IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability

1.2.1 Climate and non-climate 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. 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 NiƱo-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.

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

Non-climate driver Examples 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) 
 Urbanisation and transportation (e.g., Kalnay and Cai, 2003) Ecosystem fragmentation, deterioration of air quality, increased runoff and water pollution (e.g., Turalioglu et al., 2005) Change in albedo, urban heat island, local precipitation reduction, downwind precipitation increase, lower evaporation (e.g., Weissflog et al., 2004) 
 Afforestation (e.g., Rudel et al., 2005) Restoration or establishment of tree cover (e.g., Gao et al., 2002) Change in albedo, altered water and energy balances, potential carbon sequestration  
Land-cover modification Ecosystem degradation (desertification) Reduction in ecosystem services, reduction in biomass, biodiversity loss (e.g., Nyssen et al., 2004) Changes in microclimate (e.g., Su et al., 2004) 
Invasive species Tamarisk (USA), Alaska lupin (Iceland) Reduction of biodiversity, salinisation (e.g., Lee et al., 2006) Change in water balance (e.g., Ladenburger et al., 2006) 
Pollution Tropospheric ozone, toxic waste, oil spills, exhaust, pesticides increased soot emissions (e.g., Pagliosa and Barbosa, 2006)  Reduction in breeding success and biodiversity, species mortality, health impairment, enhanced melting of snow and ice (e.g., Lee et al., 2006)  Direct and indirect aerosol effects on temperature, albedo and precipitation