Analysis of the exposure of 185 eco-regions of exceptional biodiversity … to extreme monthly temperature and precipitation conditions in the 21st century … shows that within 60 years almost all of the regions … will experience extreme temperature conditions …
[Large-scale] loss of biodiversity is likely to occur … with climate change and high CO2 concentration driving a transition of the Earth´s ecosystems into a state unknown in human experience.
(Turn Down the Heat, 2012, p 53)
Ecosystems and Biodiversity
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- Turn Down the Heat, Potsdam Institute for Climate Impact Research and Climate Analytics, International Bank for Reconstruction and Development / World Bank, November 2012.
[There] are two international research projects that were recently initiated to quantify impacts within a sector and across sectors at different levels of global warming, including high-end scenarios. …
- the Agriculture Model Intercomparison and Improvement Project AgMIP (launched in October 2010) is bringing together a large number of biophysical and agro-economic modelling groups explicitly covering regional to global scales to compare their results and improve their models with regard to observations. …
- the first Inter-Sectoral Model Intercomparison Project (ISI-MIP) was launched in December 2011 with a fast-track phase designed to provide a synthesis of cross-sectoral global impact projections at different levels of global warming.
The overall conclusions of IPCC AR4 concerning food production and agriculture included the following:
- Crop productivity is projected to increase slightly at mid- to high latitudes for local mean temperature increases of up to 1 to 3°C depending on the crop, and then decrease beyond that in some regions (medium confidence).
- At lower latitudes, especially in seasonally dry and tropical regions, crop productivity is projected to decrease for even small local temperature increases (1 to 2°C) which would increase the risk of hunger (medium confidence).
- Globally, the potential for food production is projected to increase with increases in local average temperature over a range of 1 to 3°C, but above this it is projected to decrease (medium confidence).
Projections for food and agriculture over the 21st century indicate substantial challenges irrespective of climate change.
As early as 2050, the world’s population is expected to reach about 9 billion …
[As as consequence, global] demand for crops [is projected to increase] by about 100% from 2005 to 2050. …
[Historically,] food production has been able to increase to keep pace with demand …
[In rich countries, increases] in food production have mainly been driven by more efficient use of land [whereas in] poor countries [this has largely been achieved by] extension of arable land. …
There are some indications that climate change may reduce arable land in [poorer] low-latitude regions, with reductions most pronounced in Africa, Latin America, and India.
[Flooding] of agricultural land [in particular is] expected to severely impact crop yields in the future …
[With 1 m of sea-level rise:]
- 10.7% of South Asia´s agricultural land is projected to be exposed to inundation,
- [along with] a 10% intensification of storm surges …
Given the competition for land that may be used for other human activities (for example, urbanization and biofuel production) … it is likely that the main increase in production will have to be managed by an intensification of agriculture on the same — or possibly even reduced — amount of land.
Declines in nutrient availability (for example, phosphorus), as well as the spread in pests and weeds, could further limit the increase of agricultural productivity.
Geographical shifts in production patterns resulting from … global warming could further escalate distributional issues in the future. …
[Research since 2007 points] to a more rapidly escalating risk of crop yield reductions [with increasing temperatures] than previously predicted …
In the period since 1980 … maize has declined by 3.8% and wheat production by 5.5% …
[Three] interrelated factors are important:
- temperature-induced effect,
- precipitation-induced effect, and
- the CO2-fertilization effect.
[There are] far-reaching and uneven adverse implications for poverty in many regions arising from the macroeconomic consequences of shocks to global agricultural production from climate change.
[Even] where overall food production is not reduced or is even increased with low levels of warming, distributional issues mean that food security will remain [precarious] or worsen …
[Food] security is further challenged by a multitude of nonclimatic factors.
