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Working Group II Definitions

Likelihood of an outcome or result
Virtually certain > 99% probability of occurrence
Extremely likely > 95%
Very likely > 90%
Likely > 66%
More likely than not > 50%
Very unlikely < 10%
Extremely unlikely < 5%.

Confidence in a statement
Very high confidence: At least a 9 out of 10 chance of being correct
High confidence: About an 8 out of 10 chance
Medium confidence: About a 5 out of 10 chance
Low confidence: About a 2 out of 10 chance
Very low confidence: Less than a 1 out of 10 chance

Confidence symbols in the text
*** Very high confidence
** High confidence
* Medium confidence

Key Assumptions

The Working Group II report discusses future impacts "for the range of unmitigated climate changes projected by the IPCC over this century judged to be relevant for people and the environment." Its different scenarios of future emissions of greenhouse gases "do not include additional climate initiatives, which means that no scenarios are included that explicitly assume implementation of the United Nations Framework Convention on Climate Change or the emissions targets of the Kyoto Protocol."

North America Chapter of the IPCC's WGII Technical Report:
"Climate Change 2007: Impacts, Adaptation and Vulnerability"

On April 16, 2007, the Intergovernmental Panel on Climate Change (IPCC) released the North America chapter of the Working Group II technical report in Washington, D.C. This new chapter details the North American findings summarized in the global Summary for Policymakers released on April 6.

Key passages excerpted from this new chapter on North America as well as key passages from the Working Groups I and II Summary for Policymakers reports are available throughout this website. The Summary reports are available on the IPCC website: http://www.ipcc.ch/.

The Working Group II report, "Climate Change 2007: Impacts, Adaptation and Vulnerability," is the second in a four-part series being released in 2007, constituting the IPCC's Fourth Assessment Report (AR4).

The Working Group II report summarizes current knowledge about impacts of climate change that already have been observed. It also projects future impacts, based on scenarios in which no explicit actions are taken to address global warming and activity continues on a business-as-usual path. That is, the projections assume that climate change impacts are not mitigated by actions such as cuts in greenhouse gases nor by policies that would enhance adaptability to global warming.

In addition to key excerpts from the IPCC reports, this website offers academic and journalistic resources to help journalists localize their coverage of the four IPCC reports being issued in 2007, including regional and state resources, contact information for scientists, and recordings of regional briefings with scientists held April 2-4, 2007.

Key Passages

See regional pages for region specific excerpts:
(Midwest, Northeast, South and West)

Summary Excerpts:
[Go directly to Future Impacts, Vulnerabilities, Assumptions, Adaptation]

"North America has experienced locally severe economic damage, plus substantial ecosystem, social, and cultural disruption from recent weather-related extremes, including hurricanes, other severe storms, floods, droughts, heat waves, and wildfires [very high confidence]."

"The vulnerability of North America depends on the effectiveness and timing of adaptation and the distribution of coping capacity, which vary spatially and among sectors [very high confidence]."

"Coastal communities and habitats will be increasingly stressed by climate change impacts interacting with development and pollution [very high confidence]."

"Climate change will constrain North America's already heavily utilized water resources, increasing competition among agricultural, municipal, industrial, and ecological uses [very high confidence]."

"Without increased investments in countermeasures, hot temperatures and extreme weather are likely to cause increased adverse health impacts from heat-related mortality, pollution, storm-related fatalities and injuries, and infectious diseases [very high confidence]."

"Disturbances like wildfire and insect outbreaks are increasing and are likely to intensify in a warmer future with drier soils and longer growing seasons [very high confidence]."

Key points for describing historical experience with and future prospects for dealing with climate impacts:

Future Impacts and Vulnerabilities     [top of page]

Ecosystems, Ag/Forests/Fisheries, Health, Settlements, Tourism, Energy, Transportation, Cities

"Simulated future surface and bottom water temperatures of lakes, reservoirs, rivers, and estuaries throughout North America consistently increase from 2-7C (2xCO2 and IS92a), with summer surface temperatures exceeding 30C in Midwestern and southern lakes and reservoirs. Warming is likely to extend and intensify summer thermal stratification, contributing to oxygen depletion. A shorter ice-cover period in shallow northern lakes could reduce winter fish kills caused by low oxygen. Higher stream temperatures affect fish access, survival and spawning (e.g., west coast salmon)."

