TS4 Current knowledge about future impacts

This section summarises the main projected impacts in each system and sector (Section TS.4.1) and region (Section TS.4.2) over this century,13 judged in terms of relevance for people and the environment. It assumes that climate change is not mitigated, and that adaptive capacity has not been enhanced by climate policy. All global temperature changes are expressed relative to 1990 unless otherwise stated.14 The impacts stem from changes in climate and sea-level changes associated with global temperature change, and frequently reflect projected changes in precipitation and other climate variables in addition to temperature.

TS.4.1 Sectoral impacts, adaptation and vulnerability

A summary of impacts projected for each sector is given in Box TS.5.

Freshwater resources and their management

The impacts of climate change on freshwater systems and their management are mainly due to the observed and projected increases in temperature, evaporation, sea level and precipitation variability (very high confidence).

More than one-sixth of the world's population live in glacier- or snowmelt-fed river basins and will be affected by a decrease in water volume stored in glaciers and snowpack, an increase in the ratio of winter to annual flows, and possibly a reduction in low flows caused by decreased glacier extent or melt-season snow water storage [3.4.1, 3.4.3]. Sea-level rise will extend areas of salinisation of groundwater and estuaries, resulting in a decrease in freshwater availability for humans and ecosystems in coastal areas [3.2, 3.4.2]. Increased precipitation intensity and variability is projected to increase the risk of floods and droughts in many areas [3.3.1]. Up to 20% of the world's population live in river basins that are likely to be affected by increased flood hazard by the 2080s in the course of global warming [3.4.3].

12 WRE stabilisation profiles were used in the TAR, and a description is given in the TAR Synthesis Report.

13 Unless otherwise stated.

14 To express the temperature change relative to pre-industrial (about 1750) levels, add 0.6°C.

The number of people living in severely stressed river basins is projected to increase significantly from 1.4-1.6 billion in 1995 to 4.3-6.9 billion in 2050, for the SRES A2 scenario (medium confidence).

The population at risk of increasing water stress for the full range of SRES scenarios is projected to be: 0.4-1.7 billion, 1.02.0 billion and 1.1-3.2 billion, in the 2020s, 2050s and 2080s, respectively [3.5.1]. In the 2050s (A2 scenario), 262-983 million people are likely to move into the water-stressed category

[3.5.1]. Water stress is projected to decrease by the 2050s on 2029% of the global land area (considering two climate models and the SRES scenarios A2 and B2) and to increase on 62-76% of the global land area [3.5.1].

Semi-arid and arid areas are particularly exposed to the impacts of climate change on freshwater (high confidence).

Many of these areas (e.g., Mediterranean Basin, western USA, southern Africa, north-eastern Brazil, southern and eastern Australia) will suffer a decrease in water resources due to climate change (see Figure TS.5) [3.4, 3.7]. Efforts to offset declining surface water availability due to increasing precipitation variability will be hampered by the fact that groundwater recharge is likely to decrease considerably in some already water-stressed regions

[3.4.2], where vulnerability is often exacerbated by the rapid increase of population and water demand [3.5.1].

Higher water temperatures, increased precipitation intensity and longer periods of low flows are likely to exacerbate many forms of water pollution, with impacts on ecosystems, human health, and water system reliability and operating costs (high confidence).

These pollutants include sediments, nutrients, dissolved organic carbon, pathogens, pesticides, salt and thermal pollution [3.2, 3.4.4,3.4.5].

Climate change affects the function and operation of existing water infrastructure as well as water management practices (very high confidence).

Adverse effects of climate on freshwater systems aggravate the impacts of other stresses, such as population growth, changing economic activity, land-use change and urbanisation [3.3.2, 3.5]. Globally, water demand will grow in the coming decades, primarily due to population growth and increased affluence. Regionally, large changes in irrigation water demand as a result of climate change are likely [3.5.1]. Current water management practices are very likely to be inadequate to reduce the negative impacts of climate change on water-supply reliability, flood risk, health, energy and aquatic ecosystems [3.4, 3.5]. Improved incorporation of current climate variability into water-related management is likely to make adaptation to future climate change easier [3.6].

