To evaluate the impacts of climate change, it is essential to first understand the many ways in which our climate is changing. Indicators characterize how environmental conditions are changing over time to help communicate climate impacts, risks, and vulnerabilities. Indicators provide foundational science in support of the US Global Change Research Program’s (USGCRP) sustained assessment process, including the National Climate Assessment (NCA). In the NCA, indicators are defined as data (presented as charts or maps) based on historical observations and measurements that are used to track conditions, trends, and impacts related to our changing climate. Indicators show changes in physical, ecological, or societal systems and can represent data at varying scales, from global to local (Figure A4.1). Indicators are of most value when maintained and updated on a regular basis so that users and stakeholders can make informed decisions based on the most up-to-date information. Taken individually, each indicator depicts a specific change over time, while a broad set of indicators helps to show interconnections among the many components of global change and a warming world (Box A4.1).1 While each indicator has an important relationship to climate,2 it is beyond the scope of this appendix to explore these connections in detail. Attribution and causation related to a changing climate are discussed in other chapters throughout NCA5 (e.g., Chs. 2, 3, 15).
Indicators are needed to support decision-making and describe and monitor impacts on people. Combined with other data, such as demographic and socioeconomic information (e.g., Figures A4.4, A4.14, 11.13), their utility is ever-growing, with many advancements being made in recent years (e.g., Di Napoli et al. 2022; Kenney et al. 2020; Walsh et al. 20203,4,5). Indicators are being developed by USGCRP agencies (Box A4.1), their partners, academic institutions, and state, local, and Indigenous communities. The form, complexity, breadth, and design of indicators vary based on their intended uses, but effective indicators clearly convey information to advance understanding related to changes in key aspects of climate and the impacts on people and ecosystems. Measuring the health and societal effects of climate change is challenging because of the complex, often indirect relationships among climate drivers, environmental and social factors, and health outcomes.6,7 Increasingly, physical, ecological, and socioeconomic data are being linked together to better track impacts on human systems, allowing communities to assess risks and make informed response and adaptation decisions. In the context of human health, identifying sets of indicators along exposure pathways can help track the related nature of exposures and the prevalence of health outcomes.7,8 This holistic approach can provide insights into the extent to which changes in climate affect people and help identify opportunities for public health actions to reduce or prevent exposures and adverse health effects.
Indicators appear in every NCA, with each report offering new ways to evaluate observed changes. This appendix marks the first time a section of the NCA has been established to present and discuss nationally relevant indicators. It highlights the important role of indicators and supports the NCA with scientific evidence, using a representative set of indicators relevant to multiple chapters. Observed changes relevant to each regional chapter can be represented with indicators, as shown in Table A4.1. Indicators in this appendix are grouped into six categories (Atmosphere; Ice, Snow and Water; Ocean and Coastal; Land and Ecosystems; Health; Adaptation and Mitigation), building on NCA4’s Indicators of Change figure (Figure 1.2 in Jay et al. 20181). Examples for each category were selected to cover a diverse range of regions, highlight both existing and newly developed indicators, and focus on topics relevant to urban and rural populations as well as the natural environment. These and many other indicators in NCA5 demonstrate that climate change is happening now (Table A4.1; KM 2.1; Figures 28.1, 30.5).
Stevens, L.E., M. Kolian, D. Arndt, J. Blunden, E.W. Johnson, A.Y. Liu, and S. Spiegal, 2023: Appendix 4. Indicators. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.A4
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Both globally and across the US, temperatures are rising as a result of increasing greenhouse gases (GHGs) in our atmosphere (Figures 1.5, 2.4), primarily caused by human activities (Figures 2.1, 3.1; KMs 2.1, 3.1). Extreme events such as heatwaves, heavy downpours, and severe flooding are also increasing in frequency and intensity (KM 2.2; Figures 2.8, 21.1, A4.8). Atmospheric indicators are used to inform decision-making across a wide variety of scales (Figure A4.1) and often form the basis for assessing trends, impacts, and key risks15 among all sectors.
Greenhouse gas emissions are the primary driver of climate change (KMs 2.1, 3.1), so tracking emissions is fundamental to understanding and responding to climate change. Figure 2.1 depicts the continual increase in carbon dioxide (CO2) emissions by the greatest contributing countries and regions. The Annual Greenhouse Gas Index indicator (Figure A4.3) accounts for global emissions of CO2 and the other major long-lived greenhouse gases from 1979 to 2022 and shows how the cumulative warming effects of these GHGs have been increasing over time.