"Climate change is likely to make it more difficult to achieve existing water quality goals [high confidence]."


"Several simulations indicate that, over the 21st century, warming will lengthen growing seasons, sustaining forest carbon sinks in North America, despite some decreased sink strength resulting from greater water limitations in western forests and higher respiration in the tropics [medium confidence]. Impacts on ecosystem structure and function may be amplified by changes in extreme meteorological events, and increased disturbance frequencies. Ecosystem disturbances, caused either by humans or by natural accelerate both loss of native species and invasion of exotics."

"Overall forest growth in North America will likely increase modestly (10-20%) as a result of extended growing seasons and elevated CO2 over the next century ...."

"Bioclimate modeling based on output from five GCMs suggests that, over the next century, vertebrate and tree species richness will decrease in most parts of the conterminous U.S., even though long-term trends (millennia) ultimately favour increased richness in some taxa and locations. Based on relationships between habitat area and biodiversity, 15-37% of plant and animal species in a global sample are likely to be "committed to extinction" by 2050, although actual extinctions will be strongly influenced by human forces and could take centuries."

"Potentially more intense storms and possible changes in El Niño are likely to result in more coastal instability [medium confidence]. Damage costs from coastal storm events (storm surge, waves, wind, ice encroachment) and other factors (such as freeze-thaw) have increased substantially in recent decades and are expected to continue rising [high confidence]."

Agriculture, forestry and fisheries

"Research since the TAR supports the conclusion that moderate climate change will likely increase yields of North American rain fed agriculture, but with smaller increases and more spatial variability than in earlier estimates [high confidence]. Most studies project likely climate-related yield increases of 5-20% over the first decades of the century, with the overall positive effects of climate persisting through much or all of the 21st century. This pattern emerges from recent assessments for corn, rice, sorghum, soybean, wheat, common forages, cotton and some fruits, including irrigated grains. Increased climate sensitivity is anticipated in the south-eastern U.S. and in the U.S. Cornbelt, but not in the Great Plains."

"The critical importance of specific agro-climatic events (e.g., last frost) introduces uncertainty in future projections, as does continued debate about the CO2 sensitivity of crop growth."

"Across North America, impacts of climate change on commercial forestry potential are likely to be sensitive to changes in disturbances from insects, diseases, and wildfires [high confidence]. In the absence of dramatic increases in disturbance, effects of climate change on the potential for commercial harvest in 2040s ranged from mixed for a low emissions scenario (EPPA - LLH) to positive for a high emissions scenario (EPPA - HHL). Scenarios with increased harvest tend to lead to lower prices and, as a consequence, reduced harvests, especially in Canada. The tendency for North American producers to suffer losses increases if climate change is accompanied by increased disturbance, with simulated losses averaging US$1-2 billion/yr, over the 21st century. Increased tropospheric ozone could cause further decreases in tree growth. Risks of losses from Southern pine beetle likely depend on the seasonality of warming, with winter and spring warming leading to the greatest damage."

"Cold-water fisheries will likely be negatively affected by climate change; warm-water fisheries will generally gain; and the results for cool-water fisheries will be mixed, with gains in the northern and losses in the southern portions of ranges [high confidence]. Salmonids, which prefer cold, clear water, are likely to experience the most negative impacts."

"In the continental U.S., cold-water species will likely disappear from all but the deeper lakes, cool-water species will be lost mainly from shallow lakes, and warm water species will thrive except in the far south, where temperatures in shallow lakes will exceed survival thresholds."

"Thermal habitat suitable for yellow perch will expand, while that for lake trout will contract. While temperature increases may favour warm-water fishes like smallmouth bass, changes in water supply and flow regimes seem likely to have negative effects."

