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GAS-L: "Global Warming and Drought" - an overview

(Peter Singfield - Belize)

Global Warming and Drought -- the possible consequences to burning of
forests, creation of large cloud formations, blocking solar radiation.

One more "cascade" in a series that occurs with global warming. What are
the consequences?

"Extensive drought-induced fires burned over 475,000 acres in Florida
and cost $500M in damages. In the same year, Canada suffered its
fifth-highest fire occurrence season in 25 years. Starting in 1998,
three years of record low rainfall plagued northern Mexico. 1998 was
declared the worst drought in 70 years. It became worse as 1999
spring rainfalls were 93% below normal."


"Along with this increased vulnerability,
concern exists because some research suggests that drought in the
future may be amplified in certain areas due to changes in climate
variability and extremes resulting from global warming."


"The impact of droughts over the last few decades have shown that some
regions and sectors of the population are becoming increasingly
vulnerable to drought. Compounding these vulnerabilities is the
uncertainty of the effects of human activities and global warming
on climate in general and on drought in particular."

OK folks -- all the following excerted from:



    Drought: A Paleo Perspective -- Home Page

Cornstalk, Steinbeck quote   

...Now the wind grew strong and hard,
it worked at the rain crust
in the corn fields.

Little by little the sky
was darkened by the mixing dust,
and the wind felt over the earth,
loosened the dust and carried it away.

...from The Grapes of Wrath,
       written by John Steinbeck.

"The impact of droughts over the last few decades have shown that some
regions and sectors of the population are becoming increasingly
vulnerable to drought. Compounding these vulnerabilities is the
uncertainty of the effects of human activities and global warming
on climate in general and on drought in particular."

Droughts occur throughout North America, and in any given year, at
least one region is experiencing drought conditions. The major
drought of the 20th century, in terms of duration and spatial extent,
is considered to be the 1930s Dust Bowl drought which lasted up to 7
years in some areas of the Great Plains. The 1930s Dust Bowl drought,
memorialized in John Steinbeck's novel, The Grapes of Wrath, was so
severe, widespread, and lengthy that it resulted in a mass migration
of millions of people from the Great Plains to the western U.S. in
search of jobs and better living conditions.

Just how unusual was the Dust Bowl drought? Was this a rare event or
should we expect drought of similar magnitude to occur in the future?
Rainfall records used to evaluate drought extend back 100 years, and
are too short to answer these questions. However, these questions can
be answered by analyzing records from tree rings, lake and dune
sediments, archaeological remains, historical documents and other
environmental indicators, which can extend our understanding of past
climate far beyond the 100-year instrumental record.

This Web site was designed to explain how paleoclimatic data can
provide information about past droughts and about the natural
variability of drought over timescales of decades to millennia. We
note that droughts are a world wide phenomenon and affect the global
community. However, the focus of these Web pages is North America

The Story


Drought is an elusive climate event. The effects of drought,
economically and environmentally, are often subtle to begin with, but
can end up being incredibly costly and devastating.

How do we define drought? What have we learned about past droughts?
How can information about past drought further our understanding and
better prepare us for the droughts of the future?

These questions will answered in the following sections:

  * What is drought?
  * Why are we concerned about drought?
  * 20th Century drought
  * Paleoclimatology and drought

What is Drought?


In Drought and Its
Causes and Effects, Tannehill (1947) wrote: "We have no good
definition of drought. We may say truthfully that we scarcely know a
drought when we see one. We welcome the first clear day after a rainy
spell. Rainless days continue for a time and we are pleased to have a
long spell of such fine weather. It keeps on and we are a little
worried. A few days more and we are really in trouble. The first
rainless day in a spell of fine weather contributes as much to the
drought as the last, but no one knows how serious it will be until
the last dry day is gone and the rains have come again... we are not
sure about it until the crops have withered and died."