Long-term observations show that warming due to climate change is unambiguous but is not occurring equally or in all areas (Figures A4.1, 2.4, 2.7). Figure A4.1 shows four different temperature indicators valuable to decision-makers at multiple scales. Globally, annual average temperatures have increased almost everywhere, with the greatest increases seen across North America and the Arctic (Figure A4.1, top left). On the national scale, nights have been warming faster than days (KM 2.2), with the greatest increases in summer nighttime minimum temperatures seen across large areas of Alaska, Florida, and the western and northeastern United States (Figure A4.1, top right). On the regional scale, cooling degree days (a proxy for energy demand used to cool buildings) have increased throughout the Southwest, with California experiencing the greatest changes (Figure A4.1, bottom left). And on a local level, the number of days per year with temperatures reaching 110°F or more have increased throughout Maricopa County, Arizona, with the Phoenix area experiencing notable increases in these extremely hot days (Figure A4.1, bottom right). Additionally, in the United States, the rate at which temperatures increase differs seasonally (Figure 2.4).18 Changes in seasonal temperatures have led to shifts in the seasonality of certain events (Figure A4.13). In some areas, the combination of high humidity and high temperatures is contributing to the emergence of heat index values too severe for human tolerance (KM 2.3).19,20 As temperatures increase, people’s exposure to extreme heat becomes greater (KMs 3.12, 15.1).
These increasing temperatures directly and indirectly impact human health and societal outcomes (KM 2.2; Ch. 15). Populations directly exposed to more heatwaves experience increased heat-related illness and death.21,22 Since the 1960s, the frequency of heatwaves and the duration of heatwave seasons have steadily increased in certain areas (KM 2.2).23 Urban areas experience higher temperatures than surrounding landscapes because many structures such as roads and buildings absorb and radiate heat, exacerbating the effects of increasing temperatures (Figure A4.4; KM 12.1). Additionally, changes in land use in and around urban areas have contributed to temperature hotspots (Figure 6.3).
As temperatures rise, US precipitation patterns are changing, with long-term trends in average precipitation differing considerably by region and by season (Figure 2.4).14 Indicators can show how changes in these patterns may affect different sectors. For example, too much or too little precipitation can impact both crop (KM 4.1) and hydropower (KM 5.1) production. Seasonal activities of sensitive plant and animal species may shift as precipitation amounts and timing change (Figure 8.11). Indicators can also be used to observe how heavy precipitation events are evolving in intensity (e.g., Figure 2.8), frequency (e.g., Kunkel et al. 202214), and duration (e.g., Kunkel et al. 202025). For example, evidence suggests that in recent years, extreme single-day precipitation events are increasing in the United States.26 Heavy precipitation will become increasingly important to local engineering and community planning as the risk of flooding increases in a warmer world;25 as a result, regional- and local-scale indicators are most valuable for informing adaptive actions to protect public health and safety.
Climate change is increasing the frequency and severity of many extreme weather and climate events (Figure 1.7; KM 2.2), including heatwaves,23 heavy precipitation (Figure 2.8; KMs 4.1, 21.1; KM 22.1), drought (Section A4.3), flooding (Section A4.3; KM 4.1), wildfire (Section A4.5; KM 7.1; Focus on Western Wildfires), and tropical cyclones (KM 2.2).27 Other events, such as cold snaps, are becoming less frequent (KM 2.2). Extreme-event indicators allow communities to evaluate changes in risk, as major weather and climate disasters can threaten lives, damage property, and affect daily activities. One example is the number of disasters in the US each year that cause at least $1 billion in damages (Figures A4.5, 1.7, 2.6, 22.3). More frequent and compound extreme events (Focus on Compound Events) disproportionately impact already-overburdened groups (KM 18.2; Box 18.2), leaving communities with less time and fewer resources to respond to each disaster.
Many parts of the US are experiencing intensified droughts or reduced snowpack, which are caused or exacerbated by rising temperatures (KMs 2.2, 4.1). These changes, combined with increasing water demand from growing populations, can reduce the reliability of water supplies,31,32 and water-related indicators can help communities prepare for impacts. Also useful are indicators used to track changes in the cryosphere (the frozen parts of Earth’s surface), as melting sea and land ice (e.g., ice sheets and glaciers) and thawing permafrost can contribute to sea level rise (KM 9.1; Figures 2.5, 9.1), affect water supply (KM 4.1), and have other negative impacts on humans and ecosystems (KM 8.2; Ch. 29).