Human Health

"Severe heat waves, characterized by stagnant, warm air masses and consecutive nights with high minimum temperatures will intensify in magnitude and duration over the portions of the U.S. and Canada, where they already occur [high confidence]."

"Surface ozone concentration may increase with a warmer climate. Ozone damages lung tissue, causing particular problems for people with asthma and other lung diseases. Even modest exposure to ozone may encourage the development of asthma in children."

"For the 2050s, daily average ozone levels are projected to increase by 3.7 ppb across the eastern U.S., with the cities most polluted today experiencing the greatest increase in ozone pollution. One-hour maximum ozone follows a similar pattern, with the number of summer days exceeding the 8-hour regulatory U.S. standard projected to increase by 68%. Assuming constant population and dose-response characteristics, ozone related deaths from climate change increase by approximately 4.5% from the 1990s to the 2050s."

"Pollen, another air contaminant, is likely to increase with elevated temperature and CO2. A doubling of the atmospheric CO2 concentration stimulated ragweed-pollen production by over 50%, Ragweed grew faster, flowered earlier, and produced significantly greater above ground biomass and pollen at urban than at rural locations."

"The Northern boundary of tick-borne Lyme disease is limited by cold temperature effects on the tick, Ixodes scapularis. The northern range limit for this tick could shift north by 200 km by the 2020s, and 1000 km by the 2080s."

Human Settlements

"Since the TAR, a few studies have projected increasing vulnerability of infrastructure to extreme weather related to climate warming unless adaptation is effective [high confidence]."

"Less reliable supplies of water are likely to create challenges for managing urban water systems as well as for industries that depend on large volumes of water. U.S. water managers anticipate local, regional, or state-wide water shortages during the next ten years. Threats to reliable supply are complicated by the high population growth rates in western states where many water resources are at or approaching full utilization. Potential increases in heavy precipitation, with expanding impervious surfaces, could increase urban flood risks and create additional design challenges and costs for stormwater management."

Tourism and recreation

"Although coastal zones are among the most important recreation resources in North America, the vulnerability of key tourism areas to sea level rise has not been comprehensively assessed."

Energy demand, supply

"Recent North American studies generally confirm earlier work showing a small net change (increase or decrease, depending on methods, scenarios, and location) in the net demand for energy in buildings but a significant increase in demand for electricity for space cooling, with further increases caused by additional market penetration of air conditioning [high confidence]."

"Wind and solar resources are about as likely as not to increase [medium confidence]."

"Bioenergy potential is climate-sensitive through direct impacts on crop growth and availability of irrigation water. Bioenergy crops are projected to compete successfully for agricultural acreage at a price of US$33/Mg, or about US$1.83/109 joules."


"An increase in the frequency, intensity, or duration of heat spells could cause railroad track to buckle or kink and affect roads through softening and traffic-related rutting. Some of these problems can be ameliorated with altered road design, construction, and management, including changes in the asphalt mix or the timing of spring load restrictions."

North American cities

"Impacts of climate change in the metropolitan regions of North America will be similar in many respects."

"Since most large North American cities are on tidewater, rivers or both, effects of climate change will likely include sea level rise (SLR) and/or riverine flooding. The largest impacts are expected when SLR, heavy river flows, high tides, and storms coincide."

"Climate change will likely lead to substantial increases in electricity demand for summer cooling in most North American cities."

"North American city water supply systems often draw water from considerable distances, so climate impacts need not be local to affect cities."

Current Sensitivities/Vulnerabilities     [top of page]

Ecosystems, Coasts, Health, Settlements, Infrastructure, Energy/Industry/Trans

"Annual mean air temperature, on the whole, increased in North America for the period 1955-2005, with the greatest warming in Alaska and north-western Canada, substantial warming in the continental interior and modest warming in the south-eastern U.S. and eastern Canada."