The difficulty of recognizing the onset or end of a drought is
compounded by the lack of any clear definition of drought. Drought
can be defined by rainfall amounts, vegetation conditions,
agricultural productivity, soil moisture, levels in reservoirs and
stream flow, or economic impacts. In the most basic terms, a drought
is simply a significant deficit in moisture availability due to lower
than normal rainfall. However even this simple definition is
complicated when attempts are made to compare droughts in different
regions. For example, a drought in New Jersey would make for wet
conditions in the deserts of Arizona!

Drought, as measured by scientists, is defined by evaluating
precipitation, temperature, and soil moisture data, for the present
and past months. A number of different indices of drought have been
developed to quantify drought, each with its own strengths and
weaknesses. Two of the most commonly used are the Palmer Drought
Severity Index (PDSI) and the Standard Precipitation Index (SPI). 

Drought conditions are monitored constantly using these and
other indices to provide current information on drought-impacted
regions. For more complete information about drought definitions,
indices, and current drought conditions, see the Web pages of the
National Drought Mitigation Center or the links found at the NOAA
Drought Information Center for the U.S., and Drought Watch on the
Praries for Canada.

Because of the elusive nature of drought, we do not think of droughts
in the same way as other weather-related catastrophes, such as
floods, tornadoes, and hurricanes. However, although droughts may be
less spectacular, they are often more costly than other types of
natural disasters, and no region in North America is immune to
periodic droughts.

Why Are We Concerned About Drought?


Although the major droughts of the 20th century, the 1930s Dust Bowl
and the 1950s droughts, had the most severe impact on the central
U.S., droughts occur all across North America. Florida suffered from
the 1998 drought along with the states of Oklahoma and Texas.
Extensive drought-induced fires burned over 475,000 acres in Florida
and cost $500M in damages. In the same year, Canada suffered its
fifth-highest fire occurrence season in 25 years. Starting in 1998,
three years of record low rainfall plagued northern Mexico. 1998 was
declared the worst drought in 70 years. It became worse as 1999
spring rainfalls were 93% below normal. The government of Mexico
declared five northern states disaster zones in 1999, and nine in
2000. The U.S. West Coast experienced a six-year drought in the late
1980s and early 1990s, causing Californians to take aggressive water
conservation measures. Even the typically humid northeastern U.S.
experienced a 5-year drought in the 1960s that drained reservoirs in
New York City down to 25% of capacity. In fact, almost every year,
some region of the North America experiences drought.

Drought is a natural hazard that cumulatively has affected more
people in North America than any other natural hazard (Riebsame et
al. 1991). The cost of losses due to drought in the United States
averages $6-8 billion every year, but range as high as $39 billion
for the three year drought of 1987-1989, which was the most costly
natural disaster documented in U.S. history. Continuing uncertainty
in drought prediction contributes to crop insurance payouts of over
$175 million per year in western Canada.

Beyond the monetary costs, the impacts of drought on society, the
economy, and the natural environment are tremendous. Although
measures such as development of irrigation systems, financial aid
programs and interbasin water transfers have been undertaken to
mitigate the impacts of drought in recent decades, some regions of
the U.S. are becoming more vulnerable to the impacts of drought.

Although irrigation has made it possible to grow crops on
land that was once considered barren, this practice has led to a
reliance on ground water and surface storage in reservoirs.

Increasing demands on water have resulted in the depletion of ground
water reserves in many areas, which can make the removal of
additional water uneconomical if not impossible, especially during a
drought. In many urban areas of the semi-arid and arid western U.S.,
population growth, expansion into marginal areas, and the subsequent
development is overtaxing water supplies and heightening
vulnerability to drought. Along with this increased vulnerability,
concern exists because some research suggests that drought in the
future may be amplified in certain areas due to changes in climate
variability and extremes resulting from global warming.

Scientists have much to learn about the characteristics of drought
and the conditions that lead to the persistence of drought. Although
some progress has been made, (for instance, droughts that are related
to El Nin~o and the Southern Oscillation (ENSO)[11] are now more
predictable on a seasonal scale), scientists still cannot predict
longer, multi-year droughts.