Changes in Arctic sea ice are some of the most visible and well-known indicators of a changing climate.33,34,35 The steep decline of average Arctic sea ice extent in September, when the ice shrinks to its smallest area each year, is shown in Figure 2.3. Figure A4.6 illustrates how the total area of September sea ice has declined and how the overall length of the melt season is increasing over time. Melt location and timing is important, because as sea ice melts, it changes ocean and atmospheric circulation patterns, which can impact marine life and coastal economies (KM 10.1). Additionally, loss of sea ice is leading to increases in commercial shipping; exploration of oil, gas, and minerals; and geopolitical and global security issues (Ch. 17; KM 29.6).36
As US winters and springs warm, the amount and seasonality of snow is changing (KM 2.2). Higher temperatures cause snow to melt earlier, which affects timing and availability of water (KM 2.2).38 A variety of indicators can be used to track changes in snowpack and snow cover (Figure A4.7). These indicators focus on the western US, where millions of people depend on the melting of mountain snowpack for drinking water, crop irrigation, and hydropower (Ch. 4; KM 28.1). Changes in snowpack and snow cover affect winter recreation, tourism, plants, and wildlife.38
Indicators can be used to quantify trends in large floods, which is imperative for floodplain management and infrastructure design to maximize safety and resilience (KMs 4.2, 6.1, 12.4). For example, west of the Mississippi River, nearly 30% of monitored areas are experiencing increases in large flood frequency and/or magnitude (Figure A4.8). In contrast, other areas have recently experienced decreasing trends due to prevailing climate patterns such as the Southwest megadrought (KM 28.1).39 Flood indicators are also used to track economic damage related to flooding (Figure 4.12) and monitor flood trends of individual streams.40
The effects of drought can be far-reaching and long-lasting, posing risks to people and ecosystems and often contributing to other extreme events, such as drought-induced wildfire (KM 4.2; Focus on Western Wildfires). Several hydrologic measures exist for drought, and for certain applications, climate reanalyses can also be valuable in evaluating historical trends related to climate including drought metrics (e.g., Jasinski et al. 201942). Some drought indicators consider water availability, measured by variables such as precipitation (Figure 4.10), streamflow, groundwater and reservoir levels, or soil moisture (Figure 28.2). Other drought indicators take into account different climatic factors, such as temperature, potential evapotranspiration, and solar radiation (Figure 3.12). For example, the Standardized Precipitation Evapotranspiration Index (SPEI; Figure A4.9) measures the combination of precipitation and evapotranspiration to determine whether a certain area is experiencing extreme drought, extreme moisture, or conditions in between.43 SPEI is a valuable indicator when considering how droughts might affect activities that depend on a balance between water supply and demand, particularly those related to agriculture and ecosystems.44
Climate-driven changes to US oceans and coasts endanger marine ecosystems and coastal communities (Figures 10.1, 21.5) and threaten infrastructure and energy production (KMs 9.2, 10.1). Indicators can be used to track physical ocean conditions (e.g., sea surface temperatures, ocean acidification), ecological impacts (e.g., marine species shifts), and coastal impacts (e.g., high tide flooding). This information is used to evaluate risk, promote resilience, and increase the value of coastal and marine resources as ocean conditions change.
Global sea level is rising as warming ocean waters expand and glaciers and ice sheets melt. Along some US coastal areas, sea levels are rising faster than the global average, with the highest rates occurring along parts of the Atlantic coast and the Gulf of Mexico (Figures A4.10a, 2.5; KMs 21.2, 22.1, 26.1).46 The increase in relative sea level is driving increases in physical and societal impacts such as high tide flooding (Figure A4.10b–d; KM 9.1).
Sea surface temperatures in oceans surrounding the US have risen steadily over time, as they have in most of the world’s oceans (Figure 2.3).48 Rising temperatures in these areas contribute to increases in marine heatwave frequency (KMs 10.1, 21.2), intensity (Figure A4.11), size, and duration.49 These changes have detrimental impacts on surrounding ecosystems and economies, including shifts in the distributions of marine life (Figure A4.12; KMs 8.2, 10.1).
