Relatory of the National Climate Assessment and Development Advisory
Committee (USA)
Executive Summary
Draft for Public Comment Chapter 1
Executive Summary (v. 11 Jan 2013)
DRAFT FOR
PUBLIC COMMENT 3
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Climate change is already affecting the American people. Certain types
of weather events have become more frequent and/or intense, including heat
waves, heavy downpours, and, in some regions, floods and droughts. Sea level
is rising, oceans are becoming more acidic, and glaciers and arctic sea ice
are melting. These changes are part of the pattern of global climate change, which is primarily driven by human activity.
Many impacts associated with these changes are important to Americans’
health and livelihoods and the ecosystems that sustain us. These impacts are
the subject of this report. The impacts are often most significant for
communities that already face economic or health-related challenges, and for
species and habitats that are already facing other pressures. While some
changes will bring potential benefits, such as longer growing seasons, many
will be disruptive to society because our institutions and infrastructure
have been designed for the relatively stable climate of the past, not the
changing one of the present and future. Similarly, the natural ecosystems that
sustain us will be challenged by changing conditions. Using scientific
information to prepare for these changes in advance provides economic
opportunities, and proactively managing the risks will reduce costs over
time.
Evidence for climate change abounds, from the top of the atmosphere to
the depths of the oceans. This evidence has been compiled by scientists and
engineers from around the world, using satellites, weather balloons,
thermometers, buoys, and other observing systems. The sum total of this
evidence tells an unambiguous story: the planet is warming.
U.S. average temperature has increased by about 1.5°F since 1895; more
than 80% of this increase has occurred since 1980. The most recent decade
was the nation’s hottest on record. Though most regions of the U.S. are
experiencing warming, the changes in temperature are not uniform. In
general, temperatures are rising more quickly at higher latitudes, but there is
considerable observed variability across the regions of the U.S.
U.S. temperatures will continue to rise, with the next few decades
projected to see another 2°F to 4°F of warming in most areas. The amount of
warming by the end of the century is projected to correspond closely to the
cumulative global emissions of greenhouse gases up to that time: roughly 3°F
to 5°F under a lower emissions scenario involving substantial reductions in emissions after 2050 (referred to as the "B1 scenario"), and 5°F to
10°F for a higher emissions scenario assuming continued increases in
emissions (referred to as the "A2 scenario") (Ch. 2).
The chances of record-breaking high temperature extremes will continue
to increase as the climate continues to change. There has been an increasing
trend in persistently high nighttime temperatures, which have widespread
impacts because people and livestock get no respite from the heat. In other
places, prolonged periods of record high temperatures associated with droughts
contribute to conditions that are driving larger and more frequent
wildfires. There is strong evidence to indicate that human influence on the
climate has already roughly doubled the probability of extreme heat events
like the record-breaking summer of 2011 in Texas and Oklahoma (Ch.
2,3,6,9,20).
Human-induced climate change means much more than just hotter weather.
Increases in ocean and freshwater temperatures, frost-free days, and heavy
downpours have all been documented. Sea level has risen, and there have been
large reductions in snow-cover extent, glaciers, permafrost, and sea ice.
Winter storms along the west coast and the coast of New England have increased slightly in frequency and intensity. These changes and other climatic
changes have affected and will continue to affect human health, water supply,
agriculture, transportation, energy, and many other aspects of society (Ch.
2,3,4,5,6,10,12,16,20,24,25).
Some of the changes discussed in this report are common to many regions.
For example, very heavy precipitation has increased over the past century in
many parts of the country. The largest increases have occurred in the
Northeast, Midwest, and Great Plains, where heavy downpours have exceeded
the capacity of infrastructure such as storm drains, and have led to flooding
events and accelerated erosion. Other impacts, such as those associated with
the rapid thawing of permafrost in Alaska, are unique to one U.S. region
(Ch. 2,16,18,19,20,21,22,23).
Some impacts that occur in one region have more wide-ranging effects.
For example, the dramatic decline of summer sea ice in the Arctic – a loss
of ice cover roughly equal to half of the continental U.S. – exacerbates
global warming by reducing the reflectivity of Earth’s surface and
increasing the amount of heat the Arctic absorbs. There is some evidence that
this affects weather patterns farther south in the United States. Similarly,
wildfires in one region can trigger poor air quality in far-away regions,
and new evidence suggests the particulate matter in the atmosphere affects
global circulation, leading to more persistent periods of anomalous weather. Major storms that hit the Gulf Coast affect the entire country through their
cascading effects on oil and gas production and distribution (Ch.
2,4,16,17,18,19,20,22).
