Portland and South Portland are about to embark on a partnership whose purpose is to develop a long-range plan to do their part in combating climate change.

In the years ahead, the communities are expected to see more rainfall and less snow, increasing land and water temperatures, rising sea levels, and ocean acidification, city officials said in a joint statement.

Unless steps are taken to reduce those changes in climate, the cities say that residents will face more significant and frequent flooding from storms and tides, more heat-related illnesses such as Lyme disease, economic impacts ranging from costly property repairs to higher insurance costs, and ecological effects that will harm species currently thriving in Casco Bay. They say climate change also has the potential to disproportionately harm residents who are marginalized or more vulnerable, further increasing inequities in the Greater Portland population.

The new initiative, called One Climate Future, will be launched on June 10 in Portland at the Gulf of Maine Research Institute’s Cohen Center, 350 Commercial St. The event will start at 10:30 a.m. and feature several speakers, including the sustainability coordinators from both cities.

“This plan will help us reimagine and reinvent our cities in ways that promote economic prosperity, social equity, ways that preserve quality of life, and build climate resilience,” Portland Mayor Ethan Strimling said.

Talks between the cities about preparing for sea level rise and climate change started more than a year ago. Their plan would mirror the goals of the 2016 Paris Climate Change Agreement, which called on countries around the world to reduce carbon emissions by at least 80 percent by 2050 and limit the global average temperature rise to less than 2 degrees above pre-industrial levels.

Since January, Portland and South Portland have been working with a consulting team, city staffs and community members to produce an inventory of community-wide greenhouse gas emissions and come up with ways to better understand humans’ vulnerability to climate change. Each city kicked in $110,000 to hire a consultant.

One Climate Future has four elements that will be used to develop an action plan. They will look at using energy and buildings more efficiently, designing a more efficient transportation and land use network, minimizing waste reduction by changing how communities consume and dispose of materials, and ensuring that all neighborhoods can withstand and recover from changes to the climate.

The project will take 18 months to complete, according to the group’s website: www.oneclimatefuture.org. The final plan will be released in the the summer of 2020.

“What makes One Climate Future unique is that our neighboring cities recognize that we’re in this together, and we’re willing to tackle the problem in partnership,” South Portland Mayor Claude Morgan said.