Warming would leave great cities underwater

Hundreds of millions of urban dwellers around the world face their cities being inundated by rising seawaters if the latest United Nations warnings that the world is on course for 3 degrees C of global warming come true, according to a Guardian data analysis.

Famous beaches, commercial districts and swathes of farmland will be threatened at this elevated level of climate change, which the UN warned this week is a very real prospect unless nations reduce their carbon emissions.

Data from the Climate Central group of scientists analysed by Guardian journalists shows that 3C of global warming would ultimately lock in irreversible sea level rises of perhaps two metres.

Cities from Shanghai to Alexandria, and Rio to Osaka would be among the worst affected. Miami would be inundated – as would the entire bottom third of the US state of Florida.

In six of the coastal regions most likely to be affected, government planners are only slowly coming to grips with the enormity of the task ahead – and in some cases, they have done nothing.

This comes ahead of the latest round of climate talks in Bonn, Germany next week, when negotiators will work on ways to monitor, fund and ratchet up national commitments to cut carbon dioxide so that temperatures can rise on a safer path of between 1.5 and 2C, which is the goal of the Paris agreement reached in 2015.

The momentum for change is currently too slow, according to a report released this week by the UN Environment Programme.

Nature’s ability to help may also be diminishing. This week the World Meteorological Organisation said concentrations of CO2 in the atmosphere rose last year at a record speed to reach 403.3 parts per million – a level not seen since the Pliocene Era, three million to five million years ago.

A 3C rise would lead to longer droughts and fiercer hurricanes, and lock in sea level rises that would redraw many coastlines. Depending on the speed at which ice caps and glaciers melt, this could take decades or more than a century.

At least 275 million city dwellers live in vulnerable areas, the majority of them in Asian coastal megacities and industrial hubs such as Shanghai, Shenzhen, Bangkok and Tokyo.

Japan’s second-biggest city, Osaka, is projected to lose its business and entertainment districts of Umeda and Namba unless global emissions are forced down or flood defences are built up.

In Miami – which would be almost entirely below sea level even at 2C warming – the sense of urgency is evident at city hall, where commissioners are asking voters to approve a ‘‘Miami Forever’’ bond in a November ballot that includes US$192m for upgrading pumping stations, expanding drainage systems, elevating roads and building dykes.

Elsewhere, there is less money for adaptation, and a weaker sense of urgency.

In Rio de Janeiro, a 3C rise would flood famous beaches such as Copacabana, the waterfront domestic airport, and many of the sites for last year’s Olympic Games. But the cash-strapped city has been slow to prepare.

In Egypt, even a 0.5m sea level rise is predicted to submerge beaches in Alexandria and displace 8 million people on the Nile Delta unless protective measures are taken, according to the Intergovernmental Panel on Climate Change. But local activists say the authorities see it as a distant problem.