...the consistency between observed and modelled changes in several studies and the spatial agreement between significant regional warming and consistent impacts at the global scale is sufficient to conclude with high confidence that  anthropogenic warming over the last three decades has had a discernible influence on many physical and biological systems.

High confidence reflects an 80% chance of being correct.

Illustrative examples of global impacts projected for climate changes (and sea-level and atmospheric carbon dioxide where relevant) associated with different amounts of increase in global average surface temperature in the 21st century. Click to enlarge.

Future. The working group produced a summary report of the larger scientific tome in which they  grouped the projections of future impact into six primary areas:

Water. By mid-century, annual average river runoff and water availability are projected to decrease by 10-30% over some dry regions at mid-latitudes and in the dry tropics. Some of these are already are presently water stressed areas. Runoff and availability could increase by 10-40% at high latitudes and in some wet tropical areas.

Drought-affected areas will likely increase in extent. The US Southwest, for example, is projected to begin encountering Dust Bowl-like conditions. Heavy precipitation events, which are very likely to increase in frequency, will augment flood risk.

In the course of the century, water supplies stored in glaciers and snow cover are projected to decline, reducing water availability in regions supplied by meltwater from major mountain ranges, where more than one-sixth of the world population currently lives.

Ecosystems. The stress of climate change combined with related natural effects (e.g., flooding, drought, wildfire, insects, ocean acidification) and other anthropogenic inputs (e.g., land use change, pollution, over-exploitation of resources) is likely to push many ecosystems over the edge.

Although terrestrial ecosystems currently function as carbon sinks, their carbon uptake is likely to peak before midcentury and then weaken or even reverse, thus amplifying climate change.

Approximately 20-30% of plant and animal species assessed so far are likely to be at increased risk of extinction if increases in global average temperature exceed 1.5-2.5° C.

For increases in global average temperature exceeding 1.5-2.5° C the report projects major changes in ecosystem structure and function, species’ ecological interactions, and species’ geographic ranges, with predominantly negative consequences for biodiversity, and ecosystem goods and services—e.g., water and food supply.

The progressive acidification of oceans due to increasing atmospheric carbon dioxide is expected to have negative impacts on marine shell forming organisms (e.g., corals) and their dependent species.

Food, fiber and forest.  Crop productivity is projected to increase slightly at mid to high latitudes for local mean temperature increases of up to 1-3° C depending on the crop, and then decrease beyond that in some regions. At lower latitudes, especially seasonally dry and tropical regions, crop productivity is projected to decrease for even small local temperature increases (1-2° C), which would increase risk of hunger.  Globally, the potential for food production is projected to increase with increases in local average temperature over a range of 1-3° C, but above this it is projected to decrease.

Adaptations such as altered cultivars and planting times would allow low- and mid- to high-latitude cereal yields to be maintained at or above baseline yields for modest warming.

Increases in the frequency of droughts and floods are projected to affect local production negatively, especially in subsistence sectors at low latitudes.

Globally, commercial timber productivity rises modestly with climate change in the short- to medium-term, with large regional variability around the global trend. Regional changes in the distribution and production of particular fish species are expected due to continued warming, with adverse effects projected for aquaculture and fisheries.

Coastal systems and low-lying areas. The report projects that coasts will be exposed to increasing risks, including coastal erosion, due to climate change and sea-level rise and the effect will be exacerbated by increasing human-induced pressures on coastal areas.

Corals are vulnerable to thermal stress and have low adaptive capacity. Increases in sea surface temperature of about 1 to 3° C are projected to result in more frequent coral bleaching events and widespread mortality, unless there is thermal adaptation or acclimatization by corals.

Coastal wetlands, including salt marshes and mangroves, will be negatively affected by sea-level rise especially where they are constrained on their landward side, or starved of sediment.

Many millions more people are projected to be flooded every year due to sea-level rise by the 2080s. Those densely-populated and low-lying areas where adaptive capacity is relatively low, and which already face other challenges such as tropical storms or local coastal subsidence, are especially at risk. The numbers affected will be largest in the mega-deltas of Asia and Africa while small islands are especially vulnerable. Adaptation for coastal regions will be more challenging in developing countries than developed countries due to constraints on adaptive capacity.

