Assessing climate impacts

Abstract

Assessing climate impacts involves identifying sources and characteristics of climate variability, and mitigating potential negative impacts of that variability. Associated research focuses on climate driving mechanisms, biosphere–hydrosphere responses and mediation, and human responses. Examples of climate impacts come from 1998 flooding in the Yangtze River Basin and hurricanes in the Caribbean and Central America. Although we have limited understanding of the fundamental driving-response interactions associated with climate variability, increasingly powerful measurement and modeling techniques make assessing climate impacts a rapidly developing frontier of science.

Climate affects every aspect of our lives, yet we have relatively little understanding of the mechanisms driving the atmospheric and oceanic circulation patterns that create climate. Climate can be defined as a set of averaged quantities (variances, correlations, etc.) that characterize the structure, behavior, and interactions of the atmosphere, hydrosphere, and cryosphere over a period (1). Climate may be characterized in terms of meteorological parameters such as precipitation or temperature, or by statistical properties such as means, extremes, or cycles of varying periodicity. If climate were relatively consistent through time it would be easy to predict and adjust to. Instead, climate exhibits complex variability across a range of temporal and spatial scales. The major emphases of climate research have been to identify these scales of variability and any consistent patterns that may be present, and to determine the cause-and-effect relations associated with the variability. Understanding gained through this research may then be applied to predicting climate variability and mitigating potential negative impacts of that variability. Research related to climate impacts thus falls into three categories, focusing on climatic driving mechanisms, biosphere–hydrosphere responses and mediation, and human responses.

Research on Climate Impacts

(i) Driving mechanisms of climate variability may be associated with astronomical parameters such as the variations in the tilt of the Earth's axis, which govern the distribution of solar radiation on the planet's surface; atmospheric phenomena such as the seasonal migration of tropical cyclones; geophysical perturbations such as an increase in submarine volcanism that alters ocean temperature and circulation patterns; and biological processes such as the area covered by bogs, in which carbon is removed from the atmosphere and fixed in peat.

(ii) Climate variability may have a direct impact on humans, as when a tornado destroys a house, or the variability may be expressed indirectly through a change in the availability of water or food. Research on biosphere–hydrosphere responses and mediation examines how climate variability is filtered through surface and groundwater systems, and through biotic communities. A strong El Niño–Southern Oscillation (ENSO) circulation pattern, for example, creates drought conditions in northern Australia, New Guinea, and Indonesia, which in turn affects regional water tables and streamflow, and the ability of crops and wild plants to survive. In many of the world's arid and semiarid regions, human consumptive pressures on water supplies make it imperative to be able to predict hydroclimatic variability to store sufficient water to meet both human needs and the needs of natural communities (minimum streamflow for endangered fish, for example) during periods of drought. The response of a biotic community to climate variability and human land-use may also create an enhancing or arresting feedback with that variability, as when over-grazing and drought combine to convert savanna vegetation into desert lands, which in turn reflect more solar radiation and retain less moisture, further enhancing regional aridity.

(iii) Research on human responses to climate variability includes climate forecasting and resource planning for extreme conditions that may have an impact on our ability to grow crops, heat our dwellings, or obtain drinking water, and attempts to mitigate hazards associated with extreme storms.

Records of Climate Variability

Each of the types of climate-impact research outlined above can draw on several types of records of climate variability. Geologic records of past climate include geochemical deposits in which isotopic ratios record temperature and precipitation at the time of deposition (e.g., groundwater, glacier ice, cave deposits); fossils of plant or animal species capable of living only within a narrow range of water temperature and chemistry (marine microfauna or freshwater ostracods), or terrestrial climate (plant pollen); and sediments that record climate as expressed through lake levels, flood deposits, or desert sand dunes (2–5). Systematic records of recent and contemporary climate are direct measurements of such climate parameters as temperature, precipitation, sea surface temperature, and wind speed. As with geologic records, the length of systematic records, and their spatial and temporal resolution, vary with time and location, but the records rarely go back beyond the start of the 20th century. Computer-based simulations of climate, referred to as General Circulation Models (GCMs), have been used both to reconstruct past climate circulation patterns from geologic and systematic records and to predict future climate scenarios by using projections of such parameters as CO₂ content of the atmosphere (6). At present, GCMs are primarily boundary-value problems that focus on, for example, the statistical distribution of storms. Several important climatic controls such as an interactive biosphere have not yet been parameterized and incorporated into the models. However, simulations conducted with these models can be used to test hypotheses of what drives climate variability, and how this variability may affect the hydrosphere, biosphere, and human communities.

