Experts say it takes about three weeks to form a new habit.

If your New Year’s resolutions include being more green, then bringing your own bag to the grocery store is a good first step.

It’s pretty simple: Get some reusable bags, keep them in your car or backpack and use them. It’s not as involved as, say, starting a worm compost or buying an electric car.

Anyone can do it, whether or not any laws are in place.

Starting Friday, a new ordinance goes into effect in Hawaii County banning single-use plastic bags. These plastic bag bans have been in effect in Kauai and Maui counties since 2011.

Honolulu’s law was signed by then-Mayor Peter Carlisle in May 2012 but won’t go into effect until July 2015. The ordinance gives residents three years to wrap their brains around the idea of a ban on plastic carryout bags, and retailers the same amount of time to prepare for the change.

In a recent "Big Q" informal poll on staradvertiser.com, roughly 40 percent of readers said they are using recyclable shopping bags more often, while 60 percent are not.

I suppose there are dozens of reasons why people accept plastic bags at the checkout stand when offered.

For one thing, many stores automatically provide them. And consumers may see the bags as a convenience, so retailers offer them as a part of good customer service. People also like to reuse them to line their trash cans, pick up dog poop and handle other chores.

I wonder how many people on Oahu are hoarding plastic bags under the kitchen sink before the law goes into effect here?

Let’s take a look at the dozens of reasons why plastic is harmful to the environment, prompting these new laws.

Conventional plastic bags, typically made of petroleum, do not biodegrade, meaning they do not go away (even when burned at HPOWER, creating greenhouse gases).

Plastic bags are easily windblown, ending up in waterways, where they break down into toxic bits and enter the ocean’s ecosystem.

Scientific studies are underway at Hawaii’s universities to determine the exact impacts on fish, seabirds and, ultimately, humans. Dissections of deceased Laysan albatross chicks in the Northwestern Hawaiian Islands have time and again revealed stomachs filled with plastic.

Pieces of plastic bags are no doubt part of the Great Pacific Garbage Patch — a gyre (circular ocean current) of marine debris at least double the size of Texas — midway between Hawaii and Cali­fo­rnia.

Hawaii, if anything, is more vulnerable than other places to the harmful effects of plastic due to its geographic isolation, even more reason why these bans are necessary.

Paper bags are often given out as an alternative, but are not necessarily any better for the environment.

Some stores offer economic incentives. Whole Foods Market offers a 10-cent-per-bag credit, while Foodland offers a 5-cent credit per bag or HawaiianMiles.

It’s time to look at the bigger picture and long-term health impacts of plastic, rather than the short-term convenience. Start the new year right by taking a simple step — take your own bags to the store.

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Nina Wu writes about environmental issues. Reach her at nwu@staradvertiser.com and follow her on Twitter @ecotraveler.