Assessing Potential Climate Change Pressures 

across the Conterminous United States: 

Mapping Plant Hardiness Zones, Heat Zones, 

Growing Degree Days, and Cumulative Drought 

Severity throughout this Century

United States Department of Agriculture

Forest

Service Research Map NRS-9

Northern

Research Station March 2018
Abstract

The maps and tables presented here represent potential variability of projected climate change across the 

conterminous United States during three 30-year periods in this century and emphasizes the importance 

of evaluating multiple signals of change across large spatial domains. Maps of growing degree days, plant 

hardiness zones, heat zones, and cumulative drought severity depict the potential for markedly shifting 

conditions and highlight regions where changes may be multifaceted across these metrics. In addition to 

the maps, the potential change in these climate variables are summarized in tables according to the seven 

regions of the fourth National Climate Assessment to provide additional regional context. Viewing these 

data collectively further emphasizes the potential for novel climatic space under future projections of climate 

change and signals the wide disparity in these conditions based on relatively near-term human decisions of 

curtailing (or not) greenhouse gas emissions. 

Cover Image

A collage of four maps depicting the baseline conditions for plant hardiness zones (clockwise from top left), 

growing degree days, cumulative drought severity index, and heat zones. See text for full explanation of 

each.

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Visit our homepage at: http://www.nrs.fs.fed.us/
Assessing Potential Climate Change Pressures 

across the Conterminous United States: 

Mapping Plant Hardiness Zones, Heat Zones, 

Growing Degree Days, and Cumulative Drought 

Severity throughout this Century

Stephen N. Matthews, Louis R. Iverson, Matthew P. Peters, and Anantha M. Prasad

The Authors

Stephen N. Matthews is an assistant 

professor in the School of Environment and 

Natural Resources at Ohio State University 

and is affiliated with the Northern Institute 

of Applied Climate Science, USDA Forest 

Service, Northern Research Station in 

Delaware, OH. 

Louis R. Iverson is a research landscape 

ecologist with the Northern Institute of Applied 

Climate Science, USDA Forest Service, 

Northern Research Station in Delaware, OH. 

Matthew P. Peters is an ecologist with the 

Northern Institute of Applied Climate Science, 

USDA Forest Service, Northern Research 

Station in Delaware, OH. 

Anantha M. Prasad is a research ecologist 

with the Northern Institute of Applied Climate 

Science, USDA Forest Service, Northern 

Research Station in Delaware, OH. 
Disclaimer United States Department of Agriculture Forest Service has developed this 

information for the guidance of its employees, its contractors, and its cooperating 

Federal and State agencies. It is not responsible for the interpretation or use of this 

information by anyone except its own employees. The use of trade, firm, corporation, 

or association names in this publication is for the information and convenience of the 

reader and does not constitute an endorsement by the Department of any product or 

service. This product is reproduced from geospatial information prepared by the U.S. 

Department of Agriculture, Forest Service. Geographic information system (GIS) data 

and product accuracy may vary. They may be developed from sources of differing 

accuracy, accurate only at certain scales, based on modeling or interpretation, or 

incomplete while being created or revised. Using GIS products for purposes other 

than those or for which they were created may yield inaccurate or misleading results. 

This information was released on the indicated date. The Forest Service reserves the 

right to correct, update, modify, or replace GIS products without notification.
1

Introduction In the era of persistent climate change, it is important that we consider how 

continued perturbations to our climate system may intensify through the end 

of the century (U.S. Global Change Research Program [USGCRP] 2017). 

Further, it is important to evaluate these potential changes under alternative 

scenarios to gauge the potential magnitude of these changes. By focusing on 

four key metrics related to plant growth and survival, but also of key interest 

to human well-being, we map and summarize projections of growing degree 

days, plant hardiness zones, heat zones, and cumulative drought severity 

across the conterminous United States throughout this century. Each map 

series captures the potential shifting conditions and collectively, the maps can 

highlight broad hotspot regions for change across the variables. To evaluate 

how these combined projected pathways may influence ecological systems, 

detailed modeling is needed to capture the multidimensional influences. 

In addition, the visual contrasts between two scenarios of the IPCC 5th 

assessment (Representative Concentration Pathway [RCP] 4.5, a low 

storyline; and RCP 8.5, a high storyline) emphasize the variation of possible 

climate outcomes as a result of human decisions driving emission trajectories 

through this century (van Vuuren et al. 2011). 

Data Sources Data used for the production of growing degree days, plant hardiness zones, 

and heat zones were projected daily values of minimum and maximum 

temperature for the period 1980–2099, downscaled by Maurer et al. (2007), 

and were compiled here to address key variables related to plant growth 

and survival. To explore the potential variation in projected changes of 

these climate patterns, we evaluated each metric under two RCPs, thus 

representing “bookends” of a range of possibilities of projected climate 

change. We used the Community Climate System Model (CCSM4) (Gent 

et al. 2011), a general circulation model (GCM) with relatively low 

sensitivity to CO2 , and with the RCP 4.5 storyline of relatively rapid 

reduction of greenhouse gases so that emissions peak ~2040 (Moss et al. 

2008). As a result, CCSM4 RCP 4.5 represents the low end of model outputs 

and a “low” amount of future climate change. For the higher end of potential 

change, we used the Geophysical Fluid Dynamics Laboratory (GFDL) CM3 

model (Donner et al. 2011) with the RCP 8.5 storyline of continuing our 

current emissions path for much of this century, thus representing a relatively 

“high” projection of changes by century’s end. In this report, we will refer 
2

to these two scenarios as “low” and “high”. Global CO2 levels have been 

tracking much more closely with high than low levels in the time since these 

scenarios were generated in 2008 (Peters et al. 2013). 

These data were statistically downscaled via asynchronous regional 

regression modeling, to a resolution of one-eighth degree (~13.875 × 

13.875 km) (Maurer et al. 2007), but, as with any downscaled data, there are 

uncertainties and the uncertainty associated with daily data will be greater 

than that associated with long-term means. To address the accumulation 

of these daily errors, we report on projected change in each metric at the 

30-year average resolution for three time periods: 2010–2039, 2040–2069, 

and 2070–2099 and represented in this document as early century, midcentury, and late century, respectively. These data sets were also used to 

prepare difference maps for each time period based on the modeled baseline 

conditions (1980–2009) from each climate model. These difference maps 

provide a spatial indication of where and when change is projected. 

Additional data were required for the calculation of a cumulative drought 

severity Index (CDSI) (Peters et al. 2014), as it was performed on monthly 

projections of precipitation and temperature that were obtained from the 

National Aeronautics and Space Administration’s (NASA) Earth Exchange 

Downscaled Climate Projections (NEX-DCP30) dataset (Thrasher 

et al. 2013). GCMs within the NEX-DCP30 dataset were downscaled to 

a common grid corresponding to interpolated climate observation derived 

from the PRISM (Parameter-elevation Regressions on Independent Slopes 

Model) dataset (Daly et al. 2008) using the bias-correction and spatial 

disaggregation method (Maurer and Hildago 2008; Wood et al. 2002, 2004). 

