An Unexpected Decline in Spring Atmospheric Humidity in the Interior Southwestern United States and Implications for Forest Fires

Tess W. P. Jacobson aLamont-Doherty Earth Observatory, Columbia University, Palisades, New York
bDepartment of Earth and Environmental Sciences, Columbia University, New York, New York

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Richard Seager aLamont-Doherty Earth Observatory, Columbia University, Palisades, New York

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A. Park Williams cDepartment of Geography, University of California, Los Angeles, Los Angeles, California

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Isla R. Simpson dClimate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, Colorado

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Karen A. McKinnon eDepartment of Statistics and Data Science, University of California, Los Angeles, Los Angeles, California
fInstitute of the Environment and Sustainability, University of California, Los Angeles, Los Angeles, California
gDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California

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Haibo Liu aLamont-Doherty Earth Observatory, Columbia University, Palisades, New York

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Abstract

On seasonal time scales, vapor pressure deficit (VPD) is a known predictor of burned area in the southwestern United States (“the Southwest”). VPD increases with atmospheric warming due to the exponential relationship between temperature and saturation vapor pressure. Another control on VPD is specific humidity, such that increases in specific humidity can counteract temperature-driven increases in VPD. Unexpectedly, despite the increased capacity of a warmer atmosphere to hold water vapor, near-surface specific humidity decreased from 1970 to 2019 in much of the Southwest, particularly in spring, summer, and fall. Here, we identify declining near-surface humidity from 1970 to 2019 in the southwestern United States with both reanalysis and in situ station data. Focusing on the interior Southwest in the months preceding the summer forest fire season, we explain the decline in terms of changes in atmospheric circulation and moisture fluxes between the surface and the atmosphere. We find that an early spring decline in precipitation in the interior region induced a decline in soil moisture and evapotranspiration, drying the lower troposphere in summer. This prior season precipitation decline is in turn related to a trend toward a Northern Hemisphere stationary wave pattern. Finally, using fixed humidity scenarios and the observed exponential relationship between VPD and burned forest area, we estimate that with no increase in temperature at all, the humidity decline alone would still lead to nearly one-quarter of the observed VPD-induced increase in burned area over 1984–2019.

Significance Statement

Burned forest area has increased significantly in the southwestern United States in recent decades, driven in part by an increase in atmospheric aridity [vapor pressure deficit (VPD)]. Increases in VPD can be caused by a combination of increasing temperature and decreasing specific humidity. As the atmosphere warms with climate change, its capacity to hold moisture increases. Despite this, there is a decrease in near-surface air humidity in the interior southwestern United States over 1970–2019, which during the summer is likely caused by a decline in early spring precipitation leading to limited soil moisture and evaporation in spring and summer. We estimate that this declining humidity alone, without an increase in temperature, would cause about one-quarter of the VPD-induced increase in burned forest area in this region over 1984–2019.

© 2024 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Tess W. P. Jacobson, tessj@ldeo.columbia.edu

Abstract

On seasonal time scales, vapor pressure deficit (VPD) is a known predictor of burned area in the southwestern United States (“the Southwest”). VPD increases with atmospheric warming due to the exponential relationship between temperature and saturation vapor pressure. Another control on VPD is specific humidity, such that increases in specific humidity can counteract temperature-driven increases in VPD. Unexpectedly, despite the increased capacity of a warmer atmosphere to hold water vapor, near-surface specific humidity decreased from 1970 to 2019 in much of the Southwest, particularly in spring, summer, and fall. Here, we identify declining near-surface humidity from 1970 to 2019 in the southwestern United States with both reanalysis and in situ station data. Focusing on the interior Southwest in the months preceding the summer forest fire season, we explain the decline in terms of changes in atmospheric circulation and moisture fluxes between the surface and the atmosphere. We find that an early spring decline in precipitation in the interior region induced a decline in soil moisture and evapotranspiration, drying the lower troposphere in summer. This prior season precipitation decline is in turn related to a trend toward a Northern Hemisphere stationary wave pattern. Finally, using fixed humidity scenarios and the observed exponential relationship between VPD and burned forest area, we estimate that with no increase in temperature at all, the humidity decline alone would still lead to nearly one-quarter of the observed VPD-induced increase in burned area over 1984–2019.