Adaptation procedures and risk management practices for the water sector are being developed in some countries and regions (e.g., Caribbean, Canada, Australia, Netherlands, UK, USA, Germany) that recognise the uncertainty of projected hydrological changes (very high confidence). Since the IPCC Third Assessment, uncertainties have been evaluated and their interpretation has improved, and new methods (e.g., ensemble-based approaches) are being developed for their characterisation [3.4, 3.5]. Nevertheless, quantitative projections of changes in precipitation, river flows and water levels at the river-basin scale remain uncertain [3.3.1,3.4].

Figure TS.5. Illustrative map of future climate change impacts on freshwater which are a threat to the sustainable development of the affected regions. Background shows ensemble mean change of annual runoff, in percent, between the present (1981-2000) and 2081-2100 for the SRES A1B emissions scenario; blue denotes increased runoff, red denotes decreased runoff. Underlying map from Nohara et al. (2006) [F3.8].

Figure TS.5. Illustrative map of future climate change impacts on freshwater which are a threat to the sustainable development of the affected regions. Background shows ensemble mean change of annual runoff, in percent, between the present (1981-2000) and 2081-2100 for the SRES A1B emissions scenario; blue denotes increased runoff, red denotes decreased runoff. Underlying map from Nohara et al. (2006) [F3.8].

The negative impacts of climate change on freshwater systems outweigh its benefits (high confidence).

All IPCC regions show an overall net negative impact of climate change on water resources and freshwater ecosystems. Areas in which runoff is projected to decline are likely to face a reduction in the value of the services provided by water resources. The beneficial impacts of increased annual runoff in other areas is likely to be tempered in some areas by negative effects of increased precipitation variability and seasonal runoff shifts on water supply, water quality and flood risks (see Figure TS.5) [3.4, 3.5].

Ecosystems

Records of the geological past show that ecosystems have some capacity to adapt naturally to climate change [WGIAR4 Chapter 6; 4.2], but this resilience15 has never been challenged by a large global human population and its multi-faceted demands from and pressures on ecosystems [4.1,4.2].

The resilience of many ecosystems (their ability to adapt naturally) is likely to be exceeded by 2100 by an unprecedented combination of change in climate, associated disturbances (e.g., flooding, drought, wildfire, insects, ocean acidification), and other global change drivers (e.g., land-use change, pollution, over-exploitation of resources) (high confidence).

Ecosystems are very likely to be exposed to atmospheric CO2 levels much higher than in the past 650,000 years, and global mean temperatures at least as high as those in the past 740,000 years [WGI AR4 Chapter 6; 4.2, 4.4.10, 4.4.11]. By 2100, ocean pH is very likely to be lower than during the last 20 million years [4.4.9]. Extractive use from and fragmentation of wild habitats are very likely to impair species' adaptation [4.1.2, 4.1.3, 4.2, 4.4.5, 4.4.10]. Exceedance of ecosystem resilience is very likely to be characterised by threshold-type responses, many irreversible on time-scales relevant to human society, such as biodiversity loss through extinction, disruption of species' ecological interactions, and major changes in ecosystem structure and disturbance regimes (especially wildfire and insects) (see Figure TS.6). Key ecosystem properties (e.g., biodiversity) or regulating services (e.g., carbon sequestration) are very likely to be impaired [4.2,4.4.1,4.4.2 to 4.4.9,4.4.10, 4.4.11, F4.4,T4.1].

