Cities have long been the world's economic dynamos. They contribute roughly two thirds of global gross domestic product (GDP), and are expanding at an unprecedented rate and scale. By 2030, it is estimated that 6.3 billion people will live in urban areas. That's more than the Earth's total population was when this millennium began. So it's no great surprise that cities generate more than 70 percent of the world's CO2 emissions - and much of that from fossil fuels.
In 2012 the International Energy Agency estimated in concrete terms that to limit climate change to below the 2?C of warming, which scientists say is the maximum safe level the Earth can sustain, more than two thirds of known fossil fuel reserves need to stay in the ground.
The latest assessment by the International Panel on Climate Change (IPCC) indicates a global carbon budget, or the estimated amount of carbon available to burn if the world is to limit runaway climate change. We also know that, if action is not taken soon, all the allowable emissions would be locked in by energy infrastructure existing within five years from now. Time is short.
The good news is that we know where to take action to prevent such lock-in and build a resilient energy future. Most of today's emissions come from cities, and within that, the vast majority from three sources: the energy used to create electricity, to heat and cool buildings, and for transportation. From the Earth Hour City Challenge, a year-long programme launched by World Wide Fund For Nature (WWF) to identify and promote cities that are leading climate action, it is clear that cities, and their mayors worldwide, are already demonstrating innovative solutions to reduce dependence on fossil fuels in each of these sectors.
In Canada, the 2012 Global Earth Hour City Challenge winner Vancouver has already set clear targets and a suite of policy decisions to reduce emissions and align its investments with its ambitious Greenest City Action Plan. Their holistic and ambitious actions range from renewable energy-based neighbourhood heating and cooling, to improved electric vehicle infrastructure and the development of a fossil fuel divestment plan.
The urgency of the need to radically change global energy systems is indisputable - but it will not be quick enough unless we all press for it urgently.
In the US, a number of cities have made the decision to generate their power from renewable sources. In many cases this involves an innovative method known as Community Choice Aggregation. In short, this means groups of buyers or energy users joining together to find an alternative to private utility companies - many of which hold fossil fuel-based monopolies in the areas they serve - by going through an open market renewable energy supplier.
In South Korea, Seoul is increasing the share of safe and renewable energy sources through investment in hydropower, solar cells and biogas production - in combination with smart technology to monitor and reduce energy consumption. The city aims to increase its self-sufficiency in electricity production from 2.8 percent in 2011 to 20 percent in 2020.
Other excellent examples are to be found in the global South, where a number of cities are implementing climate action or low-carbon strategies. In India, more than 48 cities have been ear-marked as Solar Cities, as part of India's national Solar Mission. Being a Solar City involves securing major investment in solar energy infrastructure for streetlights, cookers, water heaters and traffic lights.
In South Africa, cities are combining the urgent need to address energy poverty with the opportunity to transition to renewable energy. Johannesburg's Climate Proofing for Urban Communities initiative, for example, provides low-income households with low pressure solar water heating systems.
The low-carbon solutions that are being pioneered and implemented at city level are likely the key to unlocking the answers we need globally. The leading cities are already inspiring positive discussion about what is being done, and can be done in cities, particularly to reduce greenhouse gas emissions - even though cities are hampered by a severe lack of funding and investment in renewable energies.
In addition to local action, cities need national and international partners to truly succeed. For this reason, the Nantes Declaration adopted at the World Mayors Summit in September renews the global climate advocacy of local governments - calling for greater support, at both national and international levels, to provide the urgent leadership required for a whole -scale global transformation of the world's energy systems, away from fossil fuels and towards a clean, dynamic, sustainable renewable energy-based future.
The urgency of the need to radically change global energy systems is indisputable - but it will not be quick enough unless we all press for it urgently. We are seeing signs of a shift. There is a global movement being built to divest from coal, oil and gas and to drive new investments into renewable energy, yet more needs to be done.
WWF, along with partners, is running a campaign this year, Seize Your Power, calling on governments and financial actors - including cities, major financial institutions, banks, pension funds and state agencies - to act immediately to invest more in sustainable energy powered by wind, water and the sun. They must phase out investments in coal, oil and gas and enable a just transition from the dirty and unsustainable energy of today. A bold and decisive shift in energy financing is now required to steer the world towards the necessary energy transition we so urgently need.
Jim Leape is the Director General of WWF International. Park Won Soon is the mayor of Seoul, South Korea and Chair of World Mayors Council on Climate Change. David Miller is the CEO of WWF-Canada and former Chair of C40 Climate Leadership Group. Martha Delgado is the General Director of the Secretariat of the Global Cities Covenant on Climate. Gino van Begin is the Secretary General of ICLEI - Local Governments for Sustainability.
The views expressed in this article are the authors' own and do not necessarily reflect Al Jazeera's editorial policy.