Approximately 1 billion people currently live in informal settlements, primarily in urban areas in low-and middle-income countries. Informal settlements are defined by poor-quality houses or shacks built outside formal laws and regulations. Most informal settlements lack piped water or adequate provision for sanitation, drainage, and public services. Many are on dangerous sites because their inhabitants have a higher chance of avoiding eviction. This paper considers how to build resilience to the impacts of climate change in informal settlements. It focuses on informal settlements in cities in low-and middle-income countries and how these concentrate at-risk populations. This paper also reviews what is being done to address climate resilience in informal settlements. In particular, community-and city-government-led measures to upgrade settlements can enhance resilience to climate-change risks and serve vulnerable groups. It also discusses how the barriers to greater scale and effectiveness can be overcome, including with synergies with the Sustainable Development Goals. Rapid Urbanization and Growth of Informal SettlementsThe current urban population is approximately 4.4 billion people globally. About 3.4 billion people currently live in urban centers in what the United Nations (UN) terms ''less developed regions.'' 1 UN projections suggest that urban population growth in ''less developed regions'' will be over 2 billion people by 2050 and that close to 90% of this increase will be in Asia and Africa. This means that another 2 billion urban dwellers will require housing, basic services, and resilience to climate-change impacts. 1 At present, approximately 1 billion urban dwellers live in what are termed informal settlements in poor-quality houses or shacks. 2 Informal settlements fall outside formal laws and regulations on land ownership, land use, and buildings. Their illegality makes government agencies unable or unwilling to work with them. These are settlements to which city governments have not extended what the UN Intergovernmental Panel on Climate Change (IPCC) terms risk-reducing infrastructure (paved roads, storm and surface drainage, piped water, etc.) and services relevant to resilience (including healthcare, emergency services, and rules of law). 2 Many informal settlements are ill prepared for climate change and face particularly high risks of floods and landslides as a result of poor-quality buildings and a lack of infrastructure to prevent flooding, withstand heavy storms, and cope with heat waves. 2 In the absence of more effective policies, most of the world's growth in urban population will be accommodated in informal settlements. Given the projected rates and regions of urban population growth by 2050, there is an urgent need to build resilience to climate change in these settlements and to do so at scale. There is also an urgent need to vastly expand the supply and reduce the cost of ''formal'' (i.e., legal) housing that provides low-income groups with safer and more accessible alternatives to...

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