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

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].

Figure TS.7

Figure TS.7. Sensitivity of cereal yield to climate change for maize and wheat. Responses include cases without adaptation (orange dots) and with adaptation (green dots). The studies on which this figure is based span a range of precipitation changes and CO2 concentrations, and vary in how they represent future changes in climate variability. For instance, lighter-coloured dots in (b) and (c) represent responses of rain-fed crops under climate scenarios with decreased precipitation. [F5.4]

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 100-380 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 [5.4.7]. Adaptation measures should be integrated with development strategies and programmes, country programmes and poverty-reduction strategies [5.7].

Smallholder and subsistence farmers, pastoralists and artisanal fisherfolk are likely to suffer complex, localised impacts of climate change (high confidence).

These groups, whose adaptive capacity is constrained, are likely to experience negative effects on yields of tropical crops, combined with a high vulnerability to extreme events. In the longer term, there are likely to be additional negative impacts of other climate-related processes such as snowpack decrease especially in the Indo-Gangetic Plain, sea-level rise, and a spread in the prevalence of human diseases affecting agricultural labour supply (high confidence) [5.4.7].

Globally, forestry production is estimated to change only modestly with climate change in the short and medium term (medium confidence).

The change in global forest product outputs ranges from a modest increase to a slight decrease, although regional and local changes are likely to be large []. Production increase is likely to shift from low-latitude regions in the short term, to high-latitude regions in the long term [5.4.5].

Local extinctions of particular fish species are expected at edges of ranges (high confidence).

It is likely that regional changes in the distribution and productivity of particular fish species will continue and local extinctions will occur at the edges of ranges, particularly in freshwater and diadromous species (e.g., salmon, sturgeon). In some cases, ranges and productivity are likely to increase [5.4.6]. Emerging evidence suggests concern that the Meridional Overturning Circulation is slowing down, with potentially serious consequences for fisheries [5.4.6].

Food and forestry trade is projected to increase in response to climate change, with increased food-import dependence of most developing countries (medium to low confidence).

While the purchasing power for food is likely to be reinforced in the period to 2050 by declining real prices, it would be adversely affected by higher real prices for food from 2050 to 2080 due to climate change [5.6.1, 5.6.2]. Exports of temperate-zone food products to tropical countries are likely to rise [5.6.2], while the reverse is likely in forestry in the short term [5.4.5].

Experimental research on crop response to elevated CO2 confirms TAR reviews (medium to high confidence). New results suggest lower responses for forests (medium confidence).

Recent reanalyses of free-air carbon dioxide enrichment (FACE) studies indicate that, at 550 ppm CO2, yields increase under unstressed conditions by 10 to 20% over current concentrations for C3 crops, and by 0 to 10% for C4 crops (medium confidence). Crop model simulations under elevated CO2 are consistent with these ranges (high confidence) [5.4.1]. Recent FACE results suggest no significant response for mature forest stands and confirm enhanced growth for young tree stands [5.4.1]. Ozone exposure limits CO2 response in both crops and forests [B5.2].