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2
Climate Attributes That Influence Society
Examining the ways people and their activities depend on or are
controlled by climate reveals the climate variables that have the
most influence on our lives and well-being. Our food production and
agricultural systems depend highly on local temperature,
precipitation, and the amount of sunlight during the growing
season. Our forests, which provide wood for construction as well as
habitat for our heritage of flora and fauna, depend similarly on
particular temperature and precipitation regimes. Our water
resources depend on precipitation and temperature (which controls
evaporation and melting of ice and snow) and on the natural
ecosystems and man-made features that affect water storage and
control runoff. Extremes of precipitation (or other processes in
the hydrologic system) can lead to drought or floods, often
accompanied by fire or severe erosion, which endanger natural
habitats, communities, and resources. Our coastal regions are
especially sensitive to changes in sea level; long-term changes may
involve only a slow rise in sea level, but water levels can be
raised rapidly by storm surges, inundating coastal communities and
wetlands. Our health depends directly on temperature and humidity
(both of which contribute to heat stress) and on ultraviolet
radiation (which contributes to skin cancer); indirectly, human
health is affected by temperature, precipitation, land cover, and
land use, which together contribute to determining the pathways for
disease-bearing vectors. Ecosystems on land and in the oceans
provide habitat for the biological diversity of flora and fauna
that supply us with food, medicine, recreation, and other
resources; all are dependent on a wide array of climatic variables.
Finally, climate is a fundamental driver of our economic
activities, contributing to the demands for energy, the maintenance
of our food and fiber resources, the safety of our transportation
systems, the appeal and lifestyles of different regions, the
maintenance of our natural biological resources, the availability
of outdoor recreation, and much more.
These complex and extensive interconnections point to the six
attributes influencing the Earth's climate system that appear to be
of most importance to society. These are:
• precipitation and water availability;
• temperature;
• solar radiation;
• storms;
• sea level; and
• ecosystems.
The value of their mean state or condition, how they vary over
time (on scales of days, seasons, years, decades, and centuries),
their character and extent geographically, and the frequency and
persistence of extreme values all determine the availability of
resources on which we depend, while influencing our ability to lead
healthy, productive lives. Certainly other attributes are important
to our environment and well-being, such as winds (which we treat
below, under "Storms"), air quality, and even atmospheric
composition in general. However, with regard to developing a
science strategy for understanding future climate change and
variability, these six attributes are of particular importance.
They have been demonstrated to undergo significant variability over
decade-to-century time scales in the past and are therefore likely
to do so in the future, and they are intimately entwined within the
climate system that is the focus of this science plan. Atmospheric
composition also satisfies these criteria, but it is so essential
to the Earth's radiative balance and atmospheric dynamics that we
have opted to include it in our discussions of fundamental
climate-system components in Chapter 5.
The research strategy outlined in this report focuses on the
scientific issues that must be addressed to best advance our
understanding of how these six attributes can be expected to change
in the future. Being able to predict changes in their properties,
and to recognize the conditions that indicate when significant
changes are actually underway, will enable
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society to optimize its activities and prepare for the changes
in the most cost-effective way possible. These capabilities will
enhance our environmental security and sustain our continued
economic success.
This chapter expands on some of the important, most societally
relevant influences exerted by these attributes, documents our
knowledge of how they have changed in the past, and explains why we
have chosen to focus on them now.
Precipitation and Water
Availability
Freshwater is the very basis of terrestrial life, and is
arguably its most precious natural resource. Water influences
nearly every aspect of society and day-to-day life. From the huge
amounts of freshwater required for modem industrial and
agricultural production to the bucket of clean water so highly
prized in less developed countries, the uninterrupted supply of
clean freshwater is necessary for the overall health and
continuance of our societies and economies. In addition, freshwater
distribution influences energy production and utilization, water
quality, fisheries and land ecosystems, forestry, insurance,
recreation, and transportation. The longevity of aquifers, the
reliable flow of rivers, and the fall of rain determine where
civilizations can grow and prosper. Significant investments in our
infrastructure, such as the construction of dams and levees, and
water-resource planning and management in general, are based on our
current understanding of the supply, storage, and dispersal of
freshwater. Any changes or disruptions in the freshwater cycle as
we have come to know and rely on it can thus have widespread
consequences, with implications for all levels of society and every
individual in it.
Variations in the water supply will have more serious effects on
some societies than on others. Less developed countries,
particularly those with semi-add climates, marginal agriculture,
and rigid social structures, are clearly vulnerable to
growing-season failures: The history of northeastern Brazil is
replete with examples of major failures of growing-season rainfall
(see Figure 2-1) that caused mass migrations of Nordestinos to
other parts of Brazil (Magalhaes and Magee, 1994). More developed
societies, through their economic prowess, are less vulnerable to
the year-to-year variations of precipitation. For example, the
record rainfalls over the midsection of the United States in June
to August of 1993 led to record flooding (Kunkel et al., 1994; Bell
and Janowiak, 1995); the Mississippi River was above flood stage
for almost three months at St. Louis. This resulted in
extraordinary damage (estimated at $15-20 billion—see
Changnon, 1996), yet the flood, while causing considerable local
hardship, produced only a blip in the U.S. economy. Similarly, the
record summer drought of 1988 caused an estimated $30 billion in
agricultural damage alone (Trenberth and Branstator, 1992), but the
strength of the U.S. economy (if not the balance sheets of the
people in the region) was easily able to withstand this climatic
event.
