North American Flood Activity

7/29/2019 North American Flood Activity

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SPPI&CO2SCIENCE ORIGINAL PAPER June 13, 20

NORTH AMERICAN FLOOD

ACTIVITYHas global warming led to intensified flooding in North America?


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NORTH AMERICAN FLOOD ACTIVITYCitation: Center for the Study of Carbon Dioxide and Global Change. "North American Flood Activity. Last modified

June 13, 2012. http://www.co2science.org/subject/f/summaries/floodsnortham.php.

In evaluating the climate-alarmist claim that anthropogenic-induced global warming will lead tointensified flooding around the globe, it is instructive to see how flood activity has responded to

the global warming of the past century or so, much of which is claimed by climate alarmists to

be due to anthropogenic greenhouse gas emissions. In this summary, we thus review several

studies of the subject that have been conducted in North America.

Lins and Slack (1999)1

analyzed secular streamflow trends in 395 different parts of the United

States that were derived from more than 1500 individual streamgauges, some of which had

continuous data stretching all the way back to 1914. In the mean, they found that "the

conterminous U.S. is getting wetter, but less extreme." That is to say, as the near-surface air

temperature of the planet gradually rose throughout the course of the 20th century, the United

States became wetter in the mean

but less variable at the extremes,

which is where floods and droughts

occur, leading to what could well be

called the best of both worlds, i.e.,

more water with less floods.

In a similar but more regionally-

focused study, Molnar and Ramirez

(2001)2

conducted a detailed analysis

of precipitation and streamflowtrends for the period 1948-1997 in

the semiarid Rio Puerco Basin of New

Mexico. At the annual timescale,

they reported finding "a statistically

significant increasing trend in precipitation," which was driven primarily by an increase in the

number of rainy days in the moderate rainfall intensity range, with essentially no change at the

high-intensity end of the spectrum. In the case of streamflow, however, there was no trend at

the annual timescale; but monthly totals increased in low-flow months and decreased in high-

flow months, once again reducing the likelihood of both floods and droughts, courtesy of 20th-

century warming.

Think of the implications of these findings. Increased precipitation in a semiarid region is a real

plus. Having most of the increase in the moderate rainfall intensity range also sounds like a

plus. Increasing streamflow in normally low-flow months sounds good too, as does decreasing

streamflow in high-flow months. In fact, all of the observed changes in precipitation and

1http://www.co2science.org/articles/V2/N4/C5.php.


As the near-surface air temperature of the

planet gradually rose throughout the course

of the 20th century, the United States

became wetter in the mean but less variable

at the extremes, which is where floods and

droughts occur, leading to what could wellbe called the best of both worlds, i.e., more

water with less floods.
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streamflow in this study would appear to be highly desirable, leading to more water availability

but a lowered probability of both floods and droughts, which we could now well call the best of

allworlds.

Knox (2001)3

identified an analogous phenomenon in the more mesic Upper Mississippi River

Valley, but with a slight twist. Since the 1940s and early 50s, the magnitudes of the largest daily

flows in this much wetter region have been decreasing at the same time that the magnitude ofthe average daily baseflow has been increasing, once again manifesting simultaneous trends

towards both lessened flood and drought conditions.

Much the same story is told by the research ofGarbrecht and Rossel (2002)4, who studied the

nature of precipitation throughout the U.S. Great Plains over the period 1895-1999. For the

central and southern Great Plains, the last two decades of this period were found to be the

longest and wettest of the entire 105 years of record, due primarily to a reduction in the

number of dry years and an increase in the number of wet years. Once again, however, the

number ofverywet years - which would be expected to produce flooding - "did not increase as

much and even showed a decrease for many regions."

The northern and northwestern Great Plains also experienced a precipitation increase near the

end of Garbrecht and Rossel's 105-year record; but it was primarily confined to the final decade

of the 20th century. And again, as they report, "fewer dry years over the last 10 years, as

opposed to an increase in very wet years, were the leading cause of the observed wet

conditions."

