Tim Wheeler & Joachim von Braun
Crop yields are more negatively affected across most tropical areas than at higher latitudes, and impacts become more severe with an increasing degree of climate change.
[Those] parts of the world where crop productivity is expected to decline under climate change coincide with countries that currently have a high burden of hunger.
[There] is a robust and coherent pattern on a global scale of the impacts of climate change on crop productivity and, hence, on food availability …
[Climate] change will exacerbate food insecurity in areas that already currently have a high prevalence of hunger and undernutrition.
(Climate Change Impacts on Global Food Security, Science, Vol 314, 2 August 2013, p 511)
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Noah Diffenbaugh & Christopher Field
Terrestrial ecosystems have experienced widespread changes in climate over the past century.
It is highly likely that those changes will intensify in the coming decades, unfolding at a rate that is at least an order of magnitude — and potentially several orders of magnitude — more rapid than the changes to which terrestrial ecosystems have been exposed during the past 65 million years.
In responding to those rapid changes in climate, organisms will encounter a highly fragmented landscape that is dominated by a broad range of human influences.
The combination of high climate-change velocity and multidimensional human fragmentation will present terrestrial ecosystems with an environment that is unprecedented in recent evolutionary history.
However, the ultimate velocity of climate change is not yet determined.
Although many Earth system feedbacks are uncertain, the greatest sources of uncertainly — and greatest opportunities for modifying the trajectory of change — lie in the human dimension.
As a result, the rate and magnitude of climate change ultimately experienced by terrestrial ecosystems will be mostly determined by the human decisions, innovations, and economic developments that will determine the pathway of GHG emissions.
(Changes in Ecologically Critical Terrestrial Climate Conditions, Science, Vol 314, 2 August 2013, p 490)
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Changes in Ecologically Critical Terrestrial Climate Conditions
Climate Change Impacts on Global Food Security
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- Changes in Ecologically Critical Terrestrial Climate Conditions, Science, Vol 314, 2 August 2013.
DOI: 10.1126/science.1237123. Noah S Diffenbaugh: Department of Environmental Earth System Science, Stanford University.
Christopher B Field: Department of Global Ecology, Carnegie Institution for Science.
Terrestrial ecosystems have encountered substantial warming over the past century, with temperatures increasing about twice as rapidly over land as over the oceans.
Here, we review the likelihood of continued changes in terrestrial climate, including analyses of the Coupled Model Intercomparison Project global climate model ensemble.
Inertia toward continued emissions creates potential 21st-century global warming that is comparable in magnitude to that of the largest global changes in the past 65 million years but is orders of magnitude more rapid.
The rate of warming implies a velocity of climate change and required range shifts of up to several kilometers per year, raising the prospect of daunting challenges for ecosystems, especially in the context of extensive land use and degradation, changes in frequency and severity of extreme events, and interactions with other stresses.
Phase 5 of the Coupled Model Intercomparison Project (CM1P5) includes contributions from 25 modeling centers, using models with multiple structures, parameterizations. and realizations within a given forcing pathway.
Climate forcings are provided by Representative Concentration Pathways (RCPs), which characterize the most important features of feasible alternative futures and are designed to be consistent with physical, demographic, economic, and social constraints.
The RCPs, like the Special Report on Emissions Scenarios (SRES) and other earlier scenarios, are not intended as predictions and are not assigned probabilities or other indicators of expectation.
Each RCP reaches a different level of anthropogenic radiative forcing in 2100, ranging from 2.6 W/m2 for RCP2.6 to 8.5 W/m2 for RCP8.5.
We discuss simulation results for the full range of RCPs, but with more examples from RCP8.5 because actual emissions since 2000 have been closest to RCP8.5 and RCP8.5 spans the full range of 21st-century forcing encompassed by the RCPs.
For RCP8.5, the CMIP5 ensemble exhibits substantial warming over all terrestrial regions by the 2046-2065 period.
The largest annual warming occurs over the Northern Hemisphere high latitudes, including >4°C above the 1986-2005 baseline (or about 5°C above pre-industrial temperatures).
Annual warming exceeds 2°C over most of the remaining land area in 2046-2065, including greater than 3°C over large areas of North America and Eurasia.
