Temperature Changes in the United States

185

Temperature Changes in the

United States

Climate Science Special ReportU.S. Global Change Research Program

KEY FINDINGS

1. Annual average temperature over the contiguous United States has increased by 1.2°F (0.7°C) for the

period 1986–2016 relative to 1901–1960 and by 1.8°F (1.0°C) based on a linear regression for the peri-

od 1895–2016 (very high confidence). Surface and satellite data are consistent in their depiction of rapid

warming since 1979 (high confidence). Paleo-temperature evidence shows that recent decades are the

warmest of the past 1,500 years (medium confidence).

2. There have been marked changes in temperature extremes across the contiguous United States. The

frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has in-

creased since the mid-1960s. The Dust Bowl era of the 1930s remains the peak period for extreme heat.

The number of high temperature records set in the past two decades far exceeds the number of low

temperature records. (Very high confidence)

3. Annual average temperature over the contiguous United States is projected to rise (very high confi-

dence). Increases of about 2.5°F (1.4°C) are projected for the period 2021–2050 relative to 1976–2005 in

all RCP scenarios, implying recent record-setting years may be “common” in the next few decades

(high confidence). Much larger rises are projected by late century (2071–2100): 2.8°–7.3°F (1.6°–4.1°C) in a

lower scenario (RCP4.5) and 5.8°–11.9°F (3.2°–6.6°C) in the higher scenario (RCP8.5) (high confidence).

4. Extreme temperatures in the contiguous United States are projected to increase even more than aver-

age temperatures. The temperatures of extremely cold days and extremely warm days are both expect-

ed to increase. Cold waves are projected to become less intense while heat waves will become more

intense. The number of days below freezing is projected to decline while the number above 90°F will

rise. (Very high confidence)

Recommended Citation for Chapter

Vose, R.S., D.R. Easterling, K.E. Kunkel, A.N. LeGrande, and M.F. Wehner, 2017: Temperature changes in

the United States. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles,

D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change

Research Program, Washington, DC, USA, pp. 185-206, doi: 10.7930/J0N29V45.

6 | Temperature Changes in the United States

186 Climate Science Special ReportU.S. Global Change Research Program

Introduction

Temperature is among the most important

climatic elements used in decision-making.

For example, builders and insurers use tem-

perature data for planning and risk manage-

ment while energy companies and regulators

use temperature data to predict demand and

set utility rates. Temperature is also a key

indicator of climate change: recent increases

are apparent over the land, ocean, and tropo-

sphere, and substantial changes are expected

for this century. This chapter summarizes

the major observed and projected changes in

near-surface air temperature over the United

States, emphasizing new data sets and model

projections since the Third National Climate

Assessment (NCA3). Changes are depicted

using a spectrum of observations, including

surface weather stations, moored ocean buoys,

polar-orbiting satellites, and temperature-sen-

sitive proxies. Projections are based on global

models and downscaled products from CMIP5

(Coupled Model Intercomparison Project

Phase 5) using a suite of Representative Con-

centration Pathways (RCPs; see Ch. 4: Projec-

tions for more on RCPs and future scenarios).

6.1 Historical Changes

6.1.1 Average Temperatures

Changes in average temperature are described

using a suite of observational datasets. As

in NCA3, changes in land temperature are

assessed using the nClimGrid dataset.

1, 2

Along U.S. coastlines, changes in sea surface

temperatures are quantified using a new re-

construction

that forms the ocean component

of the NOAA Global Temperature dataset.

Changes in middle tropospheric temperature

are examined using updated versions of mul-

tiple satellite datasets.

5, 6, 7

The annual average temperature of the con-

tiguous United States has risen since the start

of the 20th century. In general, temperature

increased until about 1940, decreased until

about 1970, and increased rapidly through

2016. Because the increase was not constant

over time, multiple methods were evaluated

in this report (as in NCA3) to quantify the

trend. All methods yielded rates of warming

that were significant at the 95% level. The low-

est estimate of 1.2°F (0.7°C) was obtained by

computing the difference between the average

for 1986–2016 (i.e., present-day) and the aver-

age for 1901–1960 (i.e., the first half of the last

century). The highest estimate of 1.8°F (1.0°C)

was obtained by fitting a linear (least-squares)

regression line through the period 1895–2016.

Thus, the temperature increase cited in this

assessment is 1.2°–1.8°F (0.7°–1.0°C).

This increase is about 0.1°F (0.06°C) less than

presented in NCA3, and it results from the

use of slightly different periods in each report.

In particular, the decline in the lower bound

stems from the use of different time periods

to represent present-day climate (NCA3 used

1991–2012, which was slightly warmer than

the 1986–2016 period used here). The decline

in the upper bound stems mainly from tem-

perature differences late in the record (e.g., the

last year of data available for NCA3 was 2012,

which was the warmest year on record for the

contiguous United States).

Each NCA region experienced a net warming

through 2016 (Table 6.1). The largest chang-

es were in the western United States, where

average temperature increased by more than

1.5°F (0.8°C) in Alaska, the Northwest, the

Southwest, and also in the Northern Great

Plains. As noted in NCA3, the Southeast had

the least warming, driven by a combination of

natural variations and human influences.

most regions, average minimum temperature

increased at a slightly higher rate than average

maximum temperature, with the Midwest hav-

ing the largest discrepancy, and the Southwest

and Northwest having the smallest. This differ-

ential rate of warming resulted in a continuing

6 | Temperature Changes in the United States

187 Climate Science Special ReportU.S. Global Change Research Program

Table 6.1. Observed changes in annual average temperature (°F) for each

Naonal Climate Assessment region. Changes are the dierence between

the average for present-day (1986–2016) and the average for the rst half of

the last century (1901–1960 for the conguous United States, 1925–1960 for

Alaska, Hawai‘i, and the Caribbean). Esmates are derived from the nClimDiv

dataset

1,2

NCA Region

Change in Annual

Average

Temperature

Change in Annual

Average

Maximum

Temperature

Change in Annual

Average Minimum

Temperature

Conguous U.S. 1.23°F 1.06°F 1.41°F

Northeast 1.43°F 1.16°F 1.70°F

Southeast 0.46°F 0.16°F 0.76°F

Midwest 1.26°F 0.77°F 1.75°F

Great Plains North 1.69°F 1.66°F 1.72°F

Great Plains South 0.76°F 0.56°F 0.96°F

Southwest 1.61°F 1.61°F 1.61°F

Northwest 1.54°F 1.52°F 1.56°F

Alaska 1.67°F 1.43°F 1.91°F

Hawaii 1.26°F 1.01°F 1.49°F

Caribbean 1.35°F 1.08°F 1.60°F

decrease in the diurnal temperature range that

is consistent with other parts of the globe.

