As Mayor Marty Walsh of Boston hosts the annual meeting of the United States Conference of Mayors, many of the mayors bemoan federal policies that undermine local efforts to address climate change.

The mayors are to be commended for the collaboration they have just announced to buy more renewable power, but they may not know that many of them — including all in Massachusetts — already have a stunningly effective tool to accelerate the development of renewable power like solar and wind and to reduce greenhouse gas emissions. The tool is a readily available mechanism for purchasing even larger amounts of renewable power. That tool is municipal aggregation, known in some communities as community-choice aggregation or community-choice energy.

Utilities like Eversource and National Grid deliver electricity, but they don’t generate it. Instead, they buy electricity for most residential and small business customers, who are known as “Basic Service’’ customers.

In Massachusetts, a state law authorizes municipal aggregation for cities and towns that choose it. The law gives them the right to buy electricity on behalf of the utility’s Basic Service customers. A municipal aggregation program has several advantages: It often (though not always) saves customers money; it’s a trustworthy program, vetted by the city or town; and it can offer more stable electricity prices than the electric utilities provide. And aggregation can provide communities with more renewable power than they would otherwise get from their utility.

A different state law imposes so-called renewable portfolio standards, which require Massachusetts utilities to deliver a portion of the electricity they purchase for customers from renewable sources. This year, the portion is 13 percent, and it increases 1 percent annually (with bills pending in the Legislature to increase that rate).

For several years, Massachusetts cities and towns that adopted aggregation programs — about 100 of them — did so only to try to save money. But many other cities and towns have realized that they could use the likely savings from aggregation to purchase more renewable power than utilities are required to provide. In the last few years, about 40 of the state’s communities have adopted or begun the process of adopting aggregation programs that buy more renewable power than the renewable portfolio standards require. Most of them buy 5 percent more, although Brookline’s aggregation program specifies 25 percent. Newton expects to include a sizeable renewable component in its program.

In Massachusetts, municipal aggregation is an “opt-out’’ program. Electric customers are informed of the program in a variety of ways, including by letter from their city or town. Customers who take no action are automatic participants in the aggregation at the “standard’’ level (e.g., the 5 percent, or 25 percent in Brookline). But they can choose at any time, with no fee, to opt out and remain utility Basic Service customers.

The opt-out feature of municipal aggregation, together with the renewable power component, gives the program its power to lower greenhouse gas emissions. Most electric customers do not opt out, in part because the additional renewable power that cities and towns have chosen thus far is unlikely to increase electric bills more than a few dollars per month on average, if that. In fact, the experience of several Massachusetts communities is that, at least at certain times, the cost of electricity in an aggregation is lower than the Basic Service price.

It’s hard to over-emphasize the importance of a program that drives the development of renewable electricity. Converting to cleaner electricity is the path to decarbonizing the economy.

The mayors assembling in Boston have a real opportunity. In states that do not have municipal aggregation laws, they can advocate for their adoption. In states that already have statutory authority, they can adopt municipal aggregation programs with a renewable component. The mayors have power that Washington can’t touch.

Ann Berwick is director of sustainability for the city of Newton. She was chair of the Massachusetts Department of Public Utilities and undersecretary for energy under Governor Deval Patrick.