Figure TS.6. Compendium of projected risks due to critical climate change impacts on ecosystems for different levels of global mean annual temperature rise, AT, relative to pre-industrial climate, used as a proxy for climate change. The red curve shows observed temperature anomalies for the period 1900-2005 [WGI AR4 F3.6]. The two grey curves provide examples of the possible future evolution of global average temperature change (AT) with time [WGI AR4 F10.4] exemplified by WGI simulated, multi-model mean responses to (i) the A2 radiative forcing scenario (WGI A2) and (ii) an extended B1 scenario (WGI B1+stabil.), where radiative forcing beyond2100 was kept constant at the 2100 value ¡WGI AR4 F10.4, 10.7]. White shading indicates neutral, small negative, or positive impacts or risks; yellow indicates negative impacts for some systems or low risks; and red indicates negative impacts or risks that are more widespread and/or greater in magnitude. Illustrated impacts take into account climate change impacts only, and omit effects of land-use change or habitat fragmentation, over-harvesting or pollution (e.g., nitrogen deposition). A few, however, take into account fire regime changes, several account for likely productivity-enhancing effects of rising atmospheric CO2 and some account for migration effects. [F4.4, T4.1]

Figure TS.6. Compendium of projected risks due to critical climate change impacts on ecosystems for different levels of global mean annual temperature rise, AT, relative to pre-industrial climate, used as a proxy for climate change. The red curve shows observed temperature anomalies for the period 1900-2005 [WGI AR4 F3.6]. The two grey curves provide examples of the possible future evolution of global average temperature change (AT) with time [WGI AR4 F10.4] exemplified by WGI simulated, multi-model mean responses to (i) the A2 radiative forcing scenario (WGI A2) and (ii) an extended B1 scenario (WGI B1+stabil.), where radiative forcing beyond2100 was kept constant at the 2100 value ¡WGI AR4 F10.4, 10.7]. White shading indicates neutral, small negative, or positive impacts or risks; yellow indicates negative impacts for some systems or low risks; and red indicates negative impacts or risks that are more widespread and/or greater in magnitude. Illustrated impacts take into account climate change impacts only, and omit effects of land-use change or habitat fragmentation, over-harvesting or pollution (e.g., nitrogen deposition). A few, however, take into account fire regime changes, several account for likely productivity-enhancing effects of rising atmospheric CO2 and some account for migration effects. [F4.4, T4.1]

15 Resilience is defined as the ability of a social or ecological system to absorb disturbances while retaining the same basic structure and ways of functioning, the capacity for self-organisation, and the capacity to adapt naturally to stress and change.

The terrestrial biosphere is likely to become a net carbon source by 2100, thus amplifying climate change, given continued greenhouse gas emissions at or above current rates and other unmitigated global changes, such as land-use changes (high confidence).

Several major terrestrial carbon stocks are vulnerable to climate change and/or land-use impacts [F4.1, 4.4.1, F4.2, 4.4.5,4.4.6, 4.4.10, F4.3]. The terrestrial biosphere currently serves as a variable, but generally increasing, carbon sink (due to CO2-fertilisation, moderate climate change and other effects) but this is likely to peak before mid-century and then tend towards a net carbon source, thus amplifying climate change [F4.2, 4.4.1, 4.4.10, F4.3, 4.4.11], while ocean buffering capacity begins saturating [WGIAR4, e.g., 7.3.5]. This is likely to occur before 2100, assuming continued greenhouse gas emissions at or above current rates and unmitigated global change drivers including land-use changes, notably tropical deforestation. Methane emissions from tundra are likely to accelerate [4.4.6].

Roughly 20 to 30% (varying among regional biotas from 1 % to 80%) of species assessed so far (in an unbiased sample) are likely to be at increasingly high risk of extinction as global mean temperatures exceed 2 to 3°C above pre-industrial levels (medium confidence).

Global losses of biodiversity are of key relevance, being irreversible [4.4.10, 4.4.11, F4.4, T4.1]. Endemic species richness is highest where regional palaeo-climatic changes have been muted, indicating that endemics are likely to be at a greater extinction risk than in the geological past [4.4.5, 4.4.11, F4.4, T4.1]. Ocean acidification is likely to impair aragonite-based shell formation in a wide range of planktonic and shallow benthic marine organisms [4.4.9, B4.4]. Conservation practices are generally ill-prepared for climate change, and effective adaptation responses are likely to be costly to implement [4.4.11, T4.1,4.6.1]. Although links between biodiversity intactness and ecosystem services remain quantitatively uncertain, there is high confidence that the relationship is qualitatively positive [4.1, 4.4.11,4.6,4.8].