Annual average sea surface temperature also

increased along all regional coastlines (see Fig-

ure 1.3), though changes were generally smaller

than over land owing to the higher heat capac-

ity of water. Increases were largest in Alaska

(greater than 1.0°F [0.6°C]) while increases were

smallest (less than 0.5°F [0.3°C]) in coastal areas

of the Southeast.

More than 95% of the land surface of the

contiguous United States had an increase in

annual average temperature (Figure 6.1). In

contrast, only small (and somewhat dispersed)

parts of the Southeast and Southern Great

Plains experienced cooling. From a seasonal

perspective, warming was greatest and most

widespread in winter, with increases of over

1.5°F (0.8°C) in most areas. In summer, warm-

ing was less extensive (mainly along the East

Coast and in the western third of the Nation),

while cooling was evident in parts of the

Southeast, Midwest, and Great Plains.

There has been a rapid increase in the aver-

age temperature of the contiguous United

States over the past several decades. There

is general consistency on this point between

the surface thermometer record from NOAA

and the middle tropospheric satellite re-

cords from Remote Sensing Systems (RSS),

NOAA’s Center for Satellite Applications

and Research (STAR),

and the University of

Alabama in Huntsville (UAH).

In particular,

for the period 1979–2016, the rate of warming

in the surface record was 0.512°F (0.284°C)

per decade, versus trends of 0.455°F (0.253°C),

0.421°F (0.234°C), and 0.289°F (0.160 °C) per

decade for RSS version 4, STAR version 3, and

UAH version 6, respectively (after accounting

for stratospheric influences). All trends are

statistically significant at the 95% level. For the

contiguous United States, the year 2016 was

the second-warmest on record at the surface

and in the middle troposphere (2012 was the

warmest year at the surface, and 2015 was the

warmest in the middle troposphere). Gener-

ally speaking, surface and satellite records

6 | Temperature Changes in the United States

188 Climate Science Special ReportU.S. Global Change Research Program

do not have identical trends because they

do not represent the same physical quantity;

surface measurements are made using ther-

mometers in shelters about 1.5 meters above

the ground whereas satellite measurements

are mass-weighted averages of microwave

emissions from deep atmospheric layers. The

UAH record likely has a lower trend because

it differs from the other satellite products in

the treatment of target temperatures from the

NOAA-9 satellite as well as in the correction

for diurnal drift.

Recent paleo-temperature evidence confirms

the unusual character of wide-scale warming

during the past few decades as determined

from the instrumental record. The most im-

portant new paleoclimate study since NCA3

showed that for each of the seven continen-

tal regions, the reconstructed area-weighted

average temperature for 1971–2000 was higher

than for any other time in nearly 1,400 years,

although with significant uncertainty around

the central estimate that leads to this conclu-

sion. Recent (up to 2006) 30-year smoothed

temperatures across temperate North Amer-

ica (including most of the continental Unit-

ed States) are similarly reconstructed as the

warmest over the past 1,500 years

(Figure

6.2). Unlike the PAGES 2k seven-continent

result mentioned above, this conclusion for

North America is robust in relation to the

estimated uncertainty range. Reconstruction

data since 1500 for western temperate North

America show the same conclusion at the an-

nual time scale for 1986–2005. This time period

and the running 20-year periods thereafter are

warmer than all possible continuous 20-year

sequences in a 1,000-member statistical recon-

struction ensemble.

Figure 6.1. Observed changes in annual, winter, and summer temperature (°F). Changes are the difference between

the average for present-day (1986–2016) and the average for the rst half of the last century (1901–1960 for the con-

tiguous United States, 1925–1960 for Alaska and Hawai‘i). Estimates are derived from the nClimDiv dataset.

1, 2

(Figure

source: NOAA/NCEI).

6 | Temperature Changes in the United States

189 Climate Science Special ReportU.S. Global Change Research Program

6.1.2 Temperature Extremes

Shifts in temperature extremes are examined using

a suite of societally relevant climate change in-

dices

14, 15

derived from long-term observations

of daily surface temperature.

The coldest and

warmest temperatures of the year are of par-

ticular relevance given their widespread use

in engineering, agricultural, and other sectoral

applications (for example, extreme annual

design conditions by the American Society of

Heating, Refrigeration, and Air Conditioning;

plant hardiness zones by the U.S. Department

of Agriculture). Cold waves and heat waves

(that is, extended periods of below or above

normal temperature) are likewise of great

importance because of their numerous societal

and environmental impacts, which span from

human health to plant and animal phenol-

ogy. Changes are considered for a spectrum

of event frequencies and intensities, ranging

from the typical annual extreme to the 1-in-10

year event (an extreme that only has a 10%

chance of occurrence in any given year). The

discussion focuses on the contiguous United

States; Alaska, Hawai‘i, and the Caribbean

do not have a sufficient number of long-term

stations for a century-scale analysis.