Significance Statement

Burned forest area has increased significantly in the southwestern United States in recent decades, driven in part by an increase in atmospheric aridity [vapor pressure deficit (VPD)]. Increases in VPD can be caused by a combination of increasing temperature and decreasing specific humidity. As the atmosphere warms with climate change, its capacity to hold moisture increases. Despite this, there is a decrease in near-surface air humidity in the interior southwestern United States over 1970–2019, which during the summer is likely caused by a decline in early spring precipitation leading to limited soil moisture and evaporation in spring and summer. We estimate that this declining humidity alone, without an increase in temperature, would cause about one-quarter of the VPD-induced increase in burned forest area in this region over 1984–2019.

© 2024 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Tess W. P. Jacobson, tessj@ldeo.columbia.edu
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  • Abatzoglou, J. T., and A. P. Williams, 2016: Impact of anthropogenic climate change on wildfire across western US forests. Proc. Natl. Acad. Sci. USA, 113, 11 77011 775, https://doi.org/10.1073/pnas.1607171113.

    • Search Google Scholar
    • Export Citation
  • Abatzoglou, J. T., D. S. Battisti, A. P. Williams, W. D. Hansen, B. J. Harvey, and C. A. Kolden, 2021: Projected increases in western US forest fire despite growing fuel constraints. Commun. Earth Environ., 2, 227, https://doi.org/10.1038/s43247-021-00299-0.

    • Search Google Scholar
    • Export Citation
  • Adams, D. K., and A. C. Comrie, 1997: The North American monsoon. Bull. Amer. Meteor. Soc., 78, 21972214, https://doi.org/10.1175/1520-0477(1997)078<2197:TNAM>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Awange, J. L., K. X. Hu, and M. Khaki, 2019: The newly merged satellite remotely sensed, gauge and reanalysis-based multi-source weighted-ensemble precipitation: Evaluation over Australia and Africa (1981–2016). Sci. Total Environ., 670, 448465, https://doi.org/10.1016/j.scitotenv.2019.03.148.

    • Search Google Scholar
    • Export Citation
  • Boos, W. R., and S. Pascale, 2021: Mechanical forcing of the North American monsoon by orography. Nature, 599, 611615, https://doi.org/10.1038/s41586-021-03978-2.

    • Search Google Scholar
    • Export Citation
  • Brown, P. J., and A. T. DeGaetano, 2013: Trends in U.S. surface humidity, 1930–2010. J. Appl. Meteor. Climatol., 52, 147163, https://doi.org/10.1175/JAMC-D-12-035.1.

    • Search Google Scholar
    • Export Citation
  • Byrne, M. P., and P. A. O’Gorman, 2015: The response of precipitation minus evapotranspiration to climate warming: Why the “wet-get-wetter, dry-get-drier” scaling does not hold over land. J. Climate, 28, 80788092, https://doi.org/10.1175/JCLI-D-15-0369.1.

    • Search Google Scholar
    • Export Citation
  • Chiodi, A. M., B. E. Potter, and N. K. Larkin, 2021: Multi-decadal change in western US nighttime vapor pressure deficit. Geophys. Res. Lett., 48, e2021GL092830, https://doi.org/10.1029/2021GL092830.

    • Search Google Scholar
    • Export Citation
  • Cook, B. I., J. E. Smerdon, R. Seager, and S. Coats, 2014: Global warming and 21st century drying. Climate Dyn., 43, 26072627, https://doi.org/10.1007/s00382-014-2075-y.

    • Search Google Scholar
    • Export Citation
  • Dai, A., 2006: Recent climatology, variability, and trends in global surface humidity. J. Climate, 19, 35893606, https://doi.org/10.1175/JCLI3816.1.

    • Search Google Scholar
    • Export Citation
  • Daly, C., M. Halbleib, J. I. Smith, W. P. Gibson, M. K. Doggett, G. H. Taylor, J. Curtis, and P. P. Pasteris, 2008: Physiographically sensitive mapping of climatological temperature and precipitation across the conterminous United States. Int. J. Climatol., 28, 20312064, https://doi.org/10.1002/joc.1688.

