The international climate science research community has concluded that human activities are changing the Earth’s climate in ways that increase risk to cities. This conclusion is based on many different types of evidence, including the Earth’s climate history, observations of changes in the recent historical climate record, emerging new patterns of climate extremes, and global climate models. Cities and their citizens already have begun to experience the effects of climate change.
Human activities are estimated to have caused approximately 1.0°C of global warming above pre-industrial levels, with a likely range of 0.8°C to 1.2°C. Global warming is likely to reach 1.5°C between 2030 and 2052 if it continues to increase at the current rate. Warming greater than the global annual average is being experienced in many land regions and seasons, including two to three times higher in the Arctic. Warming is generally higher over land than over the ocean.
Understanding and anticipating these changes will help cities prepare for a more sustainable future. This means making cities more resilient to climate-related disasters and managing long-term climate risks in ways that protect people and encourage prosperity. It also means improving cities’ abilities to reduce greenhouse gas emissions.
While projections for future climate change are most often defined globally, it is becoming increasingly important to assess how the changing climate will impact cities. The risks are not the same everywhere. For example, sea level rise will affect the massive zones of urbanization clustered along the world’s tidal coastlines and most significantly those cities in places where the land is already subsiding. In response to the wide range of risks facing cities and the role that cities play as home to more than half of the world’s population, urban leaders are joining forces with multiple groups including city networks and climate scientists.
Civic leaders are taking science-based actions that increase resilience and reduce greenhouse gas emissions, thus limiting the rate of climate change and the magnitude of its impacts.
In September 2015, the United Nations endorsed the new Sustainable Development Goal 11, which is to “Make cities and human settlements inclusive, safe, resilient and sustainable.” This new sustainability goal cannot be met without explicitly recognizing climate change as a key component. Likewise, effective responses to climate change cannot proceed without understanding the larger context of sustainability. As ARC3.2 demonstrates, actions take to reduce greenhouse gas emissions and increase resilience can also enhance the quality of life and social equity.
Globally, the impacts of climate-related disasters are increasing. The impacts of climate-related disasters may be exacerbated in cities due to interactions of climate change with urban infrastructure systems, growing urban populations, and economic activities. As the majority of the world’s population is currently living in cities–and this share is projected to increase in the coming decades, cities–need to focus more on climate-related disasters such as heat waves, floods, and droughts.
In a changing climate, a new decision-making framework is needed in order to fully manage emerging and increasing risks.
This involves a paradigm shift away from impact assessments that focus on single climate hazards based on past events. The new paradigm requires integrated, system-based risk assessments that incorporate current and future hazards throughout entire metropolitan regions.