Both the PRISM data and the NEX-DCP30 downscaled GCMs have a spatial 

resolution of 30 arc-seconds (~800 m) across the conterminous United States. 

To remove any bias from the GCMs, mean PRISM values for the period 

1981–2010 were adjusted using the delta method where the change between 

monthly GCM values were subtracted from the mean period 1981–2010 

from the GCM, providing the predicted change in monthly precipitation and 

temperature.

Monthly values from the PRISM and GCM datasets were aggregated to 

10 × 10 km grids to calculate a self-calibrated Palmer Drought Severity 

Index (scPDSI) using an algorithm developed by Wells et al. (2004). The 

scPDSI algorithm requires latitude, available water supply (AWS) of the 

soil, which was derived from 10-m gridded county soil survey geographic 

database (gSSURGO) (Natural Resources Conservation Service 2016), 

monthly climate normals (e.g., mean temperature of calibration period), 

monthly precipitation, and mean monthly temperature. A snowmelt function 

(Yan et al. 2014) that accumulates a snowpack when monthly temperatures 

≤0 °C and precipitation >0 mm, and then releases a portion of the snowpack 

when monthly temperatures are above freezing, was used to alter monthly 

precipitation values. Precipitation stored in the snowpack is released by 

20 percent in months when the mean monthly temperature is between 
3

0 and 5 °C; when the monthly temperature exceeds the 5 °C threshold, the 

remaining snowpack is added to the monthly precipitation. The calibration 

period was set to the baseline period 1981–2010, and monthly scPDSI values 

for the period 1980 to 2099 were calculated.

Across all of the variables presented here, the climate metrics capture 

unique areas spatially and temporally and show areas with the projected 

greatest and least change under the two climate scenarios. To facilitate 

regional comparisons, we summarized the climate variables by regions 

(Figure 1), based on the fourth National Climate Assessment (U.S. Global 

Change Research Program 2017) by presenting the mean conditions for 

each region for the baseline period and the three time periods for the low 

and high scenarios. In addition, we calculated the percent change in area 

for each climate variable by region to capture both the distributional shift 

(i.e., the range of projected change to capture the overall magnitude of shift) 

as well as a means to facilitate comparisons of change across these unique 

geographic regions. 

Map Projections All maps in this report are displayed using a geographic coordinate system 

with an angular unit in degrees and the North American Datum of 1983. 

Data for growing degree days, heat zones, and plant hardiness zones spanned 

25.125° N to 52.875° N and -124.625° E to -67.000° E, including parts of 

Mexico and Canada, while the cumulative drought severity index spanned 

24.062° N to 49.937° N and -125.021° E to -66.479° E, including only the 

United States. 

Figure 1.—Regions of the conterminous United States as defined in the National Climate Assessment (USGCRP 2017) and 

used in this study.
4

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5

Growing degree days

Growing degree days (GDD) address an important component to general 

patterns of plant growth by accumulating the degree days across the 

growing season. This metric provides a level of detail related to defining the 

growing season potential. Here, we evaluate the accumulation of growing 

degree days at or above 5 °C (41 °F), assuming that limited growth occurs 

below 5 °C (Franklin et al. 2013, Rehfeldt et al. 2006, Sork et al. 2010). 

Specifically, we calculate growing degree days by first calculating the 

average daily temperature, based on the maximum and minimum projected 

daily temperature. We then subtract 5 °C from each mean value and then 

accumulate the positive difference values for all days within each year. 

The mean GDD values for the conterminous United States during the 

baseline period ranged from <100 to over 7,000 degree days, increasing 

from north to south with highest values in the Florida panhandle, southern 

Texas, southwestern Arizona, and southeastern California (Figure 2). GDD 

projections throughout the century suggest a ubiquitous increase across the 

United States with slightly less change in the Northeast and much greater 

increases throughout the southern United States under the high scenario 

(Figure 2 mean GDD and Figure 3 change in GDD). 

By region, the mean GDD (Table 1) for the baseline period is greatest in the 

Southern Plains (4,584 degree days) and least in the Northwest (1,689 degree 

days) under the high scenario. Though the Southwest has the highest values, 

the large, cold Rocky Mountains reduces its mean to 2,884 degree days. Each 

future period increased GDD across the map (Figure 2) with an acceleration 

of change apparent starting about mid-century. For example, under the high 

scenario, the late century period showed GDD mean values of 6,272 for the 

Southeast (a gain of 1,827) and 3,053 for the Northwest (a gain of 1,364; 

Table 1). There is also a large differential between the low and high scenarios 

especially by century’s end, with a difference between scenarios of at least 

1,000 degree days for each of the regions (Table 1). 

Table 2 shows the percentage of each region within each GDD 100-degreeday change category and also clearly illustrates the aforementioned trends: 

the large change occurring, especially by mid-century, the large differential 

between scenarios, and the large geographical variation. Some locations 

in the Southeast and Southwest regions of the United States are projected 

to increase more than 2,000 GDD by the end of the century under the high 

scenario (Figure 3). Though some plants may benefit from increased GDD, 

this measure does not incorporate estimates of precipitation, which have 

greater uncertainties in climate change modeling. Many models suggest 

precipitation will occur in the form of larger extreme events (including 

greater drought in between heavy rainfall events) and with more unevenness 

in seasonality (lower proportion of rainfall in late summer—autumn). 

Moisture stress added to more heat will likely increase stress among plants 

and contribute to greater mortality (“hot droughts”) (Allen et al. 2015). The 

CDSI, which considers precipitation and temperature, captures some aspects 

of water deficits that can lead to plant stress.
6

Figure 2.—Maps of baseline and projections of mean growing degree days for conterminous United States by 30-year time 

period using a low (RCP 4.5; see text for explanation) or high (RCP 8.5; see text for explanation) climate scenario. The maps 

show an accumulation of growing degree days. Time periods: baseline (1980–2009); early century (2010–2039); 

mid-century (2040–2069); late century (2070–2099).

Baseline

Early Century Low (CCSM4 RCP 4.5)

Mid-century Low (CCSM4 RCP 4.5)

Late Century Low (CCSM4 RCP 4.5)

Early Century High (GFDL RCP 8.5)

Mid-century High (GFDL RCP 8.5)

Late Century High (GFDL RCP 8.5)

Growing Degree Days
7

Figure 3.—Mapped projections of increase in growing degree days for conterminous United States by 30-year time period and 

climate scenario. Projections suggest a ubiquitous increase across the United States, with slightly less change in the Northeast 

and much greater increases in the Southwest under the high scenario. Time periods: early century (2010–2039); mid-century 

(2040–2069); late century (2070–2099).