Figure 2-1
Northeast Brazil rainy-season (Feb-May) standardized precipitation anomalies.
(From Ward and Folland, 1991; reprinted with permission of John Wiley and Sons, Ltd.)
As the time scale of precipitation variability increases, even
the most developed countries become vulnerable. The United States
enjoys an enviable agricultural sector that has become more
efficient over the years, employing an ever-decreasing share of the
population in the task of feeding its, and the world' s, people,
and showing great resilience in recovering from the random flood or
drought, no matter how severe. But during the 1930s, when the
economy was particularly fragile, an entire decade of low rainfall
caused migrations and dislocations in the United States similar to
those of northeastern Brazil. Indeed, recent paleoclimatic evidence
from enclosed lakes (Laird et al., 1996) suggests that such
droughts were considerably more severe and longer-lived in the
past, relative to what we have experienced in the past few hundred
years (Figure 2-2). Other parts of the paleoclimate record also
suggest that such severe droughts are not unprecedented. Even
today, when the economy of the United States is far more stable
than during the Depression years, a decade of poor rainfall in the
fertile agricultural regions would lead to economic dislocations
and would place grave strains on the national and global economy.
More frequent occurrence of floods like those in the Midwest in
1993 and 1997 would have similar types of effects.
The patterns of rainfall in the Sahel region of Africa also show
decadal- and centennial-scale variability (Figure 2-3). The
devastating impact of prolonged low rainfall on the mostly nomadic
societies of sub-Saharan Africa has required massive and continuing
infusions of world resources to avoid even greater disasters
(Glantz, 1994). Such long periods of drought leave little room for
adaptation by vulnerable populations, or for future economic
development, and they affect the intellectual development of
children in ways that will echo through generations.
An even longer drought may well have spelled the doom of the
Classic Maya civilization (Hodell et al., 1995). The
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period AD 800-1000 was unusually dry, and corresponded with the
decline of the Classic Maya. It is not difficult to imagine that a
society well adapted to a given level of precipitation would find
it difficult to adapt to a long period of drought, in this case one
lasting 200 years.
The longest precipitation record comes from the Greenland ice
cores (see, e.g., Grootes, 1995), which provide information on snow
accumulation on the Greenland ice sheet for the last 100,000 years
and more. Annual snow layers are discernible for the more recent
millennia; Figure 2-4 shows snow accumulation during an 8,000-year
period during which Greenland suddenly emerged from glacial
conditions at about 15,000 BP, fell back into glacial conditions
during the Younger Dryas period, and finally emerged into our
cur-
Figure 2-2
Reconstruction of the salinity of Moon Lake in the Northern Great Plains
of the United States for the last 2,300 years. Upper panel: Logarithms
of diatom-inferred salinity; smooth curve is filtered data. Bottom panel:
Log deviation from 2300-year mean. Greater salinity implies greater
desiccation in both curves. Note that the 1930s drought (see inset) is
relatively minor relative to the more severe and long-lived droughts
of the preceding millennium. (From Laird et al., 1996; reprinted with
permission of Macmillan Magazines, Ltd.)
rent interglacial state. Superimposed on these upheavals was a
background of precipitation variability on time scales ranging from
interannual to centennial. We may safely assume that dec-cen
variability in precipitation has always been with us, and thus can
be expected in our future as well.
Temperature
Temperature probably influences our day-to-day comfort level
more than any other environmental factor. Society is especially
vulnerable to long-term changes in temperature because of the
importance of temperature conditions to crop growth, heat stress,
energy usage, and recreation. Also, the deleterious impacts of
decade-to-century-long anomalies in temperature may be compounded
by anomalous hydrologic conditions, with direct effects on
agriculture, water supply, ecosystem stability, and so on.
Agricultural vulnerability is higher in the less developed
countries, where there are few safeguards to limit the consequences
of temperature changes. However, as with freshwater, even highly
developed countries are vulnerable to temperature changes as the
duration of anomalies and magnitude of variability increase. Such
changes may increase with a changing mean climate as noted by Karl
et al. (1996), because even small shifts in the climatic mean may
lead to significant increases in the number of extreme temperature
events, such as the heat waves that sporadically strike during
summer months in the midwestern and eastern United States, or the
number of frost days. These directly influence agriculture through
a variety of means.
Temperature changes are also directly related to changes in
energy consumption for heating and cooling (Figure 2-5), which can
have immediate impacts on consumer supplies and energy costs. From
a broader perspective, because the global temperature distribution
is the engine driving the Earth' s climate system, temperature
controls the primary circulation patterns, which in turn influence
the precipitation and evaporation patterns, the tracks and
intensities of storms, and other large-scale climate patterns as
well. Consequently, while the direct impact of temperature change
on society is considerable, its indirect impacts, through its
influence on the other important climate attributes, are
enormous.