Rising temperature may [initially] increase yields at higher latitudes where low temperatures are a limiting factor on growth …
At lower latitudes, increases in temperature alone are expected to reduce yields from grain crops [because] grain crops mature earlier at higher temperatures [—] reducing the critical growth period …
(p 44, emphasis added)
Projected Impacts on Different Crops Without and With Adaptation(Adapted from Table 3: 2000-50 for warming levels of between 1.8°C and 2.8°C
without accounting for possible CO2 fertilization effects)
Without Adaptation (%) With Adaptation (%) Spring wheat –14 to –25 –4 to –10 Maize –19 to –34 –6 to –18 Soybean –15 to –30 –12 to –26
[Adaptation — ie changes] in planting and harvesting date [and] cultivar type in terms of rates of maturation [— could] reduce losses by about a factor of two for spring wheat and maize and by 15% for soybeans …
[Other potential] cultivar adaptations [include improved] heat and drought tolerance.
Recent research [indicates that] sensitivity of crop yields to temperature increases and, in particular, extreme temperature events [may be greater than previously thought.]
[Higher] yield losses per degree of regional mean warming in Australia and India [connote] an emerging risk of nonlinear effects on crop yields because of the damaging effect of temperature extremes.
Field experiments have shown that crops are highly sensitive to temperatures above certain thresholds …
[Such effects can be expected to be even more pronounced in a 4°C world.]
The total “drought disaster-affected” area is predicted to increase from currently 15.4% of global cropland to 44±6% by 2100 based on a modified Palmer Drought Severity Index.
The largest fractions of affected sown areas are expected for Africa and Oceania, reaching about 59% by 2100 in each region.
Climate projections of 20 General Circulation Models were used to estimate the change in drought disaster affected area under [emission scenarios in which] global mean temperature change in 2100 reaches … 4.9°C relative to preindustrial values.
The regions expected to see increasing drought severity and extent over the next 30 to 90 years are in
- southern Africa,
- the United States,
- southern Europe,
- Brazil, and
- Southeast Asia.
Uncertainty in CO2-fertilization Effect
[For the period] between 2000 and 2050 on the global level for a temperature increase in the range of 1.8°C to 3.4°C (SRES A1b, A2, B1, equivalent to 2.5°C to 4.1°C) [simulations indicate] a global mean increase in yields of 13% when fully accounting for the CO2 fertilization effect [versus] a decrease of 7% [without CO2 fertilization effect.]
[However, even] if such a yield increase … were achieved [it] may not be sufficient to balance population increases in several regions, including Sub-Saharan Africa, the Middle East, North Africa, South Asia, and Latin America and the Caribbean. …
[A] key constraint of the carbon fertilization effect is [the availability of essential] nutrients (for example, phosphorus and nitrogen) …
[The] response to enhanced CO2 varies across crop types, optimal temperatures for a selection of crop types (C4, for example maize) are higher than others (C3, for example rice), so that response to temperature varies as well.
The fertilization effect is … likely to be more or less offset due to higher temperatures depending on what crop is sown.
Projected Changes in Median Maize Yields under Different Management Options and(Adapted from Table 4)
Global Mean Warming Levels Above Preindustrial Temperatures with CO2 Fertilization
Experiment 1.4°C 2.4°C 3.4°C Irrigated –1.6% to –7.8% –10.2% to –16.4% –3.9% to –26.6% Rainfed +0.7% to –10.8% –5.6% to –18.1% –1.6% to –25.9%
[For] the high end of yield losses there is a consistent increase with increasing global mean warming for both rainfed and irrigated maize, with the loss larger without the CO2 fertilization effect.
[And while,] precipitation changes turn out to be more positive at one end of the probability distribution [such that] the loss in yield might be reduced above 2°C warming [— the] median estimates in all cases show increasing losses.
[There is also] a significant increase in the risk of crop failure … arising from a combination of increased heat and water stress, after taking into account the CO2 fertilization effect.
[However,] adaptation measures may be able to ameliorate many of the risks.
Implications for Economic Growth and Human Development
In a scenario that results in a 1.5°C temperature increase as soon as 2030, [the] effects on welfare as a result of the direct impact of climate change on crops will be felt most in Sub-Saharan Africa, followed by China and the United States. …
It is well established that child undernutrition has adverse implications for lifetime economic earning potential and health.