Cold extremes have become less severe over

the past century. For example, the coldest

daily temperature of the year has increased

at most locations in the contiguous United

States (Figure 6.3). All regions experienced net

increases (Table 6.2), with the largest rises in

the Northern Great Plains and the Northwest

(roughly 4.5°F [2.5°C]), and the smallest in

the Southeast (about 1.0°F [0.6°C]). In general,

there were increases throughout the record,

with a slight acceleration in recent decades

(Figure 6.3). The temperature of extreme-

ly cold days (1-in-10 year events) generally

exhibited the same pattern of increases as the

coldest daily temperature of the year. Con-

sistent with these increases, the number of

cool nights per year (those with a minimum

temperature below the 10th percentile for

1961–1990) declined in all regions, with much

of the West having decreases of roughly two

weeks. The frequency of cold waves (6-day pe-

riods with a minimum temperature below the

Figure 6.2. Pollen-based temperature reconstruction for temperate North America. The blue curve depicts the pol-

len-based reconstruction of 30-year averages (as anomalies from 1904 to 1980) for the temperate region (30°–55°N,

75°–130°W). The red curve shows the corresponding tree ring-based decadal average reconstruction, which was

smoothed and used to calibrate the lower-frequency pollen-based estimate. Light (medium) blue zones indicate 2

standard error (1 standard error) uncertainty estimations associated with each 30-year value. The black curve shows

comparably smoothed instrumental temperature values up to 1980. The dashed black line represents the average tem-

perature anomaly of comparably smoothed instrumental data for the period 2000–2006. (Figure source: NOAA NCEI).

Anomalies (°C)

(wrt 1904–1980 average)

480 600 750 900 1050 1200

Year

1350 1500 1650 1800

1950

−0.6

−0.9

−0.3

0.0

0.3

0.6

pollen D1200 Instr 1SE 2 SE 1904–1980 mean 2000–2006 mean

6 | Temperature Changes in the United States

190 Climate Science Special ReportU.S. Global Change Research Program

10th percentile for 1961–1990) has fallen over

the past century (Figure 6.4). The frequency of

intense cold waves (4-day, 1-in-5 year events)

peaked in the 1980s and then reached re-

cord-low levels in the 2000s.

Changes in warm extremes are more nuanced

than changes in cold extremes. For instance, the

warmest daily temperature of the year increased

in some parts of the West over the past century

(Figure 6.3), but there were decreases in almost

all locations east of the Rocky Mountains. In fact,

all eastern regions experienced a net decrease

(Table 6.2), most notably the Midwest (about

2.2°F [1.2°C]) and the Southeast (roughly 1.5°F

[0.8°C]). The decreases in the eastern half of Na-

tion, particularly in the Great Plains, are mainly

tied to the unprecedented summer heat of the

1930s Dust Bowl era, which was exacerbated

by land-surface feedbacks driven by springtime

Figure 6.3. Observed changes in the coldest and warmest daily temperatures (°F) of the year in the contiguous United

States. Maps (top) depict changes at stations; changes are the difference between the average for present-day (1986–

2016) and the average for the rst half of the last century (1901–1960). Time series (bottom) depict the area-weighted

average for the contiguous United States. Estimates are derived from long-term stations with minimal missing data in

the Global Historical Climatology Network–Daily dataset.

(Figure source: NOAA/NCEI).

Change in Coldest Temperature of the Year

1986–2016 Average Minus 1901–1960 Average

Change in Warmest Temperature of the Year

1986–2016 Average Minus 1901–1960 Average

−6 to −4

−4 to −2

−2 to 0

0 to 2

2 to 4

4 to 6

<−6

Difference (°F)

−6 to −4

−4 to −2

−2 to 0

0 to 2

2 to 4

4 to 6

<−6

Difference (°F)

Coldest Temperature (°F)

Warmest Temperature (°F)

−12

1900 1920 1940 1960 1980 2000 2020 1900 1920 1940 1960 1980 2000 2020

−8

−4

100

102

104

Table 6.2. Observed changes in the coldest and

warmest daily temperatures (°F) of the year for each

Naonal Climate Assessment region in the conguous

United States. Changes are the dierence between

the average for present-day (1986–2016) and the av-

erage for the rst half of the last century (1901–1960).

Esmates are derived from long-term staons with

minimal missing data in the Global Historical Clima-

tology Network–Daily dataset.

NCA Region

Change in Coldest

Day of the Year

Change in

Warmest Day

of the Year

Northeast 2.83°F −0.92°F

Southeast 1.13°F −1.49°F

Midwest 2.93°F −2.22°F

Great Plains

North

4.40°F −1.08°F

Great Plains

South

3.25°F −1.07°F

Southwest 3.99°F 0.50°F

Northwest 4.78°F −0.17°F

6 | Temperature Changes in the United States

191 Climate Science Special ReportU.S. Global Change Research Program

precipitation deficits and land mismanagement.

However, anthropogenic aerosol forcing may

also have reduced summer temperatures in the

Northeast and Southeast from the early 1950s to

the mid-1970s,

and agricultural intensification

may have suppressed the hottest extremes in the

Midwest.

Since the mid-1960s, there has been

only a very slight increase in the warmest daily

temperature of the year (amidst large interannual

variability). Heat waves (6-day periods with a

maximum temperature above the 90th percen-

tile for 1961–1990) increased in frequency until

the mid-1930s, became considerably less com-

mon through the mid-1960s, and increased in

frequency again thereafter (Figure 6.4). As with

warm daily temperatures, heat wave magnitude

reached a maximum in the 1930s. The frequency

of intense heat waves (4-day, 1-in-5 year events)

has generally increased since the 1960s in most

regions except the Midwest and the Great

Figure 6.4. Observed changes in cold and heat waves in the contiguous United States. The top panel depicts changes

in the frequency of cold waves; the middle panel depicts changes in the frequency of heat waves; and the bottom panel

depicts changes in the intensity of heat waves. Cold and heat wave frequency indices are dened in Zhang et al.,

and

the heat wave intensity index is dened in Russo et al.