There is considerable evidence documenting modern change in
temperature over decades and centuries. Of particular relevance are
those variations in the period leading up to the recent warming
trends observed in instrumental records, because they give a clear
indication of the nature and magnitude of the climate system's
natural variability. Variations during the last 150 years or so of
the modem instrumental records may reflect both natural and
anthropogenic change. Over this last 150 years, temperatures have
increased in most parts of the globe. However, this increase has
not been uniform geographically and has not been steady over time
(Figure 2-6). In the Southern Hemisphere, temperatures have
increased more or less monotonically since the turn of the century,
whereas in the Northern Hemisphere,
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Representative terms from entire chapter:
little ice
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Figure 2-3
Sahel rainfall as a function of time. Time series of yearly average normalized April-to-October departures for 20 sub-Saharan stations
located between 11º and 18º W of 10º E. (From Ropelewski et al., 1993; reprinted with permission of the American Meteorological Society.)
Figure 2-4
Snow accumulation in Greenland as a function of time from GISP 2 ice cores (bottom trace) vs. d
18O converted to temperature
(top trace). (From Kapsner et al., 1995; reprinted with permission of Macmillan Magazines, Ltd.)
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Figure 2-5
Relationship between temperature (heating degree days) and the energy
consumption for residential heating by U.S. census regions. More energy
is generally consumed for home heating in the colder regions of the United
States. Pacific includes WA, OR, CA, AK, and HI; Mountain: ID, MT, WY,
NV, UT, CO, AZ, and NM; W.N. Cent.: ND, MN, SD, IA, NE, MO, and KS;
E.N. Cent.: WI, MI, IL, IN, and OH; W.S. Cent.: OK, AR, TX, and LA; E.S.
Cent.: KY, TN, MS, and AL; New Engl.: ME, NH, VT, MA, CT, and RI; Mid.
Atl.: NY, PA, and NJ; S. Atl.: WV, MD, DE, VA, NC, SC, GA, and FL. Residential
heating data from 1993 EIA Residential Energy Consumption Survey (EIA, 1995).
(Figure courtesy of P. Schultz, National Research Council.)
Figure 2-6
Combined land-surface and sea surface temperature (ºC) from 1861 to 1994,
expressed as anomalies from the 1961-1990 average. (From IPCC, 1996a;
reprinted with permission of the Intergovernmental Panel on Climate Change.)
temperatures have risen more episodically, with the bulk of the
warming concentrated in the periods about 1920-1930 and about
1975-1995. Globally, the net increase over the last century amounts
to about 0.5º C in mean annual temperature (Ghil and Vautard,
1991; Jones and Briffa, 1992).
In the United States, the overall pattern of temperature change
(Figure 2-7) has been similar to the global pattern, but
geographically there are strong regional differences (Dettinger et
al., 1995; Karl et al., 1996). Over the last 100 years there has
been a strong increase in temperature over most of the northern and
western sections of the country, but in the southeast and south
central regions, temperatures have declined (Figure 2-8). Such
variations reflect regional circulation anomalies that are
otherwise obscured by large-scale averaging.
The period of warming in the twentieth century followed
Figure 2-7
Departures from the long-term mean of area-aver-aged annual temperature
over the contiguous United States (1900-1994). The dark line is a nine-
point binomial filter. (From Karl et al., 1996; reprinted with permission of the
American Meteorological Society.)
Figure 2-8
Trends in mean annual temperature (1900-1994) expressed as ºC per hundred years.
Closed circles represent warming, open circles cooling. (From Karl et al., 1996;
reprinted with permission of the American Meteorological Society.)
Page 14
a time of lower temperatures globally, which is often referred
to as the ''Little Ice Age'' (Jones and Bradley, 1992). Some
authors argue that this period began in the fourteenth century;
others find stronger evidence for a significant
mid-sixteenth-century decline in temperature (Bradley and Jones,
1992). Like modem warming, Little Ice Age cooling showed geographic
variability; although the period is not well documented, in mid-
and higher-latitude regions it seems to have been characterized by
episodes of temperatures generally cooler than the present, lasting
as long as a century, interrupted by only slightly warmer periods.
The generally cooler conditions had profound environmental
consequences in many parts of the world; glaciers advanced in
mountain regions and in Europe, and the much colder winters froze
rivers and canals, frequently bringing the transportation systems
of the day to a halt. Snowfall was higher and snow stayed on the
ground for longer periods in many regions (Lamb, 1972).
Historians have argued that the Little Ice Age coincided with a
time when the balance between food supply and a greatly expanded
European population was precarious, making European society highly
vulnerable to crop failures. The more frequent presence of cold air
caused unexpected freezes and heavy rains, devastating agriculture
and leading to crop failures. As a result, large numbers of people
died from malnutrition or starvation. Many countries are still
vulnerable to anomalous climatic conditions, especially where
agricultural production barely balances the needs of a growing
population.
