Africa, Asia and the Americas Face Intensifying Drought Under High-Emission Futures

Climate change may not simply expand the geographical reach of drought; it could push many already water-stressed regions toward substantially more severe conditions. A global modelling analysis finds that extreme drought expands far more rapidly under high-emission pathways, with parts of Africa, the Americas, Asia and Australia emerging as persistent centres of growing climatic stress.

The study, "Assessing Global Drought Trends Under Climate Change: A Comparative Analysis of Four Drought Indices Across SSP Scenarios," published in the journal Remote Sensing by Jingwen Cui and Yangxiaoyue Liu, examines global drought evolution from 1850 to 2100 using climate simulations from the Coupled Model Intercomparison Project Phase 6, or CMIP6. Rather than relying on a single drought measure, the researchers compare four indices designed to capture different dimensions of water stress.

Their results reveal two interconnected challenges. Future drought severity changes sharply depending on the emissions pathway, while the picture of drought itself changes depending on how it is measured. The analysis identifies the Drought Severity Index, or DSI, and Standardized Soil Moisture Index, or SSMI, as particularly informative within the modelling framework used, but for different reasons.

Drought's future is increasingly about severity, not simply spread

The study compares drought conditions under four Shared Socioeconomic Pathways ranging from SSP1-2.6, representing a relatively low-emission trajectory, to SSP5-8.5, representing very high greenhouse gas emissions. Across the projections, rising emissions are associated with progressively stronger expansion of severe and extreme drought.

Moderate drought covers a comparatively large area through much of the simulation period but changes relatively little over time. Severe and extreme drought behave differently. Their affected areas rise considerably, with extreme drought accelerating most rapidly under the highest-emission scenario.

The scale of the projected change is striking within the study's DSI calculations. Between 2015 and 2100, extreme-drought area increases at roughly 79,384 square kilometres per year under SSP1-2.6, compared with about 464,157 square kilometres per year under SSP5-8.5. SSMI produces the same overall ordering, with extreme soil-moisture drought increasing much more rapidly as emissions rise.

Those figures require careful interpretation because each climate pathway was standardized separately. Even so, the broader pattern across the modelling framework remains clear: higher-emission futures are associated with a stronger shift toward severe and extreme drought conditions, while the lowest-emission pathway produces a slower escalation.

Geographically, the projected pressure is highly uneven. By the end of the century, southern North America, northern South America, Africa, eastern Australia and parts of Asia, including southern China and Southeast Asia, emerge as major drought-sensitive regions. Many areas showing pronounced drought conditions early in the projection period continue to intensify toward 2100.

Why measuring drought differently changes the climate-risk picture

Drought cannot be reduced to rainfall alone. A region may experience insufficient precipitation, rising evaporative demand, depleted soil moisture or severe thermal stress on vegetation, and each condition can evolve on a different timescale. The study tests four widely used indicators rather than assuming that any single metric captures the full phenomenon.

The Standardized Precipitation Evapotranspiration Index, or SPEI, incorporates precipitation and potential evapotranspiration. The Temperature Condition Index uses surface temperature to represent vegetation heat stress. DSI tracks climatic water balance, while SSMI focuses directly on soil-moisture anomalies.

When the researchers compared the indices across drought-affected regions in Europe, the Horn of Africa, central Chile, the western United States and southern China, DSI and SSMI produced stronger seasonal correspondence with the drought periods examined than SPEI-12 and TCI. Their behaviour was also more internally consistent within the study's CMIP6 modelling framework.

DSI reacts comparatively strongly to changes in precipitation and potential evapotranspiration and displays larger variations in drought-affected area. SSMI changes more smoothly because it reflects the behaviour of soil moisture, making it especially informative for understanding longer-lasting land-surface water deficits.

Used together, the two indices reveal different dimensions of the same evolving hazard. DSI captures changes in climatic water balance, while SSMI exposes how moisture stored in soils responds over time. Higher-emission scenarios also amplify spatial contrasts in SSMI, with drying areas becoming drier while some regions showing wetting trends experience stronger wetting.

The comparison carries a practical implication for drought monitoring. Agricultural systems, water-resource planners and ecosystem managers may need different indicators depending on the type of stress they are trying to anticipate. A monitoring framework dominated by one metric can overlook conditions that become visible through another.

Climate hazard becomes more dangerous where vulnerability is already high

The projected drought hotspots carry consequences extending well beyond climate science. The study links worsening drought conditions with pressure on agriculture, freshwater availability and ecosystems, particularly where rising temperatures combine with reduced precipitation or stronger evaporative losses.

For developing countries, physical drought hazard is only part of the risk. Similar climatic conditions can produce radically different consequences depending on water infrastructure, agricultural dependence, income levels and institutional capacity. The study notes that regions with lower human development often have less capacity to absorb or adapt to drought shocks.

Sub-Saharan Africa and South Asia receive particular attention because climatic hazard can coincide with high population exposure and socioeconomic vulnerability. Under such conditions, worsening drought can move rapidly through food systems, rural livelihoods and water supply, turning a climatic event into a broader development crisis.

Human management of water can also amplify or reduce these pressures. Reservoir regulation, irrigation, groundwater withdrawals and land-use change influence runoff and soil moisture, while excessive extraction can intensify shortages. Conversely, storage systems and improved water allocation can provide buffers during prolonged dry periods.

The researchers caution against interpreting climate-model drought projections as complete representations of future drought risk. Their analysis captures the climate-driven component but does not directly incorporate human water management. Actual impacts will depend on how societies manage demand, infrastructure, land and water resources as climatic conditions change.

The findings point toward several adaptation priorities already discussed in the study: more efficient allocation of water, wider adoption of water-saving agricultural technologies and stronger drought resilience in urban water systems. Such measures become increasingly important where climatic drying intersects with rapid development or limited water reserves.

Powerful projections, but important uncertainties remain

The study's global scope and long historical-to-future time horizon provide a broad view of how drought could evolve under contrasting climate pathways. Its comparison of multiple indices also avoids treating drought as a single-variable phenomenon and highlights the value of combining climatic water-balance and soil-moisture information.

Methodological constraints, however, place important boundaries around the findings. Precipitation, surface temperature and upper-layer soil moisture are drawn from 16 CMIP6 models, whereas potential evapotranspiration is available from only three. DSI and SPEI therefore combine variables derived from different model populations, which may influence their calculated water balance.

The study also calculates drought indices after averaging climate-model inputs. Ensemble averaging can smooth spatial contrasts and temporal extremes that individual models produce. Because the analysis does not quantify inter-model spread, it cannot establish how many individual models support each regional pattern or projected trend.

Independent validation is another unresolved issue. Historical drought episodes are used as regional and seasonal reference points rather than as direct validation targets, and the particular ensemble constructed for the analysis was not independently tested against observational or reanalysis datasets.

Comparisons between emission pathways also require caution. DSI and SSMI were standardized separately for each SSP scenario using a combined historical and future series. Consequently, some cross-scenario differences can arise from the changing statistical reference used for standardization rather than emissions alone.

Future research could reduce these uncertainties by calculating drought indices separately for individual climate models, matching precipitation and evapotranspiration inputs from the same models and measuring the level of model agreement. A common historical baseline across scenarios would also strengthen direct comparisons between climate pathways.

Independent testing against observational datasets would help establish whether the apparent strengths of DSI and SSMI persist outside the internal CMIP6 framework. Incorporating population exposure, irrigation, groundwater extraction, reservoirs and socioeconomic vulnerability would then move the analysis from projected drought hazard toward a fuller understanding of drought risk.