Change in Growing Degree Days

Early Century Low (CCSM4 RCP 4.5)

Mid-century Low (CCSM4 RCP 4.5)

Late Century Low (CCSM4 RCP 4.5)

Early Century High (GFDL RCP 8.5)

Mid-century High (GFDL RCP 8.5)

Late Century High (GFDL RCP 8.5)
8

Table 1. The mean number of growing degree days (based on 5 oC, or 41 oF) by region for baseline (1980–2009), early (2010–2039), mid 

(2040–2069), and late century (2070–2099) climate conditions. Data are presented separately for low (CCSM4 RCP 4.5) and high (GFDL CM3 

RCP 8.5) scenarios.

Growing Degree Days

Low Scenario High Scenario

Baseline

Early 

Century

MidcenturyLate 

Century Baseline

Early 

Century

MidcenturyLate 

Century

Midwest 2,760 3,031 3,320 3,378 2,681 3,191 3,703 4,432

Northeast 2,363 2,607 2,854 2,908 2,324 2,812 3,382 4,075

Northern Plains 2,143 2,403 2,622 2,724 2,036 2,456 2,907 3,574

Northwest 1,730 1,999 2,140 2,268 1,689 2,034 2,495 3,053

Southeast 4,505 4,810 5,126 5,177 4,445 4,973 5,565 6,272

Southern Plains 4,646 4,927 5,221 5,324 4,584 5,135 5,717 6,453

Southwest 2,920 3,198 3,366 3,479 2,884 3,349 3,879 4,537

Table 2. The projected increase in mean growing degree days (GDD) are presented as a percentage of land area within each region, by time 

period. For instance, 67.4 percent of the land area in the Midwest is projected to have an increase of 500 growing degree days during the 

early century under the high scenario, and during the mid-century, 25.5 percent of the Midwest land area is projected to have an increase of 

900 growing degree days. Data are presented separately for low (CCSM4 RCP 4.5) and high (GFDL CM3 RCP 8.5) scenarios. 

Midwest Northeast Southeast Northern Plains

Change

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

GDD (Low)

0 0 0 0 0 0 0 0 0 0 0 0 0

100 0 0 0 0 0 0 0 0 0 0.5 0 0

200 29.6 0 0 51.0 0 0 6.0 0 0 31.1 0.1 0

300 70.3 0 0 49.0 0.9 0 94.0 0.2 0 68.4 4.6 0.6

400 0 1.9 0 0 25.7 14.8 0 3.9 2.8 0 19.4 5.5

500 0 42.2 16.5 0 48.7 28.2 0 5.6 4.1 0 67.7 17.9

600 0 50.3 51.0 0 24.6 50.3 0 56.3 19.9 0 8.2 68.5

700 0 5.5 31.9 0 0.1 6.7 0 34.0 65.8 0 0 7.4

800 0 0 0.6 0 0 0 0 0 7.4 0 0 0

900 0 0 0 0 0 0 0 0 0 0 0 0

GDD (High)

0 0 0 0 0 0 0 0 0 0 0 0 0

100 0 0 0 0 0 0 0 0 0 0 0 0

200 0 0 0 0 0 0 0 0 0 0.8 0 0

300 0 0 0 0.3 0 0 2.0 0 0 10.5 0 0

400 8.8 0 0 20.6 0 0 5.8 0 0 57.0 0 0

500 67.4 0 0 70.1 0 0 55.0 0 0 31.5 0.7 0

600 23.8 0 0 9.0 0 0 37.1 0 0 0.1 3.9 0

700 0 0 0 0 0 0 0 0 0 0 6.8 0

800 0 0.1 0 0 0.1 0 0 1.6 0 0 19.9 0

900 0 25.5 0 0 15.4 0 0 4.1 0 0 48.8 0.3

1,000 0 32.4 0 0 21.7 0 0 6.1 0 0 19.8 1.1

1,100 0 37.8 0 0 50.1 0 0 50.5 0 0 0.1 3.3

1,200 0 4.2 0 0 12.5 0 0 37.7 0 0 0 4.0

1,300 0 0 0 0 0.1 0 0 0 0 0 0 4.9

1,400 0 0 0 0 0 0.3 0 0 0.1 0 0 6.7

1,500 0 0 0.3 0 0 5.3 0 0 3.3 0 0 20.7

1,600 0 0 9.6 0 0 16.5 0 0 3.6 0 0 34.7

1,700 0 0 39.2 0 0 21.2 0 0 6.7 0 0 20.7

1,800 0 0 35.3 0 0 34.0 0 0 38.7 0 0 3.6

1,900 0 0 15.5 0 0 20.5 0 0 46.0 0 0 0

2,000 0 0 0 0 0 2.2 0 0 1.6 0 0 0

2,100 0 0 0 0 0 0 0 0 0 0 0 0

2,200 0 0 0 0 0 0 0 0 0 0 0 0
9

Table 2 (continued). 

Southern Plains Northwest Southwest

Change

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

GDD (Low)

0 0 0 0 0 0 0 0 0 0

100 0 0 0 0.7 0 0 1.2 0 0

200 21.4 0 0 30.3 1.0 0.1 25.9 0.4 0.1

300 78.6 0 0 68.1 14.0 1.7 64.5 7.7 1.7

400 0 1.8 0 0.9 55.2 12.1 8.4 42.1 8.5

500 0 30.1 1.3 0 29.7 37.0 0 49.0 32.0

600 0 64.7 28.3 0 0.1 42.5 0 0.8 44.1

700 0 3.4 65.0 0 0 6.6 0 0 13.6

800 0 0 5.4 0 0 0 0 0 0

900 0 0 0 0 0 0 0 0 0

GDD (High)