By 2081-2100, warming exceeds 4°C over most land areas, with much of northern North America and northern Eurasia exceeding 6°C. …
Substantial changes in annual precipitation emerge over some areas by 2046-2065 in RCP8.5, including increases over the high northern latitudes and decreases over the Mediterranean region and the mediterranean-climate regions of southwestern South America. Africa, and Australia.
These patterns intensify by 2081-2100.
Sensitivity to climate extremes can be found in tropical, temperate, and boreal ecosystems.
- [Tree] mortality in the Amazon has been linked to
- severe heat, and
- extreme wind.
- Drought and human-induced biomass burning and deforestation combine to increase tropical forest fires — and loss of tropical forest cover — during strong El Nino events.
Temperate ecosystems experience forest die-off and decreased primary production in response to severe heat and drought …
[Low] spring and summer snowmelt runoff increasing stress on mountain, riparian, and dryland ecosystems through
- increased pest pressure,
- wildfires, and
- decreased water supply [for riparian and montane ecosystems].
In the Arctic,
- extreme winter warm events can cause
- vegetation damage and reduced summer growth,
- alteration of community composition (59), and
- changes in microbial habitats (including loss of ice and thawing of permafrost) …
- whereas drought and temperature stress can limit boreal forest growth and carbon uptake.
Extreme Hot Seasons
CM1P5 projects substantial increases in the occurrence of extreme hot seasons in both RCP4.5 and RCP8.5 …
[In RCP8.5] most land areas [experience:}
- >50% of years with mean summer temperature above the late-20th-century maximum by 2046-2065 … and
- >80% of years by 2080-2099.
Extreme Dry Seasons
[Increases are projected] in the frequency of extremely dry seasons by 2080-2099, with areas of Central America, northeastern South America, the Mediterranean, West Africa, southern Africa, and south-western Australia all exhibiting >30% of years with mean seasonal precipitation below the late-20th-century minimum.
The occurrence of extremely low spring snow accumulation is also projected to increase in much of the Northern Hemisphere, including >80% of years below the baseline minimum over areas of western North America by 2080-2099.
Extreme Wet Events
The occurrence of extreme wet events has … increased globally, although not all regions exhibit uniformly increasing trends.
[Droughts] have increased in length or intensity in some regions, and the hydrologic intensity has increased over many land areas …
A Range of Possible Futures
A key source of uncertainty for ecosystem impacts is the magnitude of climate change that ecosystems will encounter in the coming decades.
Multiple factors contribute to this uncertainty, including
- the magnitude of global-scale feedbacks [such as from clouds and the carbon cycle],
- the response of certain extreme events to elevated forcing and
- the influence of internal climate variability on the local climate trend.
Although RCP2.6 is considered technically feasible, it requires economy-wide negative emissions in the second half of the 21st century, meaning that the sum of all human activities is a net removal of COs from the atmosphere.
On the other hand, a world in which all countries achieve an energy profile similar to that of the United States implies greater emissions than in RCP8.5.
Further, combustion of all remaining fossil fuels could lead to CO2 concentrations on the order of 2000 ppm, with concentrations remaining over 1500 ppm for 1000 years.
The Momentum of Climate Change and the Inertia of the Climate System
[Several sources of biogeochemical and ecological] inertia make some future climate change a virtual certainty.
- Ocean thermal inertia causes global temperature to increase even after atmospheric C02 concentrations have stabilized and regional climate to change even after emissions have ceased and global temperature has stabilized.
- Carbon-cycle inertia and ocean thermal inertia cause global temperature to remain elevated long after emissions have stopped, even as CO2 concentrations in the atmosphere decrease.
- If climate changes [trigger] widespread forest loss and/or thawing of permafrost, substantial carbon input to the atmosphere could continue even after anthropogenic CO2 emissions have ceased.
[The] human dimension of the climate system [also] creates inertia that is likely to prolong and increase the level of global warming.
- The existing fossil-fuel-based economy creates inertia toward further CO2 emissions.
The life cycle of existing infrastructure and the knowledge base for generating wealth from fossil energy resources together imply that CO2 emissions will continue for a minimum of another half-century.
- Human dynamics …
Increasing global population increases the global demand for energy, which in the current fossil-fuel-based energy system implies increasing global C02 emissions, even without economic development.