Sea level rise, combined with coastal storms, has increased the risk of
erosion, storm-surge damage, and flooding for coastal communities,
especially along the Gulf of Mexico, the Atlantic seaboard, and Alaska. In
the Southeast, coastal infrastructure including roads, rail lines, energy infrastructure, and port facilities including naval bases, are at risk from
storm surge that is exacerbated by rising sea level. Over the past century,
global sea level has risen by about 8 inches. Since 1992, the rate of global
sea level rise measured by satellites has been roughly twice the rate
observed over the last century. Sea level is projected to rise by another 1 to
4 feet in this century. A wider range of scenarios, ranging from 8 inches to
6.6 feet of rise by 2100, has been suggested for use in risk-based analyses.
In general, higher emissions scenarios that lead to more warming would be
expected to lead to sea level rise toward the upper end of the projected range.
The stakes are high, as nearly five million Americans live within four feet
of the local high-tide level (Ch. 2,4,10,16,17,20, 22,25).
In addition to changing climate, carbon dioxide from fossil fuel burning
has a direct effect on the world’s oceans. Carbon dioxide interacts with
ocean water to form carbonic acid, lowering the ocean’s pH. Ocean surface
waters have become 30% more acidic as they have absorbed large amounts of
carbon dioxide from the atmosphere. This ocean acidification reduces the
capacity of marine organisms with shells or skeletons made of calcium
carbonate (such as corals, krill, oysters, clams, and crabs) to survive,
grow, and reproduce, which in turn will affect the entire marine food chain
(Ch. 2,8,23,24,25).
Climate change produces a variety of stresses on society, affecting
human health, natural ecosystems, built environments, and existing social,
institutional, and legal agreements. These stresses interact with each other
and with other non-climate stresses, such as habitat fragmentation,
pollution, increased consumption patterns, and biodiversity loss. Addressing
these multiple stresses requires the assessment of composite threats as well
as tradeoffs among the costs, benefits, and risks of available response
options (Ch. 3,5,8,9,10,11,14,16,19,20,25,26,27,28).
Climate change will influence human health in many ways; some existing
health threats will intensify, and new health threats will emerge. Some of
the key drivers of health impacts include: increasingly frequent and intense
extreme heat, which causes heat-related illnesses and deaths and over time,
worsens drought and wildfire risks, and intensifies air pollution; increasingly
frequent extreme precipitation and associated flooding that can lead to
injuries and increases in marine and freshwater-borne disease; and rising
sea levels that intensify coastal flooding and storm surge. Certain groups
of people are more vulnerable to the range of climate change-related health
impacts, including the elderly, children, the poor, and the sick. Others are
vulnerable because of where they live, including those in floodplains,
coastal zones, and some urban areas. In fact, U.S. population growth has
been greatest in coastal zones and in the arid Southwest, areas that already
have been affected by increased risks from climate change. Just as some choices can make us more vulnerable, other choices can make us more resilient.
Maintaining a robust public health infrastructure will be critical to
managing the potential health impacts of climate change (Ch.
2,7,9,11,12,13,16,18,20,25).
Climate change affects the entire living world, including people,
through changes in ecosystems and biodiversity. Ecosystems provide a rich
array of benefits to humanity, including fisheries, drinking water, fertile
soils for growing crops, buffering from climate impacts, and aesthetic and cultural values. These benefits are not always easy to quantify, but they
translate into jobs, economic growth, health, and human well-being. Climate
change-driven perturbations to ecosystems that have direct human impacts
include reduced water supply and quality, the loss of iconic species and
landscapes, distorted rhythms of nature, and the potential for extreme events
to eliminate the capacity of ecosystems to provide benefits (Ch. 3, 6, 8,
12, 14, 23, 24).
Climate change and other human modifications of ecosystems and
landscapes often increase their vulnerability to damage from extreme events
while at the same time reducing their natural capacity to modulate the
impacts of such events. Salt marshes, reefs, mangrove forests, and barrier
islands defend coastal ecosystems and infrastructure, including roads and
buildings, against storm surges; their losses from coastal development,
erosion, and sea level rise increase the risk of catastrophic damage during
or after extreme weather events. Floodplain wetlands, although greatly
reduced from their historical extent, absorb floodwaters and reduce the effects of high flows on river-margin lands. Extreme weather events that produce
sudden increases in water flow, often carrying debris and pollutants, can
decrease the natural capacity of ecosystems to process pollutants (Ch. 3, 7,
8, 25).
As climate change and its impacts are becoming more prevalent, Americans
face choices. As a result of past emissions of heat-trapping gases, some
amount of additional climate change and related impacts is now unavoidable.