0 0 0 0 0 0 0 0 0 0

100 0 0 0 0 0 0 0 0 0

200 0 0 0 4.8 0 0 0.2 0 0

300 0 0 0 43.1 0 0 6.4 0 0

400 0.3 0 0 51.4 0.1 0 32.5 0 0

500 45.3 0 0 0.7 1.9 0 47.6 0.2 0

600 54.4 0 0 0 7.8 0 13.3 1.4 0

700 0 0 0 0 16.7 0 0 4.0 0

800 0 0 0 0 30.7 0.3 0 11.3 0

900 0 0 0 0 36.2 1.3 0 18.7 0.1

1,000 0 9.1 0 0 6.6 2.7 0 28.4 0.6

1,100 0 47.0 0 0 0 7.2 0 17.6 1.3

1,200 0 43.3 0 0 0 10.6 0 15.2 2.5

1,300 0 0.6 0 0 0 15.7 0 3.3 4.5

1,400 0 0 0 0 0 24.3 0 0 8.9

1,500 0 0 0 0 0 27.7 0 0 11.8

1,600 0 0 0 0 0 9.9 0 0 14.5

1,700 0 0 2.8 0 0 0.4 0 0 20.9

1,800 0 0 30.4 0 0 0 0 0 13.4

1,900 0 0 59.7 0 0 0 0 0 12.1

2,000 0 0 7.0 0 0 0 0 0 8.6

2,100 0 0 0 0 0 0 0 0 0.8

2,200 0 0 0 0 0 0 0 0 0
10

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Plant hardiness zones

Plant hardiness zones (PHZ) provide a general indication of the extent of 

overwinter stress experienced by plants. PHZ are based on the average 

annual extreme minimum temperatures and have been used by horticulturists 

to evaluate the cold hardiness of plants. Specifically, the value used here is 

the absolute minimum temperature achieved for each year and reported as 

the 30-year mean (Cathey 1990, Daly et al. 2012). Because they reflect cold 

tolerance for many plant species, including woody ones, hardiness zones 

are most likely to reflect plant range limits. The zonal variations caused by 

warming temperatures in the future will therefore be useful to approximately 

delineate niche constraints of many plant species and hence their future range 

potential. Plant hardiness zones and subzones were delineated according to 

the USDA definitions (Cathey 1990), which break the geography into zones 

by 10 °F (5.56 °C) increments from zone 1 (-55 to -45.6 °C) to zone 13 

(15.7 to 22 °C) of annual extreme minimum temperature. To define the 

coldest day per year, daily minimum temperatures were identified within the 

period July 1 to June 30, with the nominal year assigned to the first 6 months 

of the 12-month period. 

The high scenario captures the potential for dramatic changes in PHZ, 

evident by the mid-century time period onward (Figure 4). All regions could 

see high rates of warming, with the mean minimum temperature increasing 

(Table 3). For example, under the high scenario, mean minimum temperature 

is projected to increase by at least 5 °C in four of the regions by the end of 

the century (Table 3). This shift is illustrated in Table 4 and Figure 5 across 

the regions. For example, most of the Northeast and Midwest regions are 

projected to experience an increase in minimum temperature of at least 

8–9 °C with the entire distribution shifting toward warmer conditions. 

Likewise, Table 4 captures the progression of the temperature distributions 

across the time periods, with rapidly increasing minimum temperatures 

projected for the Northeast, Midwest, Northwest, and Northern Plains 

(Table 4). However, the low scenario is the more conservative in showing 

change from the baseline to the future. For example, though the minimum 

temperature changes are sizable for the Midwest (from -26.0 baseline to 

-21.7 °C by late century), these changes are less than 2 °C in the Northwest 

(Table 3). Thus under the low scenario, the projections show mostly a 

reshuffling of minimum temperatures within regions and a limited expansion 

of larger changes (Table 4). 
12

Figure 4.—Maps of baseline and projections of mean absolute minimum temperature (reflecting the plant hardiness zone 

metric) for conterminous United States by 30-year time period using a low (RCP 4.5; see text for explanation) or high (RCP 

8.5; see text for explanation) climate scenario. To define the coldest day per year, daily minimum temperatures were identified 

within the period July 1 to June 30, with the year assigned to the first 6 months of the 12-month period. Zone categories follow 

the USDA definitions (Cathey 1990). Time periods: baseline (1980–2009); early century (2010–2039); mid-century 

(2040–2069); late century (2070–2099).

Baseline Minimum Temperature

Early Century Low (CCSM4 RCP 4.5)

Mid-century Low (CCSM4 RCP 4.5)

Late Century Low (CCSM4 RCP 4.5)

Early Century High (GFDL RCP 8.5)

Mid-century High (GFDL RCP 8.5)

Late Century High (GFDL RCP 8.5)
13

Figure 5.—Mapped projections of changes in minimum temperature for conterminous United States by 30-year time period and 

climate scenario. Plant hardiness zones are projected to have the greatest changes in the Northeast and Northern Plains. 

Time periods: early century (2010–2039); mid-century (2040–2069); late century (2070–2099).

Change in Minimum 

Temperature (oC)

Early Century Low (CCSM4 RCP 4.5)

Mid-century Low (CCSM4 RCP 4.5)

Late Century Low (CCSM4 RCP 4.5)

Early Century High (GFDL RCP 8.5)

Mid-century High (GFDL RCP 8.5)

Late Century High (GFDL RCP 8.5)
14

Table 3. The mean minimum temperature (plant hardiness zone metric) in degrees Celsius, by region for baseline (1980–2009), early (2010–

2039), mid (2040–2069), and late century (2070–2099) climate conditions. Data are presented separately for low (CCSM4 RCP 4.5) and high 

(GFDL CM3 RCP 8.5) scenarios. 

Minimum Temperature

Low Scenario High Scenario

Baseline

Early 

Century

MidcenturyLate 

Century Baseline

Early 

Century

MidcenturyLate 

Century

Midwest -26.0 -24.8 -23.0 -21.7 -24.6 -21.7 -19.2 -15.4

Northeast -22.8 -22.6 -20.3 -19.6 -21.7 -18.6 -15.3 -12.4

Northern Plains -31.2 -30.5 -29.5 -28.1 -30.6 -28.4 -25.1 -21.9

Northwest -20.0 -20.2 -19.5 -18.3 -20.8 -18.9 -15.5 -14.3

Southeast -11.7 -11.1 -9.6 -8.4 -10.7 -9.8 -8.7 -8.1

Southern Plains -14.3 -13.4 -12.2 -10.8 -13.5 -11.9 -10.3 -9.6

Southwest -17.2 -16.6 -16.6 -15.0 -17.3 -15.9 -13.7 -12.7

Table 4. The projected changes in minimum temperature, in degrees Celsius, are presented as a percentage of land area within each region, by 

time period. For instance, 22.3 percent of the land area in the Midwest is projected to have a 2-degree (Celsius) increase in minimum 

temperature during the early century under the high scenario, and during mid-century 25.7 percent of the Midwest land area is projected to have 

a minimum temperature increase of 6 °C. Data are presented separately for low (CCSM4 RCP 4.5) and high (GFDL CM3 RCP 8.5) scenarios. 

Midwest Northeast Southeast Northern Plains

Change

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Min Temp (Low)

-3 0 0 0 0 0 0 0 0 0 0 0 0

-2 0 0 0 0 0 0 0 0 0 0 0 0

-1 0 0 0 0.8 0 0 0 0 0 2.0 0 0

0 7.0 0 0 79.3 0 0 46.5 0 0 29.9 2.0 0

1 77.1 0 0 19.9 0.2 0 53.2 4.3 0 66.1 42.8 0.5

2 16.0 13.3 0.2 0 55.7 7.8 0.3 77.7 10.8 1.9 45.4 20.2

3 0 75.4 5.0 0 41.4 54.5 0 18.0 55.5 0 9.6 52.0

4 0 10.9 60.8 0 2.6 34.4 0 0 30.7 0 0.1 26.4

5 0 0.4 34.0 0 0 3.3 0 0 3.1 0 0 0.9

6 0 0 0.1 0 0 0 0 0 0 0 0 0

7 0 0 0 0 0 0 0 0 0 0 0 0

8 0 0 0 0 0 0 0 0 0 0 0 0

9 0 0 0 0 0 0 0 0 0 0 0 0

Min Temp (High)

