IPCC Info Banner

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."

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

Issued April 6, 2007

Download the full report by the IPCC's Working Group I here:

Key passages:

Current knowledge about observed impacts - global

"With regard to changes in snow, ice and frozen ground (including permafrost), there is high confidence that natural systems are affected. Examples are:

"Based on growing evidence, there is high confidence that the following types of hydrological systems are being affected around the world:

"There is very high confidence, based on more evidence from a wider range of species, that recent warming is strongly affecting terrestrial biological systems, including such changes as:

"A global assessment of data since 1970 has shown it is likely that anthropogenic warming has had a discernible influence on many physical and biological systems. Much more evidence has accumulated over the past five years to indicate that changes in many physical and biological systems are linked to anthropogenic warming."

Current knowledge about future impacts - North America

"Moderate climate change in the early decades of the century is projected to increase aggregate yields of rain-fed agriculture by 5-20%, but with important variability among regions. Major challenges are projected for crops that are near the warm end of their suitable range or depend on highly utilised water resources [high confidence]."

"Warming in western mountains is projected to cause decreased snowpack, more winter flooding, and reduced summer flows, exacerbating competition for over-allocated water resources [very high confidence]."

"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 [very high confidence]."

"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. The growing number of the elderly population is most at risk [very high confidence]."

"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. Current adaptation is uneven and readiness for increased exposure is low [very high confidence]."

Current knowledge about future impacts - global

"Drought-affected areas will likely increase in extent. Heavy precipitation events, which are very likely to increase in frequency, will augment flood risk [high confidence]."

"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 [high confidence]."

"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 [medium confidence]. "

"For increases in global average temperature exceeding 1.5-2.5°C and in concomitant atmospheric carbon dioxide concentrations, there are projected to be 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 [high confidence]."

"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 [medium confidence]."

"Coasts are projected to 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 [very high confidence]."

"Costs and benefits of climate change for industry, settlement, and society will vary widely by location and scale. In the aggregate, however, net effects will tend to be more negative the larger the change in climate [high confidence]."

"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 urbanisation is occurring [high confidence]."

"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 [high confidence]."

"Projected climate change-related exposures are likely to affect the health status of millions of people, particularly those with low adaptive capacity, through:

Current knowledge about responding to climate change

"Very large sea-level rises that would result from widespread deglaciation of Greenland and West Antarctic ice sheets imply major changes in coastlines and ecosystems, and inundation of low-lying areas, with greatest effects in river deltas. Relocating populations, economic activity, and infrastructure would be costly and challenging. There is medium confidence that at least partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, would occur over a period of time ranging from centuries to millennia for a global average temperature increase of 1- 4°C (relative to 1990-2000), causing a contribution to sea level rise of 4-6 m or more. The complete melting of the Greenland ice sheet and the West Antarctic ice sheet would lead to a contribution to sea-level rise of up to 7 m and about 5 m, respectively. "

"A wide array of adaptation options is available, but more extensive adaptation than is currently occurring is required to reduce vulnerability to future climate change. There are barriers, limits and costs, but these are not fully understood."

"Although many early impacts of climate change can be effectively addressed through adaptation, the options for successful adaptation diminish and the associated costs increase with increasing climate change. At present we do not have a clear picture of the limits to adaptation, or the cost, partly because effective adaptation measures are highly dependent on specific, geographical and climate risk factors as well as institutional, political and financial constraints."

"However, adaptation alone is not expected to cope with all the projected effects of climate change, and especially not over the long run as most impacts increase in magnitude. "

"Sustainable development can reduce vulnerability to climate change by enhancing adaptive capacity and increasing resilience. At present, however, few plans for promoting sustainability have explicitly included either adapting to climate change impacts, or promoting adaptive capacity."

"Many impacts can be avoided, reduced or delayed by mitigation."

"A portfolio of adaptation and mitigation measures can diminish the risks associated with climate change."

"It is very likely that globally aggregated figures underestimate the [net economic costs of damages from climate change] because they cannot include many non-quantifiable impacts. Taken as a whole, the range of published evidence indicates that the net damage costs of climate change are likely to be significant and to increase over time."

Outline of IPCC's Fourth Assessment Report

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

Go back to the Home Page
About This Site