[On top of this the] demand for energy-enabled improvement in human well-being creates additional inertia, particularly given that 1.3 billion people currently lack reliable access to electricity, and 2.6 billion people rely on biomass for cooking.
- Last, the political process provides further inertia, both
- because emissions continue as political negotiations take place and
- because mitigation proposals are built around gradual emissions reductions that guarantee further emissions even if such proposals are eventually adopted. …
[There is a] very real possibility that over the coming century, atmospheric CO2 concentrations will be the highest of the past 22 million years, with the trajectory of other GHGs further enhancing the total radiative forcing.
The Velocity of Climate Change
[The] Paleocene-Eocene Thermal Maximum (PETM) encompassed warming of at least 5°C in < 10,000 years, a rate of change up to 100-fold slower than that projected for RCP8.5 …
Further, the rates of global change during the Medieval Climate Anomaly (MCA), Little Ice Age (LIA), and early Holocene were all smaller than the observed rates from 1880 to 2005 and than for the committed warming calculated to occur over the 21st century if atmospheric concentrations were capped at year-2000 levels.
- [Tree] mortality in the Amazon has been linked to
- Climate Change Impacts on Global Food Security, Science, Vol 314, 2 August 2013.
Tim Wheeler: Walker Institute for Climate System Research, Department of Agriculture, University of Reading.
Joachim von Braun: ZEF B: Center for Development Research, Department of Economic and Technical Change, University of Bonn.
Climate change could potentially interrupt progress toward a world without hunger.
A robust and coherent global pattern is discernible of the impacts of climate change on crop productivity that could have consequences for food availability.
The stability of whole food systems may be at risk under climate change because of short-term variability in supply.
However, the potential impact is less clear at regional scales, but it is likely that climate variability and change will exacerbate food insecurity in areas currently vulnerable to hunger and undernutrition.
Likewise, it can be anticipated that food access and utilization will be affected indirectly via collateral effects on household and individual incomes, and food utilization could be impaired by loss of access to drinking water and damage to health.
The evidence supports the need for considerable investment in adaptation and mitigation actions toward a "climate-smart food system" that is more resilient to climate change influences on food security.
[Substantial] progress has been made in reducing the proportion of the world's undernourished population from an estimated 980 million in 1990-92 to about 850 million in 2010-12.
[Nevertheless,] an estimated 2 billion people still suffer from micro-nutrient deficiencies today.
The long-term reduction in the prevalence of undernutrition worldwide has slowed since 2007, as a result of pressures on food prices, economic volatilities, extreme climatic events, and changes in diet …
… The complexity of global food security is illustrated by the United Nations Food and Agricultural Organization (FAO) definition:
- the availability of sufficient quantities of food of appropriate quality, supplied through domestic production or imports;
- access by individuals to adequate resources (entitlements) for acquiring appropriate foods for a nutritious diet;
- utilization of food through adequate diet, clean water, sanitation, and health care to reach a state of nutritional well-being where all physiological needs are met; and
- stability, because to be food secure, a population, household or individual must have access to adequate food at all times.
Future climate simulations [have] found that enhanced concentrations of atmospheric C02 increase the productivity of most crops through increasing the rate of leaf photosynthesis and improving the efficiency of water use.
However, more recent research has proposed that the C02 yield enhancement in crop models is too large compared with observations of crop experiments under field conditions.
[These] revised estimates [may] affect the magnitude of the previous global crop yield changes but not the spatial distribution of impacts.
[That being said,] higher concentrations of C02 in the atmosphere are already having noticeable continental level effects on plant growth in sub-Saharan Africa. …
In general, yields increased in Northern Europe, but decreased across Africa and South America. …
[Average] crop yields may decline across [Africa and South Asia] by 8% by the 2050s.
Climate change related changes in crop yields
Africa South Asia Wheat —17% Sorghum —15% —11% Millet —10% Maize —5% —16%
No mean change in yield was detected for rice.
[Evidence] for the impact of climate change on crop productivity in Africa and South Asia is
- robust for wheat maize, sorghum, and millet and
- inconclusive, absent, or contradictory for rice, cassava, and sugarcane.
Global-scale climate change impacts at a grid scale of 200 to 250 km can provide useful information on shifts in production zones and perhaps guide the focus of global crop improvement programs seeking to develop better-adapted crop varieties. …
[However,] the sheer complexity of food production systems at a very fine scale is difficult to reproduce in numerical models.
