Portland needs a solar plan. Why? Because the world’s use of coal, oil and gas is disrupting the climate.

Climate change is the greatest threat to the survival of not just the human species, but all species. It represents the principal challenge facing humanity in our day. No cause is more pressing, Pope Francis said in his 2015 encyclical on the environment and human ecology.

Burning fossil fuels generates carbon dioxide. Carbon in the atmosphere forms something like a “blanket” over the Earth that traps the sun’s heat rather than allowing it to radiate back out. This build-up has caused the average temperature of the Earth’s surface to rise almost 1.8 degrees Fahrenheit (1 degree Celsius) since the late 1800s.

Fifteen of the last 16 hottest years have happened since 2001, and scientists overwhelmingly agree that increasingly wild weather around the world is related to the global temperature rise. That’s climate disruption.

So much fossil fuel has already been burned that it’s going to take determination and commitment internationally, nationally and locally to avoid shooting past the dangerous 3.6 degrees F (2 degrees C) warming mark. That’s the commonly recognized boundary for keeping the climate compatible with human life as we know it. This means essentially stopping global CO2 emissions by 2060. That may seem like a long time in the future, but it’s within the lifetime of people under 40.

Cities are leading the transition to 100 percent clean energy in the United States. Twelve U.S. cities and counting, including San Francisco and San Diego, have already adopted ambitious 100 percent clean energy goals, and four cities in the U.S. – Aspen, Colorado; Burlington, Vermont; Greensburg, Kansas; and Kodiak Island, Alaska – have already hit their targets. These cities now generate 100 percent of the energy used community-wide from non-polluting and renewable sources.

It’s time for Portland to assume a leadership role in solar energy deployment in Maine. Solar is the best non-carbon source for urban areas: The sun’s energy is constant and plentiful. And the faster we deploy solar power, the more costs will fall, making needed changes more affordable. Mayor Ethan Strimling has said he wants to have 25 percent of Portland’s homes and businesses using solar energy within 10 years.

At the Paris climate summit, diplomats from 195 countries agreed to set a goal of preventing that 3.6 degrees F (2 degrees C) rise. Germany is already a model of national solar energy deployment despite getting less sunlight than does Maine.

In the U.S., political power struggles at the federal and state levels have prevented comprehensive, affordable solar strategies from becoming the norm. Maine, for instance, has a present solar penetration of 1 percent of peak load. This needs to be closer to 10 percent if we intend to meet the U.S. emissions reduction targets.

Completing the proposed solar installation on the Ocean Avenue landfill to supply energy for city buildings and operations would be an excellent way to demonstrate leadership. The project, planned for this year yet put in doubt by the solar bill’s defeat, would make an otherwise unusable area vital and productive.

Installing a solar array at the Ocean Avenue landfill will send a message that’s consistent with Portland’s reputation as a forward-thinking city. Yes, the Maine Legislature’s failure to override the governor’s solar bill veto has been a setback. And yes, there’s some uncertainty about how long it will take to pay ourselves back with energy savings.

But leadership requires proceeding despite setbacks and uncertainties. No energy enterprise is entirely without risk, and the risks of renewable energy inaction are far higher than the risks of forging ahead with determination and hope.

The project also makes long-term economic sense. Today’s solar arrays last at least 25 years. The reduction in energy costs will allow Portland to recoup its investment and ultimately to save millions of tax dollars.

We can’t mitigate extreme climate disruption and create a sustainable energy future without a plan. Portland shouldn’t let politics or lack of planning at higher levels stymie our doing the right thing. By moving now, Portland can show the way for others. We call on our elected and appointed officials to forge ahead on a solar plan, starting with the Ocean Avenue landfill project. The Portland Climate Action Team stands ready to assist.