Estimates are derived from long-term stations with minimal

missing data in the Global Historical Climatology Network–Daily dataset.

(Figure source: NOAA/NCEI).

Cold Spells (Days)Warm Spells (Days)

Heat Wave Magnitude

Index daily

1900 1920 1940 1960 1980 2000

2020

1900 1920 1940 1960 1980 2000

2020

1900 1920 1940 1960 1980 2000 2020

6 | Temperature Changes in the United States

192 Climate Science Special ReportU.S. Global Change Research Program

Plains.

17, 21

Since the early 1980s (Figure 6.4), there

is suggestive evidence of a slight increase in the

intensity of heat waves nationwide

as well as

an increase in the concurrence of droughts and

heat waves.

Changes in the occurrence of record-setting

daily temperatures are also apparent. Very

generally, the number of record lows has

been declining since the late-1970s while the

number of record highs has been rising.

extension, there has been an increase in the

ratio of the number of record highs to record

lows (Figure 6.5). Over the past two decades,

the average of this ratio exceeds two (meaning

that twice as many high-temperature records

have been set as low-temperature records).

The number of new highs has surpassed the

number of new lows in 15 of the last 20 years,

with 2012 and 2016 being particularly extreme

(ratios of seven and five, respectively).

6.2 Detection and Attribution

6.2.1 Average Temperatures

While a confident attribution of global tempera-

ture increases to anthropogenic forcing has been

made,

detection and attribution assessment

statements for smaller regions are generally

much weaker. Nevertheless, some detectable

anthropogenic influences on average tempera-

ture have been reported for North America and

parts of the United States (e.g., Christidis et al.

2010;

Bonfils et al. 2008;

Pierce et al. 2009

Figure 6.6 shows an example for linear trends

for 1901–2015, indicating a detectable anthropo-

genic warming since 1901 over the western and

northern regions of the contiguous United States

for the CMIP5 multimodel ensemble—a condi-

tion that was also met for most of the individual

models.

The Southeast stands out as the only

region with no “detectable” warming since 1901;

observed trends there were inconsistent with

CMIP5 All Forcing historical runs.

The cause

Figure 6.5. Observed changes in the occurrence of record-setting daily temperatures in the contiguous United States.

Red bars indicate a year with more daily record highs than daily record lows, while blue bars indicate a year with more

record lows than highs. The height of the bar indicates the ratio of record highs to lows (red) or of record lows to highs

(blue). For example, a ratio of 2:1 for a blue bar means that there were twice as many record daily lows as daily record

highs that year. Estimates are derived from long-term stations with minimal missing data in the Global Historical Clima-

tology Network–Daily dataset.

(Figure source: NOAA/NCEI).

1930 1940

1950 1960

1970 1980 1990 2000 2010

1:1

2:1

3:1

4:1

5:1

6:1

7:1

More Daily Record Highs

More Daily Record Lows

Ratio of Daily Temperature Records

Year

6 | Temperature Changes in the United States

193 Climate Science Special ReportU.S. Global Change Research Program

of this “warming hole,” or lack of a long-term

warming trend, remains uncertain, though it

is likely a combination of natural and human

causes. Some studies conclude that changes in

anthropogenic aerosols have played a crucial

role (e.g., Leibensperger et al. 2012;

29, 30

Yu et al.

2014

), whereas other studies infer a possible

large role for atmospheric circulation,

internal

climate variability (e.g., Meehl et al. 2012;

Knut-

son et al. 2013

), and changes in land use (e.g.,

Goldstein et al. 2009;

Xu et al. 2015

). Notably,

the Southeast has been warming rapidly since

the early 1960s.

35, 36

In summary, there is medium

confidence for detectable anthropogenic warm-

ing over the western and northern regions of the

contiguous United States.

6.2.2 Temperature Extremes

The Intergovernmental Panel on Climate

Change’s (IPCC’s) Fifth Assessment Report

(AR5)

concluded that it is very likely that hu-

man influence has contributed to the observed

changes in frequency and intensity of tem-

perature extremes on the global scale since the

mid-20th century. The combined influence of

anthropogenic and natural forcings was also

detectable (medium confidence) over large

subregions of North America (e.g., Zwiers et

al. 2011;

Min et al. 2013

). In general, how-

ever, results for the contiguous United States

are not as compelling as for global land areas,

in part because detection of changes in U.S.

regional temperature extremes is affected by

Assessment of Annual Surface Temperature Trends (1901–2015)

1.5

0.5

0.25

−0.25

−0.5

−1

−1.5

−2

−4

°F / 100 years

b) CMIP5 ensemble trend (1901–2015)

c) Summary trend assessment

Detectable anthro. increase,

consistent with model

Detectable increase,

less than modeled

No detectable trend; white

dots: consistent with model

Detectable anthro. increase,

greater than modeled

White stippling:

Obs. Consistent with All-Forcing

a) Observed trend (1901–2015)

Insufficient data

Assessment of Annual Surface Temperature Trends (1901–2015)

Figure 6.6. Detection and attribu-

tion assessment of trends in annual

average temperature (°F). Grid-box

values indicate whether linear trends

for 1901–2015 are detectable (that is,

distinct from natural variability) and/

or consistent with CMIP5 historical

All-Forcing runs. If the grid-box trend

is found to be both detectable and

either consistent with or greater than

the warming in the All-Forcing runs,

then the grid box is assessed as hav-

ing a detectable anthropogenic con-

tribution to warming over the period.

Gray regions represent grid boxes

with data that are too sparse for detection

and attribution. (Figure source: updated

from Knutson et al. 2013;

Meteorological Society. Used with

permission.)

6 | Temperature Changes in the United States

194 Climate Science Special ReportU.S. Global Change Research Program

extreme temperature in the 1930s.

Table 6.3

summarizes available attribution statements

for recent extreme U.S. temperature events.

