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Climate Variability and Climate ChangeWHAT IS THE DIFFERENCE?

Te summer o 2009 was one o Michigans coldest summers onrecord. In June o the same year, the U.S. Global Change ResearchProgram published a report stating: Observations show that warmingo the climate is unequivocal. How can scientists say climate ischanging i a sweatshirt is needed to stay warm on a July aternoon?

Te key to making sense o what seems like contradictory evidenceis to understand that just as the weather varies naturally, so too doesclimate. Even though scientists have no doubt that climate is warming,natural climate variability will always occur. We cannot draw conclu-sions about climate change based on one summer. Tere will be ups

and downs even i we are in the middle o a warming trend. Case inpoint: while summer 2009 was near-record cold, the summer o 2010was one o the hottest on record.

Understanding the dierence between climate variability and climatechange and how scientists study both allows us to interpret inor-mation on weather and climate and to make sense o our environment.

Climate Variability

Climate varies over seasons and years instead o day-to-day likeweather. Some summers are colder than others. Some years havemore overall precipitation. Even though people are airly perceptiveoclimate variability, it is not as noticeable as weather variabilitybecause it happens over seasons and years. Evidence includes state-

ments like: the last ew winters have seemed so short, or there seemto be more heavy downpours in recent years.

Scientists think oclimate variabilityas the way climate uctuatesyearly above or below a long-term average value. You can think o itas a story with two parts: average and range. Tese parts complementeach other; understanding the range gives context to the average andvice versa.

Average Whats Typical

Climate isnt dened by any particular timerame, however scientiststypically use average weather conditions over 30-year time intervalsto track climate. Tese 30-year averages are called climatologicalnormals, and are used to determine, monitor or represent the climate

or a specic slice o climate at a particular location. Tirty years odata is long enough to calculate an average that is not inuenced byyear-to-year variability.

Normals can be calculated or a variety o weather variables, such as

M I C H I G A N S E A G R A N T

temperature or precipitation based on data rom weather stations inthe region o interest.

Tere is signicant year-to-year variability around these 30-yearaverages. For example, in Figure 2 the normal daily maximumtemperature or February 15 in South Haven, Michigan is 34.2From 1961-1990. But or each year in that range, the daily maximumtemperature is not exactly 34.2F (e.g., in 1967 it was 50F, in 1979it was 20F). Tis year-to-year uctuation around the normal isclimate variability.

Key TermsnClimate variability Te way climate uctuates yearly above or

below a long-term average value.

nClimate change Long-term continuous change (increase ordecrease) to average weather conditions or the range o weather.

nClimatological normal 30-year average o a weather variable.

Range the Variety

Averages only tell hal the story o climate variability. Te varietyaround the average the range o weather is the other hal.When an average is calculated, the variety o the data withinis smoothed. But there is much to gain rom understandingthis variety, especially the extremes. For example, the average air

temperature in Marquette, Michigan was 42.3F in 2010. But withinthat year, Marquette residents had to deal with a high o 90F degreesand a low o -8F. I you were moving to Marquette and packedonly clothes suited or the average temperature, youd be shiveringin January and overheating in August. Studying the requency andmagnitude o extremes such as heavy downpours, cold snaps andheat waves is important because these events aect communitiesthe most.

Why Study Variability?

We use these averages and ranges to make important societaldecisions. For example, climatologal normals o precipitation andhistorical records o storm events are used to calculate probabilities

o uture rain events. Engineers can then use these data to designcommunity stormwater drainage systems.

Tese data also serve as a baseline against which to compare currentweather and climate data. Without a baseline we have no way tounderstand how current observations t into the bigger picture.

www.miseagrant.umich.edu/climate

Common drivers o climate variability include El Nio and La Nia events, which are shits o warm, tropical Pacic Ocean currents thatcan dramatically aect Michigans winters. El Nios give us milder, less snowy winters (such as the winter o 2009-2010), while La Niasgive us colder, snowier winters (such as the winter o 2007-2008). Other drivers o climate variability include volcanic eruptions andsunspots. Sometimes climate varies in ways that are random or not ully explainable.

What Causes Climate Variability?

CONTINUED ON BACK

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MICHU-11-

www.miseagrant.umich.edu/climate

Michigan Sea Grant enhances the sustainability

of Michigans coastal communities, residents, and

businesses through research, outreach and education.

For example, lets say you received a B on your latest report card. Howdo you know i this B is a sign o improvement or i your perormanceis getting worse? It depends how you did on previous report cards.Without a baseline, its impossible to tell.

Climate Change

I climate variability is year-to-year variation, what is climate change?Climate change is a long-term continuous change (increase or decrease)to average weather conditions (e.g. average temperature) OR the range

o weather (e.g. more requent and severe extreme storms). Both canalso happen simultaneously. Long-term means at least many decades.Climate change is slow and gradual, and unlike year-to-year variability,is very difcult to perceive without scientic records.