Access to food is largely a matter of household and individual-level income and of capabilities and rights. …
Climate change could transform the ability to produce certain products at regional and international levels.
If … the geography of biomass production shifts at a global scale, this
- will have production implications for all bio-based products — whether food, feed, fuels, or fiber — and
- will impinge on food trade flows, with implications for (farm) incomes and access to food.
The prices of the basic resources, such as land and water, are formed by long-term expectations, and these prices encompass expectations of climate change, such as revaluation of land with access to water.
Structural consequences can emerge, particularly when property rights are lacking and traditional land and water rights are not protected, as is the case in many developing countries with food security problems …
[Structural problems like these can erode] the assets of the poor, as seen during "land grabbing" by external and foreign interests.
[Climate] variability stresses clean drinking water availability.
Hygiene may also be affected by extreme weather events causing flooding or drought in environments where sound sanitation is absent.
[Uptake] of micronutrients is adversely affected by the prevalence of diarrheal diseases, which, in turn, [are] strongly correlated with temperature.
[Increased] costs may result from measures required to avoid food contamination stemming from ecological shifts of pests and diseases of stored crops or food.
[Good progress has been] made in improving food utilization through fortification and biofortification.
[Nevertheless, vulnerability] to food security shocks needs further research …
[Appropriately designed] programs transferring income to the poor, employment-related transfer programs, and early childhood nutrition actions may all need expanding to respond to climate-related volatilities.
New nutritional stresses are emerging [such as the so-called] "nutrition transition" …
[This is] the process by which globalization, urbanization, and changes in lifestyle are linked to excess caloric intake, poor-quality diets, and low physical activity.
[These] factors have led to rapid rises in the incidence of obesity and chronic diseases, even among the poor, in developing countries.
The nutrition transition will unfold in parallel with climate change in coming decades …
Stability of the Food System
Since 2007, the world food equation has been at a precariously low level and, consequently, even small shocks on the supply or demand side of the equation will have large impacts on prices, as experienced in 2008.
Food security of the poor is strongly affected by staple food prices, as a large part of an impoverished family's income has to be spent on staple foods.
Climate change is likely to increase food market volatility for both production and supply.
Food system stability can also be endangered by demand shocks [such as the] aggressive bioenergy subsidies and quota policies applied … in the past decade by the United States and the European Union [that were] motivated in part by energy security concerns and partly by climate mitigation objectives.
The resulting de-stabilization of food markets [contributing] to major food security problems …
The 2008 food crisis stemmed from a combination of a general reduction of agricultural productivity and acute policy failures, exacerbated by export restrictions applied by many countries, a lack of transparency in markets, and poor regulation of financial engagement in food commodity markets.
What We Know We Know — Messages for Decision-Makers
- Climate change impacts on food security will be worst in countries already suffering high levels of hunger and will worsen over time.
- The consequences for global undernutrition and malnutrition of doing nothing in response to climate change are potentially large and will increase over time.
- Food inequalities will increase, from local to global levels, because the degree of climate change and the extent of its effects on people will differ from one part of the world to another, from one community lo the next, and between rural and urban areas.
- People and communities who are vulnerable to the effects of extreme weather now will become more vulnerable in the future and less resilient to climate shocks.
- There is a commitment to climate change of 20 to 30 years into the future as a result of past emissions of greenhouse gases that necessitates immediate adaptation actions to address global food insecurity over the next two to three decades.
- Extreme weather events are likely to become more frequent in the future and will increase risks and uncertainties within the global food system.
- Change and the Integrity of Science, Science, Vol 328 no 5979 pp 689-690, 7 May 2010.
The letter, from 255 members of the National Academy of Sciences, including 11 Nobel laureates [was rejected by] the New York Times, the Washington Post, and the Wall Street Journal.
- Climate Change Scepticism: Its Sources and Strategies, AAAS Forum, Science Show, ABC Radio National, 3 April 2010.
- Public trust in science, AAAS Forum, Science Show, ABC Radio National, 6 March 2010.
Ralph Cicerone, James McCarthy and Martin Rees.
- The Scientific Consensus on Climate Change, Science, Vol 306, p 1686, 3 December 2004.
Naomi Oreskes: Department of History and Science Studies Program, University of California at San Diego, La Jolla.