"The length of the vegetation growing season has increased an average of 2 days/decade since 1950 in Canada and the conterminous U.S., with most of the increase resulting from earlier spring warming. The warming signal in North America during the latter half of the 20th century reflects the combined influence of greenhouse gases, sulphate aerosols, and natural external forcing."

"Annual precipitation has increased for most of North America with large increases in northern Canada, but with decreases in the southwest U.S., the Canadian Prairies, and the eastern Arctic. Heavy precipitation frequencies in the U.S. were at a minimum in the 1920s and 1930s, and increased to the 1990s (1895-2000)."

"Vulnerability to extended drought is increasing across North America as population growth and economic development create more demands from agricultural, municipal, and industrial uses, resulting in frequent over-allocation of water resources. Although drought has been more frequent and intense in the western part of the U.S. and Canada, the east is not immune from droughts and attendant reductions in water supply, changes in water quality and ecosystem function, and challenges in allocation."


"Global daily satellite data, available since 1981, indicate earlier onset of spring 'greenness' by 10-14 days over 19 years, particularly across temperate latitudes of the Northern Hemisphere. Field studies confirm these satellite observations."

"Net primary production (NPP) in the continental U.S. increased nearly 10% from 1982-1998, with the largest increases in croplands and grasslands of the Central Plains due to improved water balance."

Coastal regions

"Relative sea level is rising in many areas, yet coastal residents are often unaware of the trends and their impacts on coastal retreat and flooding."

"Many coastal areas in North America are potentially exposed to storm-surge flooding."

"Adaptation to coastal hazards under present climate is often inadequate and readiness for increased exposure is poor."

Human health

"Waterborne disease outbreaks from all causes in the U.S. are distinctly seasonal, clustered in key watersheds, and associated with heavy precipitation or extreme precipitation and warmer temperatures. Heavy runoff after severe rainfall can also contaminate recreational waters and increase the risk of human illness through higher bacterial counts. This association is strongest at beaches closest to rivers."

"The strain of West Nile virus (WNV) that emerged for the first time in North America during the record hot July, 1999, requires warmer temperatures than other strains. The greatest WNV transmissions during the epidemic summers of 2002-2004 in the U.S. were linked to above-average temperatures."

"Lyme disease is a prevalent tick-borne disease in the North America for which there is new evidence of an association with temperature and precipitation."

"Exposure to both extreme hot and cold weather is associated with increased morbidity and mortality, compared to an intermediate 'comfortable' temperature range. Across 12 U.S. cities, hot temperatures were associated with increased hospital admissions for cardiovascular disease. Admissions to emergency rooms were directly related to extreme heat in Toronto. Heat response plans and heat early warning systems (EWS) can save lives. After the 1995 heat wave, the City of Milwaukee initiated an 'extreme heat conditions plan' that almost halved heat-related morbidity and mortality."

Human settlements

"Among the most climate-sensitive North American communities are those of indigenous populations dependent on one or a few natural resources. Many rural settlements in North America, particularly those dependent on a narrow resource base, such as fishing or forestry, have been seriously affected by recent declines in the resource base, caused by a number of factors..."

Infrastructure and extreme events

"The devastating effects of hurricanes Ivan in 2004 and Katrina, Rita, and Wilma in 2005 . illustrate the vulnerability of North American infrastructure and urban systems that were either not designed or maintained to adequate safety margins."

"North America very likely will continue to suffer serious losses of life and property simply due to growth in property values and numbers of people at risk [very high confidence]. Of the US $19 trillion value of all insured residential and commercial property in the U.S. states exposed to North Atlantic hurricanes, US $7.2 trillion (41%) is located in coastal counties. This economic value includes 79% of the property in Florida, 63% of the property in New York, and 61% of the property in Connecticut."

"Flood hazards are not limited to the coastal zone. River basins with a history of major floods (e.g., the Sacramento, the Fraser, the Red River, the upper Mississippi), illustrate the sensitivity of riverine flooding to extreme events and highlight the critical importance of infrastructure design standards, land use planning, and weather/flood forecasts."

