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The Likely Consequences of Climate Change for the United States: Results of the US National Assessment


The Likely Consequences of Climate Change for the United States: Results of the US National Assessment Michael C. MacCracken Senior Scientist for Climate Programs at – PowerPoint PPT presentation

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Title: The Likely Consequences of Climate Change for the United States: Results of the US National Assessment

The Likely Consequences of Climate Change for
the United States Results of the US National
  • Michael C. MacCracken
  • Senior Scientist for Climate Programs at
  • The Climate Institute, Washington DC and
  • President,
  • International Association of
  • Meteorology and Atmospheric Sciences

Brief Biographical Sketch
  • Student, Princeton University, mechanical
    engineering, 1960-64
  • Ph.D. student in applied science, University of
    California Davis (climate model analysis of ice
    age hypotheses), 1964-68
  • Research scientist, Lawrence Livermore National
    Laboratory (modeling of perturbations to climate
    and air quality leader of Atmospheric and
    Geophysical Sciences Division), 1968-93
  • Executive Director , Office of the interagency
    U.S. Global Change Research Program (USGCRP),
  • Executive Director, National Assessment
    Coordination Office of the USGCRP, 1997-2001
  • Senior Scientist, Office of the USGCRP, 2001-02
    (retired, 2002)
  • Member, Integration team, Arctic Impact Climate
    Assessment, 2002-04
  • Senior Scientist for Climate Programs, Climate
    Institute, 2003-
  • President, International Association of
    Meteorology and Atmospheric Sciences, 2003-07
    (also serving on executive committees of IUGG and
    SCOR and planning team for workshop on effects of
    impacts of asteroids and comets).

The Climate Change issue is traditionally
subdivided into three components
  • 1. The Science How has the climate changed and
    how is it expected to change in the future?
  • 2. The Impacts What are the likely environmental
    and societal consequences of changes in climate
    and to what extent can we adapt?
  • 3. The Choices What are the options available
    for mitigating the changes?
  • The political process--involving leaders from
    both the public private sectors--then faces the
    challenging task of evaluating and reconciling
    the available information and deciding on the
    path to take.

Outline of talk
  • Very brief overview of scientific understanding
    about global warming (area 1)
  • Structure and format of the U.S. National
    Assessment (area 2)
  • The types of impacts that the US can be expected
    to experience (area 2)
  • Concluding remarks
  • Questions (areas 1, 2, 3 if desired)

A hundred years of climate research can be
summarized in 5 key findings
  • 1. The atmospheric concentrations of CO2, CH4,
    N2O and other gases are increasing as a result of
    human activities, primarily because of use of
    fossil fuels (coal, oil, and natural gas) and
  • 2. Higher concentrations of radiatively active
    gases like CO2, CH4, and N2O tend to enhance the
    natural greenhouse effect and warm the planet
  • 3. Human-induced changes in atmospheric
    composition have very likely contributed to
    global warming of about 0.6ºC (or more) since
  • 4. Plausible scenarios of future energy-related
    emissions are projected to cause global warming
    of 1.4 to 5.8ºC and sea-level rise of about 10 to
    90 cm by the end of the 21st century
  • 5. Moderating the rate of global warming will
    require significant reductions in emissions and
    take many decades

Several reconstructions of Northern Hemisphere
average temperature indicate that the centuries
prior to the Industrial Revolution were
relatively cool
Source Mann et al., 2003
The increase in CO2 emissions is projected to
cause a significant further increase in surface
temperature around the world (1.4 to 5.8ºC)
Projected Temperature Change
Projected CO2 Concentration
IPCC temperature increases
Source IPCC TAR, 2001
The projected increase in global average
temperature could result in global temperatures
being higher than they have been in tens of
millions of years
Prather diagram
Source IPCC TAR, 2001 and M. Prather
Global warming is shorthand for a wide array of
potential changes in the climate
  • Warming is very likely to be greater in high than
    low latitudes, with the sea ice melting back and
    land glaciers and ice sheets melting
  • Warming is very likely to be greater over land
    than over the oceans
  • Warming is very likely to be greater in winter
    than in summer in many locations, but not all
    however, the summertime heat index will make
    summertime changes feel worse, with more intense
    heat waves
  • Total evaporation is very likely to increase,
    leading to more rapid drying of soils and earlier
    onset of drought
  • Total global precipitation is very likely to
    increase, and to occur especially in relatively
    intense events (especially tropical cyclones)
    that could increase flood likelihood in some
  • The highest percentage increase in precipitation
    is likely to occur in mid- to high-latitudes
    reductions in rainfall could occur in the
  • The potential for unexpected changes will be
    increased because the balancing influences now
    acting may be disrupted and because various
    thresholds could be exceeded