Warming oceans have contributed to shifts in the geographic distribution of marine species (KM 10.1), which are extensively studied and tracked using indicators.50 It is important to track climate-driven changes in the distribution, timing, and productivity of fishery-related species that can put marine fisheries and fishing communities at risk.51 Many marine species are sensitive to environmental cues such as temperature ranges and track well with local climate velocities (the speed and direction at which species move in order to experience similar climate conditions).52 However, several other factors can influence the abundance and geographic distribution of species, such as large-scale fishing practices, ocean currents, changes in habitats, and species’ ability to adapt. Figure A4.12 depicts how multiple species adjacent to the Alaska, northeastern US, and southeastern US coasts have been shifting northward and, in some regions deeper, to cooler waters.
Earth’s land, food, and climate systems are inextricably intertwined (Figure 11.9). Climate and weather shape demand for and distribution of food, fish, and forest products. In turn, commodity production influences the climate via greenhouse gas exchange and land conservation or degradation. These feedbacks are driving the coupled climate–land system toward a host of outcomes for people and society, some undesirable.53 Indicators can help land managers and policymakers identify optimal planning and solutions in the context of changing conditions.
Seasonality refers to recurring seasonal events or processes, such as the blooming of wildflowers in spring.54 The timing, duration, and variability of many seasonal events are changing in response to changing temperature and moisture patterns (Chs. 2, 8; Figure 24.3). Indicators of seasonal change (Figure A4.13) are valuable for understanding relationships between climate and ecosystems and subsequent risks to environmental and social systems.55 Knowledge of these changes is often generated by local and Indigenous populations, who have deep connections to local ecosystems because of their cultural and subsistence practices (Ch. 16).
Wildland fires affect carbon dynamics, ecosystems, biodiversity, and human health (Ch. 7; KMs 6.1, 14.2; Figures F2.1, 28.9). The wildland–urban interface (WUI) is the area where buildings and other developments meet or mix with undeveloped natural areas, including fire-prone vegetation. Over the past several decades, the WUI has grown rapidly,56 expanding in both total area and number of homes. In addition, the annual average acreage burned by wildfires has increased since the mid-1980s.57 Together, these changes have increased risks of loss of life and property damage in many areas across the United States (Figure A4.14). Other important wildfire-related indicators include greenhouse gas emissions resulting from wildfires and prescribed fires (Figure 7.2) and related socioeconomic indicators such as federal spending on wildfire suppression.58
The climate–agriculture–food system is complex (Figure 11.9). Agricultural production and natural resources face challenges from increasing climate variability and change. Optimized management and policy decisions require an integrated indicator system that communicates climate-driven production impacts; trends in the social-ecological systems underpinning agriculture (e.g., heat-related mortality of workers; Figure 11.1); crop insurance payments (KM 11.2);5 how management performs in relation to desired social-ecological conditions;59 degree of adaptation (KM 11.1); and the relationships among climate change, consumption, and production.60 Currently, the most developed agriculture-related indicators are production-oriented (Ch. 11), such as range or crop yield, crop pathogens, animal heat stress, migration of plant hardiness zones (Figure 11.3), timing of budbreak in fruit trees, and ratios of outputs to inputs (total factor productivity).5 The productivity of rangeland vegetation provides many valued ecosystem services,61 but it has severely declined in some areas of the US in recent decades (Figure A4.15),62 with a strong correlation to regional-scale climate change exposure.63
Climate change increases risks and impacts to human health and well-being by exacerbating existing health threats and creating new challenges based on multiple factors and pathways. A variety of health outcomes are affected by climate change, including mental health challenges as well as physical health issues such as cardiorespiratory conditions from poor air quality, injuries and mortality from extreme weather events, and malnutrition from changing climate and environmental factors.15
It is important that health indicators include more than just measures of health outcomes to understand how climate impacts and exposures influence health burdens. A broader approach is helpful because of the complex, often indirect relationships among climate drivers, environmental and social factors, and health outcomes, and because of challenges with collecting and reporting health data including lag times in availability. Some widely utilized health indicators include heat-related illnesses and deaths,21,22,65 described in part in NCA4, which also details the impacts of a changing climate on vector-, water-, and foodborne diseases but without quantitative context.66 To build upon this body of knowledge and to highlight robust examples of infectious disease metrics with 1) strong science supporting the linkages among climate, environment, and human risk factors; 2) national coverage; and 3) ample temporal extent, this appendix presents indicators for three nationally notifiable infectious diseases routinely reported to the CDC (Figure A4.16).
