Changes in Precipitation Extremes in Major Global Urban Areas

Urban areas are centers of infrastructure and wealth and are home to 50% of the global population. Regional and local-scale, observation-based studies suggest that precipitation extremes have increased in the recent past and climate models suggest that these increases are likely to persist as the climate warms. Heavy to very heavy precipitation leads to flooding in urban areas and there is some evidence that suggests that urban flood losses have increased in recent years. In this study, we provide a global overview of changes in precipitation extremes in the world's 180 most populous urban centers over the past 60 years. For the period 1951-2007, we collected daily precipitation data from multiple sources for each of the urban areas, with a preference for high quality ground-based observations. We analyzed trends in annual maximum precipitation at 1, 2 and 5-day durations for each of these areas. Regional flood frequency analysis was used to distinguish changes in urban precipitation maxima for the first and second halves of the 1951-2007 period. We used the non-parametric Mann-Kendall trend analysis to estimate trends and their significance, while Bayesian change point analysis was performed to estimate the time at which changes in trends occurred. Our results suggest that many urban areas around the globe (well more than would be expected due to chance) have experienced statistically significant changes in precipitation extremes over the 58-year period of analysis, with more urban centers experiencing increases than decreases. In addition, for the urban centers that experienced statistically significant changes, positive changes tended to be larger than negative changes.