Mayors and representatives of nearly 160 cities -- ranging from Chicago and Albuquerque in the United States, to Stockholm, Athens and Moscow in Europe to Dar es Salam, Tanzania, in Africa -- signed the so-called Hamburg Declaration.
It includes an agreement to launch in the signatory cities measures to reduce greenhouse gas emissions, increase energy efficiency and boost renewable energy generation.
"Cities occupy only 2 percent of the world's land mass but are responsible for 75 percent of greenhouse gas emissions," said Anja Hajduk, the environment minister of the state of Hamburg. "This document shows that … we accept our special role in climate protection."
The declaration is also a strong call on leaders at the U.N.-organized climate conference in Copenhagen to do everything in their power to come to a binding agreement, Hajduk said.
"We call on the politicians: 'Let Copenhagen be a success,'" she said.
The signing of the paper was the final moment of the first Hamburg City Climate Conference, launched Monday in this northern German port city. It brought together nearly 400 representatives of cities from all over the world to find new ways to battle climate change in urban settings.
"We as cities can tell the story of climate change, can show leadership and initiate the change," said Marijke Vos, responsible for environment and urban planning in Amsterdam.
The Dutch city aims to satisfy 30 percent of its energy needs with renewables by 2020. New buildings planned in the inner city have to be climate-neutral for residential or climate-positive (meaning they produce more energy than they consume) for commercial construction.
The heating system of Stockholm in Sweden is already fed to 80 percent by renewables, but the city wants to have a fossil fuel-free system by 2015, said Gunnar Soederholm, the head of Stockholm's environment directorate.
In the United States, more than 1,000 cities have signed up to the Kyoto Protocol, to which the Copenhagen conference is due to find an ambitious successor. Many U.S. cities have launched programs to reduce their CO2 emissions.
Conference host city Hamburg, home to Europe's second-largest harbor, is especially sensitive to climate change.
In 1962 a storm flood that submerged large parts of the city killed more than 300 people and left tens of thousands homeless. Rising North Sea levels because of climate change mean that the city could experience heavier and more frequent flooding. Hamburg Mayor Ole von Beust recently allocated some $750 million to heighten the city's extensive dike system by around 3 feet.
But while European cities have the resources to protect themselves against climate change and initiate positive change, cities in poorer countries are often dependent on help from the rich.
Dar es Salam in Tanzania is troubled by a massive influx of migrants. Its roads are congested, leading to a surging CO2 footprint.
"We would love to have something like a subway, but we simply don't have the resources," said Adam Kimbisa, the city's mayor.
Nicolas You, of U.N. Habitat, said politicians often forget that "95 percent of urban growth will happen in developing countries … in cities you have never even heard of."
He mentioned Kisumu, in Kenya, a city with a population of around 750,000.
"It will double in size over the next five to eight years," You said, adding that Kisumu does not have resources or expertise on how to grow in a sustainable manner.
"We need cities from the North to help cities in the South. They need to tell them, 'don't copy our mistakes' … and help them not to repeat them," You said.
Copenhagen, for that matter, might just as well culminate into the very opposite -- it might pit rich against poor.
Critics say the West has been too reluctant to transfer funds to developing countries for climate-change mitigation and adaptation.
Kimbisa, from Dar es Salam, said the West would eventually have to help poor countries reduce emissions to prevent temperature rises that could end life as we know it.
"You may sleep in a big house … you may have come here in a big Mercedes, but in the end, we all breathe the same air," he said.