Investor climate activism isn’t working. Here’s what would.

When activist investors tried to put a climate-related resolution before ExxonMobil’s board, they might not have imagined that they would be the ones defending themselves. But the oil giant has sued investor group Follow This and investment adviser Arjuna Capital for pushing a motion calling on the company to speed up reductions of greenhouse gas emissions. Even after the activists dropped their motion, Exxon refused to end its suit against them.

This hardball is not surprising from an oil firm that hopes the world uses fossil fuels as long as possible. More remarkable is that these activists expected to make much progress, after bringing similar resolutions year after year, gaining little shareholder support. The lesson, for the many activists eager to force corporate America to trim greenhouse gas emissions, is that relying on inside strategies and corporations’ good will is unlikely to result in much, no matter how legitimate the cause.

If activists want to affect what matters — emissions of carbon dioxide and other greenhouse gases — they should devote most of their effort to the political process, to get policymakers to build regulations, incentives and constraints that will force companies into line.

As it stands today, even the most heralded investor activism has done next to nothing to move the needle. The divestment movement — which counts on 1,550 organizations representing more than $40 trillion in assets committed to getting rid of fossil-fuel-related investments — has proved irrelevant or even counterproductive.

It has failed at its primary goal: to hit the share prices of fossil fuel companies and raise their capital costs to the point they struggle to invest and survive. A recent study by economists at Stanford University and the University of Pennsylvania concluded that “the impact on the cost of capital is too small to meaningfully affect real investment decisions.”

Somehow activists missed that each share a green investor sells is purchased by somebody with lesser green credentials, less interest in climate change. This substitution could actually lead to more rather than fewer carbon emissions.

As the California Public Employees’ Retirement System pointed out last year when opposing a California divestment bill (which ultimately failed), “The companies in question can easily replace CalPERS with new investors, ones who are unlikely to speak up as loudly or as consistently as we have about the urgent need to move toward a low-carbon economy.”

Given divestment’s meager track record, it is no surprise that so-called impact investors have been trying something else: not selling shares but buying them to influence the board. Although that has a somewhat better track record, it has been far from a resounding success.

Shareholder preferences can induce companies to reduce emissions. One study found that companies cut emissions when the share of their equity owned by public pension funds controlled by Democrats increased but did not when the stake owned by Republican-run funds rose. This suggests that public pressure on shareholders might help at the margins to change corporate behavior.

Still, the overall track record of such pressure is unimpressive. One study of firms in the S&P 500 index concluded that companies that got the second-highest rating (a B) on carbon mitigation from CDP (formerly known as the Carbon Disclosure Project, the oldest and largest nonprofit organization that houses voluntary carbon disclosures) emitted more carbon than others.

How is that possible? Another study of highly polluting firms found little to no correlation between companies’ embrace of standard practices from the environmental good-governance tool kit — such as acknowledging climate change as a business risk — and their progress to reduce their greenhouse gas footprint. In other words, trying to seem green does not necessarily translate into actually being green.

Compare this with the impact of policy. The 2010 Greenhouse Gas Reporting Program, which required large emitters to report their plant-level emissions to federal regulators, had an immediate effect. The rule did not mandate emissions reductions. Still, power plants subject to the rule cut their carbon dioxide emissions by 7 percent, on average. Public pressure mattered, but it required regulation that mandated transparency across all companies in the sector.

To be sure, activism is not useless in all cases. Public pressure — through customers and investors — can impose costs on corporations, if only by generating stigma and weighing on brand value. This can change their cost-benefit analyses, making some climate-friendly corporate adjustments worth their while.

But environmentalists should not expect wholesale change. Pressuring the Securities and Exchange Commission to impose a rule for public companies to report corporate emissions is likely to be much more effective than activist skirmishes with ExxonMobil.