Substantial changes in structure and functioning of terrestrial and marine ecosystems are very likely to occur with a global warming of 2 to 3°C above pre-industrial levels and associated increased atmospheric CO2 (high confidence).

Major biome changes, including emergence of novel biomes, and changes in species' ecological interactions, with predominantly negative consequences for goods and services, are very likely by, and virtually certain beyond, those temperature increases [4.4]. The previously overlooked progressive acidification of oceans due to increasing atmospheric CO2 is expected to have negative impacts on marine shell-forming organisms (e.g., corals) and their dependent species [B4.4,6.4].

Food, fibre and forest products

In mid- to high-latitude regions, moderate warming benefits cereal crop and pasture yields, but even slight warming decreases yields in seasonally dry and tropical regions (medium confidence).

Modelling results for a range of sites find that, in temperate regions, moderate to medium increases in local mean temperature (1 to 3°C), along with associated CO2 increase and rainfall changes, can have small beneficial impacts on crop yields. At lower latitudes, especially the seasonally dry tropics, even moderate temperature increases (1 to 2°C) are likely to have negative yield impacts for major cereals, which would increase the risk of hunger. Further warming has increasingly negative impacts in all regions (medium to low confidence) (see Figure TS .7) [5.4].

Climate change increases the number of people at risk of hunger marginally, with respect to overall large reductions due to socio-economic development (medium confidence).

Compared with 820 million undernourished today, SRES scenarios of socio-economic development, without climate change, project 100-240 million undernourished for the SRES A1, B1 and B2 scenarios (770 million under the A2 scenario) in 2080 (medium confidence). Scenarios with climate change project 100380 million undernourished for the SRES A1, B1 and B2 scenarios (740-1,300 million under the A2 scenario) in 2080 (low to medium confidence). The ranges here indicate the extent of effects of the exclusion and inclusion of CO2 effects in the scenarios. Climate change and socio-economics combine to alter the regional distribution of hunger, with large negative effects on sub-Saharan Africa (low to medium confidence) [5.4, T5.6].

Projected changes in the frequency and severity of extreme climate events have significant consequences on food and forestry production, and food insecurity, in addition to impacts of projected mean climate (high confidence).

Recent studies indicate that increased frequency of heat stress, droughts and floods negatively affects crop yields and livestock beyond the impacts of mean climate change, creating the possibility for surprises, with impacts that are larger, and occur earlier, than predicted using changes in mean variables alone [5.4.1, 5.4.2]. This is especially the case for subsistence sectors at low latitudes. Climate variability and change also modify the risks of fires, pest and pathogen outbreaks, negatively affecting food, fibre and forestry (high confidence) [5.4.1 to 5.4.5,5.ES].

Simulations suggest rising relative benefits of adaptation with low to moderate warming (medium confidence), although adaptation may stress water and environmental resources as warming increases (low confidence).

There are multiple adaptation options that imply different costs, ranging from changing practices in place to changing locations of food, fibre and forest activities [5.5.1]. Adaptation effectiveness varies from only marginally reducing negative impacts to changing a negative impact into a positive one. On average, in cereal-cropping systems, adaptations such as changing varieties and planting times enable avoidance of a 10 to 15% reduction in yield, corresponding to 1 to 2°C local temperature increases. The benefit from adapting tends to increase with the degree of climate change [F5.2]. Changes in policies and institutions are needed to facilitate adaptation. Pressure to cultivate marginal land or to adopt unsustainable cultivation practices may increase land degradation and resource use, and endanger biodiversity of both wild and domestic species

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