Industry, Settlement and Society. The authors note that the costs and benefits of climate change for industry, settlement, and society will vary widely by location and scale. The larger the change in climate, the more negative the net effects.

The most vulnerable industries, settlements and societies are generally those in coastal and river flood plains, those whose economies are closely linked with climate-sensitive resources, and those in areas prone to extreme weather events, especially where rapid urbanization is occurring.

Poor communities can be especially vulnerable, in particular those concentrated in high-risk areas. They tend to have more limited adaptive capacities, and are more dependent on climate-sensitive resources such as local water and food supplies.

Where extreme weather events become more intense and/or more frequent, the economic and social costs of those events will increase, and these increases will be substantial in the areas most directly affected. Climate change impacts spread from directly impacted areas and sectors to other areas and sectors through extensive and complex linkages.

Health. Projected climate change-related exposures are likely to affect the health status of millions of people, particularly those with low adaptive capacity, through a variety of impacts, including increases in malnutrition and consequent disorders; increased deaths, disease and injury due to heat waves, floods, storms, fires and droughts; the increased burden of diarrhoeal disease; the increased frequency of cardio-respiratory diseases due to higher concentrations of ground level ozone related to climate change; and, the altered spatial distribution of some infectious disease vectors.

Although climate change in temperate areas is projected to bring some benefits, such as fewer deaths from cold exposure, overall these benefits will be outweighed by the negative health effects of rising temperatures world-wide, especially in developing countries.

The WG II reports also broke out the effects of climate change on a regional basis. Africa and Asia are likely to suffer some of the worst effects. 

Africa will be one of the most vulnerable continents because of multiple stresses and low adaptive capability. By 2020, between 75 and 250 million people are projected to be exposed to an increase of water stress due to climate change. Agricultural production, including access to food, in many African countries and regions is projected to be severely compromised by climate variability and change.

The area suitable for agriculture, the length of growing seasons and yield potential are expected to decrease. In some countries, yields from rain-fed agriculture could be reduced by up to 50% by 2020.

Towards the end of the 21st century, projected sea-level rise will affect low-lying coastal areas with large populations. The cost of adaptation could amount to at least 5-10% of GDP.

Asia will be hard hit by decreases in freshwater availability which, which, along with population growth and increasing demand arising from higher standards of living, could adversely affect more than a billion people by the 2050s.

Glacier melt in the Himalayas is projected to increase flooding, rock avalanches from destabilized slopes, and affect water resources within the next two to three decades. This will be followed by decreased river flows as the glaciers recede.

Coastal areas, especially heavily-populated mega-delta regions in South, East and Southeast Asia, will be at greatest risk due to increased flooding from the sea and in some mega-deltas flooding from the rivers.

Although crop yields could increase up to 20% in East and Southeast Asia, they could decrease up to 30% in Central and South Asia by the mid-21st century. Taken together and considering the influence of rapid population growth and urbanization, the risk of hunger is projected to remain very high in several developing countries.

The report expects endemic morbidity and mortality due to diarrhoeal disease primarily associated with floods and droughts to rise in East, South and Southeast Asia due to projected changes in hydrological cycle associated with global warming. Increases in coastal water temperature would exacerbate the abundance and/or toxicity of cholera in South Asia.

No region is unaffected however. Europe will suffer increased risk of inland flash floods, and more frequent coastal flooding and increased erosion (due to storminess and sea level rise). The great majority of organisms and ecosystems will have difficulties adapting to climate change. Mountainous areas will face glacier retreat, reduced snow cover and winter tourism, and extensive species losses (in some areas up to 60% under high emission scenarios by 2080).

Warming in the western mountains of North America is projected to cause decreased snowpack, more winter flooding, and reduced summer flows, exacerbating competition for over-allocated water resources.

Disturbances from pests, diseases, and fire are projected to have increasing impacts on forests, with an extended period of high fire risk and large increases in area burned.

