# Global climate: monitoring drought using the self-calibrating Palmer drought severity index

J. Barichivich, T J Osborn, I Harris, A Gollop, G van der Schrier, P D Jones

► To cite this version:

J. Barichivich, T J Osborn, I Harris, A Gollop, G van der Schrier, et al.. Global climate: monitoring drought using the self-calibrating Palmer drought severity index. Bulletin of the American Meteorological Society, 2025, 106 (8), pp.S77-S78. 10.1175/BAMS-D-25-0102.1. hal-05396898

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Submitted on 3 Dec 2025

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## 12. MONITORING DROUGHT USING THE SELF-CALIBRATING PALMER DROUGHT SEVERITY INDEX

–J. Barichivich, T. J. Osborn, I. Harris, A. Gollop, G. van der Schrier, and P. D. Jones

The self-calibrating Palmer Drought Severity Index (scPDSI) for 1e50–2024 indicates a decline in global drought severity and extent in 2024, following the record-high peak in late 2023 (Barichivich et al. 2024; Fig. 2.47). Extreme drought (scPDSI c-4) affected around 5% of the global land throughout 2024, down from over 7% in July–August 2023. Severe and extreme drought combined (scPDSI c-3) stabilized near 12% of the global land area after reaching a record 17% in July 2023. Similarly, moderate or worse drought (scPDSI c-2) affected about 23% of global land in 2024 compared to 28% in mid-2023. Most of this decline occurred in regions where drought conditions of 2023 transitioned to normal or wetter conditions, particularly in the La Plata Basin, much of non-Mediterranean Europe, and the midlatitudes of Central Asia (Fig. 2.48). Meanwhile, severe drought persisted in southwestern and northern North America, parts of tropical South America, the Mediterranean and northwest Africa, southern Africa, parts of the Middle East, southern Australia, and Mongolia (Plate 2.1w).

In Canada, 2024 ranked as the driest year on the nationally averaged yearly scPDSI for the 1e50–2024 period. In the United States, severe-to-extreme drought conditions persisted through much of Arizona and New Mexico. Mexico and most countries across Central and South America experienced a mix of moderate drought and normal conditions (Plate 2.1w). On a country-averaged basis, 2024 was the third-driest year in both Peru and Brazil, with vast areas of the Amazon basin enduring extreme drought. By mid-October, the Rio Negro at Manaus, a major tributary of the Amazon River, recorded its lowest water levels since records began in 1e02 (updated from Barichivich et al. 2018). In contrast, Chile experienced a moisture recovery in 2024 following a prolonged megadrought (Garreaud et al. 2025). In terms of average drought severity, 2024 ranked as the 18th-driest year since 1e50, following the driest years on record in 2021 and 2022.

Northwest Africa experienced record or near-record drought levels (Plate 2.1w), with 2024 being the driest year in the Canary Islands and Morocco, second driest in Algeria, and third driest in Tunisia. In southeastern Europe, it was the driest year in Greece and third driest in Bulgaria and Romania. In Italy, 2024 was ranked sixth driest for country-average drought severity, with the top six drought years all occurring in the last eight years.

Although uncertain due to sparser observations, moisture patterns in tropical Africa did not change much in 2024 with respect to 2023 (Fig. 2.48). Southern Africa experienced a continuation of drought conditions seen since

2018, and its severity continued mostly as moderate (Plate 2.1w). In Australia, the moisture pattern in 2024 also remained similar to that in 2023, with the coastal parts of the

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Fig. 2.47. Percentage of global land area (excluding ice sheets and deserts) with self-calibrating Palmer Drought Severity Index (scPDSI) indicating moderate (<-2), severe (<-3), and extreme (<-4) drought for each month of 1950-2024. Inset: each month of 2024 (shading) compared with 2023 (dashed lines).

Fig. 2.48. Change in drought categories from 2023 to 2024 (mean self-calibrating Palmer Drought Severity Index [scPDSI] for 2024 minus mean scPDSI for 2023). Increases in drought severity are indicated by negative values (brown), and decreases by positive values (green). No calculation is made where a drought index is meaningless (gray areas: ice sheets or deserts with approximately zero mean precipitation).

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country continuing under moderate drought (Plate 2.1w). Wet conditions seen through most of
India and Southeast Asia since 2022 continued during 2024. Most of the previous severe-to-ex-
treme drought conditions through China and Kazakhstan shifted to normal or wet conditions,
but Mongolia saw continued drought.

Hydrological drought results from a period of abnormally low precipitation, sometimes exac-
erbated by a concurrent increase in evapotranspiration (ET). Its occurrence can be apparent in
reduced river discharge (section 2de), groundwater storage, (section 2d10), and/or soil moisture
subsection). Here, the scPDSI (Wells et al. 2004; van der Schrier et al. 2013) is presented, using global precipitation and Penman-Monteith Potential ET from the Climatic Research Unit terrestrial series (CRU TS 4.0e) dataset (Harris et al. 2020). A simple water balance at the core of the scPDSI estimates actual evapotranspi-
ration, soil moisture content, and runoff based on the input precipitation and potential loss of
moisture to the atmosphere. Estimated soil moisture categories are calibrated over the complete
1e01-2024 period to ensure that “extreme” droughts and pluvials (wet periods) relate to events
that do not occur more frequently than in approximately 2% of the months. This calibration
affects direct comparison with other hydrological cycle variables in Plate 2.1w that use a dif-
ferent baseline period. All country rankings mentioned above are based on the ranking of spatial
averages of annual scPDSI for each country for the 1e50-2024 period; pre-1e50 data are not used
for rankings because observational coverage is poor in some countries.