Glacial deposits in the mountainous western United States and
Canada, tree-line and tree-ring studies, and anecdotal historical
evidence indicate that many parts of North America also experienced
Little Ice Age conditions similar to those of western Europe.
Indeed, the well-known abandonment of Norse settlements in
Greenland is thought to have been at least partly a consequence of
colder climatic conditions and a shorter growing season than the
original settlers had experienced in the preceding milder episode,
often referred to as the "Medieval Warm Epoch." Whether this warmer
episode was of more than regional significance remains unknown
(Hughes and Diaz, 1994).
For the Northern Hemisphere as a whole, the intervals of lowest
temperature in the last 600 years were in the late fifteenth, late
sixteenth, and entire nineteenth centuries (Figure 2-9), although
individual records show deviations from this large-scale average.
Warmer conditions were more common
Figure 2-9
Reconstruction of mean annual temperature for the Northern Hemisphere (heavy lines),
based on a network of paleoclimate data calibrated against instrumental data in the twentieth
century. The lighter lines form an envelope describing the estimated uncertainty of individual
yearly temperature estimates (at the 95 percent confidence level). The low-frequency solid
curve is a 50-year low-pass-filtered version of the individual yearly values. (After Mann et al.,
1998; reprinted with permission of Macmillan Magazines, Ltd.)
Page 15
in the early sixteenth century and in most of the eighteenth
century, although for the hemisphere as a whole, conditions
comparable to the decades from 1920 onward had not been experienced
for at least the previous several hundred years (Bradley and Jones
1993; Mann et al., 1998). The unusual nature of temperature
conditions during the twentieth century, especially over the last
20 years or so, deserves emphasis; indeed, several years in the
1990s were warmer than any other time in at least 600 years (Mann
et al., 1998). Recent low-latitude ice-core studies show in
dramatic fashion that in those areas temperatures in the last
decade were higher than they had been for at least 1,500 years in
those areas (Thompson et al., 1993b). The loss of permanent ice
caps and glaciers in the tropical Andes is of great significance
for agriculture and the water-supply networks of Ecuador, Peru,
Bolivia, and Chile. It has a negative impact on hydroelectric power
production in those countries as well. Priceless paleoclimatic
records are also melting away, often before they can be collected
and analyzed.
It is important to recognize that the globally extensive
instrumental recording of climate data, which forms the basis for
our limited view of "global warming" began during one of the
coldest periods of the last few centuries (Figure 2-9). More
extensive long-term records of global temperature changes would
allow a more rigorous statistical assessment of the current
warming. It is clear that decadal-scale (and lower-frequency)
temperature variations were also evident for many centuries prior
to the nineteenth century, while greenhouse gases exhibited
relatively little change. The fact that such temperature variations
must have been unrelated to global-scale anthropogenic effects
focuses our attention on the need to understand what drives such
"natural" variability. Better records of past climate variations
and of potentially important forcing factors, such as
solar-irradiance changes or explosive volcanic eruptions, are
needed. In particular, it would be especially valuable to examine
the enigmatic Medieval Warm Epoch—a time often cited as having
been warmer than the twentieth century, but so far not shown to
have been warmer over an entire hemisphere, much less over the
entire world (Hughes and Diaz, 1994).
Solar Radiation
Sunlight is one of the bases of life on this planet. It provides
the energy needed to warm the Earth and evaporate water; its
geographical distribution drives the Earth's climate system. Its
vertical absorption and reflection play a major role in stratifying
the atmosphere, affecting atmospheric circulation, winds, clouds,
and a multitude of energy-balance feedbacks. Sunlight is an
essential ingredient in the creation of the protective ozone layer,
and is a direct source of natural, readily harnessed energy. It
provides the light animals need to see and plants need for
photosynthesis. It is basic to the food chain of most life.
The link between solar variation and climate or weather has been
known for millennia; in the fourth century BC, Theophrastus
correlated sunspots with rainy weather. Recent measurements of
solar output have shown decadal-scale variations of less than 1
percent (Lean et al., 1992; Hoyt and Schatten, 1993; Zhang et al.,
1994), but we know that it has been orders of magnitude greater in
past eons, and presumably could be again. Variations in solar-cycle
length and Northern Hemisphere climate anomalies parallel each
other over the last 100 years (Friis-Christensen and Lassen, 1991;
Labitzke and van Loon, 1993), and solar activity and sea surface
temperature track each other well over the past 130 years (Reid,
1991; White et al., 1997b).
Solar ultraviolet radiation affects society through its role in
health. It promotes the production of vitamin D, yet it is
implicated in human skin cancer, suppression of the skin's immune
system, and damage to plants (see, e.g., Coohill, 1991, and Krupa
and Jager, 1996). The sun provides ultraviolet radiation in the
wavelength region 280-320 nm (called UV-B) that is damaging to most
plants and animals. For example, there is accumulating evidence of
damage to Antarctic ecosystems (Smith et al., 1992) in the
aftermath of the opening of the Antarctic ozone hole. Solar UV-B
radiation' s impact on ecosystems in turn affects the natural
cycles of water, carbon, and other nutrients.