Energy, industry and transportation

"North American industry, energy supply, and transportation networks are sensitive to weather extremes that exceed their safety margins. Costs of these impacts can be high. For example, power outages in the U.S. cost the economy US $30-130 billion annually. The hurricanes crossing Florida in the summer of 2004 resulted in direct system restoration costs of US $1.4 billion to the four Florida public utilities involved. From 1994-2004, fourteen U.S. utilities experienced 81 other major storms which cost an average of US $49 million/storm with the highest single storm impact of US $890 million."

"Although it was not triggered specifically by the concurrent hot weather, the 2003 summer outage in north-eastern U.S. and south-eastern Canada illustrates costs to North American society that result from large-scale power interruptions during periods of high demand. Over 50 million people were without power, resulting in US $180 million in insured losses and up to US $10 billion in total losses. Business interruptions were particularly significant, with costs incurred by the top quartile of recently surveyed companies over US $250,000/hr."

"The impacts of hurricanes Katrina, Rita, and Wilma in 2005 and Ivan in 2004 demonstrated that the Gulf of Mexico offshore oil and natural gas platforms and pipelines, petroleum refineries, and supporting infrastructure can be seriously harmed by major hurricanes which can produce national level impacts, and require recovery times stretching to months or longer."

"Hydropower production is known to be sensitive to total runoff, to its timing, and to reservoir levels."

Assumptions about Future Trends     [top of page]

"Recent climate model simulations indicate that by the 2010-2039 time slice, year-round temperatures across North America will be outside the range of natural variability..."

"For most combinations of model, scenario, season, and region, warming in the 2010-2039 time slice will be in the range of 1-3 degrees C. Late in the century, projected annual warming is likely to be 2-3C across the western, southern, and eastern continental edges, but more than 5 degrees C at high latitudes. The projected warming is greatest in the winter at high latitudes and greatest in the summer in the Southwest U.S. Warm extremes across North America are projected to become both more frequent and longer."

"In general, projected changes in precipitation extremes are larger than changes in mean precipitation."

"Recent analyses indicate no consistent future trends in El Nio amplitude or frequency."

Adaptation     [top of page]

Business, Mainstreaming, Socio-factors, Institutions, Constraints, Market Barriers

"Despite many examples of adaptive practices in North America, underinvestment in adaptation is evident in the recent rapid increase in property damage due to climate extremes and illustrates the current adaptation deficit."

"Many cities in North America have initiated "no regrets" actions based on historical experience. In the Los Angeles area, incentive and information programs of local water districts encourage water conservation. A population increase of over 35% (nearly one million people) since 1970 has only increased water use in Los Angeles by 7%. New York has reduced total water consumption by 27% and per capita consumption by 34% since the early 1980s. Vancouver's 'CitiesPLUS' 100-year plan will upgrade the drainage system by connecting natural areas and waterways, developing locally resilient, smaller systems, and upgrading key sections of pipe during routine maintenance."

Adaptation for businesses

"About 70% of businesses face some weather risk. The impact of weather on businesses in the U.S. is an estimated US$200 billion/yr. Climate change may also create business opportunities. For example, spending on storm-worthiness and construction of disaster resilient homes increased substantially after the 2004 and 2005 Atlantic hurricanes, as did the use of catastrophe bonds."

"Businesses in Canada and the U.S. are investing in climate-relevant adaptations, though few of these appear to be based on projections of future climate change. For example:

"Climate change will likely increase risks of wildfire.. the greatest reduction in risk will occur in communities that take a comprehensive approach, managing forests with controlled burns and thinning, promoting or mandating appropriate roofing materials, and maintaining defensible space around each building."

Mainstreaming Adaptation - experience and knowledge:

"The behaviour of people and systems in North America largely reflects historic climate experience, which has been institutionalized through building codes, flood management infrastructure, water systems, and a variety of other programs."

