5.4.1 Primary effects and interactions
The TAR concluded that climate change and variability will impact food, fibre and forests around the world due to the effects on plant growth and yield of elevated CO2, higher temperatures, altered precipitation and transpiration regimes, and increased frequency of extreme events, as well as modified weed, pest and pathogen pressure. Many studies since the TAR confirmed and extended previous findings; key issues are described in the following sections.
184.108.40.206 Effects of elevated CO2 on plant growth and yield
Plant response to elevated CO2 alone, without climate change, is positive and was reviewed extensively by the TAR. Recent studies confirm that the effects of elevated CO2 on plant growth and yield will depend on photosynthetic pathway, species, growth stage and management regime, such as water and nitrogen (N) applications (Jablonski et al., 2002; Kimball et al., 2002; Norby et al., 2003; Ainsworth and Long, 2005). On average across several species and under unstressed conditions, recent data analyses find that, compared to current atmospheric CO2 concentrations, crop yields increase at 550 ppm CO2 in the range of 10-20% for C3 crops and 0-10% for C4 crops (Ainsworth et al., 2004; Gifford, 2004; Long et al., 2004). Increases in above-ground biomass at 550 ppm CO2 for trees are in the range 0-30%, with the higher values observed in young trees and little to no response observed in mature natural forests (Nowak et al., 2004; Korner et al., 2005; Norby et al., 2005). Observed increase of above-ground production in C3 pastures is about +10% (Nowak et al., 2004; Ainsworth and Long, 2005). For commercial forestry, slow-growing trees may respond little to elevated CO2 (e.g., Vanhatalo et al., 2003), and fast-growing trees more strongly, with harvestable wood increases of +15-25% at 550 ppm and high N (Calfapietra et al., 2003; Liberloo et al., 2005; Wittig et al., 2005). Norby et al. (2005) found a mean tree net primary production
(NPP) response of 23% in young tree stands; however in mature tree stands Korner et al. (2005) reported no stimulation.
While some studies using re-analyses of recent FACE experimental results have argued that crop response to elevated CO2 may be lower than previously thought, with consequences for crop modelling and projections of food supply (Long et al., 2005,2006), others have suggested that these new analyses are, in fact, consistent with previous findings from both FACE and other experimental settings (Tubiello et al., 2007a, 2007b). In addition, simulations of unstressed plant growth and yield response to elevated CO2 in the main crop-simulation models, including AFRC-Wheat, APSIM, CERES, CROPGRO, CropSyst, LINTULC and SIRIUS, have been shown to be in line with recent experimental data, projecting crop yield increases of about 5-20% at 550 ppm CO2 (Tubiello et al., 2007b). Within that group, the main crop and pasture models, CENTURY and EPIC, project above-ground biomass production in C3 species of about 15-20% at 550 ppm CO2, i.e., at the high end of observed values for crops, and higher than recent observations for pasture. Forest models assume NPP increases at 550 ppm CO2 in the range 15-30%, consistent with observed responses in young trees, but higher than observed for mature trees stands.
Importantly, plant physiologists and modellers alike recognise that the effects of elevated CO2 measured in experimental settings and implemented in models may overestimate actual field- and farm-level responses, due to many limiting factors such as pests, weeds, competition for resources, soil, water and air quality, etc., which are neither well understood at large scales, nor well implemented in leading models (Tubiello and Ewert, 2002; Fuhrer, 2003; Karnosky, 2003; Gifford, 2004; Peng et al., 2004; Ziska and George, 2004; Ainsworth and Long, 2005; Tubiello et al., 2007a, 2007b). Assessment studies should therefore include these factors where possible, while analytical capabilities need to be enhanced. It is recommended that yield projections use a range of parameterisations of CO2 effects to better convey the associated uncertainty range.
