Climate and nonclimate drivers of change

Both climate and non-climate drivers affect systems, making analysis of the role of climate in observed changes challenging. Non-climate drivers such as urbanisation and pollution can influence systems directly and indirectly through their effects on climate variables such as albedo and soil-moisture regimes. Socio-economic processes, including land-use change (e.g., forestry to agriculture; agriculture to urban area) and land-cover modification (e.g., ecosystem degradation or restoration) also affect multiple systems.

12.1.1 Climate drivers of change

Climate is a key factor determining different characteristics and distributions of natural and managed systems, including the cryosphere, hydrology and water resources, marine and freshwater biological systems, terrestrial biological systems, agriculture and forestry. For example, temperature is known to strongly influence the distribution and abundance patterns of both plants and animals, due to the physiological constraints of each species (Parmesan and Yohe, 2003; Thomas et al., 2004). Dramatic changes in the distribution of plants and animals during the ice ages illustrate how climate influences the distribution of species. Equivalent effects can be observed in other systems, such as the cryosphere. Hence, changes in temperature due to climate change are expected to be one of the important drivers of change in natural and managed systems.

Many aspects of climate influence various characteristics and distributions of physical and biological systems, including temperature and precipitation, and their variability on all time-scales from days to the seasonal cycle to interannual variations. While changes in many different aspects of climate may at least partially drive changes in the systems, we focus on the role of temperature changes. This is because physical and biological responses to changing temperatures are often better understood than responses to other climate parameters, and the anthropogenic signal is easier to detect for temperature than for other parameters. Precipitation has much larger spatial and temporal variability than temperature, and it is therefore more difficult to identify the impact it has on changes in many systems. Mean temperature (including daily maximum and minimum temperature) and the seasonal cycle in temperature over relatively large spatial areas show the clearest signals of change in the observed climate (IPCC, 2001b).

Large-scale climate variations, such as the Pacific Decadal Oscillation (PDO), El Nino-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO), are occurring at the same time as the global climate is changing. Consequently, many natural and managed systems are being affected by both climate change and climate variability. Hence, studies of observed changes in regions influenced by an oscillation may be able to attribute these changes to regional climate variations, but decades of data may be needed in order to separate the response to climate oscillations from that due to longer-term climate change.

12.1.2 Non-climate drivers of change

Non-climate drivers, such as land use, land degradation, urbanisation and pollution, affect systems directly and indirectly through their effects on climate (Table 1.1). These drivers can operate either independently or in association with one another (Lepers et al., 2004). Complex feedbacks and interactions occur on all scales from local to global.

The socio-economic processes that drive land-use change include population growth, economic development, trade and migration; these processes can be observed and measured at global, regional and local scales (Goklany, 1996). Satellite observations demonstrate that land-use change, including that associated with the current rapid economic development in Asia and Latin America, is proceeding at an unprecedented rate (Rindfuss et al., 2004). Besides influencing albedo and evaporation, land-use changes hamper range-shift responses of species to climate change, leading to an extra loss of biodiversity (Opdam and Wascher, 2004). Additionally, land-use changes have been linked to changes in air quality and pollution that affect the greenhouse process itself (Pielke et al., 2002; Kalnay and Cai, 2003). Land-use and land-cover change can also strongly magnify the effects of extreme climate events, e.g., heat mortality, injuries/fatalities from storms, and ecologically mediated infectious diseases (Patz et al., 2005). Intensification of land use, as well as the extent of land-use change, is also affecting the functioning of ecosystems, and hence emissions of greenhouse gases from soils, such as CO2 and methane.

There are also a large number of socio-economic factors that can influence, obscure or enhance the observed impacts of climate change and that must be taken into account when

Table 1.1. Direct and indirect effects of non-climate drivers.

Non-climate driver


Direct effects on systems

Indirect effects on climate

Geological processes

Volcanic activity, earthquakes, tsunamis (e.g., Adams et al., 2003)

Lava flow, mudflows (lahars), ash fall, shock waves, coastal erosion, enhanced surface and basal melting of glaciers, rockfall and ice avalanches

Cooling from stratospheric aerosols, change in albedo

Land-use change

Conversion of forest to agriculture (e.g., Lepers et al., 2004)

Declines in wildlife habitat, biodiversity loss, increased soil erosion, nitrification

Change in albedo, lower evapotranspiration, altered water and heat balances (e.g., Bennett and Adams, 2004)

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