Land-use and land-cover change
土地利用和土地覆盖变化

Widespread LULCC in the US has affected the amount, configuration, and quality of habitat and has altered hydrological and climatic regimes (Tilman et al. 2001). Habitat loss, degradation, and fragmentation are leading causes of terrestrial biodiversity loss, impairment of ecosystem functioning, and associated declines in ecosystem services (IPCC 2007; Krauss et al. 2010). 

Climate change is likely to interact with LULCC in ways that further exacerbate these detrimental effects. For example, a recent meta-analysis of empirical studies found that biodiversity was more likely to be negatively affected by habitat loss in areas where precipitation rates have changed (Mantyka-Pringle et al. 2012). One empirical study of butterflies in the Sierra Nevada of California showed that both habitat loss and climate change had likely contributed to declines in species richness (Forister et al. 2010). The rapid disappearance of the green salamander (Aneides aeneus) from the southern Appalachians of the US has been attributed to changes in temperature coupled with the salamander's limited ability to disperse in landscapes modified by logging, resort development, and dams (Corser 2001). 

Only a few studies have explicitly projected future effects on species and ecosystems caused by the interactions between LULCC and climate change. Jetz et al. (2007) projected substantial range shifts for ~4.5–10% and ~10–20% of 8750 land bird species by 2050 and 2100, respectively; climate change was the primary driver of range contractions in temperate regions, whereas LULCC was the primary driver in the tropics. Similarly, the combined effects of LULCC and climate change are projected to bring about a loss of 7–24% of vascular plant diversity relative to 1995 by 2050, with climate change becoming a more important driver in the second half of this century (Van Vuuren et al. 2006). 

Land surfaces are major reservoirs of carbon (C), so LULCC could be a major driver of climate change if associated activities lead to more greenhouse-gas releases; alternatively, it could help mitigate future climate change if such activities help store or return C to the land surface (IPCC 2007). Actions such as clear-cutting or replanting major tracts of forests can also affect local weather patterns and the resulting potential capacity for biomass storage of C (Dale et al. 2011). As such, modifications to land use and land cover intended to address climate mitigation or climate adaptation may in turn interact with existing stressors or might directly affect species, populations, and ecosystems (via pathways [iii] and [iv] in Figure 1a). 

Extraction of natural resources
自然资源提取

Natural resources can be important ecosystem goods. However, their extraction can cause severe stress to species and ecosystems. Climate change can exacerbate these stresses, particularly if it further reduces the supply of a harvested resource. For example, a sufficient population growth rate in targeted fish species is one factor relevant to limiting fishery overexploitation. However, changes to the physical environment, such as water temperature, can affect individual growth, survival, and reproduction rates, and thereby rates of population replacement (Jonsson and Jonsson 2009). Likewise, water withdrawals combined with climate change are projected to have major effects on freshwater fish; for instance, Xenopoulos et al. (2005) projected that 25% of rivers could lose more than 22% of fish species by 2070 due to the combined effects of increased water withdrawal and climate change. For three out of the four rivers examined in the US, the combined effects of climate change and water withdrawal were notably greater than the effect of climate change alone (Xenopoulos et al. 2005). 

Resource extraction may also make ecosystems more vulnerable to climate change. The overharvesting of forests, for example, has the dual effect of causing local environmental damage that can decrease the resilience of an ecosystem to climate change and potentially compounding the magnitude of climate change itself by releasing stored C into the atmosphere (Hansen and Hoffman 2011). Furthermore, deforestation can lead to local warming and reductions in rainfall that can exacerbate climate impacts (Lawrence and Chase 2010). Despite the predominantly negative interactions between natural resource extraction and climate change, some interactions could have a positive impact or might make new resources available; for example, retraction of ice cover and earlier thaw in spring may result in new fishing grounds in arctic regions. In some cases, forest harvest can result in localized cooling, counteracting climate-induced increases in air temperature (Gibbard et al. 2005). One study found higher levels of butterfly diversity adjacent to irrigated fields, suggesting that irrigation may mitigate the water-limitation effects of climate change in ecosystems adjacent to agricultural fields (González-Estébanez et al. 2011). 

Biological disturbance

Biological disturbances include invasion by non-native species that have a competitive advantage, allowing them to spread rapidly; emergence of pest species that have expanded their range or are better able to survive milder winters; and disease outbreaks, whether novel, reoccurring, or introduced. Climate change will likely affect the severity, timing, and location of biological disturbances, as well as limiting the ability of the ecosystem to recover following such an event.