The stratospheric ozone layer that shields the Earth by
absorbing most of the UV-B radiation has changed. We do not have
reliable decadal records of UV-B received at the Earth's surface
and how it has varied through time. However, as Figure 2-10 shows,
we do have clear observational evidence demonstrating that lower
ozone columns result in higher UV-B irradiation, as predicted by
models (WMO, 1995). Thus it has been possible to derive accurate
calculated levels of surface UV-B irradiance, and hence of human
exposure, under clear-sky conditions (Figure 2-11); note that an
increase is evident throughout the middle latitudes.
The sun is also the source of photosynthetically active
radiation (PAR), which is in the wavelength region 0.4-0.7
µm. The amount of PAR received at the Earth's surface is
controlled by cloud cover—not only by the cloud droplet
concentration and cloud thickness, but also by the spatial and
temporal distribution of clouds, which varies considerably (see
Figure 2-12). When the amount of cloud cover over a region changes
for a long period of time, and the amount of PAR shifts, the growth
of ecosystems may be altered. Such changes are thus important for
agriculture and for natural or managed systems, such as forests.
Since the middle of this century such long-term changes in cloud
cover have in fact been observed (Figure 2-12). In addition to
trends of increasing cloud cover in some regions, concomitant
changes in other variables have also been detected, such as the
change in cloud amount that accompanied a step-like increase in
evaporation over the former Soviet Union around 1976. All such
records are subject to systematic shifts as observers and
instruments change; without knowledge of
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Figure 2-10
UV-B radiation reaching the Earth's surface as a function of ozone column, from both
measurements and models. (RAF is a radiation amplification factor.) Measurements
were made at the South Pole between February 1991 and December 1992. (From
Booth and Madronich, 1994; reprinted with permission of the American Geophysical Union.)
Figure 2-11
History of UV-B (310nm) at the Earth's surface for mid-latitudes over 15 years beginning 1979,
based on the ozone/UV-B relationship in Figure 2-10 and observed ozone-column data.
(From WMO, 1995; reprinted with permission of the World Meteorological Organization.)
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Figure 2-12
Cloud-cover changes over Australia, North America, India, and Europe since 1900, expressed as percent
deviations from the long-term mean. (From McGuffie and Henderson-Sellers, 1988; reprinted with permission
of the Canadian Meteorological and Oceanographic Society.)
these occurrences it is difficult to assemble a reliable dec-cen
record of climate change.
The ability of solar radiation to influence the Earth' s
temperature is controlled by several factors that have been shown
to vary over decades and centuries. Among them are variations in
the total solar output, changes in the amount of sunlight reflected
from clouds and aerosols in the lower atmosphere, increases in
aerosols in the atmosphere because of volcanic injections, and
trapping of the Earth's thermal radiation by the greenhouse gases.
These factors are discussed in greater detail in Chapter 5; we
present here a single example of a direct impact on society caused
by changes in total sunlight.
Like volcanic aerosols, tropospheric aerosols generated by
industrial pollution can play an important role in the amount of
radiation reaching the Earth's surface. In certain regions, such as
the Sichuan Province of China, there are clear trends of decreasing
visibility. Reduced visibility is often regarded as a
quality-of-life issue, affecting principally our appreciation of
our natural surroundings. However, in Sichuan the industrial haze
is so severe that it has led to a substantial reduction in the
annual mean solar energy reaching the surface over the past four
decades. Records from the eastern United States indicate
interdecadal changes in light extinction associated with sulfur
emissions (Figure 2-13). These changes in visibility affected the
solar flux at the ground, and thus likely also surface
temperature.
Storms
Whereas the distributions of freshwater, temperature, and
radiation affect us on a daily basis, the less frequent but more
destructive side of nature is often realized in relatively
short-lived episodes of severe weather associated with storms.
Storms generally owe their destructive power to high winds,
associated lightning, and high rates of precipitation in the form
of rain, hail, sleet, and snow. In coastal regions, the high winds
drive heavy waves and surges that cause flooding and beach erosion,
often inflicting considerable, sometimes irreversible, damage on
coastal communities and ecosystems. The entire landscape can be
permanently altered. Storms are responsible for widespread
personal-property destruction, deaths, and financial stress. The
financial stress can be both personal (uninsured loss, increased
premiums, higher taxes to cover government aid) and institutional
(massive insurance payouts, federal and state disaster relief).
Storms also influence recreation, power usage, and health. Thus,
any change in their frequency of occurrence over dec-cen time
scales can have considerable influence on economies (consider the
substantial outlay of the U.S. insurance
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Figure 2-13
Comparison of historical trends for sulfur emissions and haziness for the
northeastern (left) and southeastern (right) United States (From NAPAP, 1991.)
industry in recent years related to major storms) and
societies.
