under RCP 4.5 over the next 30 years were pre-
dicted to increase the average cost of migration
along random paths between Nogales and
Three Points by 34.1% for an adult man (1.81 ±
0.003 liters), 34.1% for a nonpregnant adult
woman (1.63 ± 0.003 liters), 33.1% for a child
(0.94 ± 0.001 liters), and 29.6% for a preg-
nant adult woman (1.90 ± 0.003 liters). Taken
together, these results indicate that undocu-
mented migration across the southwest border
of the United States will become increasingly
dangerous over the next 30 years, which
will likely result in increased mortality of
migrants.
Biophysical models such as those presented
here have been previously used to under-
stand the dynamics of thermohydric stress in
nonhuman animals in ecologically relevant
contexts. We present a new application of bio-
physical modeling to human systems, provid-
ing a methodological framework for future
studies that seek to understand the impacts
of past, present, and future climate on human
physiology, stress, and evolution [see ( 19 ) for
caveats and considerations]. Our findings dem-
onstrate the utility of social scientific inputs
to adapt these modeling tools to humans
moving through extreme landscapes. We have
used this approach to show that thermohydric
stress is a major contributor to patterns of
undocumented migrant mortality at the
southern US border. Such quantitative frame-
works for studying intersecting impacts of
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ACKNOWLEDGMENTS
We acknowledge the invaluable contributions of the men, women,
and children whose personal experiences and stories informed
the parameters of this study. We also acknowledge the innumerable
lives, both known and unknown, that have been affected by the past
and ongoing circumstances that contextualize this work. This
study would have been impossible without the bravery and sacrifice
of these individuals.Funding:Infrastructure support for this
study was provided by the University of California, Los Angeles
(UCLA); University of Idaho; and University of Wisconsin.Author
contributions:S.C.C.-S. and R.A.L. conceived of and conceptualized
this study. S.A.R., P.D.M., and W.P. developed the methodology
described herein. R.H.W., S.A.R., P.D.M., and W.P. performed
modeling and validation of the experiments performed. J.D.L.
provided ethnographic data used to parameterize the models. All
results were visualized by R.H.W. Supervision of experiments
and their contextualization were provided by R.A.L., H.L., and J.D.L.
The original draft of the manuscript was written by S.C.C.-S.,
H.L., and R.H.W., then reviewed and edited by S.C.C.-S., R.H.W.,
S.A.R., J.D.L., H.L., W.P., P.D.M., and R.A.L.Competing interests:
The authors declare that they have no competing interests.
Data and materials availability:All data are available in the main
text and supplementary materials.SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abh1924
Materials and Methods
Figs. S1 to S18
Tables S1 to S8
References ( 52 Ð 115 )
MDAR Reproducibility Checklist
Data S1 to S625 February 2021; resubmitted 4 June 2021
Accepted 21 October 2021
10.1126/science.abh19241500 17 DECEMBER 2021¥VOL 374 ISSUE 6574 science.orgSCIENCE
Fig. 4. Predicted costs of migration under current versus future climate scenarios.Distribution of
predicted water costs for diurnal (left) versus nocturnal (right) travel by four migrant demographics
traversing random paths (n= 20) between Nogales and Three Points during summer (May to September)
based on previous 30-year monthly average temperatures (“current climate”) and locally downscaled climate
projections of temperature change over the next 30 years (“future climate”). Pixel-specific values for future
climate conditions were averaged across six publicly available, locally downscaled climate models (ACCESS-1,
CanESM2, CCSM4, CNRM-CM5, CSIRO-Mk3, INM-CM4) that projected monthly maximum and minimum
temperatures in 2050 under RCP 4.5 (i.e., the intermediate mitigation scenario) ( 47 , 48 ). Predicted water
costs for migration on foot across the southern border increase significantly under this climate-change scenario
relative to current conditions for all modeled migrant demographics (linear mixed-effects model,P<0.001,
Wald test). Boxes and whiskers represent median, interquartile range, and range of predicted water loss.
RESEARCH | REPORTS