North, Gerald R.

English

Research Papers in Economics

  CHOICES  1st Quarter 2008 • 23(1) 
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Future Climate of the Continental United 
States
Gerald R. North
JEL Classification: Q54
Agriculture will be influenced by future climate changes. 
In order to see these influences and examine their implica-
tions one must obtain a climate change projection. Climate 
change projections can be obtained from Global Climate 
Models (GCMs) run under scenarios that are forced by the 
drivers of the climate system. This paper will give a very 
brief summary of the GCMs and scenarios then present 
projections for the next half-century. 
Climate Models and Their Reliability
The construction of global climate models (GCMs) began 
in the mid-seventies in parallel with the invention and pro-
liferation of high-speed computers as well as the deploy-
ment of global observing systems such as satellite and mea-
surements made directly in the atmosphere or ocean. The 
GCM is a physically-based numerical simulation model 
that includes the conservation of mass and water as well 
as momentum and energy. The dynamics of the oceans are 
coupled into the process. These models have descended 
from the numerical weather forecast models that date back 
to the 1950s. 
Changes in climate are described statistically. For pres-
ent purposes we are interested in the statistics of such quan-
tities as temperatures and precipitation over large regions. 
The most important descriptive statistics include mean val-
ues averaged over say a decade and fluctuation indicators 
such as the variance and some extreme values. Some other 
specialized statistics include the frequency and intensity of 
hurricanes, El Nino events, etc. 
Climate models strive to produce these statistics over 
historical periods when being validated and in the future 
under alternative scenarios for projections. The scenarios 
represent factors or “forcings” that affect climate. The most 
important climate forcing is the temporal evolution of the 
so-called greenhouse gases (GHGs) such as carbon dioxide, 
methane, nitrous oxide and many others. Concentrations 
of these GHGs have been observed directly for half a cen-
tury, and measurements can be taken back 650,000 years 
by measuring the amount trapped in air bubbles in polar 
ice. Other important forcings include dust left high in the 
atmosphere and lasting a few years following some volcanic 
eruptions, changing sun brightness and aerosols (tiny par-
ticles floating in the air, some manmade). 
Once a forcing scenario (e.g., steadily increasing carbon 
dioxide) is prescribed in the model simulation, feedbacks 
come into play to amplify or diminish the climate response. 
Globally the largest feedback is due to the response of wa-
ter vapor to surface warming. When the surface warms, 
more water vapor evaporates and works its way high in the 
atmosphere and this forces even more climate change. The 
water vapor feedback roughly doubles the response to the 
forcings mentioned above. Other feedbacks include 
•  Ice/snow cover (albedo) effect, which makes the planet 
less reflective as it warms, thereby enhancing the warm-
ing. 
•  Clouds, which might amplify or diminish climate ef-
fects and pose possibly the largest uncertainties in cur-
rent GCMs.
About 20 GCM modeling groups around the world 
simulate the climate response to various forcing scenarios 
for the reports produced every five years by the Intergov-
ernmental Panel on Climate Change (IPCC). The GCMs 
unanimously agree that increases in GHG emissions have 
led to a steady increase in global temperatures. Comparing 
projections suggests that if carbon dioxide were doubled 
global level temperatures would increase by about 3 deg C 
(5.4 deg F) with a range across the models of about plus/
minus 50%. At current rates of carbon dioxide emission 
increase (0.5% per year), the doubling will occur in 140 
years. When other greenhouse gases are included the effec-
tive doubling occurs in about 70 years.    1st Quarter 2008 • 23(1)  CHOICES  
Across the GCM projections sev-
eral  important  robust  and  relevant 
features emerge: 
1)  Global  average  temperatures  in-
crease  with  land  areas  leading 
ocean areas. 
2) There is more warming toward the 
poles and less in the tropical ar-
eas. 
3)  Sea  Level  will  increase  by  some 
0.30  to  0.50  meters  if  there  is 
no appreciable melting of the ice 
sheets on Greenland or the West 
Antarctic Peninsula. 
4)  Globally  precipitation  increases, 
but less than would be suggested 
by the rate of increase in atmo-
spheric water content (relative hu-
midity stays close to constant). 
5) Most of the increases in precipita-
tion  are  in  the  middle  latitudes 
such as the northern tier of the 
contiguous  United  States,  espe-
cially the Northeast. In the Conti-
nental United States expect more 
precipitation north of the Gulf of 
Mexico, less to the west of it.
6) Regions where the climate is con-
sidered ‘tropical’ will expand pole-
wards with accompanying chang-
es precipitation patterns (more on 
this later). 
7) Mountain snow packs will shrink 
and last a shorter time into the 
spring leading to changes in riv-
er flows. Snow cover on grazing 
lands  will  begin  later  and  melt 
earlier than at present. 
How much faith should we put 
in these projections? Most outspoken 
criticisms  of  the  model  simulations 
seem to be based on other than sci-
entific arguments. In the end we are 
forced to rely on expert assessments. 
In such a context the U.S. National 
Academy  of  Sciences  has  published 
many  assessment  reports  in  which 
the mainstream assertions in the last 
paragraph  are  endorsed.  The  most 
publicized  and  recent  expert  assess-
ment comes from the 2007 IPCC’s 
Fourth  Assessment  Report,  abbrevi-
ated as the AR-4. The climate science 
component of AR-4 (Working Group 
I), made use of the 20 or so GCMs in 
assembling projections. 
Climate Forcing Scenarios
In the AR-4 Working Group II Re-
port Chapter 2, the IPCC discusses 
future scenarios. All scenarios reflect 
future population projections. Some 
scenarios consider paths that empha-
size  centralized  planning  and  coop-
eration, while others take the way of 
regional differentiation. A variety of 
such scenarios are then used to gener-
ate future trends for greenhouse gas 
emissions  and  other  human-origi-
nated forcings. These are then used in 
GCM simulations for the Twenty First 
Century. One important result is that 
through the year 2030 the simulated 
results  are  insensitive  to  which  sce-
nario is employed; but after that the 
results for different scenarios begin to 
diverge. Mean warming for the globe 
by 2090-2099 relative to 1980-1999 
is projected to be 1.8, 2.8, and 3.4°C 
for three representative scenarios (B1, 
A1B, and A2). 
•  The B1 scenario is characterized 
by  population  leveling  at  2050, 
a world economy that is service 
and information based, with clean 
technology. 
•  A1B contains a market oriented, 
rapidly growing economy with the 
same population path as B1 while 
technology employs well balanced 
sources of energy.
•  A2 includes a population contin-
uously  increasing  along  with  an 
economy that is regionally orient-
ed and lowest in per capita growth 
with a technology that evolves the 
slowest with the most fragmented 
development. 
Across  these  scenarios  A2  is  the 
most  pessimistic  and  B1  the  most 
optimistic in terms of heating due to 
greenhouse gas emissions. Since these 
three scenarios are fairly representa-
tive of the spread of global outcomes 
at the end of the century, we adopt 
them as our choices for investigating 
the regional rates of change over the 
next century. 
Future Climate Projections for 
the Continental US
Following AR-4, Working Group II 
we  divide  the  Contiguous  US  into 
regions as shown in Figure 1. 
 
