Extreme Weather
Key passages from the IPCC's "Climate Change 2007"
"Climate Change 2007: The Physical Science Basis"
Issued Feb. 2
Key passages from report released April 6 (Impacts)
Key passages from report released April 16 (Mitigation)
Key passages from report released November 17th (Synthesis)
Key passages from the report by the IPCC's Working Group I:
Observed changes "At continental, regional, and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones."
"More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Increased drying linked with higher temperatures and decreased precipitation have contributed to changes in drought. Changes in sea surface temperatures (SST), wind patterns, and decreased snowpack and snow cover have also been linked to droughts."
"The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapour."
"Widespread changes in extreme temperatures have been observed over the last 50 years. Cold days, cold nights and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent."
"There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures.... There is no clear trend in the annual numbers of tropical cyclones."
Projections of future changes
"It is very likely that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent." (In IPCC terminology, "very likely" means a probability greater than 90 percent and "likely" means a probability greater than 66 percent.)
"Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs (sea surface temperatures). There is less confidence in projections of a global decrease in numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period."
"Extra-tropical storm tracks are projected to move poleward, with consequent changes in wind, precipitation, and temperature patterns, continuing the broad pattern of observed trends over the last half-century."
"Since the [2001 IPCC report] there is an improving understanding of projected patterns of precipitation. Increases in the amount of precipitation are very likely in high-latitudes, while decreases are likely in most subtropical land regions...continuing observed patterns in recent trends."
"Snow cover is projected to contract. Widespread increases in thaw depth are projected over most permafrost regions."
Scientific certainty
"There is now higher confidence in projected patterns of warming and other regional-scale features, including changes in wind patterns, precipitation, and some aspects of extremes and of ice."
"Climate Change 2007: Impacts, Adaptation and Vulnerability"
Issued April 6
Key passages:
"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]."
"[From the report's North America section:] 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]."
"[From the report's North America section:] 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]."
[top of page]North America Chapter of the IPCC's WGII Technical Report:
"Climate Change 2007: Impacts, Adaptation and Vulnerability"
Issued April 16
Key Passages:
"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). Late in the century, Chicago is projected to experience 25% more frequent heat waves annually, and the projected number of heat wave days in Los Angeles increases from 12 to 44-95."
"Future trends in hurricane frequency and intensity remain very uncertain. Experiments with climate models with sufficient resolution to depict some aspects individual hurricanes tend to project some increases in both peak wind speeds and precipitation intensities. The pattern is clearer for extra-tropical storms which are likely to become more intense, but perhaps less frequent, leading to increased extreme wave heights in the mid-latitudes."
"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]."
"El Niño events are associated with increased precipitation and severe storms in some regions, such as the U.S. Southeast, and higher precipitation in the Great Basin, but warmer temperatures and decreased precipitation in other areas such as the Pacific Northwest, western Canada, and parts of Alaska. Recent analyses indicate no consistent future trends in El Niño amplitude or frequency."
[top of page]Climate Change 2007: Synthesis Report
Issued November 17th
Key Passages:
"Advances since the TAR [Third Assessment Report] show that discernible human influences extend beyond average temperature to other aspects of climate.
"Human influences have:
- very likely contributed to sea level rise during the latter half of the 20th century
- likely contributed to changes in wind patterns, affecting extra-tropical storm tracks and temperature patterns
- likely increased temperatures of extreme hot nights, cold nights and cold days
- more likely than not increased risk of heat waves, area affected by drought since the 1970s and frequency of heavy precipitation events."
"Some systems, sectors and regions are likely to be especially affected by climate change.
"Particular ecosystems:
- terrestrial: tundra, boreal forest and mountain regions because of sensitivity to warming; mediterranean-type ecosystems because of reduction in rainfall; and tropical rainforests where precipitation declines
- coastal: mangroves and salt marshes, due to multiple stresses
- marine: coral reefs due to multiple stresses; the sea ice biome because of sensitivity to warming
- water resources in some dry regions at mid-latitudes and in the dry tropics, due to changes in rainfall and evapotranspiration, and in areas dependent on snow and ice melt
- agriculture in low-latitudes , due to reduced water availability
- low-lying coastal systems, due to threat of sea level rise and increased risk from extreme weather events
- human health in populations with low adaptive capacity.
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