220.127.116.11 Interactions of elevated CO2 with temperature and precipitation
Many recent studies confirm and extend the TAR findings that temperature and precipitation changes in future decades will modify, and often limit, direct CO2 effects on plants. For instance, high temperature during flowering may lower CO2 effects by reducing grain number, size and quality (Thomas et al., 2003; Baker, 2004; Caldwell et al., 2005). Increased temperatures may also reduce CO2 effects indirectly, by increasing water demand. Rain-fed wheat grown at 450 ppm CO2 demonstrated yield increases with temperature increases of up to 0.8°C, but declines with temperature increases beyond 1.5°C; additional irrigation was needed to counterbalance these negative effects (Xiao et al., 2005). In pastures, elevated CO2 together with increases in temperature, precipitation and N deposition resulted in increased primary production, with changes in species distribution and litter composition (Shaw et al., 2002; Zavaleta et al., 2003; Aranjuelo et al., 2005; Henry et al., 2005). Future CO2 levels may favour C3 plants over C4 (Ziska, 2003), yet the opposite is expected under associated temperature increases; the net effects remain uncertain.
Importantly, climate impacts on crops may significantly depend on the precipitation scenario considered. In particular, since more than 80% of total agricultural land, and close to 100% of pasture land, is rain-fed, general circulation model (GCM) dependent changes in precipitation will often shape both the direction and magnitude of the overall impacts (Olesen and Bindi, 2002; Tubiello et al., 2002; Reilly et al., 2003). In general, changes in precipitation and, especially, in evaporation-precipitation ratios modify ecosystem function, particularly in marginal areas. Higher water-use efficiency and greater root densities under elevated CO2 in field and forestry systems may, in some cases, alleviate drought pressures, yet their large-scale implications are not well understood (Schäfer et al., 2002; Wullschleger et al., 2002; Norby et al., 2004; Centritto, 2005).
The TAR has already reported on studies that document additional negative impacts of increased climate variability on plant production under climate change, beyond those estimated from changes in mean variables alone. More studies since the TAR have more firmly established such issues (Porter and Semenov, 2005); they are described in detail in Sections 5.4.2 to 5.4.7. Understanding links between increased frequency of extreme climate events and ecosystem disturbance (fires, pest outbreaks, etc.) is particularly important to quantify impacts (Volney and Fleming, 2000; Carroll et al., 2004; Hogg and Bernier, 2005). Although a few models since the TAR have started to incorporate effects of climate variability on plant production, most studies continue to include only effects on changes in mean variables.
18.104.22.168 Impacts on weed and insect pests, diseases and animal health
The importance of weeds and insect pests, and disease interactions with climate change, was reviewed in the TAR. New research confirms and extends these findings, including competition between C3 and C4 species (Ziska, 2003; Ziska and George, 2004). In particular, CO2-temperature interactions are recognised as a key factor in determining plant damage from pests in future decades, though few quantitative analyses exist to date; CO2-precipitation interactions will be likewise important (Stacey and Fellows, 2002; Chen et al., 2004; Salinari et al., 2006; Zvereva and Kozlov, 2006). Most studies continue to investigate pest damage as a separate function of either CO2 (Chakraborty and Datta, 2003; Agrell et al., 2004; Chen et al., 2005a, 2005b) or temperature (Bale et al., 2002; Cocu et al., 2005; Salinari et al., 2006). For instance, recent warming trends in the U.S. and Canada have led to earlier spring activity of insects and proliferation of some species, such as the mountain pine beetle (Crozier and Dwyer, 2006; see also Chapter 1). Importantly, increased climate extremes may promote plant disease and pest outbreaks (Alig et al., 2004; Gan, 2004). Finally, new studies, since the TAR, are focusing on the spread of animal diseases and pests from low to mid-latitudes due to warming, a continuance of trends already under way (see Section 5.2). For instance, models project that bluetongue, which mostly affects sheep, and occasionally goat and deer, would spread from the tropics to mid-latitudes (Anon, 2006; van Wuijckhuise et al., 2006). Likewise, White et al. (2003)
simulated, under climate change, increased vulnerability of the Australian beef industry to the cattle tick (Boophilus microplus). Most assessment studies do not explicitly consider either pestplant dynamics or impacts on livestock health as a function of CO2 and climate combined.
Impacts of climate change on managed systems, due to the large land area covered by forestry, pastures and crops, have the potential to affect the global terrestrial carbon sink and to further perturb atmospheric CO2 concentrations (IPCC, 2001; Betts et al., 2004; Ciais et al., 2005). Furthermore, vulnerability of organic carbon pools to climate change has important repercussions for land sustainability and climate-mitigation actions. The TAR stressed that future changes in carbon stocks and net fluxes would critically depend on land-use planning (set aside policies, afforestation-reforestation, etc.) and management practices (such as N fertilisation, irrigation and tillage), in addition to plant response to elevated CO2. Recent research confirms that carbon storage in soil organic matter is often increased under elevated CO2 in the short-term (e.g., Allard et al., 2004); yet the total soil carbon sink may saturate at elevated CO2 concentrations, especially when nutrient inputs are low (Gill et al., 2002; van Groenigen et al., 2006).