Cities that currently experience heat waves are expected to be further challenged by an increased number, intensity and duration of heat waves during the course of the century, with potential for adverse health impacts.

Coastal communities and habitats will be increasingly stressed by climate change impacts interacting with development and pollution. Population growth and the rising value of infrastructure in coastal areas increase vulnerability to climate variability and future climate change, with losses projected to increase if the intensity of tropical storms increases.

In Latin America by mid-century, increases in temperature and associated decreases in soil water are projected to lead to gradual replacement of tropical forest by savanna in eastern Amazonia. Semi-arid vegetation will tend to be replaced by arid-land vegetation.

In drier areas, climate change is expected to lead to salinisation and desertification of agricultural land. Productivity of some important crops are projected to decrease and livestock productivity to decline, with adverse consequences for food security.

Changes in precipitation patterns and the disappearance of glaciers are projected to significantly affect water availability for human consumption, agriculture and energy generation.

Australia and New Zealand will likely see water security problems intensify by 2030. The report projects significant loss of biodiversity to occur by 2020 in some ecologically-rich sites including the Great Barrier Reef and Queensland Wet Tropics.

Production from agriculture and forestry by 2030 is projected to decline over much of southern and eastern Australia, and over parts of eastern New Zealand, due to increased drought and fire. However, in New Zealand, initial benefits to agriculture and forestry are projected in western and southern areas and close to major rivers due to a longer growing season, less frost and increased rainfall.

Small Islands,  whether located in the Tropics or higher latitudes, have characteristics which make them especially vulnerable to the effects of climate change, sea level rise and extreme events. Coastal deterioration is projected to impact local resources as well as tourism. Sea-level rise will exacerbate inundation, storm surge, erosion and other coastal hazards.

Climate change is projected by the mid-century to reduce water resources in many small islands, e.g., in the Caribbean and Pacific, to the point where they become insufficient to meet demand during low rainfall periods.

In an editorial published in this week’s issue of Science, the two co-chairs of the Scientific Expert Group on Climate Change and Sustainable Development (SEG)  convened by Sigma Xi at the invitation of the UN Department of Economic and Social Affairs (DESA), argue for a two-pronged approach to minimizing some of the dangers and damages now more clearly outlined in the WG II report.

A sensible strategy to minimize the damages from anthropogenic climate change has two objectives: mitigate the pace and ultimate magnitude of the changes that occur and adapt to the changes that cannot be avoided.

The SEG recently submitted a report—Confronting Climate Change— to UN Secretary-General Ban Ki-moon with a call for new levels of commitment and coordination by the UN and its member states to avoid the worst climate-change dangers while there is still time.

The SEG argues for immediate action by policy makers that will limit temperature increases to 2-2.5°C above the 1750 pre-industrial level:

• Improving efficiency in the transportation sector through measures such as vehicle efficiency standards, fuel taxes, and registration fees/rebates that favor purchase of efficient and alternative fuel vehicles.

• Improving design and efficiency of commercial and residential buildings through building codes, standards for equipment and appliances, incentives for property developers and landlords to build and manage properties efficiently, and financing for energy-efficiency investments.

• Expanding the use of biofuels through energy portfolio standards and incentives to growers and consumers.

• Beginning immediately, designing and deploying only coal-fired power plants that will be capable of cost-effective and environmentally-sound retrofits for capture and sequestration of their carbon emissions.

The SEG report also argues for accelerated activity on adapting to ongoing and unavoidable changes in the climate system, and improving preparedness and response strategies.

Resources:

• Working Group II Contribution to the Intergovernmental Panel on Climate Change Fourth Assessment Report Climate Change 2007: Climate Change Impacts, Adaptation and Vulnerability (Summary for Policymakers)

• “A Two-Pronged Climate Strategy”; Rosina M. Bierbaum and Peter H. Raven; Science 6 April 2007: Vol. 316. no. 5821, p. 17 DOI: 10.1126/science.1143220

• Confronting Climate Change: Avoiding the Unmanageable and Managing the Unavoidable (SEG report)