In spite of the general

tendencies described in

these several papers, there

still were some significantfloods during the last

decade of the past

century, such as the 1997

flooding of the Red River

of the North, which

devastated Grand Forks,

North Dakota, as well as

parts of Canada. However,

as Haque (2000)5

reports,

although this particularflood was indeed the largest experienced by the Red River over the past century, it was not the

largest to occur in historic times. In 1852, for example, there was a slightly larger Red River

flood; and in 1826 there was a much larger flood that was nearly 40% greaterthan the flood of

1997. And the temperature of the globe, it should be noted, was much colderat the times of




In 1852 there was a slightly larger Red River flood;

and in 1826 there was a much larger flood that

was nearly 40% greater than the flood of 1997.And the temperature of the globe was much colder

at the times of these earlier catastrophic floods

than it was in 1997, indicating that one cannot

attribute the strength of the 1997 flood to the

degree of warmth experienced that year or

throughout the preceding decade.
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these earlier catastrophic floods than it was in 1997, indicating that one cannot attribute the

strength of the 1997 flood to the degree of warmth experienced that year or throughout the

preceding decade.

Analogously, Olsen et al. (1999)6

report that some upward trends in flood-flows have been

found in certain places along the Mississippi and Missouri Rivers, which is not at all surprising,

as there will always be exceptions to the general rule. At the same time, however, they notethat many of the observed upward trends were highly dependent upon the length of the data

record and when the trends began and ended. Hence, they say of these trends that they "were

not necessarily there in the past and they may not be there tomorrow."

Another study testing for long-term changes in flood magnitudes and frequencies in the

Mississippi River system was conducted by Pinter et al. (2008), who "constructed a hydrologic

database consisting of data from 26 rated stations (with both stage and discharge

measurements) and 40 stage-only stations." Then, to help "quantify changes in flood levels at

each station in response to construction of wing dikes, bendway weirs, meander cutoffs,

navigational dams, bridges, and other modifications," the researchers put together a geospatial

database consisting of "the locations, emplacement dates, and physical characteristics of over

15,000 structural features constructed along the study rivers over the past 100-150 years." And

as a result of these operations, Pinter et al. say that "significant climate- and/or land use-driven

increases in flow were

detected," but they indicate

that "the largest and most

pervasive contributors to

increased flooding on the

Mississippi River system

were wing dikes and related

navigational structures,followed by progressive

levee construction."

In discussing the implications

of their findings, Pinter et al.

write that "the navigable

rivers of the Mississippi

system have been intensively

engineered, and some of

these modifications areassociated with large decreases in the rivers' capacity to convey flood flows." Given such

findings, it would appear that man may indeed have been responsible for the majority of the

enhanced flooding of the rivers of the Mississippi system over the past century or so, but not in

the way suggested by the world's climate alarmists. The question that needs addressing by the

region's inhabitants, therefore, has nothing to do with CO2, but everything to do with how to


Pinteret al. write that the navigable rivers of the

Mississippi system have been intensively engineered,

and some of these modifications are associated

with large decreases in the rivers' capacity toconvey flood flows. Given such findings, it would

appear that man may indeed have been responsible

for the majority of the enhanced flooding of the

rivers of the Mississippi system over the past

century or so, but not in the way suggested by the

world's climate alarmists.
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"balance the local benefits of river engineering against the potential for large-scale flood

magnification."

Similar findings have been reported for the Upper Midwest (consisting of North Dakota, South

Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin and Illinois) byVillarini et al.

(2011)7, who "analyzed the annual maximum instantaneous flood peak distributions for 196

U.S. Geological Survey streamflow stations with a record of at least 75 years over the MidwestU.S." According to the four U.S. researchers who conducted this study, in the vast majority of

cases where streamflow changes were observed, they were "associated with change-points

(both in mean and variance) rather than monotonic trends," and they indicated that "these

non-stationarities are often associated with anthropogenic effects." But rather than associate

the increases with anthropogenic CO2 emissions, they cite such things as "changes in land

use/land cover, changes in

agricultural practice, and

construction of dams and

reservoirs" as the primary

cause(s). As a result, and, as

they note, "in agreement with

previous studies (Olsen et al.,

1999; Villarini et al., 2009),"

they conclude that "there is little indication that anthropogenic climate change has significantly

affected the flood frequency distribution for the Midwest U.S." And as they make doubly clear

in the abstract of their paper, they say that "trend analyses do not suggest an increase in the

flood peak distribution due to anthropogenic climate change."