The two major droughts of the 20th century, the 1930s Dust Bowl
drought and the 1950s drought, lasted five to seven years and covered
large areas of the continental U.S. Complete scientific understanding
of how and why these two drought episodes occurred remains elusive.
From a societal perspective, the important question is, how unusual
are these events? Most instrumental records (from thermometers and
rain gauges) are only about 100 years long, so they are too short to
answer this question. However, paleoclimatic proxy data are a
valuable tool to investigate this question by providing a longer
context within which to evaluate the reoccurrence of these major
droughts over hundreds to thousands of years.

For more complete information on the impacts of drought in North
America, see the National Drought Commission report titled "Preparing
for Drought in the 21st Century"


20th Century Drought


The Dust Bowl Drought

The Dust Bowl drought was a natural disaster that severely affected
much of the United States during the 1930s. The drought came in three
waves, 1934, 1936, and 1939-40, but some regions of the High Plains
experienced drought conditions for as many as eight years. The "dust
bowl" effect was caused by sustained drought conditions compounded by
years of land
Photos from Library of Congress and U.S. National Archives management
practices that left topsoil susceptible to the forces of the wind.
The soil, depleted of moisture, was lifted by the wind into great
clouds of dust and sand which were so thick they concealed the sun
for several days at a time. They were referred to as" black blizzards

The agricultural and economic damage devastated residents of the
Great Plains. The Dust Bowl drought worsened the already severe
economic crises that many Great Plains farmers faced. In the early
1930s, many farmers were trying to recover from economic losses
suffered during the Great Depression. To compensate for these losses,
they began to increase their crop yields. High production drove
prices down, forcing farmers to keep increasing their production to
pay for both their equipment and their land. When the drought hit,
farmers could no longer produce enough crops to pay off loans or even
pay for essential needs. Even with Federal emergency aid, many Great
Plains farmers could not withstand the economic crisis of the
drought. Many farmers were forced off of their land, with one in ten
farms changing possession at the peak of the farm transfers.
PDSI Animation, 1930s and 1950s (6 year time frame)

In the aftermath of the Dust Bowl, it was clear that many factors
contributed to the severe impact of this drought. A better
understanding of the interactions between the natural elements
(climate, plants, and soil) and human-related elements (agricultural
practices, economics, and social conditions)of the Great Plains was
needed. Lessons were learned, and because of this drought, farmers
adopted new cultivation methods to help control soil erosion in dry
land ecosystems. Subsequent droughts in this region have had less
impact due to these cultivation practices.


The 1950s Drought

Fueled by post-war economic stability and technological advancement,
the 1950s represented a time of growth and prosperity for many
Americans. While much of the country celebrated a resurgence of
well-being, many residents of the Great Plains and southwestern
United States were suffering. During the 1950s, the Great Plains and
the southwestern U.S. withstood a five-year drought, and in three of
these years, drought conditions stretched coast to coast. The drought
was first felt in the southwestern U.S. in 1950 and spread to
Oklahoma, Kansas and Nebraska by 1953. By 1954, the drought
encompassed a ten-state area reaching from the mid-west to the Great
Plains, and southward into New Mexico. The area from the Texas
panhandle to central and eastern Colorado, western Kansas and central
Nebraska experienced severe drought conditions. The drought
maintained a stronghold in the Great Plains, reaching a peak in 1956.
The drought subsided in most areas with the spring rains of 1957.

Courtesy of Baylor University, Texas Collection The 1950s drought was
characterized by both low rainfall amounts and excessively high
temperatures. Texas rainfall dropped by 40% between 1949-1951 and by
1953, 75% of Texas recorded below normal rainfall amounts. Excessive
temperatures heated up cities like Dallas where temperatures exceeded
100^*F on 52 days in the summer of 1953. Kansas experienced severe
drought conditions during much of the five-year period, and recorded
a negative Palmer Drought Severity Index [9]from 1952 until March
1957, reaching a record low in September of 1956.

A drought of this magnitude creates severe social and economic
repercussions and this was definitely the case in the southern Great
Plains region. The drought devastated the region's agriculture. Crop
yields in some areas dropped as much as 50%. Excessive temperatures
and low rainfall scorched grasslands typically used for grazing. With
grass scarce, hay prices became too costly, forcing some ranchers to
feed their cattle a mixture of prickly pear cactus and molasses. By
the time the drought subsided in 1957, many counties across the
region were declared federal drought disaster areas, including 244 of
the 254 counties in Texas.