As an example, the recent record or near-re-

cord high March–May average temperatures

occurring in 2012 over the eastern United

States were attributed in part to external

(natural plus anthropogenic) forcing;

the

century-scale trend response of temperature

to external forcing is typically a close approxi-

mation to the anthropogenic forcing response

alone. Another study found that although the

extreme March 2012 warm anomalies over the

United States were mostly due to natural vari-

ability, anthropogenic warming contributed to

the severity.

Such statements reveal that both

natural and anthropogenic factors influence

the severity of extreme temperature events.

Nearly every modern analysis of current ex-

treme hot and cold events reveals some degree

of attributable human influence.

6.3 Projected Changes

6.3.1 Average Temperatures

Temperature projections are based on glob-

al model results and associated downscaled

products from CMIP5 using a suite of Rep-

resentative Concentration Pathways (RCPs).

In contrast to NCA3, model weighting is

employed to refine projections of temperature

for each RCP (Ch. 4: Projections; Appendix

B: Model Weighting). Weighting parameters

are based on model independence and skill

over North America for seasonal temperature

and annual extremes. Unless stated other-

wise, all changes presented here represent the

weighted multimodel mean. The weighting

scheme helps refine confidence and likelihood

statements, but projections of U.S. surface air

temperature remain very similar to those in

NCA3. Generally speaking, extreme tempera-

tures are projected to increase even more than

average temperatures.

Table 6.3. Extreme temperature events in the United States for which aribuon statements have

been made. There are three possible aribuon statements: “+” shows an aributable human-induced

increase in frequency or intensity, “−” shows an aributable human-induced decrease in frequency or

intensity, “0” shows no aributable human contribuon.

Study Period Region Type Statement

Rupp et al. 2012

Angélil et al. 2017

Spring/Summer

2011

Texas Hot

Hoerling et al. 2013

Summer 2011 Texas Hot +

Dienbaugh and Scherer 2013

Angélil et al. 2017

July 2012

Northcentral

and Northeast

Hot

Caaux and Yiou 2013

Angélil et al. 2017

Spring 2012 East Hot

Knutson et al. 2013b

Angélil et al. 2017

Spring 2012 East Hot

Jeon et al 2016

Summer 2011

Texas/

Oklahoma

Hot +

Dole et al. 2014

March 2012

Upper

Midwest

Hot +

Seager et al. 2014

2011–2014 California Hot +

Wolter et al. 2015

Winter 2014 Midwest Cold −

Trenary et al. 2015

Winter 2014 East Cold 0

6 | Temperature Changes in the United States

195 Climate Science Special ReportU.S. Global Change Research Program

The annual average temperature of the contigu-

ous United States is projected to rise throughout

the century. Increases for the period 2021–2050

relative to 1976–2005 are projected to be about

2.5°F (1.4°C) for a lower scenario (RCP4.5) and

2.9°F (1.6°C) for the higher scenario (RCP8.5);

the similarity in warming reflects the similarity

in greenhouse gas concentrations during this

period (Figure 4.1). Notably, a 2.5°F (1.4°C) in-

crease makes the near-term average comparable

to the hottest year in the historical record (2012).

In other words, recent record-breaking years

may be “common” in the next few decades. By

late-century (2071–2100), the RCPs diverge sig-

nificantly, leading to different rates of warming:

approximately 5.0°F (2.8°C) for RCP4.5 and 8.7°F

(4.8°C) for RCP8.5. Likewise, there are different

ranges of warming for each scenario: 2.8°–7.3°F

(1.6°–4.1°C) for RCP4.5 and 5.8°–11.9°F (3.2°–

6.6°C) for RCP8.5. (The range is defined here as

the difference between the average increase in

the three coolest models and the average increase

in the three warmest models.) For both RCPs,

slightly greater increases are projected in sum-

mer than winter (except for Alaska), and average

maximums will rise slightly faster than average

minimums (except in the Southeast and South-

ern Great Plains).

Statistically significant warming is projected

for all parts of the United States throughout

the century (Figure 6.7). Consistent with polar

amplification, warming rates (and spatial

gradients) are greater at higher latitudes. For

example, warming is largest in Alaska (more

than 12.0°F [6.7°C] in the northern half of the

state by late-century under RCP8.5), driven in

part by a decrease in snow cover and thus sur-

face albedo. Similarly, northern regions of the

contiguous United States have slightly more

warming than other regions (roughly 9.0°F

[5.5°C] in the Northeast, Midwest, and North-

ern Great Plains by late-century under RCP8.5;

Table 6.4). The Southeast has slightly less

warming because of latent heat release from

increases in evapotranspiration (as is already

evident in the observed record). Warming is

smallest in Hawai‘i and the Caribbean (rough-

ly 4.0°–6.0°F [2.2°–3.3°C] by late century under

RCP8.5) due to the moderating effects of

surrounding oceans. From a sub-regional per-

spective, less warming is projected along the

coasts of the contiguous United States, again

due to maritime influences, although increases

are still substantial. Warming at higher eleva-

tions may be underestimated because the res-

olution of the CMIP5 models does not capture

orography in detail.

6 | Temperature Changes in the United States

196 Climate Science Special ReportU.S. Global Change Research Program

Figure 6.7. Projected changes in annual average temperatures (°F). Changes are the difference between the average

for mid-century (2036–2065; top) or late-century (2070–2099, bottom) and the average for near-present (1976–2005).

Each map depicts the weighted multimodel mean. Increases are statistically signicant in all areas (that is, more than

50% of the models show a statistically signicant change, and more than 67% agree on the sign of the change

). (Fig-

ure source: CICS-NC and NOAA NCEI).

