Climate Change Impact Analysis
The U.S. Global Change Research Program released a report summarizing the science of climate change and the impacts of climate change in the United States, now and in the future. The experts used information and events that have taken place in the past in order to make future predictions.
The critical points are:
- Precipitation has increased an average of about 5 percent over the past 50 years. Projections of future precipitation generally indicate that northern areas will become wetter, and southern areas, particularly in the West will become drier.
- The amount of rain falling in the heaviest downpours has increased approximately 20 percent on average in the past century, and this trend is very likely to continue, with the largest increases in the wettest places.
- The destructive energy of Atlantic hurricanes has increased in recent decades. The intensity of these storms is likely to increase in this century.
- In the Eastern Pacific, the strongest hurricanes have become stronger since the 1980s, even while the total number of storms has decreased.
- Sea level has risen along most of the U.S. coast over the last 50 years, and will rise more in the future.
The report specifies how nine different regions of the United States will be affected by climate change:
Northeast
Sea level in this region is projected to rise more than the global average. Rising sea level is projected to increase the frequency and severity of damaging storm surges and flooding. The densely populated coasts of the Northeast face substantial increases in the extent and frequency of storm surge, coastal flooding, erosion, property damage, and loss of wetlands.
New York alone has more than $2.3 trillion in insured coastal property and much of its coastline is exceptionally vulnerable to sea-level rise and related impacts.
Southeast
There has been an increase in heavy downpours in many parts of the region, while the percentage of the region experiencing moderate to severe drought increased over the past three decades. Sea-level rise and the likely increase in hurricane intensity and associate storm surge will be among the most serious consequences of climate change.
The destructive potential of Atlantic tropical storms and hurricanes has increased since 1970 in association with increasing Atlantic sea surface temperatures, and it is likely that hurricane rainfall and wind speeds will increase in response to global warming. The power and frequency of Atlantic hurricanes have increased substantially in recent decades. 
The intensity of Atlantic hurricanes is likely to increase during this century with higher peak wind speeds, rainfall intensity, and storm surge height and strength. Even with no increase in hurricane intensity, coastal inundation and shoreline retreat would increase as sea-level rise accelerates, which is one of the most certain and most costly consequences of a warming climate.
Compared to the present coastal situation, for which vulnerability is quite high, an increase in hurricane intensity will further affect low-lying coastal ecosystems and coastal communities along the Gulf and South Atlantic coastal margin. Wetlands will be inundated and eroded away, and low-lying areas including some communities will be inundated more frequently- some permanently- by the advancing sea. Current buildings and infrastructure were not designed to withstand the intensity of the projected storm surge, which would cause catastrophic damage.
Midwest
Heavy downpours are now twice as frequent as they were a century ago.The Midwest has experienced two record-breaking floods in the past 15 years.
Precipitation is projected to increase in winter and spring, and to become more intense throughout the year. This pattern is expected to lead to more frequent flooding, increasing infrastructure damage, and impacts on human health.
Great Plains
Relatively cold days are becoming less frequent and relatively hot days are becoming more frequent. Precipitation has also increased over most of the area. Projected changes in long-term climate and more frequent extreme events such as heat waves, droughts, and heavy rainfall will affect many aspects of life in the Great Plains. Conditions are anticipated to become wetter in the north and drier in the south.
Southwest
Record wildfires are being driven by rising temperatures and related reductions in spring snowpack and soil moisture. In general, total area burned is projected to increase. Fires in wetter, forested areas are expected to increase in frequency. The magnitude of fire damages increases as urban development increasingly impinges on forested areas. Grasslands are projected to expand, another factor likely to increase fire risk.
Increased frequency and altered timing of flooding is projected for the Southwest region. Increased flood risk in the Southwest is likely to result from a combination of decreased snow cover on the lower slopes of high mountains, and an increased fraction of winter precipitation falling as rain and therefore running off more rapidly. The increase in rain on snow events will also result in rapid runoff and flooding. Precipitation patterns are already observed to be shifting, with more rain falling in heavy downpours that can lead to flooding.
Northwest
Increases in winter precipitation are projected. Increasing winter rainfall (as opposed to snowfall) is expected to lead to more winter flooding in relatively warm watersheds on the west side of the Cascades.
Another concern is an increase in the number of wildfires. Higher summer temperatures and earlier spring snowmelt are expected to increase the risk of forest fires in the Northwest by increasing summer moisture deficits.
Sea-level rise along vulnerable coastlines will result in increased erosion and the loss of beaches and significant coastal land areas. 
An additional concern is landslides on coastal bluffs. The projected heavier winter rainfall suggests an increase in saturated soils and, therefore, an increased number of landslides. Increased frequency and/or severity of landslides is expected to be especially problematic in areas where there has been intensive development of unstable slopes. Within Puget Sound, the cycle of beach erosion and bluff landslides will be exacerbated by sea-level rise, increasing beach erosion, and decreasing slope stability.
Alaska
Over the past 50 years, Alaska has warmed at more than twice the rate of the rest of the United States’ average. The higher temperatures are already contributing to earlier spring snowmelt, reduced sea ice, widespread glacier retreat, and permafrost warming. Higher temperatures are expected to continue to reduce Arctic sea ice coverage, which increases coastal erosion and flooding associated with coastal storms. Wildfires are increasing with warming. Large areas of dead tree, such as those left behind by pest infestations, are highly flammable and thus much more vulnerable to wildfire than living trees. Under changing climate conditions, the average area burned per year in Alaska is projected to double by the middle of this
century.
Frequent storms in the Gulf of Alaska already affect the coasts during much of the year. Alaska has more coastline than the other 49 states combined. Alaska’s coastlines, many of which are low in elevation, are increasingly threatened by a combination of the loss of their protective sea ice buffer, increasing storm activity, and thawing coastal permafrost. The rate of erosion along Alaska’s northeastern coastline has doubled over the past 50 years.
Islands
The number of heavy rain events is very likely to increase. Projections suggest an increased frequency of heavy downpours and increased rainfall during summer months for the Pacific. While it might initially be seen as a benefit, increased rainfall in the Pacific Islands during summer months is likely to result in increased flooding.
Hurricane (typhoon) wind speeds and rainfall are likely to increase with continued warming. Islands and other low-lying coastal areas will be at increased risk from coastal inundation due to sea-level rise and storm surge, with major implications for coastal communities, infrastructure, natural habitats, and resources.
Flooding will become more frequent due to higher storm tides, and coastal land will be permanently lost as the sea inundates low-lying areas and the shorelines erode. Island communities, infrastructure, and ecosystems are vulnerable to coastal inundation. Critical infrastructure, including homes, airports, and roads, tend to be located along the coast. Flooding related to sea-level rise and hurricanes and typhoons negatively affects port facilities and harbors, and causes closures of airports, roads, and bridges.
As the climate continues to warm, the peak wind intensities and near-storm precipitation from future tropical cyclones are likely to increase, which, combined with sea-level rise, is expected to cause higher storm surge levels. 
Coasts
Approximately one-third of all Americans live in counties immediately bordering the nation’s ocean coasts. About half of the nation’s coastal wetlands have been lost – and most of this loss has occurred during the past 50 years.
Rising sea level is already eroding shorelines, drowning wetlands, and threatening the built environment. Significant sea-level rise and storm surge will adversely affect coastal cities and ecosystems around the nation; low-lying and subsiding areas are most vulnerable. Accelerated sea-level rise would fragment barrier islands, and place into jeopardy existing homes, businesses, and infrastructure, including roads, ports, and water and sewage systems.
Recommended Citation:
Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and
Thomas C. Peterson,
(eds.). Cambridge University Press, 2009.