-3 0 0 0 0 0 0 0 0 0 0 0 0

-2 0 0 0 0 0 0 0 0 0 0 0 0

-1 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 40.3 0.9 0.1 0 0 0

1 0.2 0 0 0.2 0 0 42.8 32.8 16.8 11.6 0 0

2 22.3 0.3 0 9.2 0 0 13.6 43.4 42.7 63.9 0 0

3 65.7 6.7 1.1 78.2 0.6 0 3.3 16.2 19.9 24.4 0.3 0

4 11.7 21.6 2.5 12.3 8.0 0.4 0 5.8 11.2 0.1 7.7 0

5 0 23.6 5.3 0 18.5 3.6 0 0.9 6.9 0 42.3 0.4

6 0 25.7 9.3 0 31.9 9.4 0 0 2.3 0 43.2 6.1

7 0 16.8 11.4 0 23.0 17.6 0 0 0.1 0 6.5 13.8

8 0 5.1 14.7 0 11.6 14.7 0 0 0 0 0 21.9

9 0 0.2 13.4 0 5.9 12.2 0 0 0 0 0 33.7

10 0 0 9.8 0 0.4 11.4 0 0 0 0 0 21.3

11 0 0 8.9 0 0 13.6 0 0 0 0 0 2.7

12 0 0 10.1 0 0 7.5 0 0 0 0 0 0.1

13 0 0 7.3 0 0 4.2 0 0 0 0 0 0

14 0 0 5.2 0 0 2.9 0 0 0 0 0 0

15 0 0 1.0 0 0 2.4 0 0 0 0 0 0

16 0 0 0 0 0 0.1 0 0 0 0 0 0

17 0 0 0 0 0 0 0 0 0 0 0 0
15

Table 4 (continued). 

Southern Plains Northwest Southwest

Change

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Min Temp (Low)

-3 0 0 0 0 0 0 0 0 0

-2 0 0 0 0.3 0 0 0 0 0

-1 0 0 0 30.3 0.2 0 0.6 0.1 0

0 19.9 0 0 59.9 46.8 0.5 35.7 17.8 0.5

1 73.4 4.2 0 9.5 52.1 45.0 60.6 59.7 22.5

2 6.7 87.4 0.1 0 1.0 38.3 3.2 20.7 37.6

3 0 8.4 53.7 0 0 15.8 0 1.7 32.4

4 0 0 45.5 0 0 0.3 0 0 6.9

5 0 0 0.8 0 0 0 0 0 0.1

6 0 0 0 0 0 0 0 0 0

7 0 0 0 0 0 0 0 0 0

8 0 0 0 0 0 0 0 0 0

9 0 0 0 0 0 0 0 0 0

Min Temp (High)

-3 0 0 0 0 0 0 0 0 0

-2 0 0 0 0 0 0 0 0 0

-1 0 0 0 0 0 0 0 0 0

0 4.4 0 0 0.1 0 0 1.8 0 0

1 46.7 0.3 0.6 25.8 0 0 57.6 0 0

2 41.9 27.8 19.2 59.7 0.5 0 37.4 19.8 3.7

3 7.0 36.3 26.5 14.3 6.3 2.8 3.0 31.1 23.5

4 0 29.9 20.7 0 19.3 5.5 0.1 28.2 23.7

5 0 5.7 18.4 0 28.8 14.8 0 16.5 24.1

6 0 0 11.5 0 35.1 20.1 0 3.4 16.3

7 0 0 3.0 0 9.6 33.5 0 0.9 5.7

8 0 0 0 0 0.4 19.8 0 0.1 2.1

9 0 0 0 0 0 3.5 0 0 0.7

10 0 0 0 0 0 0 0 0 0

11 0 0 0 0 0 0 0 0 0

12 0 0 0 0 0 0 0 0 0

13 0 0 0 0 0 0 0 0 0

14 0 0 0 0 0 0 0 0 0

15 0 0 0 0 0 0 0 0 0

16 0 0 0 0 0 0 0 0 0

17 0 0 0 0 0 0 0 0 0
16

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17

Heat Zones

Heat zones map the distribution of potential heat stress for plants and 

animals, including humans. We define heat zones as the number of days with 

maximum daily temperature >30 °C (86 °F) (Cathey 1997). Because species 

have unique adaptations and abilities to tolerate a wide variety of conditions, 

this metric is used merely as an indicator of change in “hot” conditions. 

The 30 °C value is set primarily for agricultural production and is a general 

temperature threshold at which photosynthesis can be negatively impacted 

for C3 plants (e.g., most species including trees) (Rennenberg et al. 2006), 

but it certainly also captures temperatures that induce stress in humans as 

well (Pal and Eltahir 2015). In addition, increases in temperature above these 

thresholds for longer periods, especially when accompanied with prolonged 

dry conditions, are linked to reduced performance and likely mortality of 

trees (Allen et al. 2015). Each day surpassing the 30 °C threshold was tallied 

and summed for each year and reported as the mean number of days, per 

year, over each 30-year period: baseline, early, mid, and late century. 

Maps compiling heat zones (Figure 6) show the average number of days per 

year greater than 30 °C (86 °F) across 30-year intervals. The increase in heat 

zones is most profound in the Northeast and Midwest and is apparent even by 

mid-century (Figure 7). Under the high scenario, the Northeast is projected 

to experience 96 more days by century’s end, while with the low scenario 

shows the least amount of change among the seven regions with a mean of 

29 more days above 30 °C (Table 5). The Midwest is projected to experience 

the most change under both scenarios with a mean of 95 and 42 (Table 5) 

additional days, from the high and low scenarios, respectively. Under the 

high scenario, all regions are projected to have at least a twofold increase 

in “hot” days, as compared to the low scenario (Table 5). As shown in the 

other climate metrics, the change tables, Table 6, provides an indication of 

potential for some regions to enter into novel climate space (climates not 

now experienced) as the century progresses in regards to hot days, especially 

under the high scenario. Table 6 also captures a broadening of the distribution 

shifting toward marked increases. For example in the Southeast, by the end 

of the century under the high scenario, 86 percent of the land is projected to 

experience at least an 80-day increase in hot days; this trend is even more 

extreme in the Midwest and Northeast regions, which all show the potential 

for more than 50 percent of their landscapes to exceed the 100-day increase 

threshold. These projected changes are not expected to be consistent across 

or within regions as evident by the change maps (Figure 7). Locations 

associated with high elevation (i.e., the Rocky Mountains) may have the least 

change, while areas of lower latitudes could expect the greatest changes.
18

Figure 6.—Maps of baseline and projections of mean number of days per year with temperature above 30 °C (reflecting the 

heat zone metric) for conterminous United States by 30-year time period using a low (RCP 4.5; see text for explanation) or high 

(RCP 8.5; see text for explanation) climate scenario. While 30 °C does not represent a species-specific physiological threshold, 

it is acknowledged that temperatures above this temperature exceed a general optimal threshold for photosynthesis for C3

plants (Rennenberg et al. 2006). Time periods: baseline (1980–2009); early century (2010–2039); mid-century 

(2040–2069); late century (2070–2099).