The 1987 - 1989 Drought

PDSI Instrumental Maps of 1988 Drought The three-year drought of the
late 1980s (1987-1989) covered 36% of the United States at its peak.
Compared to the Dust Bowl drought, which covered 70% during its worst
year, this does not seem significant. However, the 1980s drought was
not only the costliest in U.S. history, but also the most expensive
natural disaster of any kind to affect the U.S. (Riebsame et al.
1991). Combining the losses in energy, water, ecosystems and
agriculture, the total cost of the three-year drought was estimated
at $39 billion. Drought-related losses in western Canada exceeded
$1.8 billion dollars in 1988 alone.

The drought, beginning along the west coast and extending into the
northwestern U.S., had its greatest impact in the northern Great
Plains. By 1988, the drought intensified over the northern Great
Plains and spread across much of the eastern half of the United
States. This drought affected much of the nation's primary corn and
soybean growing areas, where total precipitation for April through
June of 1988 was even lower than during the Dust Bowl. The drought
also encompassed the upper Mississippi River Basin where low river
levels caused major problems for barge navigation. The summer of 1988
is well known for the extensive forest fires that burned across
western North America, including the catastrophic Yellowstone fire.
Forest Fire in Yellowstone. In addition to dry conditions, heat waves
during the summer of 1988 broke long-standing temperature records in
many midwestern and northeastern metropolitan areas.

The 1987-89 drought was the first widespread persistent drought since
the 1950s and undoubtedly took people by surprise. Many had not
experienced the 1950s drought and others had forgotten about the
harsh realities of drought. The financial costs of this drought were
an indication that many parts the country are now more vulnerable to
drought than ever before. This increased vulnerability was due in
part to farming on marginally arable lands and pumping of ground
water to the point of depletion. Although surplus grain and federal
assistance programs offset the impacts of the 1987-89 drought, these
types of assistance programs would be less feasible during a
lengthier drought.


Another Dust Bowl?

What is the likelihood of another Dust Bowl-scale drought in the
future? No one is yet able to scientifically predict multi-year or
decadal droughts, but the paleoclimatic record can tell us how
frequently droughts such as the 1930s Dust Bowl occurred in the past
or if droughts of this magnitude are indeed a rare event. If such
droughts occurred with some regularity in the past, then we should
expect them to occur in the future.

Paleoclimatology and Drought


World Wide Museum of Natural History Paleoclimatology is the study of
past climate. The word is derived from the Greek root paleo-, which
means ancient, and the term "climate" meaning the weather conditions
over an interval of time, usually several decades. Paleoclimate is
climate that existed before humans began collecting instrumental
measurements of weather (e.g., temperature from a thermometer,
precipitation from a rain gauge, sea level pressure from a barometer,
wind speed and direction from an anemometer). Instead of instrumental
measurements of weather and climate, paleoclimatologists use natural
environmental (or proxy) records to infer past climate conditions.
Paleoclimatology not only includes the collection of evidence of past
climate conditions, but the investigation of the climate processes
underlying these conditions.

For more information regarding Paleoclimatology, please visit NOAA's
Paleoclimatology Program's - Education and Outreach pages.

How do we reconstruct drought from paleoclimatic data?
Records of rainfall (or other variables that reflect drought, such as
changes in lake salinity, vegetation, or evidence of blowing sand)
are preserved in tree-rings, buried in the sediments of sand dunes
and lakes, contained within historical documents, and preserved in
archaeological remains. These recorders of climate are called proxy
climate data - that is they substitute for rain gauges and other
instrumental recorders of drought. By analyzing records taken from
these proxy sources of paleodrought data, scientists can extend our
records of drought far beyond the 100-year record provided by

To reconstruct drought or drought-related variables from
environmental proxy data, the proxy data are calibrated with the
instrumental record to determine how well the natural record
estimates the climate record. The mathematical relationship between
the proxy data and the climate record is defined, then used to
produce a model. The model is then used to reconstruct the
instrumental record from the proxy record for the length of the
proxy.Tony Caprio and the Laboratory of Tree Ring Research Archives

How does paleoclimatic data help us understand drought?
Proxy records from tree rings, lake and dune sediments, historical
records, and archaeological remains have all provided information
about past droughts in the United States. Each record provides a
piece of the puzzle, and together, they provide a more complete
history than any one proxy would.