Lower Scenario (RCP4.5)

Lower Scenario (RCP4.5) Higher Scenario (RCP8.5)

Higher Scenario (RCP8.5)

Mid 21st Century

Late 21st Century

Projected Changes in Annual Average Temperature

2 4 6 8 10 12 14 16 18

Change in Temperature (°F)

Projected Changes in Annual Average Temperature

6 | Temperature Changes in the United States

197 Climate Science Special ReportU.S. Global Change Research Program

Table 6.4. Projected changes in annual average temperature (°F) for each Naonal Climate Assessment region in

the conguous United States. Changes are the dierence between the average for mid-century (2036–2065) or

late-century (2071–2100) and the average for near-present (1976–2005) under the higher scenario (RCP8.5) and

a lower scenario (RCP4.5). Esmates are derived from 32 climate models that were stascally downscaled using

the Localized Constructed Analogs technique.

Increases are stascally signicant in all areas (that is, more than

50% of the models show a stascally signicant change, and more than 67% agree on the sign of the change

NCA Region

RCP4.5

Mid-Century

(2036–2065)

RCP8.5

Mid-Century

(2036–2065)

RCP4.5

Late-Century

(2071–2100)

RCP8.5

Late-Century

(2071–2100)

Northeast 3.98°F 5.09°F 5.27°F 9.11°F

Southeast 3.40°F 4.30°F 4.43°F 7.72°F

Midwest 4.21°F 5.29°F 5.57°F 9.49°F

Great Plains North 4.05°F 5.10°F 5.44°F 9.37°F

Great Plains South 3.62°F 4.61°F 4.78°F 8.44°F

Southwest 3.72°F 4.80°F 4.93°F 8.65°F

Northwest 3.66°F 4.67°F 4.99°F 8.51°F

6.3.2 Temperature Extremes

Daily extreme temperatures are projected to

increase substantially in the contiguous Unit-

ed States, particularly under the higher sce-

nario (RCP8.5). For instance, the coldest and

warmest daily temperatures of the year are ex-

pected to increase at least 5°F (2.8°C) in most

areas by mid-century,

rising to 10°F (5.5°C) or

more by late-century.

In general, there will be

larger increases in the coldest temperatures of

the year, especially in the northern half of the

Nation, whereas the warmest temperatures

will exhibit somewhat more uniform changes

geographically (Figure 6.8). By mid-century,

the upper bound for projected changes (i.e.,

the average of the three warmest models) is

about 2°F (1.1°C) greater than the weighted

multimodel mean. On a regional basis, annual

extremes (Table 6.5) are consistently projected

to rise faster than annual averages (Table 6.4).

Future changes in “very rare” extremes are

also striking; by late century, current 1-in-20

year maximums are projected to occur every

year, while current 1-in-20 year minimums are

not expected to occur at all.

The frequency and intensity of cold waves is

projected to decrease while the frequency and

intensity of heat waves is projected to increase

throughout the century. The frequency of cold

waves (6-day periods with a minimum tem-

perature below the 10th percentile) will de-

crease the most in Alaska and the least in the

Northeast while the frequency of heat waves

(6-day periods with a maximum temperature

above the 90th percentile) will increase in all

regions, particularly the Southeast, Southwest,

and Alaska. By mid-century, decreases in the

frequency of cold waves are similar across

RCPs whereas increases in the frequency of

heat waves are about 50% greater in the high-

er scenario (RCP8.5) than the lower scenario

(RCP4.5).

The intensity of cold waves is pro-

jected to decrease while the intensity of heat

waves is projected to increase, dramatically so

under RCP8.5. By mid-century, both extreme

cold waves and extreme heat waves (5-day,

1-in-10 year events) are projected to have

temperature increases of at least 11.0°F (6.1°C)

nationwide, with larger increases in northern

regions (the Northeast, Midwest, Northern

Great Plains, and Northwest; Table 6.5).

There are large projected changes in the num-

ber of days exceeding key temperature thresh-

olds throughout the contiguous United States.

6 | Temperature Changes in the United States

198 Climate Science Special ReportU.S. Global Change Research Program

Figure 6.8. Projected changes in the coldest and warmest daily temperatures (°F) of the year in the contiguous

United States. Changes are the difference between the average for mid-century (2036–2065) and the average for

near-present (1976–2005) under the higher scenario (RCP8.5). Maps in the top row depict the weighted multimodel

mean whereas maps on the bottom row depict the mean of the three warmest models (that is, the models with the

largest temperature increase). Maps are derived from 32 climate model projections that were statistically down-

scaled using the Localized Constructed Analogs technique.

Increases are statistically signicant in all areas (that

is, more than 50% of the models show a statistically signicant change, and more than 67% agree on the sign of the

change

). (Figure source: CICS-NC and NOAA NCEI).

Table 6.5. Projected changes in temperature extremes (˚F) for each Naonal Climate Assessment region in the

conguous United States. Changes are the dierence between the average for mid-century (2036–2065) and the

average for near-present (1976–2005) under the higher scenario (RCP8.5). Esmates are derived from 32 climate

models that were stascally downscaled using the Localized Constructed Analogs technique.

Increases are sta-

scally signicant in all areas (that is, more than 50% of the models show a stascally signicant change, and

more than 67% agree on the sign of the change

NCA Region

Change in Coldest

Day of the Year

Change in Coldest

5-Day 1-in-10 Year

Event

Change in Warmest

Day of the Year

Change in Warmest 5-Day

1-in-10 Year Event

Northeast 9.51°F 15.93°F 6.51°F 12.88°F

Southeast 4.97°F 8.84°F 5.79°F 11.09°F

Midwest 9.44°F 15.52°F 6.71°F 13.02°F

Great Plains North 8.01°F 12.01°F 6.48°F 12.00°F

Great Plains South 5.49°F 9.41°F 5.70°F 10.73°F

Southwest 6.13°F 10.20°F 5.85°F 11.17°F

Northwest 7.33°F 10.95°F 6.25°F 12.31°F

6 | Temperature Changes in the United States

199 Climate Science Special ReportU.S. Global Change Research Program

For instance, there are about 20–30 more days

per year with a maximum over 90°F (32°C)

in most areas by mid-century under RCP8.5,

with increases of 40–50 days in much of the

Southeast (Figure 6.9). The upper bound for

projected changes is very roughly 10 days

greater than the weighted multimodel mean.