Mean Days >30 °C Baseline

Early Century Low (CCSM4 RCP 4.5)

Mid-century Low (CCSM4 RCP 4.5)

Late Century Low (CCSM4 RCP 4.5)

Early Century High (GFDL RCP 8.5)

Mid-century High (GFDL RCP 8.5)

Late Century High (GFDL RCP 8.5)
19

Figure 7.—Mapped projections of changes in number of days per year with temperature above 30 °C for conterminous 

United States by 30-year time period and climate scenario. Heat zones are projected to expand everywhere, but particularly 

in the Midwest, parts of the Northeast, and the Rocky Mountains by the end of the century, especially under the high climate 

scenario. Time periods: early century (2010–2039); mid-century (2040–2069); late century (2070–2099).

Increase in Days >30 °C

Early Century Low (CCSM4 RCP 4.5)

Mid-century Low (CCSM4 RCP 4.5)

Late Century Low (CCSM4 RCP 4.5)

Early Century High (GFDL RCP 8.5)

Mid-century High (GFDL RCP 8.5)

Late Century High (GFDL RCP 8.5)
20

Table 5. The mean number of days above 30 °C (heat zone metric) by region for baseline (1980–2009), early (2010–2039), mid (2040–2069), 

and late century (2070–2099) climate conditions. Data are presented separately for low (CCSM4 RCP 4.5) and high (GFDL CM3 RCP 8.5) 

scenarios. 

Heat Zone

Low Scenario High Scenario

Baseline

Early 

Century

MidcenturyLate 

Century Baseline

Early 

Century

MidcenturyLate 

Century

Midwest 40 59 78 82 36 69 99 131

Northeast 17 29 43 46 18 42 78 114

Northern Plains 38 54 69 73 34 58 80 108

Northwest 24 36 45 49 25 40 58 81

Southeast 106 124 138 143 102 133 160 186

Southern Plains 128 142 154 160 123 152 175 200

Southwest 73 87 96 100 73 97 121 145

Table 6. The projected increase in days with temperatures above 30 °C is presented as a percentage of land area within each region, by time 

period. For instance, 31.2 percent of the land area in the Midwest is projected to have 30 additional days with temperatures above 30 °C 

during the early century under the high scenario, and during mid-century 39.7 percent of the Midwest land area is projected to have 60 more 

days with temperature above 30 °C. Data are presented separately for low (CCSM4 RCP 4.5) and high (GFDL CM3 RCP 8.5) scenarios. 

Midwest Northeast Southeast Northern Plains

Change

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Heat Zone (Low)

0 1.8 0.3 0.2 16.2 2.5 1.5 0.4 0.2 0.1 9.0 4.1 3.2

10 17.3 1.1 0.6 43.7 16.4 15.3 32.8 0.4 0.2 15.1 4.2 3.5

20 74.0 3.7 2.9 36.8 22.7 19.1 60.3 10.2 1.2 75.8 6.1 4.5

30 6.9 16.6 9.5 3.3 29.2 24.7 6.5 64.6 50.4 0 53.0 12.4

40 0 69.8 39.8 0 23.5 28.5 0 18.6 34.4 0 32.6 74.9

50 0 8.6 46.9 0 5.6 10.8 0 6.0 12.1 0 0 1.4

60 0 0 0.1 0 0 0.1 0 0 1.6 0 0 0

70 0 0 0 0 0 0 0 0 0 0 0 0

80 0 0 0 0 0 0 0 0 0 0 0 0

90 0 0 0 0 0 0 0 0 0 0 0 0

Heat Zone (High)

0 0.4 0 0 2.1 0 0 0.2 0 0 5.3 1.6 0.1

10 2.4 0.2 0 20.6 0.2 0 0.4 0 0 8.2 2.4 0.3

20 15.0 0.1 0 24.2 1.3 0 19.8 0.1 0 27.9 3.1 0.5

30 31.2 0.7 0 31.0 4.7 0 58.1 0.1 0 54.9 5.1 0.8

40 51.0 3.7 0.1 18.9 10.0 0.2 20.4 0.3 0 3.7 22.9 1.5

50 0.1 13.2 0.1 3.2 12.3 0.2 1.1 49.0 0 0 46.9 3.3

60 0 39.7 0.1 0 20.4 1.1 0 34.7 0 0 17.9 6.7

70 0 34.9 0.4 0 37.9 4.0 0 11.0 13.7 0 0.1 37.9

80 0 7.3 10.5 0 12.9 9.4 0 4.7 54.6 0 0 36.5

90 0 0 34.2 0 0.2 20.4 0 0.1 19.2 0 0 12.3

100 0 0 49.2 0 0 41.0 0 0 7.2 0 0 0

110 0 0 5.3 0 0 21.3 0 0 3.6 0 0 0

120 0 0 0 0 0 2.3 0 0 1.7 0 0 0

130 0 0 0 0 0 0 0 0 0 0 0 0

140 0 0 0 0 0 0 0 0 0 0 0 0
21

Table 6 (continued). 

Southern Plains Northwest Southwest

Change

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Heat Zone (Low)

0 0 0 0 23.6 11.9 9.4 10.7 7.3 6.6

10 68.2 0.1 0 37.4 14.3 11.2 36.4 8.1 3.3

20 31.8 41.4 2.4 39.0 29.3 14.4 52.9 38.0 21.2

30 0 56.6 77.4 0 43.6 47.1 0 40.4 46.6

40 0 2.0 20.2 0 0.9 17.9 0 6.2 22.4

50 0 0 0 0 0 0 0 0 0.1

60 0 0 0 0 0 0 0 0 0

70 0 0 0 0 0 0 0 0 0

80 0 0 0 0 0 0 0 0 0

90 0 0 0 0 0 0 0 0 0

Heat Zone (High)