Recent projections of the consequences of a warming of [2.7°C to 3.2°C (relative to preindustrial temperatures)] by the 2050s … indicate substantial increases, particularly [of] severe stunting in Sub-Saharan Africa (23 percent) and South Asia (62 percent).
(p 46, emphasis added)
Changes to Levels of Precipitation and Water Stress in a 2°C World and in a 4°C+ World
Drier conditions are projected for southern Europe, Africa (except some areas in the northeast), large parts of North America and South America, and Australia …
Wetter conditions are projected for the northern high latitudes, that is, northern North America, northern Europe, and Siberia.
[Mean] annual runoff [is projected to decrease] in a 2°C world … in the
- Danube [30%],
- Mississippi [20%],
- Amazon [40%], and
- Murray Darling [20%] river basins …
[These] changes are approximately doubled in magnitude in a 4°C world.
In a 2°C world … changes in water stress would mostly be dominated by population changes, not climate changes.
Increasing water demand would exacerbate water stress in most regions, regardless of the direction of change in runoff.
However, in a 4°C world, climate changes would become large enough to dominate changes in water stress in many cases.
Again, water stress is expected to increase in southern Europe, the United States, most parts of South America, Africa, and Australia, while it is expected to decrease in high latitude regions. …
In five of the six major river basins … the seasonality of runoff increases along with global warming, that is, wet seasons become wetter and dry seasons become drier.
This means that while an increase in annual mean runoff, for example, in the Nile or the Ganges basin may appear beneficial at first sight, it is likely to be distributed unevenly across the seasons, possibly leading to increased flooding in the high-flow season, while hardly improving water stress in the low-flow season.
This would have severe adverse consequences for affected populations, especially if the seasonality of runoff change would be out of phase with that of demand, such as for crop growing or the cooling of thermal power plants.
Major investments in storage facilities would be required in such cases in order to control water availability across the year and actually reap the local benefits of any increases in runoff.
For such basins as the Ganges, another reason to strengthen water management capacities is that hydrological projections for the Indian monsoon region are particularly uncertain because of the inability of most climate models to simulate accurately the Indian monsoon.
(p 47, emphasis added)
[About] 50% of the runoff changes in either direction expected with warming of 4°C could be avoided if warming were constrained to 2°C.
In terms of water stress, however, the difference appears to be smaller.
In a 2°C world, about 20 to 30% less people globally are expected to be affected by increased water stress, based on per-capita availability, than in a 4°C world.
Moreover, based on the ratio of water withdrawals to availability, about 15 to 47% less people would be affected. …
Thus, when it comes to the difference between a 2°C world and a 4°C world, much more uncertainty is associated with the actual societal impacts of climate change than with the physical change in runoff.
The Availability of Water for Food Production
[Projections] for the 2080s [at +4°C compared to preindustrial temperatures] find that 43 to 50% of the global population will be living in water-scarce countries, compared to 28% today.
A Note of Caution: Limits to Anticipating Water Insecurity in a 4°C World
The climate impact on global water resources will likely be …
- increasing water availability mainly in the high latitudes of the Northern Hemisphere, and
- decreasing water availability in many regions across the tropics and subtropics, including large parts of Africa, the Mediterranean, the Middle East, and parts of Asia. …
Climate change is expected to alter the seasonal distribution of runoff and soil water availability, likely increasing the number of such extreme events as floods and droughts, both of which can have devastating effects, even if annual mean numbers remain unchanged. …
In many countries, particularly in the developing world, the adverse impacts of decreasing runoff and total water availability would probably be greatly exacerbated by high rates of population growth and by the fact that many of these countries are already water scarce and thus have little capacity to satisfy the growing demand for water resources.
Conversely, positive impacts of climate change are expected to occur primarily in countries that have higher adaptive capacities and lower population growth rates.