Uncertainty remains with respect to several key issues such as the impacts of increased frequency of extremes on the stability of carbon and soil organic matter pools; for instance, the recent European heatwave of 2003 led to significant soil carbon losses (Ciais et al., 2005). In addition, the effects of air pollution on plant function may indirectly affect carbon storage; recent research showed that tropospheric ozone results in significantly less enhancement of carbon-sequestration rates under elevated CO2 (Loya et al., 2003), because of the negative effects of ozone on biomass productivity and changes in litter chemistry (Booker et al., 2005; Liu et al., 2005).
Within the limits of current uncertainties, recent modelling studies have investigated future trends in carbon storage over managed land by considering multiple interactions of climate and management variables. Smith et al. (2005) projected small overall carbon increases in managed land in Europe during this century due to climate change. By contrast, also including projected changes in land use resulted in small overall decreases. Felzer et al. (2005) projected increases in carbon storage on croplands globally under climate change up to 2100, but found that ozone damage to crops could significantly offset these gains.
Finally, recent studies show the importance of identifying potential synergies between land-based adaptation and mitigation strategies, linking issues of carbon sequestration, emissions of greenhouse gases, land-use change and long-term sustainability of production systems within coherent climate policy frameworks (e.g., Smith et al., 2005; Rosenzweig and Tubiello, 2007).
5.4.2 Food-crop farming, including tree crops
As noted in Section 5.1.3, the TAR indicated that impacts on food systems at the global scale might be small overall in the first half of the 21st century, but progressively negative after that. Importantly, crop production in (mainly low latitude) developing countries would suffer more, and earlier, than in (mainly mid- to high-latitude) developed countries, due to a combination of adverse agro-climatic, socio-economic and technological conditions (see recent analyses in Alexandratos, 2005).
5.42.1 What is new since the TAR?
Many studies since the TAR have confirmed key dynamics of previous regional and global projections. These projections indicate potentially large negative impacts in developing regions, but only small changes in developed regions, which causes the globally aggregated impacts on world food production to be small (Fischer et al., 2002b, 2005b; Parry, 2004; Parry et al., 2005). Recent regional assessments have shown the high uncertainty that underlies such findings, and thus the possibility for surprises, by projecting, in some cases, significant negative impacts in key producing regions of developed countries, even before the middle of this century (Olesen and Bindi, 2002; Reilly et al., 2003). Many recent studies have contributed specific new knowledge with respect to several uncertainties and limiting factors at the time of the TAR, often highlighting the possibility for negative surprises, in addition to the impacts of mean climate change alone.
New Knowledge: Increases in frequency of climate extremes may lower crop yields beyond the impacts of mean climate change.
More frequent extreme events may lower long-term yields by directly damaging crops at specific developmental stages, such as temperature thresholds during flowering, or by making the timing of field applications more difficult, thus reducing the efficiency of farm inputs (e.g., Antle et al., 2004; Porter and Semenov, 2005). A number of simulation studies performed since the TAR have developed specific aspects of increased climate variability within climate change scenarios. Rosenzweig et al. (2002) computed that, under scenarios of increased heavy precipitation, production losses due to excessive soil moisture would double in the U.S. by 2030 to US$3 billion/yr. Monirul and Mirza (2002) computed an increased risk of crop losses in Bangladesh from increased flood frequency under climate change. In scenarios with higher rainfall intensity, Nearing et al. (2004) projected increased risks of soil erosion, while van Ittersum et al. (2003) simulated higher risk of salinisation in arid and semi-arid regions, due to more water loss below the crop root zone. Howden et al. (2003) focused on the consequences of higher temperatures on the frequency of heat stress during growing seasons, as well on the frequency of frost occurrence during critical growth stages.
New Knowledge: Impacts of climate change on irrigation water requirements may be large.
