More Intense Precipitation Extremes: Are They Linked to Human Activity?

Two papers published in early 2011, together with several other papers published in the last few years, suggest a strong linkage between more intense precipitation extremes human-added CO₂ in the atmosphere. The first paper by Min et al. (2011) compares observed and simulated changes in extreme precipitation based on annual maxima of daily precipitation amount and also five-day consecutive amount over the Northern Hemisphere land-area, using precipitation data for the period 1951-1999 from the Hadley Centre (UK) global land-based gridded climate extreme dataset. Multi-model simulations were obtained from the CMIP3 (Coupled Model Intercompariosn Project Phase 3) archive and also from individual climate modeling centers. The models included historical natural forcing (solar and volcanic) and also anthropogenic forcing, due to greenhouse gases. According to Min et al., their study reveals that "human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events over approximately two-thirds of data-covered parts of Northern Hemisphere land-areas. The study further suggests that (future) extreme precipitation projected by models may be underestimated because most models seem to underestimate observed increases in precipitation due to warming"

The second paper by Pall et al. (2011) focuses on a major flood event in England and Wales in autumn 2000 and strongly implicates 'world-wide increased greenhouse gas emissions' as providing an increased risk of flood occurrence there. The paper generates several thousand climate model simulations of autumn 2000 weather, both under realistic conditions and under conditions as they might have been had these greenhouse gas emissions and the resulting large-scale warming never occurred. The model simulations suggested, according to the authors, that "the 20th century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%"

The two above studies add to a large body of alarmist literature on precipitation extremes linking their occurrences to increasing anthropogenic greenhouse gas emissions. However, there are still large uncertainties associated with many of these types of climate modeling studies, which do not consider impact of other processes (be they natural or human-induced) on precipitation extremes. For example, a paper by Kishtawal et al. (2010) documented an urbanization signature in the observed heavy rainfall climatology of summer (June-September) monsoon over India. Many large cities in India (with population over 5 million) have witnessed an increasing frequency of extreme precipitation which Kishtawal et al. attributed to UHI (Urban Heat Island) induced mesoscale convergence. In another recent paper (Kunkel et al., 2010), an increasing frequency of extreme precipitation in the US was linked to tropical cyclones and more landfalling of hurricanes, especially in the southeastern US over the period 1994-2008. In the monsoonal climate of India, many extreme precipitation events (as high as 200 mm/day) have been documented during the early part of the 20th century, when anthropogenic greenhouse gas emissions were much lower than present (Dhar and Nandargi 1993). Another paper by Kunkel (2003) examined North American trends in extreme precipitation and documented high EPI (extreme precipitation index) values during the decade 1891-1900 and also the recent decade 1991-2000, thus implying a role for natural variability in producing extreme precipitation events.

In summary, the modeling studies reported above neglect to adequately consider other processes, including natural variability, that may explain observed extreme precipitation trends in many land-areas of the Northern Hemisphere. A close inspection of long-term climatologies, especially for the monsoonal climate region of south Asia, for example, reveals large numbers of extreme precipitation events occur primarily due to natural variability, while the role of anthropgenic forcing appears to be a distant secondary influence.

Additional References
Dhar, O.N. and Nandargi, S. 1993. The zones of severe rainstorm activity over India. International Journal of Climatology 13: 301-311.

Kishtawal, C.M., Niyogi, D., Tewari, M., Pielke, Sr., R.A. and Sheppard, J.M. 2010. Urbanization signature in the observed heavy rainfall climatology over India. International Journal of Climatology 30: 1908-1916, doi:10.1002/joc.2044.

Kunkel, K.E. 2003. North American trends in extreme precipitation 2003. Natural Hazards 29: 291-305.

Kunkel, K.E., Easterling, D.R., Kristovitch, D.A.R., Gleason, B., Stoecker, L. and Smith, R. 2010. Recent increases in US heavy precipitation associated with tropical cyclones. Geophysical Research Letters 37: L24706 doi:10.1029/2010GL045164.