One of the delights of the tropics is the delicious cool air in the evening. As the sun goes down, people move gratefully to their gardens or balconies, or flock to parks to enjoy the remains of the day—but not in Singapore. The city is too hot, especially in the evening, with temperatures that often remain in the high 20’s or low 30’s Celsius (82° – 86°F) throughout the night. This is not just, because Singapore has a tropical climate, but because the city has grown warmer in recent years due to the “Urban Heat Island” effect (UHI). The reasons for UHI are well known. Cities have less vegetation than rural areas to provide shade and to cool the air; they consume huge amounts of energy in electricity and fuel; they trap the sun’s radiation in deep urban canyons between high buildings; and they contain massive amounts of steel and concrete that store the sun’s warmth. To be clear, UHI is not the same as climate change, although both cause Singapore to get warmer. According to a recent study by Singapore’s meteorological service, the Centre for Climate Research Singapore and the UK’s Met Office Hadley Centre, by the end of the century, average temperatures will rise by between 1.4°C and 4.6°C. In response to the global concern about climate change, Singapore has developed strong policies for understanding and addressing the potential problems. In particular, it is a signatory of the Paris Agreement that calls for capping the rise in global average temperature to, “well below 2 °C above pre-industrial levels,” while pursuing efforts to limit the increase to 1.5 °C. Few would dispute that this concern for climate change is right and responsible. However, the effect of climate change upon temperatures in Singapore is currently much smaller than that of the Urban Heat Island effect. While the effects of UHI vary across the city, the difference compared to rural areas averages 3°C to 4°C, and in the evening can be as much as 8°C. Such elevated temperatures should also be a matter for concern. Higher temperatures not only reduce thermal comfort, but also discourage people from outdoor activities such as walking and cycling. As the temperature rises, so does the demand for air-conditioning, which now accounts for over half of all electricity consumed in commercial buildings and a large percentage of residential buildings. Urban warming also has unwanted environmental effects, such as reducing the availability of water and trapping pollution and; therefore, reducing air quality. It also increases the occurrence of heavy storms and flash floods. If nothing is done to curb it, Singapore’s UHI will only get worse as the population grows and the size and density of buildings increase. Indeed, average temperatures could well increase by 6°C by the end of the century through a combination of climate change and a growing UHI. Such an increase would severely impair the quality of life for Singaporeans, as well as pose a threat to the country’s environmental sustainability and economic activity. Thankfully, further warming of Singapore is not inevitable. There are many ways to mitigate the UHI effect, some of which have been known for centuries. Before the advent of air conditioning, builders showed great skill in designing structures that remained cool even in the hottest climates, in an approach known as “passive cooling.” A large suite of tried and tested measures is known, including green roofs, green corridors, passive cooling systems, reflective materials, permeable pavements, and so on. Today, we also have modelling tools that enable us to assess the contribution of these different measures to thermal comfort. In addition, new technologies offer exciting new possibilities, such as novel coating materials that reduce the absorption of solar energy or promote the re-radiation of energy. However, despite all of these promising technological advances, there are important gaps in our knowledge. For instance, how do we estimate the cumulative effect of many small measures upon the overall climate of the city? What steps should take the highest priority if we are to achieve a significant reduction in urban temperatures? Many people believe it is becoming urgent for Singapore to find answers to these questions and develop a concerted strategy to reducing its UHI. Such a strategy would be a major undertaking, and would affect many planning functions, including urban planning, building, transport, energy, and the environment, among others. To contribute to such a strategy, a research project, “Cooling Singapore,” was recently launched with the aim of providing actionable knowledge for policymakers. The project forms part of the National Research Foundation’s CREATE program and brings together research teams from the Singapore-ETH Centre, SMART, TUM CREATE and NUS. One of its goals is to build an expert community within academia and government that can help guide policy in mitigating UHI in the longer term. To meet this goal, it has set up a ‘UHI task force’ composed of representatives from government agencies and universities to facilitate the exchange of knowledge and to jointly develop an effective and realistic policy ‘roadmap’. The challenge of reversing urban warming is both daunting and rewarding. There is much to be done, but if we succeed, we can make future cities places where people feel comfortable outdoors as well as indoors, encouraging them to pursue more active lifestyles, while using less energy for cooling and transport. Best of all, we can enjoy sitting on the balcony with a cool beer, watching the sun go down! About the author: Peter Edwards is a Professor Emeritus of ETH Zurich and former Director of the Singapore-ETH Centre, which manages two major programs: The Future Cities Laboratory and Future Resilient Systems. He was professor of Plant Ecology at ETH Zurich and an adjunct professor in the Asian School of the Environment at the Nanyang Technological University (NTU) in Singapore.