Drought Frequency Explorer
What will changing temperature and precipitation patterns mean for future drought and its frequency?
Explore: Moderate Drought
Drag the slider to compare historical drought frequency with the Moderate Drought frequencies expected under a RCP4.5 (intermediate emissions) climate scenario. On the left are historical observations (1473 to 2005) of drought return periods with a specified duration of at least four years or a Palmer Drought Severity Index (PDSI) of -2.5 or below. On the right are the RCP4.5 climate scenario projections for return periods for drought with the same characteristics. The return periods are shown in years, where dark orange represents 10-25 years and dark yellow indicates a return period of 25-50 years.
How does the drought return period change under a high emissions scenario? Drag the slider to compare the RCP8.5 climate scenario projections to historical values.
How does the emissions scenario used affect the projected changes in drought frequency?
Drag the slider to compare results for the RCP4.5 scenario (left) with those for the RCP8.5 scenario (right).
Explore: Extreme Drought
Drag the slider to compare historical drought values with the frequencies for Extreme Drought (duration ≥ 4 years and PDSI ≤ -2.5) expected under a RCP4.5 (intermediate emissions) climate scenario. The blues and light yellow shading indicate Extreme Drought return periods on the order of 50 to 500 years in the historical record (left). Projected return periods are shown on the right, where the dark yellow color corresponds to return periods of 25-50 years.
How does the return period for Extreme Drought change under a high emissions scenario? Drag the slider to compare the projections to historical values.
How does the emissions scenario used affect the projected changes in drought frequency?
Drag the slider to compare results for the RCP4.5 scenario (left) with those for the RCP8.5 scenario (right).
What Do the Results Show?
The analysis and projections reveal that droughts are becoming more frequent. Historical observations (1473 to 2005) of drought return periods with a specified duration of at least four years and/or a severity (PDSI) of -2.5 are shown on the left. The years from 1473 to 2005 were selected as the common period of record for the Western United States in the analysis; significant drought events also occurred before the start of this analysis period. Shifts in the colors on the maps from blues and light yellow to dark orange and red indicate shortening return periods, corresponding to more frequent drought.
Projected drought return periods for 2006 to 2099 indicate that for nearly the entire domain, future Moderate Droughts meeting the criteria specified are expected to recur with an increased frequency, on average once every 10-25 years.
Comparing results for the RCP4.5 and RCP8.5 climate scenarios reveals that water-limited conditions occur in an intermediate emissions environment, and even when looking at only Moderate Drought criteria.
Extreme droughts will also occur more often. Droughts lasting at least four years and having severity (PDSI) of -2.5 based on observations from 1473 to 2005 have occurred about once every 100-500 years, on average, for the majority of the domain. The projected return periods based on the RCP4.5 scenario indicate that the specified drought conditions are expected to become more frequent, occurring, on average, once every 25-50 years. Under the higher (RCP8.5) emissions scenario, in some areas, the Extreme Drought return period shortens to 10-25 years.
Note that the information shown in the maps is for two examples using specified criteria for drought duration and severity. Drought events with different thresholds will have different return periods, and the changes in those return periods will also vary. The two example events, and the return periods for those events based on climate conditions projected from RCP4.5 and RCP8.5 emissions scenarios, none-the-less provide us illustrative information.
About the Analysis
Analyzing changes in drought frequency depends on looking at changes in the return period, or interarrival times, of drought that meet specified criteria. For this study, the Palmer Drought Severity Index (PDSI) is the main tool for determining drought severity. [ Learn more about Drought Definitions and Characteristics .]
In the animation below, see how Drought Duration and Drought Severity can be used to help define Drought Interarrival Times (which relate directly to Drought Frequency).
