composition (Chapter 5) elements and other national and international programs that contribute to climate observations and research.

Water is a key element in all five Climate Variability and Change questions. As noted in Chapter 7, the water cycle is an integral part of the Earth's climate system (through processes involving, for example, evaporation, clouds, precipitation, snow packs, groundwater, floods, and droughts, and through feedbacks and interactions involving them). Atmospheric Composition and the Carbon Cycle are also key elements for Questions 1, 2, and 3. Interactions with other research elements, specifically Ecosystems (Chapter 10), Land Use/Land Cover Change (Chapter 8), and Human Contributions and Responses (Chapter 11), are also required to successfully implement the Climate Variability and Change research agenda. Moreover, internationally coordinated research programs such as the World Climate Research Programme (WCRP) and its projects Climate Variability and Predictability (CLIVAR), Stratospheric Processes and their Role in Climate (SPARC), Climate and Cryosphere (CliC), the Global Energy and Water Cycle Experiment (GEWEX); as well as the International Geosphere-Biosphere Programme (e.g., PAGES paleoscience project), are critical for developing global infrastructure and research activities designed to ensure that global aspects of climate variability and change are addressed.