Have precipitation extremes and annual totals been increasing in the world's dry regions over the last 60 years?

TL;DR

This study reevaluates claims of increasing extreme rainfall in dry regions over 60 years, highlighting the importance of data processing choices and dryness definitions in trend detection.

Contribution

It demonstrates that previous reported trends are overestimated due to statistical artifacts and that definitions of dry regions significantly influence trend assessments.

Findings

No significant increase in extreme rainfall in dry regions after accounting for artifacts.

Data processing methods critically affect trend detection in climate studies.

Proper dryness definitions are essential for robust trend analysis.

Abstract

Daily rainfall extremes and annual totals have increased in large parts of the global land area over the last decades. These observations are consistent with theoretical considerations of a warming climate. However, until recently these global tendencies have not been shown to consistently affect land regions with limited moisture availability. A recent study, published by Donat et al. (2016, Nature Climate Change, doi:10.1038/nclimate2941), now identified rapid increases in globally aggregated dry region daily extreme rainfall and annual rainfall totals. Here, we reassess the respective analysis and find that a) statistical artifacts introduced by the choice of the reference period prior to data standardization lead to an overestimation of the reported trends by up to 40%, and also that b) the definition of `dry regions of the globe' affect the reported globally aggregated trends in extreme rainfall. Using the same observational dataset, but accounting for the statistical artifacts and using alternative, well-established dryness definitions, we find no significant increases in heavy precipitation in the world's dry regions. Adequate data pre-processing approaches and accounting for uncertainties regarding the definition of dryness are crucial to the quantification of spatially aggregated trends in the world's dry regions. In view of the high relevance of the question to many potentially affected stakeholders, we call for a cautionary consideration of specific data processing methods, including issues related to the definition of dry areas, to guarantee robustness of communicated climate change relevant findings.