"Decisions by community water managers and set-back regulations in coastal areas also account for historic experience but rarely incorporate information about climate change or sea-level rise. In general, decision makers lack the tools and perspectives to integrate future climate, particularly events that exceed historic norms."

"Examples of adaptive behaviour influenced exclusively or predominantly by projections of climate change are largely absent from the literature, but some early steps toward planned adaptation have been taken by the engineering community, insurance companies, water managers, public health officials, forest managers, and hydroelectric producers."

"Adaptive behaviour is typically greater in the communities that recently experienced a natural disaster. But the near absence of any personal preparedness following the 2003 blackout in eastern North America demonstrated that adaptive actions do not always follow significant emergencies."

Socio-economic factors

"The poor and marginalized in Canada and the U.S. have historically been most at risk from weather shocks, with vulnerability directly related to income inequality. Differences in individual capacity to cope with extreme weather were evident in New Orleans during and after Hurricane Katrina, when the large majority of those requiring evacuation assistance were either poor or in groups with limited mobility - including elderly, hospitalized, and disabled citizens."

Political and institutional capacity

"Public officials in Canada and the U.S. typically provide early and extensive assistance in emergencies. Nevertheless, emergency response systems in the U.S. and Canada are based on the philosophy that households and businesses should be capable of addressing their own basic needs for up to 72 hours after a disaster."

Constraints and opportunities

"An active dialogue among stakeholders and political institutions has the potential for clarifying the opportunities for adaptation to changing climate. However, public discussion about adaptation is at an early stage in the U.S. and Canada, largely because national governments have focused public discussion on mitigation, with less attention to adaptation. Some public funds have been directed to research on impacts and adaptation, and both countries have undertaken national assessments with a synthesis of the adaptation literature, but neither country has a formal adaptation strategy."

"High adaptive capacity, as in most of North America, should be an asset for coping with or benefiting from climate change. Capacity, however, does not ensure positive action or any action at all.. In North America, information about climate change is usually not 'mainstreamed' or explicitly considered in the overall decision-making process. This can lead to actions that are maladapted, for example, development near floodplains or coastal areas known to be vulnerable to climate change. Water managers are unlikely to use climate forecasts, even when they recognize the vulnerability, unless the forecast information can fit directly into their everyday management decisions."

"A major challenge is the need for efficient technology and knowledge transfer. In general, questions about responsibility for funding research, involving stakeholders, and linking communities, government and markets have not been answered."

Financial and market barriers

"In the U.S., recent spending on adaptation to extremes has been a sound investment, contributing to reduced fatalities, injuries and significant economic benefits. The Multihazard Mitigation Council (2005) found that US$3.5 billion in spending between 1993 and 2003 on programs to reduce future damages from flooding, severe wind, and earthquakes contributed US$14 billion in societal benefits... Adaptation also benefited government as each dollar of spending resulted in US$3.65 in savings or increased tax revenue. This is consistent with earlier case studies; the CAD $65 million invested in 1968 to create the Manitoba Floodway has prevented several billion dollars in flood damage."

"Climate change creates a broad range of difficult challenges that influence the attainment of sustainability goals. Several of the most difficult emerge from the long time scale over which the changes occur and the possible need for action well before the magnitude (and certainty) of the impacts is clear. Other difficult problems arise from the intrinsic global scale of climate change. Because the drivers of climate change are truly global, even dedicated action at the regional scale has limited prospects for ameliorating regional scale impacts. These two sets of challenges, those related to time scale and those related to the global nature of climate change, are not in the classes that have traditionally yielded to the free market mechanisms and political decision making that historically characterize Canada and the U.S. Yet, the magnitude of the climate change challenge calls for proactive adaptation and technological and social innovation, areas where Canada and the U.S. have abundant capacity."

Outline of the IPCC's Fourth Assessment Report (AR4)

AR4 Working Group II report: "Climate Change 2007: Impacts, Adaptation and Vulnerability"

AR4 Working Group I report: "Climate Change 2007: The Physical Science Basis"

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