    • Search Google Scholar
    • Export Citation
  • Douglas, M. W., R. A. Maddox, K. Howard, and S. Reyes, 1993: The Mexican monsoon. J. Climate, 6, 16651677, https://doi.org/10.1175/1520-0442(1993)006<1665:TMM>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Douville, H., S. Qasmi, A. Ribes, and O. Bock, 2022: Global warming at near-constant tropospheric relative humidity is supported by observations. Commun. Earth Environ., 3, 237, https://doi.org/10.1038/s43247-022-00561-z.

    • Search Google Scholar
    • Export Citation
  • Finco, M., B. Quayle, Y. Zhang, J. Lecker, K. A. Megown, and C. K. Brewer, 2012: Monitoring Trends and Burn Severity (MTBS): Monitoring wildfire activity for the past quarter century using Landsat data. Moving from Status to Trends: Forest Inventory and Analysis (FIA) Symp. 2012, Newtown Square, PA, U.S. Department of Agriculture, Forest Service, Northern Research Station, 222–228, https://www.nrs.fs.usda.gov/pubs/gtr/gtr-nrs-p-105papers/35finco-p-105.pdf.

  • Gelaro, R., and Coauthors, 2017: The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). J. Climate, 30, 54195454, https://doi.org/10.1175/JCLI-D-16-0758.1.

    • Search Google Scholar
    • Export Citation
  • Gimeno, L., and Coauthors, 2020: Recent progress on the sources of continental precipitation as revealed by moisture transport analysis. Earth-Sci. Rev., 201, 103070, https://doi.org/10.1016/j.earscirev.2019.103070.

    • Search Google Scholar
    • Export Citation
  • Harris, I., T. J. Osborn, P. Jones, and D. Lister, 2020: Version 4 of the CRU TS monthly high-resolution gridded multivariate climate dataset. Sci. Data, 7, 109, https://doi.org/10.1038/s41597-020-0453-3.

    • Search Google Scholar
    • Export Citation
  • Held, I. M., and B. J. Soden, 2006: Robust responses of the hydrological cycle to global warming. J. Climate, 19, 56865699, https://doi.org/10.1175/JCLI3990.1.

    • Search Google Scholar
    • Export Citation
  • Held, I. M., M. Ting, and H. Wang, 2002: Northern winter stationary waves: Theory and modeling. J. Climate, 15, 21252144, https://doi.org/10.1175/1520-0442(2002)015<2125:NWSWTA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Hersbach, H., and Coauthors, 2020: The ERA5 global reanalysis. Quart. J. Roy. Meteor. Soc., 146, 19992049, https://doi.org/10.1002/qj.3803.

    • Search Google Scholar
    • Export Citation
  • Homer, C. H., J. A. Fry, and C. A. Barnes, 2012: The National Land Cover Database. U.S. Geological Survey Fact Sheet, 4 pp., https://pubs.usgs.gov/fs/2012/3020/fs2012-3020.pdf.

  • Jacobson, T. W. P., R. Seager, A. P. Williams, and N. Henderson, 2022: Climate dynamics preceding summer forest fires in California and the extreme case of 2018. J. Appl. Meteor. Climatol., 61, 9891002, https://doi.org/10.1175/JAMC-D-21-0198.1.

    • Search Google Scholar
    • Export Citation
  • Juang, C. S., A. P. Williams, J. T. Abatzoglou, J. K. Balch, M. D. Hurteau, and M. A. Moritz, 2022: Rapid growth of large forest fires drives the exponential response of annual forest-fire area to aridity in the western United States. Geophys. Res. Lett., 49, e2021GL097131, https://doi.org/10.1029/2021GL097131.

    • Search Google Scholar
    • Export Citation
  • Kobayashi, S., and Coauthors, 2015: The JRA-55 reanalysis: General specifications and basic characteristics. J. Meteor. Soc. Japan, 93, 548, https://doi.org/10.2151/jmsj.2015-001.

    • Search Google Scholar
    • Export Citation
  • Lehner, F., C. Deser, I. R. Simpson, and L. Terray, 2018: Attributing the U.S Southwest’s recent shift into drier conditions. Geophys. Res. Lett., 45, 62516261, https://doi.org/10.1029/2018GL078312.

