Using human observations with instrument-based metrics to understand changing rainfall patterns

Abstract

Shifting precipitation regimes are a well-documented and pervasive consequence of climate change. Subsistence-oriented communities worldwide can identify changes in rainfall patterns that most affect their lives. Here we scrutinize the importance of human-based rainfall observations (collated through a literature review spanning from 1994 to 2013) as climate metrics and the relevance of instrument-based precipitation indices to subsistence activities. For comparable time periods (1955-2005), changes observed by humans match well with instrumental records at same locations for well-established indices of rainfall (72% match), drought (76%), and extreme rainfall (81%), demonstrating that we can bring together human and instrumental observations. Many communities (1114 out of 1827) further identify increased variability and unpredictability in the start, end, and continuity of rainy seasons, all of which disrupt the cropping calendar, particularly in the Tropics. These changes in rainfall patterns and predictability are not fully captured by existing indices, and their social-ecological impacts are still understudied.

Fig. 1. General overview of human observations of rainfall changes in comparison with instrumental observations of precipitation.

A Ecozonal distribution of selected rainfall observation types made in subsistence-oriented communities and their agreement with instrument-based metrics; (B) Distribution of subsistence-oriented communities reporting changes in rainfall patterns (black circles), superposed on the decadal trend in Dimensionless Seasonality Index (DSI)^, (1955–2005). Solid grey lines indicate −23.5 and 23.5 degrees latitude (Tropics). Dashed lines indicate −35 and 35 degrees latitude (Subtropics); (C) Distribution of subsistence-oriented communities reporting changes in rainfall predictability (black circles), superposed on the decadal trend in DSI^, (1955–2005). Solid grey lines indicate −23.5 and 23.5 degrees latitude (Tropics). Dashed lines indicate −35 and 35 degrees latitude (Subtropics). TP Total Precipitation, R95p Annual total precipitation from days >95 percentile, CDD Consecutive Dry Days, PDSI Palmer Drought Severity Index. Human observations are provided as source data.

Fig. 2. Geographical distribution of human observations that match (in black) or do not match (in purple) with precipitation indices.

A TP (GPCC version 7; resolution: 2.5 × 2.5) with human observations of rainfall; (B) R95p (GHCNDEX; resolution: 2.5 × 2.5) with human observations of extreme rainfall; (C) CDD (GHCNDEX; res. 2.5 × 2.5) with human observations of drought; (D) PDSI (Dai et al.; resolution: 2.5 × 2.5) with human observations of drought. Note: there is a match if the sign of the climate trend at each location is consistent with the sign of the human observation. TP Total Precipitation, R95p Annual total precipitation from days >95 percentile, CDD Consecutive Dry Days, PDSI Palmer Drought Severity Index, GPCC Global Precipitation Climatology Centre, GHCNDEX gridded temperature and precipitation climate extremes indices from the Global Historical Climatology Network (GHCN) dataset, HadEX3 dataset of gridded station-based climate extremes indices. Human observations are provided as source data.

Fig. 3. Aggregated percent agreement in gridboxes for three time periods between human observations of rainfall changes and annual Total Rainfall (TP).

A Human observations and TP calculated for the period 1955–2005; (B) Human observations and TP calculated for the period 1975–2005; (C) Human observations and TP calculated for the period 1985–2005. Interestingly, there are fewer gridboxes with agreement for the shorter time series (1985–2005) and some shifting of gridboxes between the bottom two panels (1975–2005 and 1985–2005), but overall not much change in the percent agreement. Note: Human observations are provided as source data.

Fig. 4. Distribution of the observations of rainfall predictability.

The maps on the left (and on the background on the right) show the (A) Annual Relative Entropy; (B) Annual Rainfall; and (C) DSI (Dimensionless Seasonality Index)^, calculated over the period 1955–2005 as decadal trends. Note: Only values around zero indicate no or minor changes. “Match” means that the human observation category “Rainfall - increased variability/changed patterns” matched the direction of the indexes for the plots on the right. Human observations are provided as source data.