0 0 0 0 13.3 7.1 1.8 6.5 2.7 0.8

10 0 0 0 26.5 6.6 3.4 3.7 1.9 0.7

20 12.2 0 0 51.7 8.9 3.2 42.1 1.5 0.5

30 85.7 0.1 0 8.5 15.0 2.9 34.3 8.8 0.6

40 2.1 4.1 0 0 49.3 4.4 13.3 24.8 0.8

50 0 69.8 0.1 0 13.1 11.3 0 30.9 8.8

60 0 25.6 1.8 0 0 50.3 0 17.0 23.0

70 0 0.3 39.3 0 0 19.1 0 8.3 25.0

80 0 0.1 53.3 0 0 3.5 0 4.0 17.5

90 0 0 5.2 0 0 0 0 0.1 10.1

100 0 0 0.2 0 0 0 0 0 6.6

110 0 0 0.1 0 0 0 0 0 4.2

120 0 0 0 0 0 0 0 0 1.3

130 0 0 0 0 0 0 0 0 0.2

140 0 0 0 0 0 0 0 0 0
22

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23

Cumulative drought severity index

Cumulative drought severity index (CDSI) is a weighted value derived from 

the occurrence and intensity of monthly drought events, where the frequency 

of moderate (scPDSI -2 to -2.9), severe (scPDSI -3 to -3.9), and extreme 

(scPDSI ≤ -4) conditions were weighted 1, 2, or 3 times, respectively, and 

tallied for each year (Peters et al. 2014, 2015). The yearly CDSI values were 

then accumulated over each of the 30-year periods and mapped. The range 

of CDSI values for the 360 months in a 30-year period can range from 0 (no 

months with any drought) to 1080 (every month has an extreme drought, 

with a weighted score of 12 × 30 × 3 = 1080). The cumulative drought 

severity index (CDSI) is derived from monthly self-calibrated Palmer 

Drought Severity Index (scPDSI) values which used the baseline calibration 

period (1981–2010). 

Each future 30-year interval is generally expected to experience more 

drought events compared to the baseline, according to the two scenarios 

used in this study (Figure 8). However, as precipitation patterns and 

temperatures change among the 30-year periods, new normal conditions will 

be established, influencing the calibration of scPDSI values. Additionally, 

due to the weighting of drought intensity, regions that generally experience 

few severe to extreme droughts could see CDSI values increase if many 

moderate droughts or a few more severe to extreme events are predicted to 

occur. Under both scenarios, some portions of the regions are predicted to 

experience fewer drought events or less intense ones than the baseline period 

(Figure 9); one example is the Southeast during the mid-century period 

(Table 7). However, most regions are projected to have more frequent or 

more intense droughts by end of century regardless of scenario (Table 7). 

Certainly, there are major differences between the low and high scenarios, 

especially by end of century (Figure 9). Across five of the seven regions 

(Midwest, Northeast, Southern Plains, Northwest, and Southwest), more 

than 45 percent of the area in each region is projected to have an increase of 

at least 700 CDSI under the high scenario (Table 7). These increases have 

the potential to place additional stress on vegetation leading to increased 

mortality (Clark et al. 2016).
24

Figure 8.—Maps of baseline and projections of Cumulative Drought Severity Index (CDSI) for conterminous United States 

by 30-year time period using a low (RCP 4.5; see text for explanation) or high (RCP 8.5; see text for explanation) climate 

scenario. The yearly CDSI values were accumulated over each of the 30-year periods. The range of CDSI values for the 

360 months in a 30-year period can range from 0 (no months with any drought) to 1080 (every month has an extreme drought). 

Time periods: baseline (1980–2009); early century (2010–2039); mid-century (2040–2069); late century (2070–2099).

CDSI

Months

Baseline

Early Century Low (CCSM4 RCP 4.5)

Mid-century Low (CCSM4 RCP 4.5)

Late Century Low (CCSM4 RCP 4.5)

Early Century High (GFDL RCP 8.5)

Mid-century High (GFDL RCP 8.5)

Late Century High (GFDL RCP 8.5)
25

Figure 9.—Mapped projections of changes in CDSI for conterminous United States by 30-year time period and climate 

scenario. Each future 30-year interval is generally expected to experience more drought events (increased index score), 

according to the two scenarios used in this study. However, most regions could see more frequent or more intense droughts 

by end of century, regardless of scenario. More than 45 percent of the area in the Midwest, Northeast, Southern Plains, 

Northwest, and Southwest regions is projected to see an increase of at least 700 CDSI under the high scenario. 

Time periods: early century (2010–2039); mid-century (2040–2069); late century (2070–2099).

Change in CDSI

Early Century Low (CCSM4 RCP 4.5)

Mid-century Low (CCSM4 RCP 4.5)

Late Century Low (CCSM4 RCP 4.5)

Early Century High (GFDL RCP 8.5)

Mid-century High (GFDL RCP 8.5)

Late Century High (GFDL RCP 8.5)
26

Table 7. The projected changes in CDSI are presented as a percentage of land area within each region, by time period. For instance, 

1.7 percent of the land area in the Midwest is projected to experience an increase of 200 CDSI during the early century under the high 

scenario, and during the mid-century, 8.9 percent of the Midwest land area is projected to experience an increase of 200 CDSI. CDSI values 

represent the occurrence of drought conditions weighted by intensity during a 30-year period for each region; a positive change (increase) in 

CSDI values indicate an increase in droughty conditions. Data are presented separately for low (CCSM4 RCP 4.5) and high (GFDL CM3 RCP 

8.5) scenarios. 

Midwest Northeast Southeast Northern Plains

Change

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

CDSI (Low)

-400 0 0 0 0 0 0.1 0 0 0 0 0 0

-300 0.1 0 0 0.2 0 0.8 0.3 0.1 0.1 0.1 0.1 0.1

-200 1.0 0.1 0 5.9 1.1 6.1 9.7 6.5 10.8 5.2 1.8 0.9

-100 8.7 1.2 0 15.9 3.2 14.5 27.5 24.1 28.9 13.8 2.2 1.2

-50 31.5 17.2 3.5 40.2 18.0 37.0 46.0 54.2 34.5 37.6 12.0 4.7

50 14.1 11.4 3.5 10.1 10.0 7.5 6.9 7.1 6.7 13.7 13.4 5.0

100 22.5 18.1 13.1 11.0 16.1 9.7 5.8 4.4 7.0 14.7 25.3 13.7

200 12.8 13.1 15.5 6.9 11.2 7.4 2.5 1.9 4.6 6.5 15.8 16.1

300 6.9 9.4 16.3 5.1 7.8 5.4 1.0 1.0 3.7 3.5 10.8 17.8

400 2.4 7.8 21.1 2.9 6.2 4.1 0.2 0.4 2.0 2.2 7.1 14.8

500 0.1 6.0 13.0 0.9 5.4 3.2 0.1 0.2 0.9 1.5 4.4 10.3

600 0 5.0 6.7 0.7 5.4 2.0 0 0.1 0.4 0.9 2.6 6.8

700 0 5.7 4.5 0.2 6.3 1.1 0 0 0.2 0.3 1.8 4.4

800 0 4.0 2.1 0.1 6.2 0.6 0 0 0.1 0 1.3 2.5

900 0 1.0 0.6 0 2.9 0.2 0 0 0 0 1.2 1.6

1000 0 0.1 0 0 0.2 0 0 0 0 0 0 0

1100 0 0.1 0 0 0.2 0 0 0 0 0 0 0

CDSI (High)