This is due to the long-lived nature of many of these gases, the amount of heat absorbed and retained by the oceans, and other
responses within the climate system. However, beyond the next few decades,
the amount of climate change will still largely be determined by choices
society makes about emissions. Lower emissions mean less future warming and
less severe impacts; higher emissions would mean more warming and more severe impacts. The choices about emissions pathway fall into a category of response
options usually referred to as "mitigation" – ways to reduce the amount
and speed of future climate change by reducing emissions of heat-trapping
gases (Ch. 2, 26, 27).
The other major category of response options is known as
"adaptation" and refers to changes made to better respond to new
conditions, thereby reducing harm or taking advantage of opportunity.
Mitigation and adaptation are linked, in that effective mitigation reduces the
need for adaptation. Both are essential parts of a comprehensive response
strategy. The threat of irreversible impacts makes the timing of mitigation
efforts particularly critical. This report includes chapters on Mitigation,
Adaptation, and Decision Support that offer an overview of the kinds of
options and activities being planned or implemented around the country as governments
at local, state, federal, and tribal levels, businesses, other
organizations, and individuals begin to respond to climate change (Ch. 26,
27, 28).
Large reductions in global emissions, similar to the lower emissions
scenario (B1) analyzed in this assessment, would be necessary to avoid some
of the worst impacts and risks of climate change. The targets called for in
international agreements would require even larger reductions than those
outlined in scenario B1 (Figure 1). Meanwhile, global emissions are still
rising, and are on track to be even higher than the high emissions scenario
(A2) analyzed in this report.
The current U.S. contribution to global
emissions is about 20%. Voluntary efforts, the recent shift from coal to
natural gas for electricity generation, and governmental actions in city,
state, regional, and federal programs under way and have contributed to
reducing U.S. emissions in the last few years. Some of these actions are
motivated by climate concerns, sometimes with non-climate co-benefits, while
others are motivated primarily by non-climate objectives. These U.S. actions
and others that might be undertaken in the future are described in the
Mitigation chapter of this report; at present they are not sufficient to
reduce total U.S. emissions to a level that would be consistent with
scenario B1 or the targets in international agreements (Ch. 2, 4, 29 27).
With regard to adaptation, the pace and magnitude of observed and
projected changes emphasize the need for being prepared for a wide variety
and intensity of climate impacts. Because of the influence of human
activities, the past climate is no longer a sufficient indicator of future conditions. Planning and managing based on the climate of the last century
means that tolerances of some infrastructure and species will be exceeded.
For example, building codes and landscaping ordinances will likely need to
be updated not only for energy efficiency, but also to conserve water
supplies, protect against insects that spread disease, reduce susceptibility to
heat stress, and improve protection against extreme events. The knowledge
that climate change is real and accelerating points to the need to develop
and refine approaches that enable decision-making and increase flexibility,
robustness, and resilience in the face of ongoing and future impacts. Being
prepared for such events paves the way for economic opportunities (Ch. 2, 3, 5,
41 9, 11, 13, 26, 27, 28).
Adaptation considerations include local, state, regional, national, and
international jurisdictional issues. For example, in managing water supplies
to adapt to a changing climate, the implications of international treaties
should be considered in the context of managing the Great Lakes, the Columbia
River, and the Colorado River to deal with increased drought risk. Both
"bottom up" community planning and "top down" national
strategies may help regions deal with impacts such as increases in electrical
brownouts, heat stress, floods, and wildfires. Such a mix of approaches will
require cross-boundary coordination at multiple levels as operational agencies
integrate adaptation planning into their programs (Ch. 3, 7, 9, 10, 18, 20,
21, 26, 28).
Proactively preparing for climate change can reduce impacts, while also
facilitating a more rapid and efficient response to changes as they happen.
The Adaptation chapter in this report highlights efforts at the federal,
regional, state, tribal, and local levels, as well as initiatives in the corporate and non-governmental sectors to build adaptive capacity and
resilience towards climate change (Ch. 28).
This report identifies a number of areas for which improved scientific
information or understanding would enhance the capacity to estimate future
climate change impacts. For example, knowledge of the mechanisms controlling
the rate of ice loss in Greenland and Antarctica is limited, making it
difficult for scientists to narrow the range of future sea level rise. Research on ecological responses to climate change is limited, as is
understanding of social responses and how ecological and social responses
will interact (Ch. 29).
There is also a section on creating a sustained climate assessment
process to more efficiently collect and synthesize the rapidly evolving
science and to help supply timely and relevant information to decision-makers.
Results from all of these efforts will continue to build our understanding
of the interactions of human and natural systems in the context of a changing
climate (Ch. 30).
