Introduction to the Special Issue on Earth’s Climate and Weather: Dominant Variability and Disastrous Extremes

We live on beautiful Planet Earth, which was born 4.6 billion years ago along with the rest of the solar system. Earth consists of an atmosphere, biosphere, hydrosphere and geosphere. Most of the time, Earth’s weather and climate are benevolent. We can hike in the forest, stand on a mountain top, lie down on the beach, and surf on the sea. With the advances of modern science and technology, we can also explore the poles and Mount Everest, dive into the Challenger Deep in the Mariana Trench, and even take measurements of Earth from space. However, once in a while, extreme weather and climate events hit the world, such as tropical cyclones, winter blizzards, summer heat waves, tornadoes, floods, and droughts. Sometimes, a mega-drought can last for decades or centuries. Throughout history, persistent mega-droughts drove the migration of early humans out of Africa (Scholz et al., 2007), the Migration Period and collapse of Western Roman Empire in Europe (Buntgen et al., 2011), the collapse of Maya civilization in the Americas (Kennett et al., 2012), and the collapses of several dynasties in China (Zhang et al., 2008). As we enter the new millennium, weatherand climate-related disasters are still frequently causing large numbers of fatalities and costly property damage (e.g. Goodkind & West, 2001; United Nation News, 2013). Three of the deadliest heat waves of the last decade caused 128,500 fatalities in France, Russia, India and Pakistan (Haider & Anis, 2015; McMichael & Lindgren, 2011; Reuters, 2015; Robine et al., 2008). Cyclone Nargis killed 138,000 people in Bangladesh in 2008 (Fritz et al., 2009). In the United States, $268 billion in damage was caused by three major hurricanes in 2017 (NOAA NCEI, 2018), which wiped out 60% of the country’s annual GDP growth in that year. The large number of fatalities and high cost of damage in recent years reflect the persistent difficulty in predicting weatherand climate-related disasters. For example, we still cannot predict the rapid intensification of hurricanes (Cangialosi, 2020), the occurrence of deadly tornadoes (Brooks et al., 2019), and the occurrence of widespread droughts around the world. The Earth’s climate is a complex system with strong feedbacks among its different components, i.e. the atmosphere, ocean, land, sea ice and biogeochemistry. Earth’s climate also covers a very wide range of time and spatial scales, from billions of years to seconds and from the whole globe to 10 metres. The key phenomena are summarized in Figure 1. Generally, weather refers to the shortterm, small-scale phenomena that last for seconds up to about half a month, and occur over one community, one state or one country, while climate refers to long-term, large-scale phenomena that last for one season up to 4.5 billion years, and occur over one continent, one ocean basin or the whole globe. The intraseasonal variability lies in between and connects weather and climate. In order to predict weatherand climate-related disasters successfully, research must move through three important stages: (i) comprehensive observations of the phenomena, (ii) deep understanding of the physical mechanisms, and (iii) accurate prediction. We have made significant progress on the first stage, thanks to the excellent works of many scientists, are now working on the second, and will eventually advance to the third. Figure 2 provides a brief overview of climate and weather research, including the history of observations and the histories of theories, modelling and predictions. The purpose of this special issue is to present a set of detailed review papers on some of the current frontiers in climate and weather research. They summarize the comprehensive observations obtained to date, the current understanding of the physical mechanisms and the major challenges/ difficulties remaining, and the state-of-the-art of modelling