First published in IGBP's Global Change magazine Issue 82, May 2014

Assessing risk
Risk assessments for coastal megacities have not yet fully encapsulated the dynamic interactions between environmental and anthropogenic forces. This is because of the complexity introduced by, for example, non-linear responses and feedback loops that can rapidly trigger potentially unexpected outcomes. One way of unpacking this complexity is to conceptualise risk as the nexus of three domains. The domain of environmental quality includes aspects such as resource depletion and ecosystem degradation. Natural hazard encompasses a wide range of hazards at a variety of scales. Socio-political and institutional context represents such pressures as population growth, governance regimes, political stability/instability and trade; these determine particular modes of urban development and patterns of vulnerability.

Urban population growth (largely because of migration) provides an example of how the different domains interact to generate risk. Population growth generates demand for food, water, energy and land. This may translate into the depletion of coastal ecosystems such as fish stocks and the destruction of wetland habitats. The degradation of mangroves and salt flats removes what would have served as a buffer against storms and coastal surges; it thereby heightens social and economic risk.

Whereas impacts can be highly localised, they can also aggregate and operate at a much larger scale. For example, models indicate that New York City and Tokyo modify the atmospheric flow within their confines and thereby affect regional weather systems. Such processes may introduce new drivers of risk in the context of climate change. However, such outcomes are not readily predicted from historical observations and the modelling of risk at the megacity-coast interface is thus difficult.

Globalisation introduces its own complexities. Megacities are connected to and co-dependent on other regions of the world. A crop failure or extreme-weather event in one part of the world can have drastic impacts on global markets, with knock-on effects on economic and social vulnerability of megacities.

Responding to the challenges
The complex challenges confronting megacities demand policy and research agendas that are innovative and forward thinking. We need to develop an international, integrated data archive for coordinated risk management and knowledge sharing. This should be coupled with long-term monitoring and model refinement. Finally, we need to better understand how global connectivity as well as feedbacks between natural and social systems could amplify risks and increase their unpredictability. These recommendations hold for other settings too, but the stakes in megacities are particularly high given the rapid increase in populations and assets at risk, the combination of local and global environmental change, and the potential for dangerous feedback loops.

In short, effective governance is a prerequisite to managing the risks that cities face and are perhaps most acute on the coast. Aspects such as health, economy, poverty, education, demography, gender, culture and the environment will need to be considered simultaneously. Because of the interlinkages between the risk domains discussed above, urban planning that prioritises either the environment or the society will not be sustainable. A holistic approach is called for.

Fortunately, megacities are well placed for the development of innovative solutions. In addition to being well connected, such cities possess a concentration of assets and expertise, and are hubs of political, academic and private-sector activity. Hence, they offer fresh opportunities for collaborative and complementary adaptation responses.

Megacities and global sustainability
As sites of consumption and nodes in the networks of global trade, large cities occupy a pivotal position in struggles towards global sustainability. There is no denying that the tremendous social and environmental impacts of the dominant model of mega-urbanisation present a hurdle to sustainable development.

Economic growth of an individual megacity might, at first glance, appear to be a positive step towards sustainability. Sanitation improves over time and air pollution decreases. However, many of the improvements are possible only by load displacement (eg, Hornborg 2011): this is the process whereby environmental gains in one region come at the expense of pollution elsewhere (or even globally). Many of the gains in average wellbeing might reflect the reliance on cheap, migrant labour that often struggles to fully benefit from the gains.

We are beginning to see that this model might not be sustainable in the long term. The collective contribution of coastal megacities to global environmental change is in some ways reflected in their increased susceptibility to sea-level rise and other hazards. Uneven and unplanned growth has translated into the poor living in low-lying, hazard-prone locations and severe modification of natural drainage (for example, the Mithi River in Mumbai). Even in high-income nations the increased interconnectivity of energy, water, transport, communication and security infrastructure has led to compounding risk.

Cities have already transformed their local environments. Adapting to live with global change will likely usher in a new era of environmental and social transformation as the desires for growth and security are rebalanced. Understanding better the interactions between ecological, physical, socio-economic and political processes is a starting point for making transformations that can be deliberate – rather than forced – and, through this process, for enhancing informed and transparent decision-making.