How much difference can one decision make? According to a new report with huge implications for Maine, a whole lot.

The report, published in the journal PLOS ONE, looks at nearly 700 marine species on the North American continental shelf, and tries to predict how climate change will affect their movement through the next century based on two scenarios — one with low greenhouse gas emissions and one with higher emissions.

The lower emissions are tied to the Paris climate accord, the landmark 2015 pact signed by 195 nations. Under the accord, the United States — the world’s second-highest polluter — committed to lowering its emissions significantly below 2005 levels, and spending up to $3 billion in aid to poorer countries by 2020.

Last June, President Donald Trump pulled the United States out of the Paris deal, saying it was bad for the country’s economy, and that he would negotiate a better deal.

The high-end emission standard used in the report is based on a world without the Paris accord, and the difference is night and day for anyone concerned with Maine fisheries.

It is clear that marine species are on the move, and water temperature caused by climate change is the chief driver. Fishermen see that every day in the Gulf of Maine, one of the fastest-warming bodies of water on the planet, where lobster and other species are migrating north to colder water, and new species are appearing from the south.

It’s a significant disruption in the fisheries, and it will have to be managed. Species will appear where they haven’t been before, and others will move on from places where they have long been part of the culture and economy. In Maine, that will affect lobster, scallops, shrimp and other important species.

This disruption will undoubtedly lead to conflicts — where people can fish will change, as will what they can fish for and how much they can take in. Regional and state officials will have to work it all out.

How much they have to work out, and the time they have to do it, however, varies widely depending on emission levels.

The report looked at how far each species is liable to shift geographically. It found that the shifts will be two, three or even four times the magnitude under the high emission standard than under the Paris goal.

That means instead of a species moving 50 to 100 kilometers, it may shift up to 300 kilometers. Under this scenario, the report says, most coastal areas can expect a large loss of biodiversity among its marine species, as well as a net loss in fisheries productivity for most areas.

If emissions are curtailed, however, the report found that large-scale species distribution can be mostly avoided.

Under one scenario, future generations of Maine fishermen, everyone in the supply chain, and the communities that the fisheries support, would have to cope with a massive shift in how they survive. Under another, the massive shift would be reduced to some manageable growing pains.

That’s a big difference. The right decision is clear — for the sake of the fisheries, and many other reasons, the United States should rejoin the Paris accord, and retake our place as a world leader on climate change.