Report Findings
1. Global climate is changing, and this is apparent across the U.S. in a
wide range of observations. The climate change of the past 50 years is due
primarily to human activities, predominantly the burning of fossil
fuels.
U.S. average temperature has increased by about 1.5°F since 1895, with
more than 80% of this increase occurring since 1980. The most recent decade
was the nation’s warmest on record. Because human-induced warming is
superimposed on a naturally varying climate, rising temperatures are not
evenly distributed across the country or over time (Ch. 2).
2. Some extreme weather and climate events have increased in recent
decades, and there is new and stronger evidence that many of these
increases are related to human activities.
Changes in extreme events are the primary way in which most people
experience climate change. Human-induced climate change has already
increased the frequency and intensity of some extremes. Over the last 50
years, much of the U.S. has seen an increase in prolonged stretches of
excessively high temperatures, more heavy downpours, and in some regions more severe droughts (Ch. 2, 16, 17, 18, 19, 20, 23).
3. Human-induced climate change is projected to continue and accelerate
significantly if emissions of heat-trapping gases continue to
increase.
Heat-trapping gases already in the atmosphere have committed us to a
hotter future with more climate-related impacts over the next few decades.
The magnitude of climate change beyond the next few decades depends
primarily on the amount of heat-trapping gases emitted globally, now and in
the future (Ch. 2, 27).
4. Impacts related to climate change are already evident in many sectors
and are expected to become increasingly challenging across the
nation throughout this century and beyond.
Climate change is already affecting human health, infrastructure, water
resources, agriculture, energy, the natural environment, and other factors –
locally, nationally, and internationally. Climate change interacts with
other environmental and societal factors in a variety of ways that either
moderate or exacerbate the ultimate impacts. The types and magnitudes of
these effects vary across the nation and through time. Several populations – including children, the elderly, the sick, the poor, tribes and other
indigenous people – are especially vulnerable to one or more aspects of
climate change. There is mounting evidence that the costs to the nation are
already high and will increase very substantially in the future, unless
global emissions of heat-trapping gases are strongly reduced (Ch. 3, 4, 5, 6,
7, 8, 9, 10, 32 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25).
5. Climate change threatens human health and well-being in many ways,
including impacts from increased extreme weather events, wildfire,
decreased air quality, diseases transmitted by insects, food, and
water, and threats to mental health.
Climate change is increasing the risks of heat stress, respiratory
stress from poor air quality, and the spread of waterborne diseases. Food
security is emerging as an issue of concern, both within the U.S. and across
the globe, and is affected by climate change. Large-scale changes in the
environment due to climate change and extreme weather events are also
increasing the risk of the emergence or reemergence of unfamiliar health
threats (Ch. 2, 6 , 9, 11, 12, 16, 19, 20, 22, 23).
6. Infrastructure across the U.S. is being adversely affected by
phenomena associated with climate change, including sea level rise, storm
surge, heavy downpours, and extreme heat.
Sea level rise and storm surges, in combination with the pattern of
heavy development in coastal areas, are already resulting in damage to
infrastructure such as roads, buildings, ports, and energy facilities.
Infrastructure associated with military installations is also at risk from
climate change impacts. Floods along the nation’s rivers, inside cities, and on
lakes following heavy downpours, prolonged rains, and rapid melting of
snowpack are damaging infrastructure in towns and cities, farmlands, and a
variety of other places across the nation. Extreme heat is damaging
transportation infrastructure such as roads, rail lines, and airport runways. Rapid warming in Alaska has resulted in infrastructure impacts due to
thawing of permafrost and the loss of coastal sea ice that once protected
shorelines from storms and wave-driven coastal erosion (Ch. 2, 3, 5, 6, 11,
16, 17, 18, 19, 20, 21, 22, 23, 25).
7. Reliability of water supplies is being reduced by climate change in a
variety of ways that affect ecosystems and livelihoods in many
regions, particularly the Southwest, the Great Plains, the Southeast,
and the islands of the Caribbean and the Pacific, including the state of
Hawai`i.
Surface and groundwater supplies in many regions are already stressed by
increasing demand for water as well as declining runoff and groundwater
recharge. In many regions, climate change increases the likelihood of water
shortages and competition for water among agricultural, municipal, and
environmental uses. The western U.S. relies heavily on mountain snowpack for
water storage, and spring snowpack is declining in most of the West. There is
an increasing risk of seasonal water shortages in many parts of the U.S.,
even where total precipitation is projected to increase. Water quality
challenges are also increasing, particularly sediment and contaminant
concentrations after heavy downpours (Ch. 2, 3, 12, 26 16, 17, 18, 19, 20, 21,
23).