    • Search Google Scholar
    • Export Citation
  • Mankin, J. S., R. Seager, J. E. Smerdon, B. I. Cook, and A. P. Williams, 2019: Mid-latitude freshwater availability reduced by projected vegetation responses to climate change. Nat. Geosci., 12, 983988, https://doi.org/10.1038/s41561-019-0480-x.

    • Search Google Scholar
    • Export Citation
  • Mantua, N. J., S. R. Hare, Y. Zhang, J. M. Wallace, and R. C. Francis, 1997: A Pacific interdecadal climate oscillation with impacts on salmon production. Bull. Amer. Meteor. Soc., 78, 10691080, https://doi.org/10.1175/1520-0477(1997)078<1069:APICOW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Martens, B., and Coauthors, 2017: GLEAM v3: Satellite-based land evaporation and root-zone soil moisture. Geosci. Model Dev., 10, 19031925, https://doi.org/10.5194/gmd-10-1903-2017.

    • Search Google Scholar
    • Export Citation
  • McKinnon, K. A., A. Poppick, and I. R. Simpson, 2021: Hot extremes have become drier in the United States southwest. Nat. Climate Change, 11, 598604, https://doi.org/10.1038/s41558-021-01076-9.

    • Search Google Scholar
    • Export Citation
  • O’Gorman, P. A., and C. J. Muller, 2010: How closely do changes in surface and column water vapor follow Clausius–Clapeyron scaling in climate change simulations? Environ. Res. Lett., 5, 025207, https://doi.org/10.1088/1748-9326/5/2/025207.

    • Search Google Scholar
    • Export Citation
  • Scheff, J., and J. C. Burroughs, 2023: Diverging trends in US summer dewpoint since 1948. Int. J. Climatol., 43, 41834195, https://doi.org/10.1002/joc.8081.

    • Search Google Scholar
    • Export Citation
  • Seager, R., and N. Henderson, 2013: Diagnostic computation of moisture budgets in the ERA-interim reanalysis with reference to analysis of CMIP-archived atmospheric model data. J. Climate, 26, 78767901, https://doi.org/10.1175/JCLI-D-13-00018.1.

    • Search Google Scholar
    • Export Citation
  • Seager, R., and M. Hoerling, 2014: Atmosphere and ocean origins of North American droughts. J. Climate, 27, 45814606, https://doi.org/10.1175/JCLI-D-13-00329.1.

    • Search Google Scholar
    • Export Citation
  • Seager, R., and Coauthors, 2014: Dynamical and thermodynamical causes of large-scale changes in the hydrological cycle over North America in response to global warming. J. Climate, 27, 79217948, https://doi.org/10.1175/JCLI-D-14-00153.1.

    • Search Google Scholar
    • Export Citation
  • Seager, R., A. Hooks, A. P. Williams, B. Cook, J. Nakamura, and N. Henderson, 2015: Climatology, variability, and trends in the U.S. vapor pressure deficit, an important fire-related meteorological quantity. J. Appl. Meteor. Climatol., 54, 11211141, https://doi.org/10.1175/JAMC-D-14-0321.1.

    • Search Google Scholar
    • Export Citation
  • Seager, R., T. J. Osborn, Y. Kushnir, I. R. Simpson, J. Nakamura, and H. Liu, 2019: Climate variability and change of Mediterranean-type climates. J. Climate, 32, 28872915, https://doi.org/10.1175/JCLI-D-18-0472.1.

    • Search Google Scholar
    • Export Citation
  • Seager, R., M. Ting, P. Alexander, J. Nakamura, H. Liu, C. Li, and I. R. Simpson, 2022: Mechanisms of a meteorological drought onset: Summer 2020 to spring 2021 in southwestern North America. J. Climate, 35, 73677385, https://doi.org/10.1175/JCLI-D-22-0314.1.

    • Search Google Scholar
    • Export Citation
  • Simpson, I. R., K. A. McKinnon, D. Kennedy, D. M. Lawrence, F. Lehner, and R. Seager, 2023: Observed humidity trends in dry regions contradict climate models. Proc. Natl. Acad. Sci. USA, 121, e2302480120, https://doi.org/10.1073/pnas.2302480120.

