Multiple Dimensions of Climate Change and Their Implications for Biodiversity

Background Changes in Earth’s climate over time can be measured in many ways. The different metrics available represent alternative dimensions of climate change, each with distinct implications for biodiversity conservation and other sectors. However, this diversity is rarely recognized. At any given locality, average temperature or precipitation can increase or decrease, extreme values can become more intense or frequent, and the timing of specific climatic events can shift. At the same time, climatic conditions are redistributed at broader spatial extents. Across sets of localities, particular climatic conditions can become more or less available and can shift closer or farther in position at different velocities. Metrics quantifying these and other dimensions of change are commonly used in basic and applied sciences. In ecological contexts, individual metrics have helped to explain the role of past climate changes in driving species diversity or extinctions and to forecast the exposure of biodiversity to future climate changes. Yet, a comparison of the many alternative metrics in use is lacking to gain understanding of their properties and guide their use in biodiversity assessments. The diversity of metrics of climate change. Climate parameters, such as temperature or precipitation, can change at individual localities over time (left), whereas shifts in the distribution of climatic conditions across sets of localities can also occur (right). Local metrics can quantify changes in the magnitude of average or extreme values, as well as shifts in the timing of climatic events. Regional metrics describe how specific climatic conditions may increase or decrease in area, become more dissimilar to past climatic conditions, or move in space. These and other commonly used metrics of climate change describe different dimensions of change and are expected to relate to distinct challenges for biodiversity. Different metrics thus provide complementary information when describing future climates and their potential effects. Examining metrics in combination can show how they interact to exacerbate or lessen species’ exposure to climate change. Advances Our review demonstrates that six commonly used metrics of climate change show contrasting patterns under 21st-century climate forecasts across the world. For example, whereas polar climates are projected to warm and shrink in area, the tropics see the emergence of novel climatic conditions and undergo local changes in average climates beyond past variability. To help interpret metrics of climate change, our review critically assesses the ecological implications of different metrics. Supported by examples of empirical links between observed changes in biological systems and different dimensions of climate change, we outline a conceptual framework for classification of climate change metrics according to the types of threat and opportunity they are likely to impose on biodiversity. Climate changes at the locality level are often associated with demographic threats and opportunities at the population level, whereas changes across localities can have positive or negative implications for the size and the position of species’ ranges. Outlook Forecasting the long-term impacts of future climate changes on biodiversity is challenging, not least because the responses of organisms are contingent on demographic, physiological, and evolutionary mechanisms, as well as on the interaction with other human-induced stressors such as habitat fragmentation. Lack of data for the majority of species on Earth further hampers the use of available bioclimatic modeling methods. By contrast, the use of simple metrics of climate change is more easily scalable to wholesale biodiversity. When appropriately implemented, such examination can provide a first-order assessment of the challenges that species are potentially exposed to, and in many circumstances, it might be the only option available. Interpreting Climate Change Metrics While forecasts of climate change effects on biodiversity rely mostly on bioclimatic modeling approaches of varying complexity, an alternative to existing models is to use simple metrics to quantify the exposure of regions to climate changes over time and relate them to different threats and opportunities for biodiversity. It remains poorly understood how existing metrics differ in the information they provide, specifically in the context of biodiversity. Garcia et al. (p. 10.1126/science.1247579) review the variety of metrics commonly used to describe climate change in biodiversity-impact assessments covering local changes in climate averages and extremes, regional changes in the availability and position of climates, and the velocity of climate change. While metrics are often arbitrarily chosen in studies of ecology and evolution, and interchangeably used as synonyms of climate change, they capture different dimensions of change and reveal contrasting spatial patterns across the world. Defining the links between climate change dimensions and the challenges they represent to species leads to a framework for interpreting climate change metrics. The 21st century is projected to witness unprecedented climatic changes, with greater warming often reported for high latitudes. Yet, climate change can be measured in a variety of ways, reflecting distinct dimensions of change with unequal spatial patterns across the world. Polar climates are projected to not only warm, but also to shrink in area. By contrast, today’s hot and arid climates are expected to expand worldwide and to reach climate states with no current analog. Although rarely appreciated in combination, these multiple dimensions of change convey complementary information. We review existing climate change metrics and discuss how they relate to threats and opportunities for biodiversity. Interpreting climate change metrics is particularly useful for unknown or poorly described species, which represent most of Earth’s biodiversity.