Moving across the continent, Villarini and Smith (2010) "examined the distribution of flood

peaks for the eastern United States using annual maximum flood peak records from 572 U.S.

Geological Survey stream gaging stations with at least 75 years of observations." This workrevealed that, "in general, the largest flood magnitudes are concentrated in the mountainous

central Appalachians and the smallest flood peaks are concentrated along the low-gradient

Coastal Plain and in the northeastern United States." They also found that "landfalling tropical

cyclones play an important role in the mixture of flood generating mechanisms, with the

frequency of tropical cyclone floods exhibiting large spatial heterogeneity over the region." And

they additionally write that "warm season thunderstorm systems during the peak of the warm

season and winter-spring extratropical systems contribute in complex fashion to the spatial

mixture of flood frequency over the eastern United States."

Of even greater interest to the climate change debate, however, were their more basic findingsthat (1) "only a small fraction of stations exhibited significant linear trends," that (2) "for those

stations with trends, there was a split between increasing and decreasing trends," and that (3)

"no spatial structure was found for stations exhibiting trends." Thus, they concluded, most

importantly of all, that (4) "there is little indication that human-induced climate change has

resulted in increasing flood magnitudes for the eastern United States," providing no support for


There is little indication that anthropogenic

climate change has significantly affected the

flood frequency distribution for the Midwest U.S.
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the claim that global warming will lead to more frequent, more widespread and more serious

floods.

Much the same was reported for Canada by Cunderlik and Ouarda (2009), who evaluated

trends in the timing and magnitude of seasonal maximum flood events across that country,

based on pertinent data obtained from 162 stations of the Reference Hydrometric Basin

Network established by Environment Canada over the 30-year period 1974 to 2003. In spite ofthe supposedly unprecedented warming experienced over the period of time they studied, the

Canadian researchers report finding that "only 10% of the analyzed stations show significant

trends in the timing of snowmelt floods during the last three decades (1974-2003)," and they

say these results imply that "the occurrence of snowmelt floods is shifting towards the earlier

times of the year," as would be expected in a warming world. However, they note that mostof

the identified trends "are only weakly or medium significant results," and they add that "no

significant trends were found in the timing of rainfall-dominated flood events."

With respect to flood magnitudes, the two scientists state that the trends they observed "are

much more pronounced than the trends in the timing of the floods," but they say that most of

these trends "had negative signs, suggesting that the magnitude of the annual maximum floods

has been decreasing over the last three decades." In addition, they found that "the level of

significance was also higher in these trends compared to the level of significance of the trends

in the timing of annual maximum floods."

Expanding the temporal scope of the subject somewhat, there are a number of studies that

have examined floods over much longer intervals of time. Wolfe et al. (2005)8, for example,

conducted a multi-proxy hydro-ecological analysis of Spruce Island Lake (5851'N, 11129'W), a

shallow, isolated, upland lake in a bedrock basin located in the northern Peace sector of the

Peace-Athabasca Delta in northern Alberta, Canada, in an attempt to assess the impacts of both

natural variability and anthropogenic change on the hydro-ecology of the region over the past300 years. Specifically, their research was designed to answer the following three questions: (1)

Have hydro-ecological conditions in Spruce Island Lake since 1968 (the year in which river flow

became regulated from hydroelectric power generation at the headwaters of the Peace River)

varied beyond the range of natural variation of the past 300 years? (2) Is there evidence that

flow regulation of the Peace River has caused significant changes in hydro-ecological conditions

in Spruce Island Lake? (3) How is hydro-ecological variability at Spruce Island Lake related to

natural climatic variability and Peace River flood history?

Wolfe et al.'s research efforts revealed that hydro-ecological conditions varied substantially

over the past 300 years, especially in terms of multi-decadal dry and wet periods. With respectto the three research questions posed above, for example, the authors found for question #1

that hydro-ecological conditions after 1968 have remained well within the broad range of

natural variability observed over the past 300 years, with both "markedly wetter and drier

conditions compared to recent decades" having occurred prior to the time of Peace River flow

regulation. With respect to question #2, they note that the current drying trend is not the

product of Peace River flow regulation, but rather the product of an extended drying period

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that was initiated in the early to mid-1900s. Lastly, with respect to question #3, Wolfe et al.

showed that the multi-proxy hydro-ecological variables they analyzed were well correlated with

other reconstructed records of natural climate variability, indicating a likely climatic influence

on Spruce Island Lake hydro-ecological conditions over the period of record.

