New York's new mayor must build on Michael Bloomberg's green legacy

During his 12 years as mayor of New York city, billionaire businessman Michael Bloomberg has had an indelible effect on America's largest metropolis.

As his tenure comes to a close, both the media and New Yorkers are analysing his legacy, from his aggressive reform of the school system to his controversial stop-and-frisk police policy. Rarely mentioned, however, is Bloomberg's dogged effort to make New York a sustainable city by reducing its greenhouse gas emissions and protecting its 8.4 million residents from the perils of climate change.

Bloomberg's sustainability push began quietly in 2005 when demographers projected that New York City would grow by 1 million people by 2030. His staff immediately started searching for ways to fit the extra residents into an already cramped and overstrained city. The solution, they eventually realised, was sustainability. By conserving water and energy resources, slashing air pollution and bolstering public transportation, the city could make life better and healthier for its growing population—while also dealing with global warming.

For more than a year, hundreds of city staffers, local scientists, environmentalists and community leaders hammered out a sweeping sustainability agenda, known as PlaNYC. Released in April 2007, it included 127 initiatives that would transform New York into a greener city and a leader in the fight against climate change.

PlaNYC suffered some big setbacks in its first few years. New York state legislators rejected Bloomberg's much-lauded plan to charge drivers a fee to enter parts of Manhattan during rush hour—similar to an existing programme in London. The 2008 financial crisis forced budget cuts that delayed or scaled-back some projects. Real estate developers and property owners at first opposed stricter energy efficiency standards and green building codes.

But the Bloomberg administration pressed ahead, and New York was gradually and dramatically transformed.

Bloomberg and his staff created hundreds of acres of new parks, planted nearly 1 million trees and replenished wetlands. They turned concrete lots into green play spaces, built hundreds of miles of bike lanes and poured billions of dollars into cleaning up the city's waterways. A programme to swap high-sulfur heating oil with cleaner alternatives helped cut soot pollution 23% and sulphur dioxide 69%. The city reached the halfway point in its greenhouse gas emission reduction goal.

Despite this progress, however, little attention was paid to adaptation measures—often more costly and large-scale than mitigation—to protect residents and infrastructure along the city's 520-mile coastline from the impacts of climate change.

All that changed when superstorm Sandy slammed into New York City on 29 October, 2012.

Hurricane-force winds ripped across the city, tearing trees from their roots and snapping utility poles in half. Surges of water turned streets into rivers, flooding subway tunnels and washing away homes, storefronts and cars. Overnight, the city's worst climate-related fears came true. Sandy was a wake-up call that even after five years of work New York had a long way to go.

Scientists warned the risk would only worsen in the coming decades.

New York City has already experienced sea level rise nearly twice the global average since 1900, climbing 1.2 inches per decade. By the 2050s, sea level in New York harbour will likely rise 11 to 24 inches—or maybe as much as 31 inches. As ocean heights rise, even a storm much smaller than Sandy could cause the same amount of damage.

In the wake of Sandy, Bloomberg and his staff rushed to create a $19.5bn (£12bn) rebuilding plan that would not only help New York recover from the storm, but rebuild stronger and smarter so it would be better prepared for future climate threats. Finished in June 2013, the strategy, known as the special initiative on rebuilding and resiliency, required a massive overhaul of transportation, energy, parks, building and insurance programmes, as well as the creation of a sweeping coastal protection system. The plan includes 257 initiatives spread across the city's five boroughs.

Bloomberg and his staff have tried to finish or at least start as many of these initiatives as possible before the mayor leaves office this month. But responsibility for the majority of the plan—as well as for all of Bloomberg's sustainability and climate initiatives—will rest with future New York City leaders, beginning with Bloomberg's successor, Mayor-elect Bill de Blasio.

Under Bloomberg, New York has proven that local governments can help fill the void left by fragmented or non-existent national and global policies. It has created a roadmap for other cities looking to reduce their emissions and adapt to a warming climate. It has also shown the need for powerful, strong-willed leaders who understand that protecting their cities from climate change is a worthy pursuit. As Bloomberg told InsideClimate News in September, "You gotta lead from the front."

• Katherine Bagley and Maria Gallucci are the authors of Bloomberg's Hidden Legacy: Climate Change and the Future of New York City , published by InsideClimate News.