Consistent with widespread warming, there

are 20–30 fewer days per year with a mini-

mum temperature below freezing in the north-

ern and eastern parts of the nation, with de-

creases of more than 40–50 days in much the

West. The upper bound for projected changes

in freezing events is very roughly 10–20 days

fewer than the weighted multimodel mean in

many areas.

Figure 6.9. Projected changes in the number of days per year with a maximum temperature above 90°F and a min-

imum temperature below 32°F in the contiguous United States. Changes are the difference between the average for

mid-century (2036–2065) and the average for near-present (1976–2005) under the higher scenario (RCP8.5). Maps

in the top row depict the weighted multimodel mean whereas maps on the bottom row depict the mean of the three

warmest models (that is, the models with the largest temperature increase). Maps are derived from 32 climate model

projections that were statistically downscaled using the Localized Constructed Analogs technique.

Changes are sta-

tistically signicant in all areas (that is, more than 50% of the models show a statistically signicant change, and more

than 67% agree on the sign of the change

). (Figure source: CICS-NC and NOAA NCEI).

0 10 20 30 40 50 60 70

Weighted Multi-Model Mean Weighted Multi-Model Mean

Projected Change in Number of Days Above 90°F

Mid 21st Century, Higher Scenario (RCP8.5)

Projected Change in Number of Days Below 32°F

Mid 21st Century, Higher Scenario (RCP8.5)

0 10 20 30 40 50 60 70

Mean of Three Warmest Models

−70 −60 −50 −40 −30 −20 −10 0

Mean of Three Warmest Models

−70 −60 −50 −40 −30 −20 −10 0

200 Climate Science Special ReportU.S. Global Change Research Program

6 | Temperature Changes in the United States

TRACEABLE ACCOUNTS

Key Finding 1 Annual average temperature over the

contiguous United States has increased by 1.2°F (0.7°C)

for the period 1986–2016 relative to 1901–1960 and by

1.8°F (1.0°C) based on a linear regression for the period

1895–2016 (very high condence). Surface and satellite

data are consistent in their depiction of rapid warming

since 1979 (high condence). Paleo-temperature evi-

dence shows that recent decades are the warmest of

the past 1,500 years (medium condence).

Description of Evidence Base

The key nding and supporting text summarize exten-

sive evidence documented in the climate science liter-

ature. Similar statements about changes exist in other

reports (e.g., NCA3;

Global Climate Change Impacts in

the United States;

SAP 1.1: Temperature trends in the

lower atmosphere

Evidence for changes in U.S. climate arises from mul-

tiple analyses of data from in situ, satellite, and other

records undertaken by many groups over several de-

cades. The primary dataset for surface temperatures

in the United States is nClimGrid,

1, 2

though trends

are similar in the U.S. Historical Climatology Network,

the Global Historical Climatology Network, and other

datasets. Several atmospheric reanalyses (e.g., 20th

Century Reanalysis, Climate Forecast System Reanal-

ysis, ERA-Interim, Modern Era Reanalysis for Research

and Applications) conrm rapid warming at the surface

since 1979, with observed trends closely tracking the

ensemble mean of the reanalyses.

Several recently im-

proved satellite datasets document changes in middle

tropospheric temperatures.

5, 6, 7

Longer-term changes

are depicted using multiple paleo analyses (e.g., Wahl

and Smerdon 2012;

Trouet et al. 2013

Major Uncertainties

The primary uncertainties for surface data relate to

historical changes in station location, temperature

instrumentation, observing practice, and spatial sam-

pling (particularly in areas and periods with low station

density, such as the intermountain West in the early

20th century). Satellite records are similarly impacted

by non-climatic changes such as orbital decay, diurnal

sampling, and instrument calibration to target tem-

peratures. Several uncertainties are inherent in tem-

perature-sensitive proxies, such as dating techniques

and spatial sampling.

Assessment of condence based on evidence and

agreement, including short description of nature

of evidence and level of agreement

Very high (since 1895), High (for surface/satellite agree-

ment since 1979), Medium (for paleo)

Likelihood of Impact

Extremely Likely

Summary sentence or paragraph that integrates

the above information

There is very high condence in observed changes in av-

erage temperature over the United States based upon

the convergence of evidence from multiple data sourc-

es, analyses, and assessments.

Key Finding 2

There have been marked changes in temperature ex-

tremes across the contiguous United States. The fre-

quency of cold waves has decreased since the early

1900s, and the frequency of heat waves has increased

since the mid-1960s. The Dust Bowl era of the 1930s re-

mains the peak period for extreme heat. The number of

high temperature records set in the past two decades

far exceeds the number of low temperature records.

(Very high condence)

Description of Evidence Base

The key nding and supporting text summarize exten-

sive evidence documented in the climate science liter-

ature. Similar statements about changes have also been

made in other reports (e.g., NCA3;

SAP 3.3: Weather and

Climate Extremes in a Changing Climate;

IPCC Special

Report on Managing the Risks of Extreme Events and Di-

sasters to Advance Climate Change Adaptation

Evidence for changes in U.S. climate arises from mul-

tiple analyses of in situ data using widely published

climate extremes indices. For the analyses presented

201 Climate Science Special ReportU.S. Global Change Research Program

6 | Temperature Changes in the United States

here, the source of in situ data is the Global Historical

Climatology Network–Daily dataset,

with changes

in extremes being assessed using long-term stations

with minimal missing data to avoid network-induced

variability on the long-term time series. Cold wave fre-

quency was quantied using the Cold Spell Duration

Index,

heat wave frequency was quantied using the

Warm Spell Duration Index,

and heat wave intensity

were quantied using the Heat Wave Magnitude Index

Daily.