The word "storm" refers to an array of extreme weather phenomena
that display a wide range of spatial scales and owe their existence
to a variety of physical mechanisms (see, e.g., Palmen and Newton,
1969). Consequently, their impact is manifested through a variety
of means and geographic extents. For example, in the mid-latitudes,
wintertime extra-tropical cyclonic storms extend over hundreds to
thousands of kilometers, although their severe weather is generally
associated with smaller-scale features such as fronts and squall
lines embedded in the large-scale disturbance (see, e.g., Wallace
and Hobbs, 1977). In the subtropical and mid-latitude land areas,
severe storms occur in all seasons (including summer) as a result
of mesoscale (length scale of tens of kilometers) organization of
convection. Forming relatively localized features, such as squall
lines and the so-called multi- and super-cell storms, these storms
are accompanied by strong wind gusts, tornadoes, lightning, hail,
and flash floods. Tropical ocean regions spawn cyclones known as
hurricanes and typhoons. These storms are somewhat smaller in size
than mid-latitude cyclones, but their intensity tends to exceed
that of the latter. Because they have a relatively large radius of
influence where high winds combine with large amounts of
precipitation, tropical storms are among the most dreaded natural
phenomena. The largest destructive potential of these storms is in
coastal regions, where population density is often high.
Several studies, focusing predominantly on the Atlantic where
the historical records are longest, have indicated the existence of
decadal-to-centennial fluctuations in storm intensity and
distribution in the tropics and extra-tropics. Multidecadal changes
in Atlantic hurricane activity were noted by Gray (1990) and
Landsea et al. (1996). These changes consist of a
higher-than-normal occurrence of intense hurricanes and hurricane
days between the mid-1940s and the mid-1960s, and lower-than-normal
occurrence between the mid-1960s and the early 1990s (Figure
2-14a,b). Despite the general decrease in the number of Atlantic
hurricanes, the damage totals from these storms have increased
dramatically since the 1940s (Figure 2-14c). The increasing damages
are related to the greater vulnerability arising from the growing
population densities and property values in coastal regions (Pielke
and Landsea, 1998). While monetary damages have increased,
hurricane-related deaths have decreased over the past century
(Figure 2-14c). This decline is
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Figure 2-14
(a) Number of intense Atlantic hurricanes per year since 1944. The squares show the 5-year
running mean. (After Landsea and Gray, 1992; reprinted with permission of the American
Meteorological Society). (b) Atlantic hurricane tracks for the periods 1947-1969 and 1970-
1987. (From Gray, 1990; reprinted with permission of the American Association for the
Advancement of Science.) (c) Hurricane-related damage costs and deaths in the U.S. since
1900; damages normalized to 1990 dollars. (From Hebert et al., 1996; reprinted with permission of NOAA.)
directly related to improvements in the National Weather
Service's ability to forecast hurricane tracks and intensities, as
well as to improvements in the dissemination speed and coverage of
hurricane warnings. The declining number of intense hurricanes has
also contributed to the decrease in mortality.
In the extra-tropics, historical evidence of long-term changes
in storminess is found in written records and in pat-
Page 20
terns of beach erosion and destruction in western European
countries bordering the Atlantic (Lamb, 1995, and references
therein). At the end of the medieval times and during the Little
Ice Age—that is, essentially between the early 1200s and the
late 1600s—coastal flooding and reports of other coastal
damage suggest abnormally strong (in both frequency and intensity)
storm activity in the eastern North Atlantic. Increases in North
Atlantic wave heights since the early 1960s (Günther et al.,
1998) provide evidence of more intense storminess (Figure
2-15).
These observations complement the increased frequency of reports
of extremely low sea-level pressure, high wind conditions, and
large ocean waves encountered at sea and along western European
shores since the 1960s (Lamb, 1995). The change in ocean storminess
is supported by observations of increases of the wintertime mean
north-south pressure gradient across the North Atlantic since 1960
or so (Flohn et al., 1990; Bacon and Carter, 1993; Hurrell, 1995;
WASA Group, 1998), and the link between the large-scale pressure
gradient and wave statistics (Bacon and Carter,
Figure 2-15
Annual wave-height maxima, means, and 99th and 90th percentiles at three North
Atlantic sites, as derived from wave hindcasts. Note that the mean remains fairly
constant, but the maxima show a marked rise and suggest a decadal pattern. (From Günther
et al., 1998; reprinted with permission of the Gordon and Breach Publishing Group.)
1993; Kushnir et al., 1997). Long-term changes in storminess
have also been observed over North America (Hayden, 1981). Such
changes have also been linked to large-scale climate variability
(Dickson and Namias, 1976).
