Drought Variability

Drought variability is of crucial importance not only to farmers but also to people throughout the world who rely on food and resources from a region. Unfortunately, instrumental records provide only a limited picture of the size and severity of historical droughts as well as the factors impacting drought variability throughout the world.

Tree-ring data have been used to extend records of past drought. The tree-ring reconstructions show a more complete range of drought variability than is provided by the 20th century instrumental record. The extended records are useful for placing the instrumental period droughts into a longer time frame and for evaluating the rarity of the major 20th century, 1930s, and 1950s droughts in a broader temporal context.

The drought data are in the form of summer (June–August)​ averages of the Palmer Drought Severity Index (PDSI) or Palmer Modified Drought Index (PMDI). Negative Palmer Index values indicate dry conditions, while positive values indicate wet conditions. Palmer Drought Index values generally fall between -6 and +6.

Palmer Drought Index Studies

• Living Blended Drought Atlas (LBDA) - Version 2
  The half degree gridded Jun-Aug PMDI reconstructions from Cook et al. (2010) were recalibrated using the Global Historical Climatology Network (GHCN) 5km grid PMDI data. The 5km data were first upscaled to match the original half-degree grid. The recalibration was performed using a kernel density distribution mapping (KDDM) technique outlined in McGinnis et al. (2015) using an R-package provided by Seth McGinnis. We used a 50-year recalibration period covering 1929–1978 CE. We also adjusted each grid point's mean PMDI value for the recalibration period to be zero to avoid importing wet or dry bias into the recalibration. The recalibrated data set covers the continental United States just as the GHCN instrumental data does. Since instrumental data was used for 1979–2005 CE in the Cook dataset, recalibration applied only to the years 0–1978 CE. The 1979–2017 instrumental years are filled in using the post-recalibration period values from the GHCN PMDI data. The data are available in a year per row text file, a data point per row text file, and a netCDF collection. There is also a directory of annual PMDI maps.
• North American Drought Atlas (NADA) (Version 2a)
  Cook et al. 2008 reconstruction of North American summer PDSI using a 2.5° x 2.5° grid. Updated PDSI values from Cook et al. 2004. Temporal coverage is 0–2006 AD. The full dataset is also available for download as a netCDF collection.
• North American Drought Atlas
  Cook et al. 2004 reconstruction of summer PDSI for most of North America using a 2.5° x 2.5° grid. Temporal coverage is 0–2003 AD. The full dataset is also available for download as a netCDF collection.
• Living Blended Drought Atlas (LBDA) PDSI Dataset
• Mexican Drought Atlas (MXDA) PDSI Dataset
• Monsoon Asia Drought Atlas (MADA) PDSI Dataset
• Old World Drought Atlas (OWDA) PDSI Dataset
• Eastern Australia and New Zealand Drought Atlas (ANZDA) PDSI Dataset

Additional Palmer Drought Severity Index Studies

• Cook et al. 1999 PDSI Reconstruction
  Covers the coterminous United States using a 155-point grid of 2° latitude x 3° longitude. Temporal coverage is 1700–1978 AD.
• Zhang et al. 2004 PDSI Reconstruction
  Uses the same coterminous United States grid and has the same temporal coverage as Cook et al. 1999.

Other Data Archives

• NCEI Climate Monitoring
  Explore NCEI's efforts to monitor and assess the state of the Earth's climate, including current levels of drought coverage.
• North American Drought Atlas (2004 Version) Home Page
  Lamont-Doherty Earth Observatory Tree Ring Lab archive of NADA drought reconstuctions. Includes additional PDSI maps and animations.
• National Integrated Drought Information System (NIDIS)
  Data portal that monitors current drought conditions and provides risk and impact assessment tools.