Examples of Climate Variability and Human Response

The summer of 1998 was characterized by high-intensity rainfall that lasted for a long period over large areas of China. The Yangtze and Song Hua River Basins were particularly affected by this rainfall. The Yangtze, which drains one-fifth of China's land area, had eight flood peaks, which killed an estimated 4,150 people and caused $30.7 billion US in losses. The heavy precipitation and flooding resulted from anomalously warm sea surface temperatures associated with the 1997/1998 ENSO circulation. Increased sea surface temperatures led to above-average snowfall on the Tibetan plateau, which in turn caused a later, weaker east Asian summer monsoon, and a stronger subtropical high pressure belt. This allowed warm air from the southwest to mix with cold air from the northwest over the Yangtze River Basin, leading to increased precipitation.

The destruction associated with the 1998 flooding highlights the importance of China's research programs (1986–2000) focusing on assessing the effects of climate change on water resources and on adaptive responses. These programs are organized on the conceptual model shown in Fig. 1. Effective implementation of this model requires (i) an understanding of hydrologic response to climate change, (ii) identifying regions vulnerable to climate impacts (water resource use and flood control), and (iii) developing adaptive solutions (response activities). Techniques particularly important to implementation, which are still being developed, include (i) downscaling methods that permit continental- or global-scale GCMs to be scaled to the level of drainage basin hydrologic models, (ii) evaporation models, (iii) distributed hydrologic models employing daily or monthly time steps at a spatial resolution of 30 × 30 km or smaller, and (iv) models of climate impacts on water quality.

Figure 1.

Conceptual model for China's research program on assessing the impacts of climate change on water resources.

A second example of climate variability having an impact on humans comes from hurricanes in the Caribbean, Central America, and the United States. Hurricanes in this region are not evenly distributed in time, as evidenced by decadal-scale periods of high (e.g., 1930s, 1940s) and low (e.g., 1970s, 1980s) hurricane activity. However, human vulnerability to hurricane hazards can change even without changes in hurricane magnitude and frequency. Vulnerability to climate is constructed from the following: (i) societal and environmental factors that precondition the degrees of impact; (ii) the exposure or location of property in regions of risk; (iii) the timing, magnitude, and duration of physical risks; (iv) the capacity to respond and recover; and (v) the conjunction of two or more of these factors (7, 8). A key question that must be asked when assessing hurricane or other climate impacts is whether the strategies and assumptions for hazard mitigation are appropriate, given the actual event frequency and social vulnerability. Lessons that can be drawn from past events may help us to answer this question, and may demonstrate how choices for hazard mitigation may be constrained by the physical environment at a given stage of technology (theoretical constraints), and by existing culture and institutions (practical constraints).

Assessing Climate Impacts: A Frontier of Science

Assessing climate impacts may be thought of as a frontier of science in that, although we do not yet understand many of the fundamental driving-response interactions associated with much climate variability, increasingly powerful techniques for measuring and modeling large-scale climate parameters, and for reconstructing past climate conditions, are rapidly being developed. These techniques of paleoclimatic and geochronologic research, remote sensing, and GCMs are improving our understanding of nonlinear, chaotic climate patterns operating across large time and space scales just as human vulnerability to climate impacts is escalating. Rapid population growth and movement into areas exposed to floods, landslides, coastal storm surges, tornadoes, or droughts has produced increasing loss of life and economic losses in most countries during recent decades. Effective mitigation of hazards associated with climate variability will thus require continued development of our understanding of mechanisms driving climate variability, the effects of this variability on such resources as water supplies and biotic communities, and the responses of human communities to climate-related hazards.