Station-based index values were averaged into

4° grid boxes, which were then area-averaged into a

time series for the contiguous United States. Note that

a variety of other threshold and percentile-based indi-

ces were also evaluated, with consistent results (e.g.,

the Dust Bowl was consistently the peak period for ex-

treme heat). Changes in record-setting temperatures

were quantied as in Meehl et al. (2016).

Major Uncertainties

The primary uncertainties for in situ data relate to his-

torical changes in station location, temperature instru-

mentation, observing practice, and spatial sampling

(particularly the precision of estimates of change in

areas and periods with low station density, such as the

intermountain West in the early 20th century).

Assessment of condence based on evidence and

agreement, including short description of nature

of evidence and level of agreement

Very high

Likelihood of Impact

Extremely likely

Summary sentence or paragraph that integrates

the above information

There is very high condence in observed changes in

temperature extremes over the United States based

upon the convergence of evidence from multiple data

sources, analyses, and assessments.

Key Finding 3

Annual average temperature over the contiguous

United States is projected to rise (very high con-

dence). Increases of about 2.5°F (1.4°C) are projected

for the period 2021–2050 relative to 1976–2005 in all

RCP scenarios, implying recent record-setting years

may be “common” in the next few decades (high con-

dence). Much larger rises are projected by late century

(2071–2100): 2.8°–7.3°F (1.6°–4.1°C) in a lower scenario

(RCP4.5) and 5.8°–11.9°F (3.2°–6.6°C) in a higher scenar-

io (RCP8.5) (high condence).

Description of Evidence Base

The key nding and supporting text summarize ex-

tensive evidence documented in the climate science

literature. Similar statements about changes have also

been made in other reports (e.g., NCA3;

Global Climate

Change Impacts in the United States

). The basic physics

underlying the impact of human emissions on climate

has also been documented in every IPCC assessment.

Projections are based on global model results and as-

sociated downscaled products from CMIP5 for RCP4.5

(lower scenario) and RCP8.5 (higher scenario). Model

weighting is employed to rene projections for each

RCP. Weighting parameters are based on model inde-

pendence and skill over North America for seasonal

temperature and annual extremes. The multimodel

mean is based on 32 model projections that were sta-

tistically downscaled using the Localized Constructed

Analogs technique.

The range is dened as the dier-

ence between the average increase in the three coolest

models and the average increase in the three warmest

models. All increases are signicant (i.e., more than 50%

of the models show a statistically signicant change,

and more than 67% agree on the sign of the change

Major Uncertainties

Global climate models are subject to structural and

parametric uncertainty, resulting in a range of esti-

mates of future changes in average temperature. This is

partially mitigated through the use of model weighting

and pattern scaling. Furthermore, virtually every en-

semble member of every model projection contains an

increase in temperature by mid- and late-century. Em-

pirical downscaling introduces additional uncertainty

(e.g., with respect to stationarity).

202 Climate Science Special ReportU.S. Global Change Research Program

6 | Temperature Changes in the United States

Assessment of condence based on evidence and

agreement, including short description of nature

of evidence and level of agreement

Very high for projected change in annual average tem-

perature; high condence for record-setting years be-

coming the norm in the near future; high condence

for much larger temperature increases by late century

under a higher scenario (RCP8.5).

Likelihood of Impact

Extremely likely

Summary sentence or paragraph that integrates

the above information

There is very high condence in projected changes in av-

erage temperature over the United States based upon

the convergence of evidence from multiple model sim-

ulations, analyses, and assessments.

Key Finding 4

Extreme temperatures in the contiguous United States

are projected to increase even more than average tem-

peratures. The temperatures of extremely cold days and

extremely warm days are both expected to increase.

Cold waves are projected to become less intense while

heat waves will become more intense. The number of

days below freezing is projected to decline while the

number above 90°F will rise. (Very high condence)

Description of Evidence Base

The key nding and supporting text summarize exten-

sive evidence documented in the climate science liter-

ature (e.g., Fischer et al. 2013;

Sillmann et al. 2013;

Wuebbles et al. 2014;

Sun et al. 2015

). Similar state-

ments about changes have also been made in other

national assessments (such as NCA3) and in reports by

the Climate Change Science Program (such as SAP 3.3:

Weather and Climate Extremes in a Changing Climate

Projections are based on global model results and as-

sociated downscaled products from CMIP5 for RCP4.5

(lower scenario) and RCP8.5 (higher scenario). Model

weighting is employed to rene projections for each

RCP. Weighting parameters are based on model inde-

pendence and skill over North America for seasonal

temperature and annual extremes. The multimodel

mean is based on 32 model projections that were sta-

tistically downscaled using the Localized Constructed

Analogs technique.

Downscaling improves on the

coarse model output, establishing a more geographi-

cally accurate baseline for changes in extremes and the

number of days per year over key thresholds. The up-

per bound for projected changes is the average of the

three warmest models. All increases are signicant (i.e.,

more than 50% of the models show a statistically sig-

nicant change, and more than 67% agree on the sign

of the change

Major Uncertainties

Global climate models are subject to structural and

parametric uncertainty, resulting in a range of esti-

mates of future changes in temperature extremes. This

is partially mitigated through the use of model weight-

ing and pattern scaling. Furthermore, virtually every

ensemble member of every model projection contains

an increase in temperature by mid- and late-century.

Empirical downscaling introduces additional uncer-

tainty (e.g., with respect to stationarity).

Assessment of condence based on evidence and

agreement, including short description of nature

of evidence and level of agreement

Very high

Likelihood of Impact

Extremely likely

Summary Sentence

There is very high condence in projected changes in

temperature extremes over the United States based

upon the convergence of evidence from multiple mod-

el simulations, analyses, and assessments.

6 | Temperature Changes in the United States

203 Climate Science Special ReportU.S. Global Change Research Program

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