How do scientists detect climate change? Tey look or long-termcontinuous changes (trends) in climatological averages and normalsand the variety around these averages. Climate in the Great Lakesregion is generally highly variable in the short term, which makesit difcult to tease apart natural variability rom long-term change.However, looking at data since the late 1800s reveals some signicantshits in temperature, total precipitation, and extreme events in recentdecades in the Great Lakes region. Scientists use this evidence toconclude that climate is indeed changing.

Climate change occurs because o changes to Earths environment,like changes in its orbit around the sun or human modication o theatmosphere. Tere is nothing inherently wrong with climate change. Ithas happened in the past and will happen again. Te current concernstems rom the rate o change how quickly changes are happening.Scientists have ound that the current rate o temperature increase ishigher than any previously seen in the last 800,000 years. Evidencestrongly indicates that human-driven changes in the atmosphere arecontributing to the unprecedented rate o temperature increase.

Using historical weather data rom a weather station in South Haven,Michigan, a coastal community on the Lake Michigan shoreline, wecan graphically illustrate the climate change and variability conceptsusing a real lie example:

n Figure 1 shows how day-to-day weather data is averaged tobecome climate data.

n Figure 2 illustrates climatological normal, climate variability,extreme events and range.

n Figure 3 depicts long-term historical mean and climate change.

Constant Variability

Tere will always be natural climate variability at many scales decadal, yearly and short-term extreme events. Tis means that overthe long-term record, there will be ups and downs with the yearly and30-year averages, even i climate is getting warmer. We cannot expect

every summer to be warmer than the previous one, but we can expectand plan or variability.

Main conceptual Graphic:Tree panels o graphs,oriented vertically one overtop the other, tracking a single

weather point through all theother graphs. Keely will workon this

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DAILY HIGH TEMPERATURE, SOUTH HAVEN, MIJanuaryMarch 1981

DATE

DAILY: The high temperature on

February 15, 1981 was 46F.

Each daily high is weather.

MONTHLY AVERAGE: The

average daily high temperature

forFebruary 1981 was 37.4F.

Monthly averages are used to

trackclimate.

SEASONAL AVERAGE: The average

daily high temperature forWinter 1981

was 36.9F. Seasonal averages are used

to trackclimate.Feb 15, 1981

TEMPER

ATURE(F)

JANUARY FEBRUARY MARCH

510

15

20

25

30 5

10

15

20

25 5

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15

20

25

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HIGH TEMPERATURE - FEBRUARY 15, SOUTH HAVEN, MI1961-1990

YEAR

30-year

highest high

30-year

lowest high

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

The average daily high temperature for

Feb 15 from 1961-1990 was 34.2F. This is

the 30-year average, orclimatological normal,

for 1961-1990, indicated by the dashed line.

The nearly

flat trendline

indicates that

over this

30-year period,

there is nosignificant

warming or

cooling trend.

The variety of data points above and below the

climatological normal is climate variability.

The range indicates how much above or below

the year-to-year data differs from the normal.

Feb 15, 1981

TEMPERATURE(F)

18

96

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19

04

19

08

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12

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16

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20

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24

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28

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32

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36

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40

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44

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48

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52

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56

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60

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64

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68

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72

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76

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80

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84

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88

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92

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96

20

00

20

04

20

08

33.3F

(1921-1950)

33.1F

(1911-1940)

30.6F

(1901-1930)

33.9F

(1931-1960)

34.3F

(1941-1970)

34.1F

(1951-1980)

34.2F

(1961-1990)

35.3F

(1971-2000)

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20

30

40

50

60

70

HIGH TEMPERATURE - FEBRUARY 15, SOUTH HAVEN, M1896-2010

YEAR

30-year Normals

Record:

lowest

daily high

Record: highest

daily high

Thelong-term mean

since 1896 is 33.3F,

indicated by the

dashed line.

The variety of data

points above and below

the the long-tem mean

is climate variability.

The upward sloping trendline

indicates a warming trend since

1896. This is climate change.

Feb 15, 1981

TEMPERATURE(F)

ReferencesClimate Literacy: Te Essential Principles of ClimateSciences. U.S. Global Change Research Program.March 2009. Available at: http://www.climate.noaa.gov/education/pds/ClimateLiteracy-8.5x11-March09FinalHR.pd

Global Climate Change Impacts in the United States.2009. U.S. Global Change Research Program.Available at: http://globalchange.gov/publications/reports/scientic-assessments/us-impacts

All Michigan-specic data reerenced in this actsheetwas obtained rom the Midwest Regional ClimateCenter: http://mrcc.isws.illinois.edu

om Worko, Midwest Regional Climate Center.Personal Communication, 2011.

Elizabeth LaPorte, Michigan Sea Grant(734) 647-0767, elzblap@umich.edu

Contact

Author: Keely Dinse,Michigan Sea Grant