氣候變遷如何影響颶風、颱風和氣旋？

Hurricane Milton is making its way toward Florida, and forecasters are warning that it could bring extremely dangerous winds, significant storm surges, and heavy rainfall. This storm is expected to impact Tampa Bay, a densely populated area, just a short time after Hurricane Helene caused considerable damage in the state. Milton is the ninth hurricane of the 2024 Atlantic hurricane season, which runs until the end of November. Hurricanes are powerful storms that form over warm tropical ocean waters. In different parts of the world, these storms are referred to as cyclones or typhoons. Collectively, they are known as tropical cyclones. In the Atlantic and northeast Pacific, they are called hurricanes, while in the northwest Pacific, they are known as typhoons, and in the southwest Pacific and Indian Ocean, they are called cyclones. These storms are characterized by very high wind speeds, heavy rainfall, and short-term rises in sea levels, which can lead to widespread damage and flooding. Hurricanes are categorized based on their peak sustained wind speeds. Major hurricanes are classified as category three and above, meaning they have wind speeds of at least 111 miles per hour. The Saffir-Simpson scale categorizes hurricanes as follows: category one has peak sustained winds of 74 miles per hour; category two has winds above 96 miles per hour; category three has winds above 111 miles per hour; category four has winds above 130 miles per hour; and category five has winds above 157 miles per hour. Hurricanes, typhoons, and cyclones begin as weather systems, such as a tropical wave or an area of low pressure where thunderstorms and clouds develop. As warm, moist air rises from the ocean surface, the winds in the storm clouds start to spin. This spinning is influenced by the Earth's rotation, particularly in tropical regions near the equator. For a hurricane to develop and maintain its spin, the sea surface temperature generally needs to be at least 27 degrees Celsius to provide sufficient energy, and the winds must not vary significantly with height. When these conditions align, a powerful hurricane can form, although the specific causes of individual storms can be complex. Over the past century, the overall frequency of tropical cyclones has not increased globally, although long-term data is limited in some areas. However, it is likely that a greater proportion of tropical cyclones are reaching category three or higher, indicating they have the highest wind speeds. The United Nations climate body has reported medium confidence that there has been an increase in the average and peak rainfall rates associated with tropical cyclones. The frequency and intensity of rapid intensification, where maximum wind speeds increase quickly, can be particularly dangerous. Additionally, there appears to be a shift in the average location where tropical cyclones reach their peak intensity, which can expose new communities to these hazards. The increased intensity of hurricanes in the United States has resulted in more damage. Assessing the precise impact of climate change on individual tropical cyclones is challenging due to their localized and short-lived nature, as well as their variability. However, rising temperatures have measurable effects on these storms. Warmer ocean waters contribute to stronger storms, leading to higher wind speeds. Record high sea surface temperatures were a significant factor in the intensity of Hurricane Harvey in 2017. These high temperatures are primarily due to long-term greenhouse gas emissions. Additionally, a warmer atmosphere can hold more moisture, resulting in more intense rainfall. Climate change made the extreme rainfall from Hurricane Harvey approximately three times more likely. The rising sea levels, caused by a combination of melting ice and thermal expansion, mean that storm surges occur on top of already elevated sea levels. For instance, it is estimated that flood heights from Hurricane Katrina in 2005 were significantly higher than they would have been under the climate conditions of 1900. Overall, there is high confidence that human activities have contributed to increases in precipitation associated with tropical cyclones, and medium confidence that humans have increased the likelihood of tropical cyclones being more intense. Looking to the future, the number of tropical cyclones globally is unlikely to increase, according to climate experts. However, as global temperatures rise, it is very likely that these storms will experience higher rates of rainfall and reach greater top wind speeds. This means that a larger proportion of storms will reach the most intense categories, four and five. The more global temperatures rise, the more extreme these changes will be. The proportion of tropical cyclones reaching categories four and five may increase by around 10% if global temperature rises are limited to 1. 5 degrees Celsius, increasing to 13% at 2 degrees Celsius and 20% at 4 degrees Celsius, according to the IPCC, although the exact figures remain uncertain.