Doll (2002) considered direct impacts of climate change on crop evaporative demand (no CO2 effects) and computed increases in crop irrigation requirements of +5% to +8% globally by 2070, with larger regional signals (e.g., +15%) in South-East Asia, net of transpiration losses. Fischer et al. (2006) included positive CO2 effects on crop water-use efficiency and computed increases in global net irrigation requirements of +20% by 2080, with larger impacts in developed versus developing regions, due to both increased evaporative demands and longer growing seasons under climate change. Fischer et al. (2006) and Arnell (2004) also projected increases in water stress (the ratio of irrigation withdrawals to renewable water resources) in the Middle East and South-East Asia. Recent regional studies have also found key climate change and water changes in key irrigated areas, such as North Africa (increased irrigation requirements; Abou-Hadid et al., 2003) and China (decreased requirements; Tao et al., 2003).
New Knowledge: Stabilisation of CO2 concentrations reduces damage to crop production in the long term.
Recent work further investigated the effects of potential stabilisation of atmospheric CO2 on regional and global crop production. Compared to the relatively small impacts of climate change on crop production by 2100 under business-as-usual scenarios, the impacts were only slightly less under 750 ppm CO2 stabilization. However, stabilisation at 550 ppm CO2 significantly reduced production loss (by -70% to -100%) and lowered risk of hunger (-60% to -85%) (Arnell et al., 2002; Tubiello and Fischer, 2006). These same studies suggested that climate mitigation may alter the regional and temporal mix of winners and losers with respect to business-as-usual scenarios, but concluded that specific projections are highly uncertain. In particular, in the first decades of this century and possibly up to 2050, some regions may be worse off with mitigation than without, due to lower CO2 levels and thus reduced stimulation of crop yields (Tubiello and Fischer, 2006). Finally, a growing body of work has started to analyse potential relations between mitigation and adaptation (see Chapter 18).
TAR Confirmation: Including effects of trade lowers regional and global impacts.
Studies by Fischer et al. (2005a), Fischer et al. (2002a), Parry (2004) and Parry et al. (2005) confirm that including trade among world regions in assessment studies tends to reduce the overall projected impacts on agriculture compared to studies that lack an economic component. Yet, despite socio-economic development and trade effects, these and several other regional and global studies indicate that developing regions may be more negatively affected by climate change than other regions (Olesen and Bindi, 2002; Cassman et al., 2003; Reilly et al., 2003; Antle et al., 2004; Mendelsohn et al., 2004). Specific differences among studies depend significantly on factors such as projected population growth and food demand, as well as on trends in production technology and efficiency. In particular, the choice of the SRES scenario has as large an effect on projected global and regional levels of food demand and supply as climate change alone (Parry et al., 2004; Ewert et al., 2005; Fischer et al., 2005a; Tubiello et al., 2007a).
22.214.171.124 Review of crop impacts versus incremental temperature change
The increasing number of regional and global simulation studies performed since the TAR make it possible to produce synthesis graphs, showing not only changes in yield for key crops against temperature (a proxy for both time and severity of climate change), but also other important climate and management factors, such as changes in precipitation or adaptation strategies. An important limitation of these syntheses is that they collect single snapshots of future impacts, thereby lacking the temporal and causal dynamics that characterise actual responses in farmers' fields. Yet they are useful to summarise many independent studies.
Figure 5.2 provides an example of such analyses for temperature increases ranging from about 1-2°C, typical of the next several decades, up to the 4-5°C projected for 2080 and beyond. The results of such simulations are generally highly uncertain due to many factors, including large discrepancies in GCM predictions of regional precipitation change, poor representation of impacts of extreme events and the assumed strength of CO2 fertilisation (5.4.1). Nevertheless, these summaries indicate that in mid- to high-latitude regions, moderate to medium local increases in temperature (1°C to 3°C), across a range of CO2 concentrations and rainfall changes, can have small beneficial impacts on the main cereal crops. Further warming has increasingly negative impacts (medium to low confidence) (Figure 5.2a, c, e). In low-latitude regions, these simulations indicate that even moderate temperature increases are likely to have negative yield impacts for major cereal crops (Figure 5.2b, d, f). For temperature increases more than 3°C, average impacts are stressful to all crops assessed and to all regions (medium to low confidence) (Figure 5.2). The low and mid-to-high latitude regions encompass the majority of global cereal production area. This suggests that global production potential, defined by Sivakumar and Valentin (1997) as equivalent to crop yield or Net Primary Productivity (NPP), is threatened at +1°C local temperature change and can accommodate no more that +3°C before beginning to decline. The studies summarised in Figure 5.2 also indicate that precipitation changes (and associated changes in precipitation:evaporation ratios), as well as CO2 concentration, may critically shape crop-yield responses, over and above the temperature signal, in agreement with previous analyses (Section 5.4.1). The effects of adaptation shown in Figure 5.2 are considered in Section 5.5.