-400 0 0 0 0 0 0 0 0 0 0 0 0

-300 0.1 0.1 0 0.4 0.1 0 0 0 0 0.3 0.4 0.1

-200 1.7 3.3 0 5.0 0.4 0.1 5.7 2.2 0.1 16.2 14.8 0.7

-100 16.4 16.6 0 11.5 1.4 0 23.7 14.4 0.7 24.2 25.5 1.1

-50 61.5 38.0 1.4 31.8 12.5 0.9 50.0 53.3 11.4 36.5 31.3 2.2

50 11.3 10.3 0.6 12.6 9.1 0.8 10.4 15.2 8.1 7.4 8.4 2.5

100 7.1 14.6 3.6 16.7 18.7 2.2 6.8 8.8 18.1 7.6 8.0 15.4

200 1.7 8.9 7.5 11.6 15.5 3.2 2.8 2.8 16.1 3.6 4.0 16.5

300 0.1 4.5 10.3 5.4 11.1 5.4 0.5 1.8 11.7 1.7 2.3 13.0

400 0.1 1.9 9.1 2.7 9.1 7.0 0.1 0.8 11.2 1.0 1.5 10.0

500 0 1.2 11.0 1.4 6.8 8.4 0 0.2 8.2 0.9 1.1 9.8

600 0 0.5 14.4 0.6 7.0 9.2 0 0.2 6.5 0.6 0.8 10.0

700 0 0.1 16.8 0.2 4.1 13.5 0 0.1 4.1 0.2 0.7 7.1

800 0 0 14.2 0.1 2.9 23.9 0 0.1 2.9 0 0.6 6.0

900 0 0 10.1 0 1.3 23.6 0 0.1 1.1 0 0.6 5.4

1000 0 0 1.1 0 0 1.9 0 0 0 0 0 0.2

1100 0 0 1.0 0 0 1.9 0 0 0 0 0 0.2
27

Table 7 (continued). 

Southern Plains Northwest Southwest

Change

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

Early 

Century

MidcenturyLate 

Century

CDSI (Low)

-400 0 0 0 0 0 0 0 0 0

-300 0 0 0 0.4 0.7 0.4 0 0.1 0

-200 3.3 1.9 0 7.5 8.2 4.1 1.3 3.1 1.1

-100 7.6 11.6 0 10.1 8.3 4 4.2 5.4 2.4

-50 25.1 28.4 2 28.2 20.5 12.4 20.8 16.3 12.4

50 15.1 12.6 3.8 12.2 9.7 7.7 14.5 9.2 8.3

100 21.7 20.3 14.4 16.4 16.3 17.2 25.4 16.8 16.1

200 12.7 9.7 22.3 8.4 10.1 15 15.7 12.4 15.4

300 6.8 4.9 22.9 6.3 7.3 11.8 8.4 10.1 13.4

400 3.9 3.6 15.1 3.9 4.7 8.4 4.4 8.2 10.3

500 3.0 3.9 8.9 3.4 3.5 5.7 2.3 6.8 7.9

600 0.6 2.4 4.6 2.1 3.0 4 1.6 5.3 5.7

700 0.1 0.5 3.2 1.0 2.7 3.9 0.9 3.6 3.5

800 0 0.1 2.3 0 3.0 3.2 0.3 1.8 2.3

900 0 0 0.5 0 1.9 2 0 0.8 1.1

1000 0 0 0 0 0.1 0.1 0 0.1 0.1

1100 0 0 0 0 0 0 0 0.1 0.1

CDSI (High)

-400 0 0 0 0 0 0 0 0 0

-300 0 0 0 0.4 0.8 0.1 0 0.1 0

-200 0.1 0.5 0 7.9 8.4 0.6 0.7 0.9 0

-100 0.7 5.2 0 12.2 8.5 0.9 3.3 1.7 0.1

-50 14.5 12.9 0.1 29.3 18.0 3.7 20.4 4.0 0.4

50 10.6 6.8 0.3 11.0 8.0 1.8 12.9 2.3 0.3

100 24.6 12.2 1.4 15.1 14.2 3.9 19.6 6.5 0.7

200 20.7 15.8 3.7 9.1 11.4 3.6 17.8 8.1 0.9

300 18.7 16.1 4.4 5.9 9.0 4.1 12.0 10.5 1.5

400 8.0 12.9 5.3 3.7 7.3 5.5 5.8 11.7 2.1

500 1.9 7.0 7.5 2.7 5.4 8.3 3.5 13.3 4.1

600 0.2 4.8 8.2 2.0 3.4 11.4 2.2 11.8 7.4

700 0 3.8 10.3 0.9 2.5 13.7 1.4 10.9 12.7

800 0 1.8 21.6 0 2.0 22.9 0.5 11.1 24.3

900 0 0.2 33.7 0 1.1 19.3 0.1 6.9 41.6

1000 0 0 3.7 0 0 0.4 0 0.2 3.9

1100 0 0 3.5 0 0 0.3 0 0.2 3.8
28

Conclusions The maps depict the potential for a changing climate across the conterminous 

United States by the end of the century. The magnitude of these shifts 

is strongly influenced by the emissions pathways evaluated, and thus 

the decisions of the world’s policy makers and citizens with regard to 

its energy future. The combination of climate conditions showing a 

widening of the range of conditions within each region and a directional 

shift toward increases in each of the metrics under the higher emissions 

scenario represents a clear picture of the multifaceted way global climate 

change can perturb our climate as well as ecological, social, and economic 

systems. Individually, each of these variables has the potential to influence 

and reshape competitive interactions of species. Collectively, they further 

indicate the potential for novel conditions likely to emerge across all 

regions. Understanding these potential patterns as well as advancing our 

understanding of how species respond to climate and other global change 

pressures will be essential to plan for resilience and adaptation in our forests 

and other aspects of biological conservation and society as a whole.
29

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Matthews, Stephen N.; Iverson, Louis R.; Peters, Matthew P.; Prasad, Anantha M. 

2018. Assessing potential climate change pressures across the 

conterminous United States: mapping plant hardiness zones, heat zones, 

growing degree days, and cumulative drought severity throughout this 

century. RMAP-NRS-9. Newtown Square, PA: U.S. Department of Agriculture, 

Forest Service, Northern Research Station. 31 p. 

https://doi.org/10.2737/NRS-RMAP-9. 

The maps and tables presented here represent potential variability of projected 

climate change across the conterminous United States during three 30-year 

periods in this century and emphasizes the importance of evaluating multiple 

signals of change across large spatial domains. Maps of growing degree days, 

plant hardiness zones, heat zones, and cumulative drought severity depict the 

potential for markedly shifting conditions and highlight regions where changes may 

be multifaceted across these metrics. In addition to the maps, the potential change 

in these climate variables are summarized in tables according to the seven regions 

of the fourth National Climate Assessment to provide additional regional context. 

Viewing these data collectively further emphasizes the potential for novel climatic 

space under future projections of climate change and signals the wide disparity in 

these conditions based on relatively near-term human decisions of curtailing (or not) 

greenhouse gas emissions. 

Key words: climate change indices, regional change, novel climate conditions

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