New York (CNN Business) New York City Mayor Bill de Blasio and London Mayor Sadiq Khan have an urgent message to the world's cities: Ditch your fossil fuel investments now to avert a climate catastrophe.

The leaders unveiled Tuesday what amounted to how-to kits that would allow mayors to follow in their footsteps by divesting from oil, natural gas and coal investments.

The divestment push adds to the mounting financial, legal and political pressure on the fossil fuels industry. The momentum is clearly in favor of renewable energy.

"Removing fossil fuel assets from our portfolios and increasing climate-friendly investments are not easy steps, but they are absolutely necessary," de Blasio wrote.

The toolkit offers tips on how to communicate with voters, finance officials and the media about divestment. It provides responses to likely concerns and provides case studies from recent campaigns.

The effort is being led by C40 , a network of nearly 100 global cities that have committed to fighting climate change.

In 2018, New York became the first major American city to set a goal of divesting from fossil fuels. The city is aiming to divest its $195 billion pension portfolio by the end of 2020 . New York has also pledged to double its investments in climate change solutions to $4 billion by 2021.

Other major cities have recently pledged to exit fossil fuel investments, including Melbourne, Berlin and Stockholm.

"Every major city in the world needs to step up and do its bit if we are to deliver on our shared ambition of safeguarding our planet," Khan wrote.

ExxonMobil XOM BP BP Royal Dutch Shell RDSA The London Pensions Fund Authority has sold its stakes inand. Its holdings in extractive fossil fuels have been cut from 1% of total assets in 2017 to 0.4% today. The LPFA's holdings in green investments have climbed to nearly 3% of total assets.

Institutions with $12 trillion have promised to divest

Despite the divestment push by cities, it's important to note that municipal pension funds remain a relatively small piece of the overall pie. Private investors have much greater sway.

Still, it's clear that the fossil fuels industry is increasingly being viewed by a segment of investors as toxic. That is especially true for coal, the dirtiest of the fuel options.

Public, private and nonprofit institutions controlling more than $12 trillion of assets have announced partial or full commitments to dump fossil fuels investments, according to a tally by environmental group 350.org. That's a startling increase from just $52 billion in 2014.

Some of those divestment promises have been made by public institutions, including state pension funds in California.

Even the $1 trillion sovereign wealth fund of Norway -- a country whose wealth was largely built on oil, is gradually exiting its investments in exploration and production companies.

Beyond the push by some major cities to exit fossil fuels, the energy industry is feeling the heat from the rise of socially-conscious investing.

In the United States, 26% of all professionally managed debt and equity investments are ESG-oriented funds, according to Raymond James. In general, ESG funds promise to back companies with robust environmental, social and governance practice.

Although some ESG funds still invest in oil and gas companies , many shy away from the fossil fuels industry because of concerns about the carbon emissions at the center of the climate crisis.

Responding to climate concerns

For oil and gas companies to remain "relevant and investable" to ESG-oriented funds, they must make stark changes to their businesses, according to Pavel Molchanov, an energy analyst at Raymond James.

"ESG is not going to go away. To the contrary, it will get larger and larger with time," he said.

The American Petroleum Institute, an industry lobby, is fighting back by launching a national advertising campaign that calls for common ground in the energy debate. The seven-figure ad blitz includes messages on television, websites, radio and outdoor billboards.

"Clean natural gas and oil power our modern world -- and there has never been a better time to invest in it," the American Petroleum Institute, an industry lobby, said in a statement to CNN Business. "Bold and achievable action on climate change at the global level is essential, and America's natural gas and oil industry is committed to being a part of the solution."

Fossil fuels aren't going extinct right now

The toolkit announced by de Blasio and Khan offers city leaders a way to combat concerns that divesting away from fossil fuels will hurt pension fund performance.

The report argues that none of the city pension funds that have divested have suffered a negative impact on portfolio performance, including Berlin, Oslo and Stockholm.

That makes sense given the horrible recent performance of energy stocks . Last decade, the energy sector was easily the biggest loser among the 11 sectors in the S&P 500, according to Refinitiv. In fact, the next closed sector, materials, climbed five times as much. In other words, avoiding fossil fuel stocks would have been a great strategy.

Of course, there is no guarantee that the trend will continue.

Oil and gas stocks closely track the price of underlying commodities. A spike in oil prices, for instance one caused by a conflict in the Middle East, could instantly change the fortunes of struggling energy companies.

Despite the divestment campaign by New York and London, fossil fuels aren't going away any time soon. Solar and wind energy are rapidly gaining traction, yet coal remains America's second-leading source of electric power. Natural gas, a cleaner burning fossil fuel, is No. 1.

"Fossil fuels are the dominant portion of global energy supply," said Molchanov of Raymond James. "And it will remain that way, for decades in the future."