Climate protesters have disrupted the tennis at Wimbledon, thrown tomato soup at the glass protecting famous paintings, sprayed orange powder on Stonehenge and blocked traffic. In response, European governments have been cracking down on environmental protesters with detentions and fines, and, in one case, with a five-year prison sentence for advocating civil disobedience in a Zoom call.

Whether a protest is ethical is distinct from whether it is legal. As Martin Luther King, Jr., argued in his “Letter From a Birmingham Jail,” protesters who engage in civil disobedience show their respect for the law when they behave nonviolently and accept the penalty that the law imposes.

We agree that nonviolence is a crucial element in an ethical protest and that protesters who live in a democracy with safeguards against excessive penalties for nonviolent political actions should be willing to accept the penalty that the law imposes. But what counts as an excessive penalty depends on how much inconvenience protesters can ethically inflict on the public.

We suggest adapting the internationally recognized principle of proportionality in war. That principle prohibits military actions when the harm to civilians is disproportionate to the military advantage gained. To target a single enemy commander, while foreseeing that the method used will kill hundreds of innocent civilians, is wrong.

Applying this approach to nonviolent protests in liberal democracies has four aspects. First, a “protest proportionality principle” should attempt to balance the annoyance to others against the number of citizens protesting. If a million people join a protest march, it is inevitable that they will fill the street, stopping traffic.

Princeton University professor Zeynep Tufekci is correct that contemporary mass protests have lost gravitas because social media has made them so much easier to organize. Instead of investing years building bonds between like-minded individuals and then leveraging such grassroots efforts to culminate in a national protest, organizers can simply send a tweet with a time and place to meet.

Yet such ease of communication does not invalidate the difference between five protesters and 500,000. People still have to be willing to give up their time and make the effort to turn out. Vast numbers of protesters tell a democracy that something may be fundamentally wrong.

The second component of the protest proportionality principle is the importance of the issue relative to the inconvenience caused. Just because protesters are few in number does not necessarily make their actions wrong. The art-targeting climate activists are trying to spur us to action against a slow-motion crisis that has already harmed millions, and without drastic action, will alter our planet’s climate in ways that harm billions. Even a small chance of waking people up to the need for change outweighs the disappointment of many tourists who are temporarily unable to see the Mona Lisa.

Of course, protesters tend to believe that their cause is crucial. If they are wrong, the protest may be unethical, even if it causes only minor inconvenience.

The third consideration is whether activists intend to inflict significant inconvenience or seek to minimize disruption. Protesters feel they are trapped in an activist’s dilemma: The easiest way to gain publicity is to protest in a disruptive way that might turn public opinion against their cause. To break through the media fog, they believe they must cause mass inconvenience.

It can be hard to think of creative ways to let others know about injustice without disturbing their lives, but it can be done. Instead of disrupting a Christmas tree lighting for kids, protesters could hold a rogue tree-planting event in a public park. The originality of such a protest could go viral, while also ensuring that any response by bureaucrats, such as removing the newly planted trees, would only add to the media coverage.

Intentionally inflicting inconvenience is not necessarily unethical; it must be weighed in relation to the other protest proportionality factors, including who is being inconvenienced. This is the fourth factor. A dozen climate protesters blocking thousands of people from driving their kids to school or going to work is more ethically worrisome than disrupting the routine of those with the power to reduce greenhouse-gas emissions.

It is more ethical to inconvenience politicians and bureaucrats in their official capacity than the public. For democracies to function, public officials must know the views of citizens. Yet protesters need to consider the importance of the services to be disrupted. Interrupting a politicians’ meeting with corporate lobbyists is easier to justify than blocking the provision of public services.

Whether we are assessing international conflicts or nonviolent protests, the principle of proportionality will not yield precise answers. Yet ethical considerations remain vital. Unethical activism, even when nonviolent, can easily spur counterprotests that spiral into violence. The principle of proportionality can serve as a useful guide to the factors that protesters should consider, enabling us to defend the right to protest while also specifying protesters’ ethical responsibilities.

Peter Singer, Martin Skladany

Peter Singer, founder of the organization The Life You Can Save, is a professor of bioethics at Princeton University. Martin Skladany is a professor of law at Penn State. The views expressed here are the writers’ own. -- Ed.

(Project Syndicate)