As a result, climate change could lead to a range
of important impacts
Health Impacts Weather-related mortality/heat
stress Infectious diseases Air quality-induced
respiratory effects
Agriculture Impacts Crop yields and commodity
prices Irrigation demands Pests and weed
Climate Changes
Forest Impacts Change in forest composition Shift
geographic range of forests Forest health and
Water Resource Impacts Changes in water supply
and timing Water quality Increased competition
for water
Sea Level Rise
Coastal Area Impacts Erosion of
beaches Inundation of coastal wetlands Costs to
defend coastal communities
Ecosystem Impacts Shifts in ecological zones Loss
of habitat and species Coral reefs threatened
Adapted from EPA
The IPCC has provided a summary of how climate
change is likely to affect peoples and
environments around the world
While the IPCC Impacts Assessment is a First
Step, Much More Refined Studies Are Needed
  • Changes in climate are most readily studies and
    estimated by starting at the global level and
    working to finer scales (top-down)
  • The consequences of climate change are most
    appropriately studied by starting from the local
    environment and aggregating to larger scales
  • The key challenge is in building a strong
    interface between the two type of efforts
  • The latest versions of climate models are now
    providing sufficient resolution for starting to
    seriously examine local and regional consequences
  • What is now most needed is an intensive effort to
    put the translate and apply these results to
    examine potential environmental, resource and
    societal consequences--and this is starting to
    happen (Canada, UK, US, Arctic, etc.)

Model with 300 km grid
Model with 75 km grid
Model with 50 km grid
The first US National Assessment took place from
1997 to 2001 and focused on evaluating potential
consequences and response options
  • Regional workshops and assessments examined the
    most important issues in 20 regions that covered
    the country
  • Sectoral workshops and assessments examined the
    array of issues that could affect agricultural
    productivity, forest health and productivity,
    water resources, human health, and coastal areas
    and marine resources. More recent studies are
    starting to look at transportation, air quality,
    water quality, etc.
  • A National Synthesis provided an overview of the
    range of issues facing the Nation as a whole,
    finding that the potential impacts on ecosystems
    and water resources were likely to be the most

While much more could be done now with the very
new capabilities that are being developed,
unfortunately the US research program is
presently stuck on evaluating remaining
uncertainties about how much the climate change
will be
Regional workshops were held in 1997-98 to
identify potential impacts meriting further
A special workshop was also held in 1998 to
address issues relating to Native Peoples/Native
Workshop and assessment reports can be viewed
under Assessments at http//
The participating groups in the US National
Assessment were asked to evaluate the
vulnerability of the environment and society to
three classes of climate outcomes
  • Continuation of the types and range of climate
    conditions that prevailed during the 20th century
  • Results of simulations from leading climate
    models assuming continued reliance on fossil
    fuels for most of the worlds energy needs
    (leading to an emissions rate of about 20 GtC/yr
    in 2100)
  • The possibility that nonlinear changes could
    occur, for example if the threshold for an impact
    were to be exceeded (and then examined model
    simulations and proxy indicators to estimate
    possible likelihood)

Climate models were found to be reasonably able
to represent the large-scale distribution of
annual average temperature (ºF) for the present
climate of the US
Canadian Model
Hadley (UK) Model
Climate models were also found to be reasonably
able to represent the seasonal temperature range
(ºF) for the present climate of the US
Canadian Model
Hadley (UK) Model
Climate models were also able to simulate some
features of the monthly average precipitation
(in./mo), but were not able to adequately resolve
the complex effects of mountain ranges
Canadian Model
Hadley (UK) Model
The two climate model simulations that were
available to be used in the US National
Assessment projected a 21st century warming of
about 5 - 10ºF
Canadian model scenario for increase by 2100
Hadley model scenario for increase by 2100
Find the Hadley and Canadian model estimates of
change in temp
Plots show the projected change in annual average
temperature over the 21st century
Projections of the change in precipitation were
less certain, indicating the possibility of
either hotter and drier or warmer and
moister conditions
Canadian model scenario for change by 2100
Hadley model scenario for increase by 2100
Plots show the projected percentage change in
annual precipitation over the 21st century (in )
Note More recent results form a wider selection
of models suggest that the apparent increase in
winter precipitation projected for the
southwestern US may not be robust
Model projections indicated that more of the
precipitation will occur as part of intense
rainfall events
More intense events could increase the potential
for flooding, but with likelihood of greater
drying and lower river and lake levels between
The prospective changes in climate would be
roughly equivalent to imposing climatic
conditions many hundreds of miles away
Preliminary indications were that the El Niño-La
Niña cycle would be likely to intensify
More hurricanes strike the US under La Niña than
El Niño conditions
A greater range would lead to increased
interannual variability over the US
Changes in precipitation were projected to alter
the availability of water resources
  • Storm intensity, track, and number would be
    likely to shift, although most types of changes
    remain uncertain
  • Warmer conditions will lead to reduced springtime
    snowpack and altered timing of river runoff (with
    less in summer)
  • A larger fraction of the precipitation is likely
    to come in intense storms, continuing the 20th
    century trend
  • Warmer conditions will lead to warmer water
    temperatures and faster drying of soils between
  • Average river flows and lake levels will be
    lower, especially in summer creating greater
    competition for water resources)