Historical records, such as diaries and newspaper accounts, can
provide detailed information about droughts for the last two hundred
(mid-western and western U.S.) or three hundred (eastern U.S.) years.
Tree-ring records can extend back 300 years in most areas, and
thousands of years in some regions. In trees that are sensitive to
drought conditions, tree rings provide a record of drought for each
year of the tree's growth. For records longer than those provided by
trees and historical accounts (and for regions where we may not find
trees and/or historical accounts), scientists turn to sediments from
dunes and lakes.

Lake sediments, if the cores of the sediments are sampled at very
frequent intervals, can provide information about variations
occurring at frequencies less than a decade in length. Lake level
fluctuations can beCalifornia Academy of Sciences Diatom Collection
recorded as geologic bath tub rings as beach material sediments are
deposited either high (further from the center under wetter
conditions) or lower (closer to the center under drier conditions)
within a basin as the water depth and thus lake level changes in
response to drought. Droughts can increase the salinity of lakes,
changing the species of small, lake-dwelling organisms that occur
within a lake.

Pollen grains get washed or blown into lakes and accumulate in
sediments. Different types of pollen in lake sediments reflect the
vegetation around the lake and the climate conditions that are
favorable for that vegetation. So, a change in the type of pollen
found in sediments from, for example, an abundance of grass pollen to
an abundance of sage pollen, can indicate a change from wet to dry
conditions. courtesy of Eric Grimm

Records of more extreme environmental changes can be found by
investigating the layers within sand dunes. The sand layers are
interspersed among layers of soil material produced under wetter
conditions, between the times when the sand dune was active. For a
soil layer to develop, the climate needs to be wet for an extended
period of time, so these layers reflect slower, longer-lasting

Taken together, these different proxies record variations in drought
conditions on the order of single seasons to decadal and
century-scale changes, providing scientists with the information
about both rapid and slow changes, and short and long periods of
drought. These records are needed to put individual droughts in
perspective, as well as to characterize droughts of the 20th century.

The Data

Instrumental records of drought for the United States extend back
approximately 100 years. These records capture the major 20th century
droughts, but are too short to assess the reoccurrence of major
droughts such as those of the 1930s and 1950s. As droughts continue
to have increasingly costly and devastating impacts on our society,
economy and environment, it is becoming even more important to put
the severe droughts of the 20th century into a long-term perspective.
This perspective can be gained through the use of paleoclimatic
records of drought.

Scientists have developed paleoclimatic records of drought from a
variety of types of proxy data that span the past hundreds to tens of
thousands of years, and longer. These records demonstrate patterns of
natural drought variability and allow us to compare 20th century
droughts with those of the past. These records can also be examined
in light of what we know about the circulation features that are
important to drought today, such as ENSO. Research using both
paleoclimatic records of drought and circulation features can
determine how slowly changing climate conditions may influence
periods of long or more frequent droughts.

The sections below highlight some of the data and studies for four
catagories of time, beginning with the 20th century instrumental
record of drought, and ending with paleoclimatic records of drought
more than two thousand years ago. These studies have yielded much
information about climate and drought conditions of the past and
demonstrate the usefulness and importance of paleoclimate data.

