Shannon Stapleton / Reuters A child cools off from the hot weather at Domino Park in the Williamsburg section of Brooklyn.

A top New York City lawmaker announced a bill Monday to mandate dramatic energy use cuts in big buildings, by far the biggest source of carbon dioxide, in a historic move that could set a new standard for cities around the world.

The legislation plans to require the city’s largest buildings to reduce energy use by 20 percent by 2030, as well as to set a framework for increasing the cuts by 40 percent to 60 percent by 2050. Combined with projected increases for renewable energy capacity on the power grid, the city could reduce its climate-warming emissions by 80 percent. Electricity and heating in buildings make up nearly 70 percent of the city’s climate pollution, with luxury towers producing the lion’s share.

“The low-hanging fruit is gone,” City Councilman Costa Constantinides, a Queens legislator who leads the council’s Committee on Environmental Protection, said Monday morning on the steps of City Hall. “If we are going to make a real impact on climate change, it’s going to be on buildings.”

The legislation, which is not yet complete, would make the nation’s largest and most economically influential metropolis among the first major cities in the world to mandate strict retrofits on existing buildings to reduce planet-warming emissions. The proposals outlined in the bill came from an unprecedented first agreement, released last week, between environmental groups, affordable housing advocates, unions and the city’s real estate lobby on a set of policies to slash buildings’ carbon pollution 80 percent by 2050.

The only thing similar to the proposal is a cap-and-trade policy in Tokyo that allows big commercial landlords in the Japanese capital to buy and sell a limited and shrinking number of CO2 pollution permits, said Russell Unger, executive director of the Urban Green Council. Besides that, the activists and industry representatives his group convened to negotiate the proposal only had the last set of New York laws to upgrade green building requirements to use as legal guidelines.

“Nobody has done this in the world,” Pete Sikora, a senior adviser at the grassroots nonprofit New York Communities for Change, said ahead of Monday’s press conference. “This is new stuff.”

Climate policies tend to ripple far beyond New York’s five boroughs. The city is a financial and cultural giant with a gross domestic product big enough to rank among the world’s top 20 economies.

In January, Mayor Bill de Blasio announced a city lawsuit against five major oil companies over infrastructure damage from climate change; New York became one of the biggest municipalities to mount such a legal challenge. The mayor also unveiled plans to divest roughly $5 billion in fossil fuel investments from the city’s five pension plans. Other cities quickly followed suit, and, in July, Rhode Island became the first state to sue big oil firms.

The agreement last week came after months of talks between environmental groups and the powerful Real Estate Board of New York, which spent a record $10 million through its super PAC in 2013 on typically sleepy citywide races. In the end, the industry agreed to a legally enforceable 2030 target in exchange for more flexibility on how to reach the 2050 goals.