8. Adverse impacts to crops and livestock over the next 100 years are
expected. Over the next 25 years or so, the agriculture sector is
projected to be relatively resilient, even though there will be
increasing disruptions from extreme heat, drought, and heavy downpours.
U.S. food security and farm incomes will also depend on how agricultural systems adapt to climate changes in other regions of the world.
Near-term resilience of U.S. agriculture is enhanced by adaptive
actions, including expansion of irrigated acreage in response to drought,
regional shifts in crops and cropped acreage, continued technological
advancements, and other adjustments. By mid-century, however, when
temperature increases and precipitation extremes are further intensified,
yields of major U.S. crops are expected to decline, threatening both U.S.
and international food security. The U.S. food system also depends on
imports, so food security and commodity pricing will be affected by
agricultural adaptation to climate changes and other conditions around the
world (Ch. 2, 6, 12, 13, 14, 18, 19).
9. Natural ecosystems are being directly affected by climate change, including
changes in biodiversity and location of species. As a result, the capacity of
ecosystems to moderate the consequences of disturbances such as
droughts, floods, and severe storms is being diminished.
In addition to climate changes that directly affect habitats, events
such as droughts, floods, wildfires, and pest outbreaks associated with
climate change are already disrupting ecosystem structures and functions in a
variety of direct and indirect ways. These changes limit the capacity of
ecosystems such as forests, barrier beaches, and coastal- and freshwater-wetlands to adapt and continue to play important roles in reducing the impacts
of these extreme events on infrastructure, human communities, and other
valued resources (Ch. 2, 3, 10 6, 7, 8, 10, 11, 14, 15, 19, 25).
10. Life in the oceans is changing as ocean waters become warmer and
more acidic.
Warming ocean waters and ocean acidification across the globe and within
U.S. marine territories are broadly affecting marine life. Warmer and more
acidic waters are changing the distribution of fish and other mobile sea
life, and stressing those, such as corals, that cannot move. Warmer and more
acidic ocean waters combine with other stresses, such as overfishing and
coastal and marine pollution, to negatively affect marine-based food
production and fishing communities (Ch. 2, 23, 24, 25).
11. Planning for adaptation (to address and prepare for impacts) and
mitigation (to reduce emissions) is increasing, but progress with
implementation is limited.
In recent years, climate adaptation and mitigation activities have begun
to emerge in many sectors and at all levels of government; however barriers
to implementation of these activities are significant. The level of current
efforts is insufficient to avoid increasingly serious impacts of climate
change that have large social, environmental, and economic consequences.
Well-planned and implemented actions to limit emissions and increase resilience to impacts that are unavoidable can improve public health, economic
development opportunities, natural system protection, and overall quality of
life (Ch. 6, 7, 8, 9, 10, 13, 15, 27 26, 27, 28).
Table 1.1: Regional Observations of Climate Change
|
Northeast
|
Heat waves, coastal flooding due to sea level rise and storm surge,
and river flooding due to more extreme precipitation events are affecting
communities in the region.
|
Southeast
|
Decreased water availability, exacerbated by population growth and land-use
change, is causing increased competition for water; risks associated with
extreme events like hurricanes are increasing.
|
Midwest
|
Longer growing seasons and rising carbon dioxide levels are increasing
yields of some crops, although these benefits have already been offset in
some instances by occurrence of extreme events such as heat waves, droughts,
and floods.
|
Great Plains
|
Rising temperatures are leading to increased demand for water and
energy and impacts on agricultural practices.
|
Southwest
|
Drought and increased warming have fostered wildfires and increased
competition for scarce water resources for people and ecosystems.
|
Northwest
|
Changes in the timing of streamflow related to earlier snowmelt have
already been observed and are reducing the supply of water in summer, causing
far-reaching ecological and socioeconomic consequences.
|
Alaska
|
Summer sea ice is receding rapidly, glaciers are shrinking, and
permafrost is thawing, causing damage to infrastructure and major changes to
ecosystems; impacts to Alaska native communities are increasing.
|
Hawaii
|
Increasingly constrained freshwater supplies, coupled with increased
temperatures, are stressing both people and ecosystems, and decreasing food
and water security.
|
Coasts
|
Coastal lifelines, such as water supply infrastructure and evacuation
routes, are increasingly vulnerable to higher sea levels and storm surges,
inland flooding, and other climate-related changes.
|
Oceans
|
The oceans are currently absorbing about a quarter of human-caused
carbon dioxide emissions to the atmosphere and over 90% of the heat
associated with global warming, leading to ocean acidification and the
alteration of marine ecosystems.
|