[In] a rapidly warming world, the most adverse impacts on water availability associated with a 4°C world may coincide with maximum water demand as world population peaks [in the second half of this century.]
Ecosystems and Biodiversity
Approximately 20 to 30% of plant and animal species … are likely to be at increased risk of extinction, if increases in global average temperature exceed of 2—3° above preindustrial levels.
[Major] changes are projected in
- ecosystem structure and function,
- species' ecological interactions and
- shifts in species' geographical ranges,
- biodiversity and
- ecosystem goods and services, such as water and food supply.
[While it is] important to recall that there remain many uncertainties … most model projections agree on major adverse consequences for biodiversity in a 4°C world.
With high levels of warming, coalescing human induced stresses on ecosystems have the potential to trigger large-scale ecosystem collapse. …
In a scenario of 2.5°C warming, severe ecosystem change, based on absolute and relative changes in carbon and water fluxes and stores, cannot be ruled out on any continent. …
Considerable change is projected for cold and tropical climates already at 3°C of warming.
At greater than 4°C of warming, biomes in temperate zones will also be substantially affected. …
Increasing vulnerability to heat and drought stress will likely lead to increased mortality and species extinction. …
Climate change also has the potential to facilitate the spread and establishment of invasive species (pests and weeds) …
Human land-use changes are expected to further exacerbate climate change driven ecosystem changes …
[In the tropics, in particular,] rising temperatures and reduced precipitation are expected to have major impacts … such as forest loss resulting from droughts and wildfire exacerbated by land use and agricultural expansion. …
[Furthermore,] shifts in the fire regime are … powerful drivers of biome shifts, potentially resulting in considerable changes in carbon fluxes over large areas …
[Poleward] latitudinal biome shifts of up to 400 km are possible in a 4°C world.
In the case of mountaintop ecosystems, for example, such a shift is not necessarily possible …
[Likewise, species] that dwell at the upper edge of continents or on islands would face a similar impediment to adaptation …
[Of] 5197 African plant species studied, 25-42% could lose all suitable range by 2085.
There is also an increased risk of extinction for herbivores in regions of drought-induced tree dieback, owing to their inability to digest the newly resident C4 grasses.
Heat and drought related die-back has already been observed in substantial areas of North American boreal forests …
At 4°C warming [the global] extent of humid tropical forest … is expected to contract to approximately 25% of its original size, while at 2°C warming, more than 75% of the original land can likely be preserved.
Current projections indicate that fire occurrence in the Amazon could double by 2050 …
A decrease in precipitation over the Amazon forests may therefore result in forest retreat or transition into a low biomass forest.
[In Amazonia] more than 90% of the original humid tropical forest niche … is likely to be preserved in the 2°C case, compared to just under half in the 4°C warming case.
Sea-level rise can cause a loss of mangroves by cutting off the flow of fresh water and nutrients and drowning the roots.
By the end of the 21st century, global mangrove cover is projected to experience a significant decline because of heat stress and sea-level rise.
[Mangroves] would need to geographically move on average about 1 km/year to remain in suitable climate zones. …
With mangrove losses resulting from deforestation presently at 1 to 2% per annum, climate change may not be the biggest immediate threat to the future of mangroves. …
The Great Barrier Reef … has been estimated to have lost 50% of live coral cover since 1985, which is attributed in part to coral bleaching because of increasing water temperatures.
Under atmospheric C02 concentrations that correspond to a warming of 4°C by 2100, reef erosion will likely exceed rates of calcification …
[Indeed,] bleaching events under global warming in even a 2°C world [are] projected to exceed the ability of coral reefs to recover.
The extinction of coral reefs would be catastrophic for entire coral reef ecosystems and the people who depend on them for food, income and shoreline. …
Loss of those species presently classified as 'critically endangered' would lead to mass extinction on a scale that has happened only five times before in the last 540 million years.
The loss of those species classified as ‘endangered’ and ‘vulnerable’ would confirm this loss as the sixth mass extinction episode.