Droughts, Floods, and Global Warming

1. Droughts, Floods, and Global Warming Shaw Chen Liu Research Center for Environmental Change Academia Sinica Symposium and Review Panel Meeting Seoul, Korea Aug. 4-6, 2013

2. Acknowledgment A significant part of this talk is based on: Liu, S. C., C. Fu, C-J Shiu, J-P Chen, and F. Wu, GRL, 2009. Shiu, C-J, S. C. Liu & J-P Chen, Journal of Climate, 2009. Shiu, C. J., S. C. Liu, C. Fu, A. Dai, and Y. Sun, GRL, 2012. Also recent collaborative work with Prof. Yuanhang Zhang, Jun Li and Run Liu (Peking Univ.) Appreciate the help with data analysis: Ms. You-Yu Mao and Mr. Chih-Wei Wan

3. Increases in very heavy precipitation, and often with decreases in light precipitation have been reported in recent years over most land areas (e.g. Karl & Knight, 1998; Manton et al., 2001; Klein Tank and Können, 2003; Fujibe et al., 2005; Goswami et al. 2006; Liu et al. 2009) as well as the tropical oceans (Lau and Wu, 2007). • Increases in heavy precipitation can lead to more and worse floods and mudslides. • Light and moderate precipitation is a critical source of soil moisture and ground water, its reduction increases the risk of droughts.

4. Linear trends of precipitations ( in mm/4 hours) at different intensities in Japan, (a) for 3 time periods, (b) for 4 seasons, (c) for 3 regions, and (d) for 3 urban population ranges. (Fujibe et al., 2005)

5. Changes (%) in precipitation intensity (10 blue bins) and consecutive dry days in Taiwan for each degree warming in global temperature(Based on Liu et al. GRL2009) method2

6. Changes in global precipitation intensity from climate models and observations for one degree warming in global temperature(Liu et al. GRL2009)

7. Two mechanisms, global warming and effects of anthropogenic aerosols,have been proposed as possible causes of the increases of heavy precipitation and decreases of light precipitation. • Which is the primary cause?

8. Dynamically how should the precipitation intensity change in a warming globe? • Trenberthet al. (2003) hypothesized that heavy precipitation of large storms should increase at about the same rate as atmospheric moisture, i.e. about 7 %/K according to the Clausius-Clapeyronequation. • The increase of heavy precipitation could even exceed the moisture increase because additional latent heat released from the increased water vapor could invigorate the storms. • In turn, the invigorated storms can increase the stability of the atmosphere, thereby suppressing light and moderate precipitation.

9. From IPCC 2007 Total column water vapor has increased over the global oceans by about 1.2% per decade from 1988 to 2004, consistent with the assumption of near-constant RH. Column water vapor is from the SSM/I: Special Sensor Microwave/Imager

10. less polluted activation more polluted activation Indirect Effects of Aerosols • The aerosol indirect effect is the change in cloud properties caused by a change in the Cloud Condensation Nuclei (CCN) (Twomey, 1977). • There are two kinds of indirect effects: • Albedo, surface area increase (First indirect effect) • Cloud lifetime increase (Second indirect effect) • Possible impacts on precipitation: • Delay and/or suppression of precipitation by aerosols (Twomey et al. 1984; Albrecht, 1989; Rosenfeld and Lensky, 1998).

11. Numerous recent works on aerosol effects on precipitation, especially in the Amazon • Aerosols invigorate large convections by suppressing onset of precipitation, pulling in more moisture, releasing more latent heat, pushing moisture to higher altitude and forming more ice clouds (Andreae et al., 2004) • For small, low clouds the action stops at suppressing onset of precipitation and burning off the clouds (Koren et al., 2008). An overall net effect is to increase the precipitation intensity.

12. We examine data from urban and rural stations in China and Taiwan, as well as global data (GPCP) to show that global warming rather than aerosol effects is the primary cause of the increases of heavy precipitation and decreases of light precipitation.

13. AOD around Taiwan from MODIS(Li et al., 2004)

14. Taiwan: West (polluted) compared to East (clean) Changes of bottom 10% light precipitation in summer

15. Taiwan: West (polluted) compared to East (clean) Changes of top 10% heavy precipitation in summer

16. Geographical distribution of 109 surface meteorological stations in Mainland China with average annual precipitation over 500 mm, gray dots :40 urban stations, green dots: 69 rural stations

17. Amounts of annual bottom 10% light precipitation (penal a) and top 10% heavy precipitation (penal b), colored shades denote 1-standard deviations.

18. Changes (%) in precipitation intensities in southeastern China (blue), land area in 20N-45N (green), and 10S-10N oceanic area (yellow) for each degree warming of the temperature in the 30S-30N zone

19. A brief conclusion • The large increases in precipitation intensity, including the increases of heavy precipitation and decreases of light precipitation, are most likely driven by global warming rather than aerosols.

20. Explore a causal link among global warming, changes in precipitation extremes, and higher risk of floods and droughts.

21. Changes of dry days and light rain days in Taiwan Total changes from 1961 to 2011 Light-rain days: -27 days (141 days in 1960) Heavy-rain days: +5.5 days (13.5 days in 1960) Dry days: +21.5 days (207 days in 1960)

22. Changes (%) in precipitation intensity (10 blue bins) and consecutive dry days in Taiwan for each degree warming in global temperature(Based on Liu et al. GRL2009) method2

24. Changes in global precipitation intensity from climate models and observations (mostly from satellites) for one degree warming in global temperature(Liu et al. GRL2009)

25. Conclusions • Evidence supports a causal link starting with global warming, to increase in precipitation intensity, to changes in precipitation extremes, and to the increased risk of floods and droughts. • In southeastern China there have been a severe increase of about 75% in annual days with top 1% heavy precipitation, an increase of 35% in the annual occurrence of 10 or more consecutive days without precipitation, and an astonishing increase of about 20 times in the annual occurrence of PDSI (Palmer Drought Severity Index) <= -3 during the period 1955-2011. • Greater changes are found at lower latitudes for heavy precipitation, e.g. about 100% increase in the top 10% heavy precipitation in Taiwan in the last 50 years (200% increase in the top 1% heavy precipitation).

26. Thank you for your attention!

27. Annual total precipitation in China averaged over 40 urban stations (red) and 69 rural stations (blue), Colored shades denote 1-standard deviations.