It is important to note that there is nothing unusual about recent trends in the hydro-ecology of

the Spruce Island Lake region. Wet and dry conditions of today fall well within the range ofnatural variability and show no fingerprint of anthropogenic global warming. What is more,

they even bear no fingerprint of anthropogenic flow control on the Peace River since 1968,

demonstrating, in the words of

the authors, that "profound

changes in hydro-ecological

conditions are clearly a natural

feature of this ecosystem,

independent of human

influence or intervention."

Moving southward, Shapley et

al. (2005)9

developed a 1000-

year hydroclimate

reconstruction from local bur oak (Quercus macrocarpa) tree-ring records and lake sediment

cores from the Waubay Lake complex located in eastern South Dakota. During the 1990s, broad

areas of the US Northern Great Plains experienced notable lake highstands, including Waubay

Lake, which rose by 5.7 meters and more than doubled in area from 1993 to 1999, severely

flooding roads, farms and towns, and prompting the Federal Emergency Management Agency

to declare the region a disaster area on 1 June 1998. In their paper, therefore, Shapley et al. set

out to determine the historical context of that 1990s lake-level rise.

In doing so, the researchers found that "prior to AD 1800, both lake highstands and droughts

tended towards greater persistence than during the past two centuries," such that "neither

generally low lake levels occurring since European settlement (but before the recent flooding)

nor the post-1930s pattern of steadily increasing water availability and favorableness for tree

growth are typical of the long-term record." In this particular part of the world, therefore, it is

clear that longer-lasting floods and droughts of equal or greater magnitude than those of

modern times occurred repeatedly prior to 1800.

In another study,Fye et al. (2003)10

developed multi-century reconstructions of summer (June-

August) Palmer Drought Severity Index over the continental United States from annual proxiesof moisture status provided by 426 climatically-sensitive tree-ring chronologies. This exercise

indicated that the greatest 20th-century wetness anomaly across the United States was the 13-

year pluvial that occurred in the early part of the century, when it was considerably colder than

it is now. In addition, Fye et al.'s analysis revealed the existence of a 16-yearpluvial from 1825



There is nothing unusual about recent trends in

the hydro-ecology of the Spruce Island Lake

region. Wet and dry conditions of today fall well

within the range of natural variability and show

no fingerprint of anthropogenic global warming.
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to 1840 and a prolonged 21-year wet period from 1602 to 1622, both of which anomalies

occurred during the Little Ice Age, when, of course, it was colder still.

St. George and Nielsen (2002)11

likewise used "a ringwidth chronology developed from living,

historical and subfossil bur oak (Quercus macrocarpa (Michx.)) in the Red River basin to

reconstruct annual precipitation in southern Manitoba since A.D. 1409." Their analysis

indicated, in their words, that "prior to the 20th century, southern Manitoba's climate wasmore extreme and variable, with prolonged intervals that were wetter and drier than any time

following permanent Euro-Canadian settlement."

Also working with tree-ring chronologies, Ni et al. (2002)12

developed a 1000-year history of

cool-season (November-April) precipitation for each climate division in Arizona and New

Mexico, USA. In doing so, they found that several wet periods comparable to the wet conditions

seen in the early 1900s and post-1976 occurred in 1108-20, 1195-1204, 1330-45 (which they

denominate "the most persistent and extreme wet interval"), the 1610s, and the early 1800s,

all of which wet periods are embedded in the long cold expanse of the Little Ice Age.

