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This has been a dizzying year for those anxious about climate change. On the policy front, President Trump has worked to unravel President Obama's progress on climate, reversing two of his signature achievements first by in June and then in October.

Meanwhile, after , natural disasters ravaged the United States in 2017. Hurricanes battered , , and ; wildfires torched communities in San Francisco and Los Angeles. All told, natural disasters in 2017, the most expensive year on record. And 2018 is already off to a dreadful start, with mudslides killing more than 20 people near .

In the face of ever-greater urgency to act on climate, and few prospects for action at the federal level, . Last week, Mayor Bill de Blasio built upon New York City's with two big announcements: first, New York City would . And second, the city would —BP, Chevron, Conoco Phillips, Exxon, and Royal Dutch Shell—for their contributions to climate change and efforts to .

Why Divest?

In committing to sell off its fossil fuel investments, New York joins of institutional investors that have committed to divest in full or part. Cities like Paris, Capetown, and San Francisco; pension funds like the California Public Employees' Retirement System; faith-based groups like the Church of England; philanthropies like the Rockefeller Family Fund; and universities like Oxford, Stanford, and Georgetown have mobilized to divest over $6 trillion in total assets from the fossil fuel industry.

Of his decision to divest, Mayor de Blasio : "This is a case where the prudent action to protect city pensions also lines up perfectly with good social policy to fight climate change." While future action on climate could imperil fossil fuel profitability down the road, professor Jeffrey Sachs, director of Columbia University's Center on Sustainable Development, that divestment is primarily about "sending a powerful message to the major oil companies" and removing their social license to operate without regard for human health and safety.

A new paper points out that this political process is already underway, with the divestment movement increasing attention and support for climate policies like a carbon tax.

While divestment is first and foremost a political tactic, pioneered by critics of the Apartheid regime in South Africa, the fossil fuel divestment movement has also risen to prominence alongside a broader conversation on the ethical implications of finance. According to , one in every five dollars invested through professional managers was devoted to ethically-screened investments in 2016. That's more than double 2012 levels. Once on the fringe of the investing world, the idea of socially responsible investing has reached the mainstream: on Tuesday, the investment giant BlackRock announced that companies would in order to earn a slice of the firm's $6 trillion.

Divestment cannot replace government action on climate. Nor is it meant to; in contrast, its proponents hope to spur regulatory action. But, it is an important step in the transition away from fossil fuels toward a clean energy future. Institutions should use all available tools—including investment funds and the courts—to press for a just and sustainable future.

This story is republished courtesy of Earth Institute, Columbia University .