Long-term changes in precipitation extremes are of great importance to the welfare of human beings as well as the entire ecosystem. Increases in heavy precipitation can lead to more and worse floods, while persistent chronic decreases of light and moderate precipitation pose a serious threat to the drought problem because light and moderate precipitation are a critical source of water for the replenishment and retention of soil moisture.

　　In regard to the impact of climate changes, most people mainly notice the increasing global temperature and rising sea level. Many people also think that these changes are gradual, long term, and even remote. They often overlook that extreme weather events can also be affected by climate changes. One of the reasons is that extreme weather events affected by climate changes are very difficult to forecast, especially on the regional scale. As a result the United Nations 2007 IPCC Report (Intergovernmental Panel on Climate Change) was very conservative in the projection of changes in extreme weather events due to global warming.

　　Significant increases of the very heavy precipitation and decreases of light and moderate precipitation have been observed over most land areas of the globe in the last few decades. Over the oceans, analyses of satellite data at tropical/low latitudes in 1979-2003 also found similar changes. The widespread increases of heavy precipitation have been attributed to global warming by Trenberth et al. [2003]. These authors point out that, according to the Clausius-Clapeyron equation (C-C), tropospheric warming can lead to an increase of about 7% K-1 in the atmospheric water-holding capacity. This is supported by available observations of recent decades. They hypothesized that the precipitation intensity should increase at about the same rate as atmospheric moisture because precipitation rates from storms were determined by low-level moisture convergence. Furthermore, they argued that the increase in heavy rainfall could even exceed the moisture increase because additional latent heat released from the increased water vapor could feed back and invigorate the storms. However, they did not give a quantitative estimate of the changes in precipitation extremes due to global warming. Furthermore their hypothesis was not corroborated by a study of results from an ensemble of 17 latest generation climate models used by IPCC, which shows that, under global warming driven by increasing greenhouse gases in the models, the global mean increase of precipitation intensity is only about 2% K-1 for the ensemble average, substantially less than the 7% K-1 of C-C. This is another reason that IPCC has been very conservative in assessing the risk of changes in precipitation extremes due to global warming.

　　By using a new analysis methodology, this work finds that the top 10% bin of precipitation intensity increases by about 95% for each degree Kelvin (K) increase in global mean temperature, while lower 30%-60% bins decrease by about 20% K-1 (Figure 1, red bars). The global average precipitation intensity increases by about 23% K-1, qualitatively consistent with the hypothesis that the precipitation intensity should increase by more than the 7% K-1 water vapor increase because of the additional latent heat released from the increased moisture. However an ensemble of 17 latest generation climate models estimates an increase of only about 2% K-1 in precipitation intensity (blue bars), about one order of magnitude smaller than our value, suggesting that the risk of extreme precipitation events due to global warming is substantially greater than that estimated by the climate models. In this regard, our findings will enable a significant improvement in the assessment of the risk of changes in precipitation extremes in the next IPCC report.

　　For Taiwan, this study finds that the top 10% bin of precipitation intensity increases by about 140% for each degree increase in global temperature, while the lower 20% bins decrease by about 70% K-1(Figure 2). With global temperature increases by about 0.7K in the last 45 years, the top 10% bin of precipitation intensity in Taiwan has increased by about 100% in that period, and the corresponding decrease in bottom 20% light precipitation has been about 50%. Moreover, since the global temperature will increase by another 0.7K in about 30 years, another round of similar changes are expected. This implies that heavy rain in typhoons has doubled in last 45 years, and will triple in about 30 years, severely increasing the risk of floods and mudslides. Droughts will get worse and more frequent too. Since mitigation of the greenhouse warming by reducing CO2 emissions will take decades to be effective (because the CO2 residence time in the atmosphere is about 80 years), it is imperative that adaptation strategies in Taiwan such as flood control, water resource policy and land use plans are developed and implemented quickly.