HOW IS CLIMATE EXPECTED TO CHANGE
IN THE FUTURE?
Continued greenhouse gas emissions at or above current rates are
expected to cause further warming and to induce many changes
during the 21st century that will very likely be larger than those
of the last century.

• Temperature
– All of North America is very likely to warm during this
century, and to warm more than the global average
increase in most areas.
– Nearly all the models used in the IPCC assessment project
that average warming in the United States will exceed
3.6°F, with five out of 21 models projecting that average
warming will exceed 7.2°F.
– Abnormally hot days and nights and heat waves are very
likely to become more frequent, and cold days and cold
nights are very likely to become much less frequent over
North America.

• Precipitation and Drought

– In the 21st century, precipitation over North America is projected to be less frequent but more intense.

– Most climate models project an increase in winter precipitation in the northern tier of states and a decrease in portions of the Southwest.

– It is likely that droughts will continue to be exacerbated by earlier and possibly lower spring snowmelt runoff in the mountainous West, which results in less water available in late summer.

• Snow and Ice

– Snow cover is projected to continue to decrease as the climate warms.

– Results from multiple model simulations indicate that an Arctic Ocean free of summer ice is likely by the end of the century, with some models suggesting that this could occur as soon as 2040.

– Glaciers and terrestrial ice sheets are projected to continue to lose mass. This will contribute to sea-level rise.

• Sea-Level Rise
– Future sea-level rise is virtually certain to cause some areas of dry and in the United States to become inundated.

– The IPCC projects that global sea level will rise between 7 and 23 inches by the end of the century (2090-2099).

– Potential accelerations in ice flow of the kind recently observed in some Greenland outlet glaciers and West Antarctic ice streams could substantially increase the contribution from the ice sheets to sea level, a possibility not reflected in the aforementioned projections.
Understanding of these processes is limited and there is no consensus on their magnitude and thus on the upper bound of sea-level rise rates.

– Storm surge levels are expected to increase due to projected sea-level rise.

• Atlantic Hurricanes
– For North Atlantic, it is likely that hurricane rainfall and wind speeds will increase in response to human-caused warming. There is less confidence in the projected changes in the number of hurricanes.

WHAT ARE THE MAJOR IMPACTS OF CLIMATE CHANGE FOR THE UNITED STATES?

It is very likely that temperature increases, increasing carbon
dioxide levels, and altered patterns of precipitation are already
affecting U.S. water resources, agriculture, land resources,
biodiversity, and human health, among other things. And it is
very likely that climate change will continue to have significanteffects on these resources over the next few decades and beyond.
A very brief summary of the key findings of climate impacts on
the United States follows.

The Natural Environment
Ecosystems provide society with a number of goods (e.g., food, fiber, fuel, pharmaceutical products) and services (e.g., cycling of water and nutrients, removal of waste products, sustaining biological diversity) and are essential to human health and wellbeing:

• The resilience of many ecosystems is likely to be exceeded this century by an unprecedented combination of climate change, associated disturbances (e.g., flooding, drought, wildfire, insects, ocean acidification), and other global change drivers (e.g., land-use change, pollution).

• In North America, warming has generally resulted in and is expected to continue to result in shifts of species ranges poleward and to higher altitudes.

Agriculture
The many U.S. crops and livestock (valued at about $200 billion in 2002) are strongly affected by climate:

• The marketable yield of many horticultural crops (e.g., tomatoes, onions, and fruits) is very likely to be more sensitive to climate change than grain and oilseed crops.

• Higher temperatures will very likely reduce livestock production during the summer season, but these losses will very likely be partially offset by warmer temperatures during the winter season.

Water

Although U.S. water management practices are generally quite advanced, particularly in the West, the reliance on past conditions as the foundation for current and future planning and practice will no longer be tenable as climate change and variability increasingly create conditions that are well outside of historical parameters, eroding predictability:

• Stream temperatures are likely to increase as the climate warms and are very likely to have effects on aquatic ecosystems and water quality.

• Where earlier snowmelt peaks and reduced low flows in the
summer and fall have already been detected, continuing shifts
in this direction are very likely and may substantially affect the
performance of reservoir systems.

Population and Society

While it may appear that industrialized countries like the United
States are well equipped to cope with gradual climate change at a
national level, at a local level there may be substantial variability
in climate effects and capacities to adapt; on the other hand, some
U.S. communities may find opportunities in climate change:

• Population growth is generally shifting toward areas (e.g.,
coastal regions) more likely to be vulnerable to the effects of
climate change.

• For small islands, particularly in the Pacific, some studies
suggest that sea-level rise could reduce island size, raising
concerns for parts of Hawaii and other U.S. territories.

• Wildfires have increased in extent and severity in recent years
and are very likely to intensify in a warmer future. At the same
time, the population has been expanding into fire-prone areas.

Health

Climate variability and change can affect health through the effects
of temperature changes on the body. Climate change can also
make it possible for animal-, water-, and food-borne diseases to
spread or emerge in areas where they had been limited or had not existed, or it can make it possible for such diseases to disappear by
making areas less hospitable to the disease carrier or pathogen:

• An increased frequency and severity of heat waves is expected,
leading to more illness and death, particularly among the
young, elderly, frail, and poor.

• Increases in extreme weather (e.g., storms, flooding) and
accompanying events (e.g., wildfire resulting from prolonged
drought) may lead to increases in deaths, injuries, infectious
diseases, interruptions of medical care for chronic disease
treatment, and stress-related disorders and other adverse
effects associated with social disruption and migration.

Energy
To date, most discussions on energy and climate change have
focused on mitigating human effects on climate. However, along
with this role as a driver of climate change, the energy sector will
be subject to the effects of climate change:

• Direct impacts from increased intensity of extreme weather
events.

• Reduced water supplies in regions dependent on water
resources for hydropower and/or thermal power plant cooling.

• Positive or negative impacts on production of biomass, wind
power, or solar energy where climate conditions change.

Transportation

Increasing global temperatures, rising sea levels, and changing
weather patterns pose significant challenges to the Nation’s roads,
airports, railways, transit systems, and ports. The transportation
network is vital to the U.S. economy and quality of life:

• Increasing frequency, intensity, or duration of heat spells could cause railroad tracks to buckle or kink and could affect roads through softening and traffic-related rutting.

• Coastal and riverine flooding and landslides are very likely to cause negative impacts on roads, rails, and ports.

• Warmer or less snowy winters are likely to improve ground and air transportation reliability, and decrease the need for winter road maintenance. However, more intense winter storms could increase risks for traveler safety and require increased localized snow removal.

scientific assessment
This fact sheet was generated by the Climate Change Science Program Office
For further information, see <www.climatescience.gov>.