Whether a region was relatively wet or dry,
potential water-related impacts were of concern
across the United States
Even with increased wintertime precipitation,
warmer winters and a rising snowline are
projected to lead to significant reductions in
springtime snowpack in the western US, greatly
impacting water resource management in these
The costs of extreme weather events have been
increasing, probably reflecting both the
increased exposure of society and perhaps the
slow change in the number and/or intensity of
extreme events
The present distribution of ecosystems is a
result of plants and wildlife adapting to
prevailing climatic and soil conditions
Ecosystem Impacts by 2100 Even accounting for
the effects of CO2 fertilization and enhancement,
changes in the climate will lead to regional
changes in the predominant vegetation types and,
as a result, in the extant wildlife. Some of the
transition is likely to occur relatively rapidly
as a result of fires and other disturbances
Overall forest productivity is expected to rise,
although the predominant tree species in a
location may change. In some areas, drier summers
are likely to raise the potential for fires.
-100 -50 No change 50 100-200 gt200
Simulated change in biomass burnt by wildfires
between the 20th and 21st centuries (per cent)
For hot, dry summertime conditions (Canadian
model scenario), forest mass burned increases
significantly, especially in the western US and
the lower Mississippi-central Appalachians. For
warm, moist conditions (Hadley scenario),
increased fire likelihood occurs mainly in the
western US.
The effects of climate change on terrestrial and
marine ecosystems and wildlife are projected to
vary across the country
The US agricultural sector is likely to be able
to adapt to climate change. Overall productivity
is likely to increase due to the higher CO2 level
and improved water use efficiency.
  • With increased production
  • Commodity prices would be expected to fall,
    benefiting consumers, but causing economic harm
    in marginal farming areas
  • What happens in other world grain-growing
    regions will have large effect
  • Issues of changes in pests and plant diseases
    have yet to be evaluated.

For agriculture, a key factor will be the
lengthening of the frost free season, continuing
a trend that is already underway
Global sea level is projected to rise by about 9
to 88 cm during the 21st century, with a
mid-range value likely (or even higher if
augmented by accelerated melting of the Greenland
ice sheet )
Contributions to global sea level rise are
projected to come mainly from thermal expansion
of ocean waters and melting of mountain glaciers.
Changes in Greenland and Antarctica are projected
to approximately balance.
Global sea level rose 10-20 cm during the 20th
The area of Greenland experiencing summertime
melting appears to be increasing, with some edge
regions showing very rapid deterioration. With
only a little more global warming, seasonal
melting could spread across the plateau.
Orange is area experiencing melting in 1992 red
is additional area experiencing melting in 2002.
US coastal regions will be exposed to rising sea
level and higher storm surges, especially if
hurricanes become more intense and possibly more
Mid-range (50 probability) estimates of
effective sea level rise by 2100 and by 2200
(i.e., accounting for changes in coastal
Portland, ME 19 43
Seattle, WA 19 42
New York, NY 22 48
San Francisco, CA 15 36
Los Angeles 13 32
Charleston, SC 25 53
Miami Beach, FL 20 44
Grand Isle, LA 55 112
Estimates are in inches.
Source U.S. EPA (1995).
The Gulf and East coasts are particularly
vulnerable to projected sea level rise
Coastal islands and wetlands are the shock
absorbers in the event of hurricanes
Coral reefs are threatened by both ocean warming
and the change in ocean acidity caused by the
rising CO2 concentration
The potential consequences for human health are
of several interacting types
The nighttime minimum temperature is projected to
increase more than the daytime maximum
The potential impacts include higher daytime peak
electricity demand, longer periods of high
electricity demand, and reduced combustion