Climate change is expected to exacerbate the impacts of many introduced plant and animal species. Evidence suggests that some of these species have already responded to recent changes in the atmosphere and to climate (Walther et al. 2009), and future changes are likely to increase the ranges of several invasive plant species across the US (Bradley et al. 2010), potentially expanding their impact. In particular, changes in fire regimes will affect interactions among native and non-native species (Keith et al. 2008). Extreme climatic events that stress or kill native species are thought to temporarily increase communities' susceptibility to invasion (Diez et al. 2012); these events are projected to become more frequent with climate change. Although climatic changes may have strong effects on species ranges in the future, changes in the extent of different habitat types within a region have the potential to exert as much or more influence over the abundance of some invasive plant species (Ibáñez et al. 2009).

Climate change is also affecting the geographic ranges and virulence of many pests, pathogens, parasites, and disease vectors, and enhancing their ability to spread. For example, climate-induced habitat change has expanded the range of fungal pathogens that cause amphibian mortality (Pounds et al. 2006). The near-epidemic spread of pine and bark beetles in western US states (Bentz et al. 2010) and the northward expansion of the oyster diseases “MSX” and “dermo” to Nova Scotia (Ford and Smolowitz 2007) may also be linked to climate. On the basis of an assumption of a 2°C warming and changes in precipitation, Benning et al. (2002) found that extant Hawaiian honeycreepers may be driven to extinction through the combined effects of climate change, introduced avian malaria (spread by introduced mosquitoes), and historical land-use changes. As warming continues, hemlock woolly adelgids (Adelges tsugae), insect pests that have killed many eastern hemlocks (Tsuga canadensis) in recent years, are likely to expand their ranges northward (Dukes et al. 2009). Climate-change impacts on pests may have cascading effects: projected increases in temperature could increase the frequency and severity of insect outbreaks, and the resulting increase in tree mortality may in turn promote wildfires.

Forecasting changes in impacts from pests, pathogens, or invasive plant species is often fraught with uncertainties beyond those associated with forecasting climate change alone; often, too little is known about the climatic tolerances or responses of the species of concern to make confident projections under a given scenario (Dukes et al. 2009). Indeed, climate change may not always increase the net impact of introduced species, and some have argued it could be associated with the decline of pathogens, vectors, and hosts (Lafferty 2009). In areas that become climatically suitable for a new pest, pathogen, or host, other factors – such as competition, physical barriers, or predation – may still limit its range. Furthermore, climate change may reduce climatic suitability for a species or induce other habitat changes that are less favorable to its spread (Slenning 2010).

Pollution

Climate change may magnify the adverse environmental effects of pollutants, including metals, pesticides, organic material, nutrients, endocrine disruptors, and atmospheric ozone (O₃; Hansen and Hoffman 2011). It also alters temperature, pH, dilution rates, salinity, and other environmental conditions that modify the availability of pollutants, the exposure and sensitivity of species to pollutants, and the transport patterns, uptake, and toxicity of pollutants (Noyes et al. 2009).

Climate change is affecting where and when pollutants are found in the environment. For example, changes in transport patterns, such as currents, wind, and river flows, may enable environmental pollutants to accumulate in new places, exposing biota to risk in different habitats. Some contaminants thought to be diminishing in concern, such as polychlorinated biphenyls, are being remobilized in the environment as a result of climate change. Persistent organic pollutants, deposited in glaciers during the period of heavy use in the mid-20th century, are now being released due to climate-induced ice melt (Blais et al. 2001). Adélie penguins (Pygoscelis adeliae) in western Antarctica, for example, have continued to bioaccumulate DDT over the past 30 years, most likely via DDT release from melting glaciers (Geisz et al. 2008). Altered pH can make heavy metals more biologically available in aquatic systems, thereby increasing their impact on the environment.

Climate change is also intensifying the effects of some pollutants. Rising temperatures, for instance, can enhance exposure to metals by increasing the respiration rates of fish (Ficke et al. 2007). Higher temperatures resulted in greater mortality rates in metal-exposed ectotherms in 80% of the cases examined, largely associated with increased biological uptake and accumulation of metals (Sokolova and Lannig 2008). Higher temperatures can also exacerbate hypoxic conditions because warmer water holds less dissolved oxygen than cooler water and accelerates the bacterial decay of organic matter, which in turn consumes more oxygen (Rabalais et al. 2009). More frequent extreme rainfall events can further exacerbate hypoxia by increasing the runoff of nitrogen and phosphorus (P) into waterways.

Finally, climate change may cause increases in some pollutants, most notably ground-level O₃. Many regions of the world are projected to have higher O₃ concentrations by the end of the 21st century (Sitch et al. 2007). Few studies have examined the potential consequences of O₃ pollution for biodiversity, and predictions are confounded by interactions across trophic levels. However, reductions in wild plant productivity as a result of O₃ exposure suggest that climate-induced enhancement of O₃ concentrations could affect biodiversity (Wittig et al. 2009). Furthermore, ground-level O₃ damage will likely offset some productivity gains in plants due to rising atmospheric carbon dioxide levels, thus reducing C storage on land and possibly contributing further to climate change (Sitch et al. 2007).