Changes in temperature, precipitation patterns, and extreme events can alter the seasonality, distribution, and prevalence of vector-, water-, and foodborne diseases (KM 15.1).7 West Nile virus (WNV) neuroinvasive disease and Lyme disease are impacted by climate change through complex shifts in land use, vector ecology, and human behavior (Chs. 8, 15). Vibriosis, linked to warming marine and coastal waters, is an illness contracted through exposure to Vibrio bacterial species from contaminated seafood or from open skin wounds exposed to contaminated water.7
Adaptation to promote climate resilience of populations, ecosystems, and infrastructure as well as mitigation to reduce emissions are critical, particularly for protecting human well-being and the environment (Chs. 31, 32). Indicators of adaptation and mitigation are important tools that help track and assess progress70,71,72, as well as evaluate adaptation decisions and improve resilience (KM 31.5). This can be done, for example, by aggregating the number of documented adaptation activities by state over a certain time period (Figures 1.3, 31.1, 32.20). Although valuable for decision-making and evaluating effectiveness, indicators of resilience, adaptation responses, and adaptive capacity remain relatively limited.72,73,74 Figure A4.17 is an example of a mitigation indicator showing how US energy production from renewables has increased in recent years (KM 32.1; Figures 26.6, 32.3).
Climate change disproportionately impacts certain communities and populations (Ch. 20). Social, environmental, and economic factors76,77,78 contribute to disparities experienced by groups at greater risk of climate change stressors (KM 15.2). Indices that combine multiple variables have been developed to capture complex issues affecting communities that are overburdened (e.g., Figures 15.5, 22.12). Furthermore, indicators that couple human and social dimensions with climate data (e.g., Figures 11.13, 12.6, 22.18) are necessary to better assess who is at highest risk from impacts and to prioritize and evaluate response decisions.
It is vital to recognize data specific to Indigenous communities to adequately address the disproportionate impacts of climate change (Ch. 16).79 Indicators drawing from Indigenous Knowledge (KM 16.3)80 and focusing on the concept of cultural keystone indicator species81 may better represent the perspectives of Indigenous Peoples affected by climate change than the indicators featured in this appendix.
Indicators are used to evaluate community response and preparedness,82 as well as the capacity for socioecological systems to build resilience.83 However, it is difficult to incorporate consistent indicators of resilience and adaptation (e.g., Brooks 2014; Keenan and Maxwell 202184,85). Distilling best practices at the community scale remains a challenge.86 The emerging understanding of compound events and their impacts (Ch. 18) will likely inform new indicator development. Confidence in attributing outcomes to climate change varies among physical climate indicators, especially for societal and ecosystem indicators (e.g., IPCC 202215).
Data sharing and transparency standards arising from the Information Quality Act (IQA; App. 2), are well established for geophysical information and are reliably compiled in several recurring volumes,2,9,87 whereas biological and health information is typically built on local and less-federated data. For health, limitations in sharing data, due to privacy concerns or cost, hinder the creation of nationally consistent indicators.8 Advances in applying IQA standards to nonphysical data will increase the availability and credibility of this information.
Newer observing systems and sensors and community-led (“citizen”) science bring additional data options. In recent years, broader public participation in data collection and curation has played an increasingly important role in contributing to existing or potential indicators. Such efforts include improvements in documenting physical climate variables at finer scales,88 capturing the impacts on or responses of ecosystems, and recording climate-related influences on human health.89
While indicators provide valuable information on past changes, it is important that they be well positioned to provide information on how these changes may continue in the future, to assist with planning, adaptation, and strategic policy decisions. For example, national surveillance systems, such as the National Notifiable Diseases Surveillance System,90 could integrate indicators into existing data collection and analysis processes to advance interpretation of observed data, trends, and impacts. New indicators that track how compound events are changing over time would potentially help communities become more climate resilient (see Focus on Compound Events).
Looking ahead, indicator systems that reflect the coupled nature of climate systems and management systems will be needed for optimal planning and policymaking. This will require integration across disciplines, stakeholder groups, government agencies, and nations.91
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