Projections are for very large increases in the
number and intensity of extreme heat episodes
An increase in the average of a distribution can
lead to very large changes in the probability
that cold and hot extremes may be experienced.
The average summertime heat index is projected to
increase by much more than the temperature
Peak loads are likely to become higher,
increasing need for electric reserves
The probability that 4 days in June will exceed
a citys temperature threshold for heat stress
could increase significantly

St. Louis
New York
Kansas City
Los Angeles
San Francisco
Source Probabilities of Temperature Extremes in
the U.S.A, Version 1, NCDC, 1999
Kalkstein, 1989
Average Annual Excess Weather-Related Mortality
for 1993, 2020 and 2050 Climate
GFDL Climate Change Scenario
New York City
Los Angeles
Sources Kalkstein and Green (1997)
Chestnut et al.(1995)
Note Includes both summer and winter mortality.
Assumes full acclimation to changed
climate. Includes population growth.
Strengthening public health and community
planning is likely to help moderatesome types of
potential consequences
Community design and standards can limit mosquito
Air-conditioning and better housing can limit
heat stress
Some types of changes in climate will affect
particular regions
  • Coastal regions will be affected by
  • Rising sea level
  • Higher storm surges
  • Increased peak hurricane winds and rainfall rates
  • Warmer and saltier estuarine waters
  • More stagnant summer conditions
  • Various regions of the country will be affected
  • Drier forest conditions that make them more
    susceptible to fires
  • Increasingly difficult restoration of
    pre-disturbance ecosystems
  • Altered regions of high winds
  • Shifting landscapes and potential for biomass

Examples of Potential Coupling of Transportation
Issues with Climate Variability and Change
  • Examples of potential location-based issues
  • Great Lakes (changes/variations in lake ice
    duration and extent, lake levels, outflow through
    the St. Lawrence River, hydropower, etc.)
  • Mississippi-Missouri-Ohio River system
    (changes/variations in summer/fall- and
    winter/spring-time river flow, water supplies for
    cities, etc.
  • Coastal waterways (changes/variations in ocean
    levels, rates of erosion and changes in barrier
    islands, etc.)
  • Panama Canal (changes/variations in water flow
    due to El Nino)
  • Arctic/Northwest and Northeast Passages (less
    summertime sea ice fragile environment in case
    of accidents)
  • Examples of potential event-based issues
  • Hurricanes (frequency, locations, and intensity
    may change)
  • Warmer temperatures (generally make combustion
    engines less efficient, increase A/C demand,
  • Ice storms (likely to have poleward shift to
    regions not used to them)

Examples of key regional consequences within the
Region Environmental Consequence Economic Consequence Consequence to People
Northeast Wetland inundation Reduced wintertime recreation Rising summertime heat index
Southeast Loss of coastal ecosystems changing forests Increasing productivity of hardwood forests Increased coastal flooding longer, hotter summers
Midwest Higher lake and river temperatures alter fish species Increasing agricultural productivity Lowered lake and river levels hotter summers
Great Plains Warmer winters allow more invasive species Increasing agricultural productivity Worsened climatic extremes in spring/summer
West Altered ecosystems, and more fire Rising snowline intensifies water problems Shift toward warm season recreation greater fire danger
Northwest Stress to cold/cool water ecosystems and fish Earlier winter runoff tightens water supplies Shift to warm season recreation coastal erosion
The Arctic is a particularly vulnerable region
Alaska is threatened by bothcoastal erosion and
permafrost melting
  • Meltback of sea ice endangers wildlife, impacts
    subsistence harvests, and allows increased
    coastal erosion
  • Melting of permafrost undermines structures and
    weakens forests, with trees later attacked by
    pests and made more vulnerable to fires

Trend in the Bering Sea
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As the sea ice melts back, there will be
increased opportunity for seasonal shipping
through the Arctic Ocean, but doing it safely is
likely to require great care and preparation.
While it is important to look closely at regional
consequences, the closer the examination of
potential regional and sectoral impacts in the
US, the more couplings appear to worldwide impacts
  • Economic and Market Couplings imported products,
    export markets, investments, international
  • Shared Resources and Environments water
    resources, hydropower, fisheries, migrating
    species, ocean resources, biodiversity
  • Human Health disease prevalence and vectors,
    health of visitors, disease level of countries
    where visit and do business
  • International Security and Well-Being pressures
    for immigration, environmental refugees, levels
    of international conflict and requirements for
    peace-keeping, concern for relatives and peoples

If done in the context of the multiple stresses
facing society, preparing for climate change can
improve resilience and reduce potential adverse
Addressing the potential impacts of climate
change in the context of other stresses can
reduce overall vulnerability and limit exposure
to multiple stresses
Copies of the US National Assessment reports are
available from Cambridge University Press and at
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