In a study designed to determine the environmental origins of extreme flooding events

throughout the southwestern United States, Ely (1997) wrote that "paleoflood records from

nineteen rivers in Arizona and southern Utah, including over 150 radiocarbon dates and

evidence of over 250 flood deposits, were combined to identify regional variations in the

frequency of extreme floods," which information "was then compared with paleoclimatic data

to determine how the temporal and spatial patterns in the occurrence of floods reflect the

prevailing climate." The results of this comparison indicated that "long-term variations in the

frequency of extreme floods over the Holocene are related to changes in the climate and

prevailing large-scale atmospheric circulation patterns that affect the conditions conducive to

extreme flood-generating storms in each region," which changes, in Ely's view, "are very

plausibly related to global-scale changes in the climate system."

With respect to the Colorado River watershed, for example, which integrates a large portion of

the interior western United States, she writes that "the largest floods tend to be from spring

snowmelt after winters of heavy snow accumulation in the mountains of Utah, western

Colorado, and northern New Mexico," such as occurred with the "cluster of floods from 5 to 3.6

ka," which occurred in conjunction with "glacial advances in mountain ranges throughout the

western United States" during the "cool, wet period immediately following the warm mid-

Holocene."

The frequency of extreme floods also increased during the early and middle portions of the firstmillennium AD, many of which coincided "with glacial advances and cool, moist conditions both

in the western U.S. and globally." Then came a "sharp drop in the frequency of large floods in

the southwest from AD 1100-1300," which corresponded, in her words, "to the widespread

Medieval Warm Period, which was first noted in European historical records." With the advent

of the Little Ice Age, however, there was another "substantial jump in the number of floods in


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the southwestern U.S.," which was "associated with a switch to glacial advances, high lake

levels, and cooler, wetter conditions." And in distilling her findings down to a single succinct

statement, and speaking specifically of the southwestern United States, Ely states that "global

warm periods, such as the Medieval Warm Period, are times of dramatic decreases in the

number of high-magnitude floods in this region."

In another investigation,Schimmelmann et al. (2003)13

analyzed gray clay-rich flood deposits inthe predominantly olive varved sediments of the Santa Barbara Basin off the coast of California,

USA, which they accurately dated by varve-counting. Their analysis indicated that six prominent

flood events occurred at approximately AD 212, 440, 603, 1029, 1418 and 1605, "suggesting," in

their words, "a quasi-periodicity of ~200 years," with "skipped" flooding just after AD 800, 1200

and 1800. They further note that "the floods of ~AD 1029 and 1605 seem to have been

associated with brief cold spells," that "the flood of ~AD 440 dates to the onset of the most

unstable marine climatic interval of the Holocene (Kennett and Kennett, 2000)," and that "the

flood of ~AD 1418 occurred at a time when the global atmospheric circulation pattern

underwent fundamental reorganization at the beginning of the 'Little Ice Age' (Kreutz et al.,

1997; Meeker and Mayewski, 2002)." As a result, they hypothesize that "solar-modulated

climatic background conditions are opening a ~40-year window of opportunity for flooding

every ~200 years," and that "during each window, the danger of flooding is exacerbated by

additional climatic and environmental cofactors." They also note that "extrapolation of the

~200-year spacing of floods into the future raises the uncomfortable possibility for historically

unprecedented flooding in southern California during the first half of this century."

Consequently, if such flooding does occur in the near future, there will be no need to suppose it

came as a consequence of what climate alarmists call the unprecedented warming of the past

century, although they will surely claim it did.

In doubling the length of time investigated in the prior study, Campbell (2002)14

analyzed the

grain sizes of sediment cores obtained from Pine Lake, Alberta, Canada, to provide a non-vegetation-based high-resolution record of streamflow variability for this part of North America

over the past 4000 years. This work revealed that the highest rates of stream discharge during

this period occurred during the Little Ice Age, approximately 300-350 years ago, at which time

grain sizes were about 2.5 standard deviations above the 4000-year mean. In contrast, the

lowest rates of streamflow were observed around AD 1100, during the Medieval Warm Period,

when median grain sizes were nearly 2.0 standard deviations belowthe 4000-year mean.