Sea Level
Coastal flooding by seawater inundation is one of the most
direct and threatening impacts of raised sea level. This point was
dramatically illustrated in 1953 when a combination of high tides
and strong winds forced a local sea-level rise that inundated
approximately one-sixth of the Netherlands, flooding land 64 km
from the coast, killing approximately 2,000 people and leaving
another 100,000 homeless (Schneider, 1997). Today, the Netherlands
expends U.S. $30 million a year defending itself against such
encroachment by the sea (de Ronde, 1993), in addition to the
tremendous infrastructure already in place for this defense. South
Florida, Chesapeake Bay, and several other U.S. locations are also
losing land to immersion, including some settled islands. Sandy
beaches can be especially vulnerable to a small sea-level rise,
depending on the offshore depth profile. Bruun's rule (1962), which
allows one to estimate beach erosion as a function of sea-level
rise, suggests that a rise of 50 cm can erode approximately 50 m of
beach, roughly the width of Virginia Beach. Between 1962 and 1985
the U.S. Army Corps of Engineers renourished over 700 km of beaches
at a cost of $8 billion (Schwartz and Bird, 1990).
The physical effects of sea-level rise are not limited to simple
inundation, however. In many cases sea-level rise exacerbates other
changes (natural or anthropogenic) occurring in the coastal
environment. Consequently, rising (relative) sea level affects
coastal communities via inundation, erosion of land, saltwater
intrusion, elevated water tables, and increased flooding and storm
damage (Nicholls and Leatherman, 1994). In fact, the IPCC (IPCC,
1996b) estimates that worldwide approximately 46 million people a
year experience flooding due to storm surges, and this number would
double if there were a 50 cm rise in sea level. For example,
consider the impact of erosion in this country due to changes in
relative sea level. The Mississippi delta has the largest rate of
land loss in the United States. Wetlands have been lost there at a
rate of up to 100 km2 per year in
this century, primarily through loss of sediment and land
subsidence. Sea-level rise will only aggravate this problem.
In addition to causing direct flooding, a higher mean sea level
elevates the base for storm surges, which can have devastating
impacts on human life and property. For example, 300,000
Bangladeshis died in a 1970 storm surge. Sea-level rise in the
Ganges-Brahmaputra delta of Bangladesh has been exacerbated by
local subsidence of the land, which can exceed 20 mm per year
(Alam, 1996). Higher sea level also slows rainwater drainage,
leading to increased risk of riverine flooding. In September 1987
river flooding affected nearly half of Bangladesh, where population
and poverty
Page 21
compound the problem—land is so scarce that people
repopulate land on which the previous tenants have died in a flood.
Maldives, a small island country in the Indian Ocean lying mostly
less than two meters above sea level, could be totally inundated,
as could numerous other small island countries scattered throughout
the tropical and subtropical world.
Furthermore, saltwater can intrude landward either through the
ground or up waterways, though the latter process is much more
rapid. Both processes are sensitive to sea level and to the supply
of freshwater at the coast. During the droughts of the 1960s
saltwater advanced 53 km up the Delaware River, forcing some
industries near Philadelphia to seek water imported from the
Susquehanna River Basin. Today, farmers along the edges of
Chesapeake Bay are losing one to two rows of corn per year to
saltwater intrusion resulting from land subsidence and sea-level
rise. A concern in the Netherlands is that in drought summers the
freshwater needed to combat saltwater intrusion and flush the land
of salt may not be available. Likewise, in Bangladesh, heavy
monsoon rainfall is needed to desalinate the topsoil so that rice
can be grown in a region occupied by saltwater during the dry
season.
These examples illustrate the vulnerability of societies
worldwide to variations in sea level. Recent analyses suggest that
global sea level has been rising at a rate of 1.2-2.0 mm per year
for at least a century (Douglas, 1995; Unal and Ghil, 1995),
amounting to a rise of up to 20 cm. While there has been no
detectable acceleration of the rise in this century, there is
evidence that the rate of rise was smaller in previous millennia
(Gornitz, 1995). Considerable regional variability is superimposed
on the global sea-level-rise trend; some regions have experienced
falling sea level (e.g., Scandinavia) and others have seen a rise
larger than the global mean (e.g., the southeast coast of the
United States).
Decadal-scale regional variations during this century can also
be seen. Such variations are apparent in the tide-gauge records for
individual locations such as San Francisco (Figure 2-16). Records
like these may reflect a number of processes, including vertical
movements of land, redistribution of water in the oceans, and
global sea-level changes. Decadal fluctuations tend to be
synchronous along long sections of a particular coast, and can
sometimes be linked to wind-stress variability (Sturges and Hong,
1995). High-frequency fluctuations are often associated with local
winds, though oceanic wave propagation allows a larger spatial
influence (Sturges, 1987). Low-frequency atmospheric variability
influences coastal sea level on longer time scales, and planetary
waves can propagate these changes over very large spatial scales,
influencing regions well away from the local source of the
disturbance. For example, sea-level changes
Figure 2-16
Variations in sea level at San Francisco since 1855, from data in Spencer
and Woodworth (1993). (Figure courtesy of M. Winton, NOAA/GFDL.)