5.423 Research tasks not yet undertaken - ongoing uncertainties
Several uncertainties remain unresolved since the TAR. Better knowledge in several research areas is critical to improve our ability to predict the magnitude, and often even the direction, of future climate change impacts on crops, as well as to better define risk thresholds and the potential for surprises, at local, regional and global scales.
In terms of experimentation, there is still a lack of knowledge of CO2 and climate responses for many crops other than cereals, including many of importance to the rural poor, such as root crops, millet, brassica, etc., with few exceptions, e.g., peanut (Varaprasad et al., 2003) and coconut (Dash et al., 2002). Importantly, research on the combined effects of elevated CO2 and climate change on pests, weeds and disease is still insufficient, though research networks have long been put into place and a few studies have been published (Chakraborty and Datta, 2003; Runion, 2003; Salinari et al., 2006). Impacts of climate change alone on pest ranges and activity are also being increasingly analysed (e.g., Bale et al., 2002; Todd et al., 2002; Rafoss and Saethre, 2003; Cocu et al., 2005; Salinari et al., 2006). Finally, the true strength of the effect of elevated CO2 on crop yields at field to regional scales, its interactions with higher temperatures and modified precipitation regimes, as well as the CO2 levels beyond which saturation may occur, remain largely unknown.
1 Forb: a broad-leaved herb other than grass.
In terms of modelling, calls by the TAR to enhance crop model inter-comparison studies have remained unheeded; in fact, such activity has been performed with much less frequency after the TAR than before. It is important that uncertainties related to crop-model simulations of key processes, including their spatial-temporal resolution, be better evaluated, as findings of integrated studies will remain dependent upon the particular crop model used. It is still unclear how the implementation of plot-level experimental data on CO2 responses compares across models; especially when simulations of several key limiting factors, such as soil and water quality, pests, weeds, diseases and the like, remain either unresolved experimentally or untested in models (Tubiello and Ewert, 2002). Finally, the TAR concluded that the economic, trade and technological assumptions used in many of the integrated assessment models to project food security under climate change were poorly tested against observed data. This remains the situation today (see also Section 5.6.5).
Pastures comprise both grassland and rangeland ecosystems. Grasslands are the dominant vegetation type in areas with low rainfall, such as the steppes of central Asia and the prairies of North America. Grasslands can also be found in areas with higher rainfall, such as north-western and central Europe, New Zealand, parts of North and South America and Australia. Rangelands are found on every continent, typically in regions where temperature and moisture restrictions limit other vegetation types; they include deserts (cold, hot and tundra), scrub, chaparral and savannas.
Pastures and livestock production systems occur under most climates and range from extensive pastoral systems with grazing herbivores, to intensive systems based on forage and grain crops, where animals are mostly kept indoors. The TAR identified that the combination of increases in CO2 concentration, in conjunction with changes in rainfall and temperature, were likely to have significant impacts on grasslands and rangelands, with production increases in humid temperate grasslands, but decreases in arid and semiarid regions.
5.43.1 New findings since TAR
New Knowledge: Plant community structure is modified by elevated CO2 and climate change.
Grasslands consisting of fast-growing, often short lived species, are sensitive to CO2 and climate change, with the impacts related to the stability and resilience of plant communities (Mitchell and Csillag, 2001). Experiments support the concept of rapid changes in species composition and diversity under climate change. For instance, in a Mediterranean annual grassland after three years of experimental manipulation, plant diversity decreased with elevated CO2 and nitrogen deposition, increased with elevated precipitation and showed no significant effect from warming (Zavaleta et al., 2003). Diversity responses to both single and combined global change treatments were driven mainly by significant gains and losses of forb1 species (Zavaleta et al., 2003). Elevated CO2 influences plant species
(a) Maize, mid- to high-latitude
(b) Maize, low latitude
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