Focusing on the northern Uinta Mountains of northeastern Utah, Carson et al. (2007)15

developed a Holocene history of flood magnitudes from reconstructed cross-sectional areas of

abandoned channels and relationships relating channel cross-sections to flood magnitudesderived from modern stream gage and channel records. Of most interest to the present subject,

Carson et al. report that over the past 5,000 years, the record of bankfull discharge

"corresponds well with independent paleoclimate data for the Uinta Mountains," and that

"during this period, the magnitude of the modal flood is smaller than modern during warm dry



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intervals and greater than modern during cool wet intervals," noting most particularly that "the

decrease in flood magnitudes following 1000 cal yr B.P. corresponds to numerous local and

regional records of warming during the Medieval Climatic Anomaly."

Based upon the three researchers' graphical results, the three largest negative departures from

modern bankfull flood magnitudes (indicating greater than modern warmth) range from

approximately 15-22%, as best as can be determined from visual inspection of their plotteddata; and they occur between about 750 and 600 cal yr B.P., as determined from radiocarbon

dating of basal channel-fill sediments.

In addition to demonstrating that the degree of natural variability in northeastern Utah flood

magnitudes throughout the Holocene has been much larger (in both positive and negative

directions) than what has been observed in modern times (which demonstrates that possible

future occurrences of greater- or smaller-than-modern floods in the region ought not be

regarded as "unprecedented," as climate alarmists are typically prone to claim), Carson et al.'s

findings demonstrate that the portion of the Medieval Warm Period between about AD 1250

and 1400 was likely significantly warmer than it is at present (which demonstrates that since

something other than high concentrations of atmospheric CO2 was responsible for the region's

earlier greater-than-present warmth, one need not invoke today's much higher CO2

concentrations as the reason for our actually lowercurrent temperatures).

Further extending the temporal scope of review, Brown et al. (1999)16

analyzed various

properties of cored sequences of hemipelagic muds deposited in the northern Gulf of Mexico

for evidence of variations in Mississippi River outflow over the past 5300 years. This group of

researchers found evidence of seven large megafloods, which they describe as "almost certainly

larger than historical floods in the Mississippi watershed." In fact, they say these fluvial events

were likely "episodes of multidecadal duration," five of which occurred during cold periods

similar to the Little Ice Age.

Last of all, in a study that covered

essentially the entire Holocene,

Noren et al. (2002)17

employed

several techniques to identify and

date terrigenous in-wash layers

found in sediment cores extracted

from thirteen small lakes

distributed across a 20,000-km2

region in Vermont and eastern New York that depict the frequency of storm-related floods.Their results indicated, in their words, that "the frequency of storm-related floods in the

northeastern United States has varied in regular cycles during the past 13,000 years (13 kyr),

with a characteristic period of about 3 kyr." Specifically, they found there were four major

peaks in the data during this period, with the most recent upswing in storm-related floods

beginning "at about 600 yr BP [Before Present], coincident with the beginning of the Little Ice



Taken together, the research described

suggests that, if anything, North American

flooding tends to become both less frequent

and lesssevere when the planet warms.
http://www.co2science.org/articles/V2/N23/C3.phphttp://www.co2science.org/articles/V2/N23/C3.phphttp://www.co2science.org/articles/V2/N23/C3.phphttp://www.co2science.org/articles/V2/N23/C3.phphttp://www.co2science.org/articles/V5/N47/C1.phphttp://www.co2science.org/articles/V5/N47/C1.phphttp://www.co2science.org/articles/V5/N47/C1.phphttp://www.co2science.org/articles/V5/N47/C1.phphttp://www.co2science.org/articles/V5/N47/C1.phphttp://www.co2science.org/articles/V2/N23/C3.php


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Age." In addition, they note that several "independent records of storminess and flooding from

around the North Atlantic show maxima that correspond to those that characterize our lake

records [Brown et al., 1999; Knox, 1999; Lamb, 1979; Liu and Fearn, 2000; Zong and Tooley,

1999]."

Taken together, the research described above suggests that, if anything, North American

flooding tends to become both less frequent and less severe when the planet warms, althoughthere have been some exceptions to this general rule. And although there could also be

exceptions to this rule in the future, it is more likely than not that any further warming of the

globe would tend to further reduce both the frequency and severity of flooding in North

America, which is just the opposite of what the world's climate alarmists claim would occur

under such conditions.

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Cover photo of a river flowing through the Grand Teton

National Park in Wyoming, USA provided by Microsoft.

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North American Flood Activity