    • Search Google Scholar
    • Export Citation
  • Smith, A., N. Lott, and R. Vose, 2011: The Integrated Surface Database: Recent developments and partnerships. Bull. Amer. Meteor. Soc., 92, 704708, https://doi.org/10.1175/2011BAMS3015.1.

    • Search Google Scholar
    • Export Citation
  • Swann, A. L. S., F. M. Hoffman, C. D. Koven, and J. T. Randerson, 2016: Plant responses to increasing CO2 reduce estimates of climate impacts on drought severity. Proc. Natl. Acad. Sci. USA, 113, 10 01910 024, https://doi.org/10.1073/pnas.1604581113.

    • Search Google Scholar
    • Export Citation
  • Ting, M., R. Seager, C. Li, H. Liu, and N. Henderson, 2018: Mechanism of future spring drying in the southwestern United States in CMIP5 models. J. Climate, 31, 42654279, https://doi.org/10.1175/JCLI-D-17-0574.1.

    • Search Google Scholar
    • Export Citation
  • Turco, M., J. T. Abatzoglou, S. Herrera, Y. Zhuang, S. Jerez, D. D. Lucas, A. AghaKouchak, and I. Cvijanovic, 2023: Anthropogenic climate change impacts exacerbate summer forest fires in California. Proc. Natl. Acad. Sci. USA, 120, e2213815120, https://doi.org/10.1073/pnas.2213815120.

    • Search Google Scholar
    • Export Citation
  • Westerling, A. L., H. G. Hidalgo, D. R. Cayan, and T. W. Swetnam, 2006: Warming and earlier spring increase western U.S. forest wildfire activity. Science, 313, 940943, https://doi.org/10.1126/science.1128834.

    • Search Google Scholar
    • Export Citation
  • Williams, A. P., and Coauthors, 2014a: Causes and implications of extreme atmospheric moisture demand during the record-breaking 2011 wildfire season in the southwestern United States. J. Appl. Meteor. Climatol., 53, 26712684, https://doi.org/10.1175/JAMC-D-14-0053.1.

    • Search Google Scholar
    • Export Citation
  • Williams, A. P., and Coauthors, 2014b: Correlations between components of the water balance and burned area reveal new insights for predicting forest fire area in the southwest United States. Int. J. Wildland Fire, 24, 1426, https://doi.org/10.1071/WF14023.

    • Search Google Scholar
    • Export Citation
  • Williams, A. P., J. T. Abatzoglou, A. Gershunov, J. Guzman-Morales, D. A. Bishop, J. K. Balch, and D. P. Lettenmaier, 2019: Observed impacts of anthropogenic climate change on wildfire in California. Earth’s Future, 7, 892910, https://doi.org/10.1029/2019EF001210.

    • Search Google Scholar
    • Export Citation
  • Williams, A. P., and Coauthors, 2020: Large contribution from anthropogenic warming to an emerging North American megadrought. Science, 368, 314318, https://doi.org/10.1126/science.aaz9600.

    • Search Google Scholar
    • Export Citation
  • Williams, A. P., B. I. Cook, and J. E. Smerdon, 2022: Rapid intensification of the emerging southwestern North American megadrought in 2020–2021. Nat. Climate Change, 12, 232234, https://doi.org/10.1038/s41558-022-01290-z.

    • Search Google Scholar
    • Export Citation
  • Wills, R. C. J., R. H. White, and X. J. Levine, 2019: Northern Hemisphere stationary waves in a changing climate. Curr. Climate Change Rep., 5, 372389, https://doi.org/10.1007/s40641-019-00147-6.

    • Search Google Scholar
    • Export Citation
  • Xia, Y., and Coauthors, 2012: Continental-scale water and energy flux analysis and validation for the North American Land Data Assimilation System project phase 2 (NLDAS-2): 1. Intercomparison and application of model products. J. Geophys. Res., 117, D03109, https://doi.org/10.1029/2011JD016048.

    • Search Google Scholar
    • Export Citation
  • Zhuang, Y., R. Fu, B. D. Santer, R. E. Dickinson, and A. Hall, 2021: Quantifying contributions of natural variability and anthropogenic forcings on increased fire weather risk over the western United States. Proc. Natl. Acad. Sci. USA, 118, e2111875118, https://doi.org/10.1073/pnas.2111875118.

    • Search Google Scholar
    • Export Citation
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