Page 22
on the order of 10-50 cm are associated with the El
Niño/Southern Oscillation (ENSO) phenomenon, and satellite
altimetry has documented the propagation of these changes across
the Pacific (Jacobs et al., 1994). At Bermuda (Figure 2-17),
decadal variations on the order of 10 cm have been traced to
variability of wind-stress curl over the open ocean to the east
(Sturges and Hong, 1995).
Ecosystems
Ecosystems influence society directly and indirectly; our very
existence depends in fundamental ways on the ecosystems of the
Earth, of which we are a part. In fact, the dependence of all life
forms on the physical environment, as well as their
interdependence, are embodied in the concept of ecosystems,
reflecting intricate communities of primary producers, grazers, and
predators that are adapted to each other as well as to particular
regimes of temperature, moisture, radiation, and other factors. The
ecosystems in turn influence, both locally and globally, the
physical and chemical environment in which they exist. For example,
they moderate the flow of radiatively active gases, sequester
carbon, and alter the atmospheric moisture content and the Earth's
albedo; all of these functions directly influence other climate
attributes. Ecosystems also influence society through other means.
Our food consists of organisms taken from terrestrial and marine
habitats. Ecosystems harbor the genetic diversity of life; the
continuity of our food supply partially depends on the existence of
wild strains that may possess resistance against emerging plant
diseases and pests. Wild varieties offer the potential for higher
crop yields, for new cultivars adapted to changed climatic
conditions, or for entirely new crops. New medicines have often
been developed from existing biological compounds.
The carriers of human illness respond to climate change when the
ecosystem in which they live is affected by that change. It is
believed that the outbreak of the deadly Hantavirus in 1993 in the
southwestern United States was caused by a period of drought
succeeded by heavy rains. The drought induced a decline in all
animal populations, but deer mice, carriers of the virus, recovered
far more quickly than their predators (they increased tenfold when
the rains returned); this greatly increased their contact with
humans (Levins et al., 1994). Coastal algal blooms, a breeding
ground for the bacterium Vibrio cholera that sometimes finds
its way into the food chain, have been linked to the cholera
outbreak in South America in 1992 (Epstein, 1993). A threefold
increase in malaria incidence in Rwanda in 1987 has been ascribed
to record high temperatures and rainfall (Loevinsohn, 1994).
Other important contributions of ecosystems to our well-being
are the creation of soils over thousands of years, and the
establishment of an environment that is varied and enjoyable. The
latter often translates directly into economic value via money
spent on recreation. Soils are, of course, indispensable for
agriculture, but they also function as reservoirs of water, carbon,
and nutrients. However, ecosystems did not come into existence
solely for the benefit of humans; they also harbor pathogens and
toxic compounds, and climate-related changes in the soils may lead
to changes in these threats.
Ecosystems change over most time scales. On decade-to-century
time scales, there is considerable natural variability as well as
anthropogenic change. For example, in southern Ontario the analysis
of pollen has shown that dominant beech
Figure 2-17
Sea-level fluctuations for Bermuda. The solid line is the observed record, adjusted to
constant atmospheric pressure. The dashed line is a hindcast made by a simple model
forced with wind-stress observations. (From Sturges and Hong, 1995; reprinted with permission
of the American Meteorological Society.)
Page 23
trees were replaced first by oak, then by pine trees, during the
Little Ice Age, only a few hundred years ago (Campbell and
McAndrews, 1993). Also, the monitoring of fluctuations in
Northwestern salmon catch since the early part of this century (see
Figure 2-18) has revealed a remarkable coincidence with decadal
changes in the North Pacific sea surface temperature (Mantua et
al., 1997). Decadal-scale changes in the extent of the Sahel have
also been documented
Figure 2-18
Salmon catch as a function of time, showing decadal variation in yield. The top panel shows the Pacific
(inter)Decadal Oscillation (PDO) index. The panels below show catch records for various types of Pacific
salmon. For Alaska catches, the black (grey) bars denote values that are greater (less) than the long-term
mean. The shading convention is reversed for the bottom two panels. Light, dotted vertical lines indicate
the PDO reversal times at 1925, 1947, and 1977. (From Mantua et al., 1997; reprinted with permission of the
American Meteorological Society.)
Page 24
(see, e.g., Tucker et al., 1991). Barry et al. (1995) document a
shift in the community structure of invertebrate fauna in a
California rocky intertidal zone consistent with observations of
warming sea surface temperature between 1932 and 1993. In addition
to these regional changes, the observed increase in the
intra-annual amplitude of atmospheric CO2 concentration may indicate that the
cycle of net primary production and respiration is increasing in
vigor on a hemispheric scale (Keeling et al., 1996a).
Natural and anthropogenic habitat destruction and fragmentation
are the greatest contributors to the extinction of species.
Consider the entirely new ''ecosystem'' that has recently grown to
occupy several percent of the land area of the United States. It is
composed of a patchwork of asphalt, concrete, and structures, with
some accompanying reforestation; all of these influence and are
influenced by other ecosystems, society, and climate. Fire and
other disasters such as floods and storms are part of a naturally
varying environment to which ecosystems will adapt, but this new
"ecosystem" will be more challenging.