  *  The Instrumental Record
    This section highlights aspects of drought concerning varying
    patterns of drought and the relationship of ENSO to patterns of
    drought. Links to other web pages on current drought, climate,
    and ENSO conditions are included as well as information about
    obtaining these records for the 20th century.
  * The Last 500 Years
    Droughts of the last four centuries are well documented in
    paleoclimatic proxies such as historical documents and tree
    rings. Spatial patterns of drought for every year since 1700 have
    been generated from a gridded network of tree-ring
    reconstructions and are featured in this section. Highlighted in
    this section are those periods with droughts that appear to have
    been more severe than any we have experienced in the 20th
  * The Last 2000 Years
    A number of tree-ring records exist for the last two millennia
    which suggest that 20th century droughts may be mild when
    evaluated in the context of this longer time frame. The evidence
    from long tree-ring records is augmented with paleodrought
    records from other proxies, such as lake sediments that reflect
    changes in salinity and precipitation/evaporation balances.
  * Even Longer Records
    Paleoclimatic data in this section document drought conditions
    back to the beginning of the Holocene (10,000 years before
    present). These records demonstrate that North America
    experienced periods of extremely dry conditions that were severe
    and sustained enough to result in the eastward expansion of
    prairie into forested areas, fluctuations in lake levels, and
    mobilization of sand dunes over large areas of the Great Plains
    which are now covered with vegetation. These changes are also
    reflected in salinity and chemistry records from sediments of
    lakes in the northern Great Plains.

A Final Word


How is the paleoclimatic record of drought relevant for
understanding or predicting drought today, or in the future?
The North American record of past drought allows us to determine what
has been the range of natural variability of drought over hundreds if
not thousands of years. This long-term perspective is important
because although severe droughts have occurred in the 20th century, a
more long-term look at past droughts, when climate conditions appear
to have been similar to today, indicates that 20th century droughts
do not represent the possible range of drought variability.

The paleoclimatic record of past droughts is a better guide than what
is provided by the instrumental record alone of what we should expect
in terms of the magnitude and duration of future droughts. For
example, paleoclimatic data suggest that droughts as severe at the
1950s drought have occurred in central North America several times a
century over the past 300-400 years, and thus we should expect (and
plan for) similar droughts in the future. The paleoclimatic record
also indicates that droughts of a much greater duration than any in
20th century have occurred in parts of North American as recently as
500 years ago. These data indicate that we should be aware of the
possibility of such droughts occurring in the future as well. The
occurrence of such sustained drought conditions today would be a
natural disaster of a magnitude unprecedented in the 20th century.

In addition to establishing a baseline of drought variability over
the long term, the paleoclimatic record of drought provides
information about drought under a range of naturally varying climate
conditions, some of which are the same as the climate of today and
some which are quite different. This paleoclimatic perspective can be
used to learn about the underlying process and characteristics of
drought under very different future climate conditions.

The impact of droughts over the last few decades have shown that some
regions and sectors of the population are becoming increasingly
vulnerable to drought. Compounding these vulnerabilities is the
uncertainty of the effects of human activities and global warming
on climate in general and on drought in particular. A number of
climate model simulations for doubled CO2 conditions suggest an
increased frequency of drought in midcontinental regions (e.g.
Gregory et al, 1997, Mearns et al, 2000 whereas other model
simulations and recent decadal trends in the instrumental record
suggest wetter conditions, at least in the short term, due to an
intensification of the hydrologic cycle associated with warmer sea
surface temperatures. Better constrained answers to the question of
the severity of future droughts requires improved understanding and
modeling of the processes underlying the drought behavior exhibited
in both the instrumental and the paleoclimate records.

What can we do to better understand past droughts and predict future

Our understanding of what causes drought conditions to persist for
years and decades is far from complete. Much work is needed to
comprehensively understand drought and the causes of drought, and to
improve drought prediction capabilities. Putting together the pieces
of past droughts through the use of paleoclimatic data is a vital
part of building this understanding and developing an improved
capacity to anticipate droughts in the future.

Focused efforts are needed to bring together paleoclimatic records of
past droughts with scientists working to better understand the
workings of the climate system. Currently, scientists are working on
this sort of focused effort for western Canada. In the Prarie Drought
Paleolimnology Project, paleoecological reconstructions will be
incorporated into novel models specifically developed for use with
long-term climatic data. The models will be used to predict drought
frequency, duration and intensity over the next 5-50 years. More such
efforts are needed to understand the drought across all of North



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