Figure 1. The regions used in describing climate change rates over the next 
century (adapted from AR-4, Working Group II, Chapter 2) . (Published with 
the permission of the Intergovernmental Panel on Climate Change, see Carter et 
al., 2007) 8  CHOICES  1st Quarter 2008 • 23(1) 
The  trends  in  temperature  and 
precipitation  and  precipitation  for 
different  seasons  are  shown  in  the 
table  (taken  from  figures  in  AR-4, 
Working Group II, Chapter 2:)
Recent  research  (Seager  et  al., 
2007) completed too late to be in-
cluded in the AR-4 suggests that the 
U.S. Southwest using the same AR-4 
models will be particularly dryer per-
haps to the extent that normal pre-
cipitation  minus  evaporation  might 
compare with the record droughts of 
the 1930s and 1950s. 
Summary
We can summarize the results most 
relevant  to  U.S.  agriculturalists  via 
the  following  statements.  It  is  very 
likely  that  the  Continental  United 
States  will  be  30C  (5.40F)  warmer 
plus or minus 1.50C (2.70F). There 
are likely to be more heat waves with 
more mid-latitude drying in summer 
and  an  increased  risk  of  prolonged 
droughts  (and  their  consequences, 
fires, etc.). Precipitation in the Unit-
ed States will be mixed: a) the East-
ern Sector will likely have more rain 
than now because mid-latitude storm 
tracks are likely to edge northwards. 
b) The Southwest is likely to be much 
drier as the storm tracks move north-
wards and the tropical summers expe-
rienced in that region are likely to be 
longer. Most models suggest that the 
multi-year swings of wet and dry pe-
riods will be more pronounced than 
those of today’s climate. Sea level will 
rise a foot or two under the conser-
vative  assumptions  that  melting  of 
the big ice sheets on Greenland and 
Antarctica  does  not  accelerate  cata-
strophically.
For More Information
Richard Seager, Mingfang Ting, Isaac 
Held,  Yochanan  Kushnir,  Jian 
Lu,  Gabriel  Vecchi,  Huei-Ping 
Huang,  Nili  Harnik,  Ants  Leet-
maa, Ngar-Cheung Lau, Cuihua 
Li,  Jennifer  Velez,  Naomi  Naik, 
(2007): Model Projections of an 
Imminent Transition  to  a  More 
Arid  Climate  in  Southwestern 
North America. Science. 25 May 
2007  316:  1181-1184.  [DOI: 
10.1126/science.1139601]
Carter, T.R., R.N. Jones, X. Lu, S. 
Bhadwal, C. Conde, L.O. Mearns, 
B.C. O’Neill, M.D.A. Rounsevell 
and M.B. Zurek, 2007: New As-
sessment Methods and the Char-
acterisation of Future Conditions. 
Climate  Change  2007:  Impacts, 
Adaptation and Vulnerability. Con-
tribution of Working Group II to 
the Fourth Assessment Report of the 
Intergovernmental Panel.
Gerald R. North is distinguished pro-
fessor of atmospheric sciences at Texas 
A&M University, College Station, TX 
77843-3150,  e-mail:  g-north@tamu.
edu,  http://www.met.tamu.edu/people/
faculty/north.php
Temperature Change (deg C per century) same as deg F per fifty years.
  DJF  MAM  JJA  SON
Western N. Amer.(WNA)   to   2 to   3 to 8.  2 to .
Central N. Amer. (CAN)   to   2 to 8  3 to   2. to 8
Central America (CAM)  1. to .  2 to .  2 to .0  2. to .0
Precipitation  Change (% per century) half it per fifty years. 
  DJF  MAM  JJA  SON
Western N. Amer.(WNA)  - to 20  - to +1  -1 to +10  -30 to +10
Central N. Amer. (CAN)  -10 to 10  - to +1  -20 to +10  -30 to +
Central America (CAM)  - to -1  -0 to +1  - to +1  -3 to +20

Future Climate of the Continental United States

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