This week, California became the first state to ban single-use plastic bags. Going forward, customers at grocery stores, pharmacies, and convenience marts will have two options for bagging their purchases: they can pay ten cents for a paper bag, or they can bring a reusable bag from home.

the type of bag you use is less important than what you put in it

Supporters argued that the ban would help cut down on plastic litter — particularly in the oceans, where the bags can pose a threat to sea turtles and other marine life.

But this is also part of a bigger trend. Dozens of cities — including Chicago, Dallas, and DC — either tax plastic bags or ban them outright. And among shoppers, bringing a reusable bag from home has become the most visible symbol of environmental-friendliness. If recycling was the quintessential environmental activity of the the 1970s and 80s, toting a reusable bag may be today's.

There's nothing wrong with any of this. It's a mildly positive step for the planet — especially if the goal is to cut down on plastic waste. But the disproportionate emphasis on plastic bags among people who care about the environment is also a bit misplaced. If you want to use your shopping choices to benefit wildlife and the environment as a whole, the type of bag you use is far less important than what you put inside it.

Plastic bags are a tiny fraction of the trash we make

The main problem with plastic bags — and the one that gets all the attention — is waste.

Most bags end up in landfills, but some don't. Because some people carelessly drop them all over the place, they're a major source of litter. In San Jose, for instance, — which surveyed plastic litter prior to implementing a ban in 2012 — bags previously made up about 8 percent of litter in local creeks. They also made up about 15 percent of the trash in nearby Monterrey Bay. Perhaps most troublingly, they contribute to the patches of garbage that swirl in each of the world's oceans and are a threat to wildlife.

Eliminating the bags will certainly cut down on this litter. But at the same time, it won't substantially cut down on the total amount of trash we make. It's estimated that the US throws out roughly 100 billion plastic bags annually, generating about 3.3 million tons of trash (only about 1 to 2 percent are recycled).

This is a huge amount. But it's actually a pretty small percentage of the total amount of plastic we throw away: 31.8 million tons annually. And this total is still smaller than the amount of food waste we throw away — 36.4 million tons per year.

In other words, we throw away ten times as much food as plastic bags. If we care about reducing garbage, fixating on the huge amount of food waste might make more sense.

Of course, all waste is not the same. While paper and food waste are thought to biodegrade within a few months, plastic bags may take anywhere from 500 to 1000 years to get broken down — we don't really know, because they simply haven't been around that long.

But there are a few reasons why this distinction matters less than you might think. One minor point is that in the actual landfill environment, food waste and paper biodegrade at considerably slower rates than under ideal experimental conditions. Michigan State researchers, for instance, dug into layers of landfill from the early 70s and found readable newspapers, intact grass clippings, and five preserved hot dogs.

running out of space for garbage is not a pressing environmental problem

But the bigger thing to note here is that running out of space for garbage, despite what you might imagine, is not actually a pressing environmental problem. Although worldwide trash production is projected to continue increasing for the foreseeable future, we're nowhere near running out of room for landfills. They can be put virtually anywhere — including otherwise unusable land — and once they're sealed, we can do all sorts of things with the land on top of them, like establish nature preserves or build football stadiums or airports.

Now, burying landfills all over our planet is certainly not an ideal use of it. But doing so just causes localized, limited environmental disruption. In 500 or 1000 years — when those bags are still around — it's exceedingly unlikely that the biggest problem facing our planet's ecosystems will be buried islands of plastic, decomposing at a snail's pace. We know exactly what the problem will be, and we're doing pretty much nothing to stop it.

Trash isn't the real problem. Climate change is

In 500 to 1000 years, the primary concern for pretty much every ecosystem on earth will be global warming.

The facts on this are pretty clear. If we don't significantly cut back on greenhouse gas emissions very soon, the world will get hotter, sea levels will rise, and the oceans will turn more acidic, among other problems. If we let truly drastic levels of warming occur — and at this point, there's no sign we're doing anything to stop it — scientists warn that profound disruptions to both modern human society and the natural world are very likely.

when it comes to climate change, eliminating plastic bags is a symbolic move

What does all this have to do with plastic bags? When it comes to greenhouse gases, they're once again dramatically less important than the products we buy and put inside them.

In 2002, when considering a tax on plastic bags, the Australian government conducted one of the most detailed studies of their lifetime environmental costs. They calculated that an average bag has .48 megajoules (MJ) of energy embodied in it — that is, .48 MJ of energy went into producing the plastic polymer, manufacturing the bag, and transporting the various inputs to make it all possible. More energy means more greenhouse gas emissions.

For comparison, making a one-liter plastic bottle — the kind used to hold water, juice, or soda — requires 3.4 MJ. A can of corn requires about 12.82 MJ. A single order of french fries uses up about 3.7 MJ, and a quarter-pound hamburger about 19.88 MJ.

These numbers can vary based on agricultural techniques, shipping methods, and other factors, but when you compare plastic bags with food, it's not even close. Yet for whatever reason, we associate plastic bags — but not food production — with environmental degradation. If we care about climate change, cutting down on food waste would be many, many times more beneficial than worrying about plastic bags.

Moreover, when food waste is dumped in landfills, it releases substantial amounts of methane — a greenhouse gas — as it as it decomposes. (It doesn't if it's composted, because it undergoes aerobic decomposition, rather than anaerobic.) This is more than a symbolic problem: landfills as a whole account for about 18 percent of all US methane emissions, with food waste responsible for a substantial portion.

On top of all this, if plastic bag bans like California's end up causing people to use more paper bags — instead of bringing their reusable ones to the store — it'll certainly end up being worse for the environment. Research shows that making a paper bag consumes about four times more energy than a plastic bag, and produces about four times more waste if it's not recycled.

When it comes to both climate change and trash production, eliminating plastic bags is a symbolic move, not a substantial one. Encouraging people to cut down on food waste, on the other hand, would actually mean something.

So what should we be focusing on?

Despite all this, you might say that a small, symbolic step is better than no step at all. A heated-up planet with fewer plastic bags on it, in theory, is still better than one with more of them.

The major benefit of the former scenario is reduction of plastic litter in marine environments, and the benefit to wildlife. But at the same time, it'd be extremely hard to find a marine biologist willing to argue that garbage patches are more of a threat to ocean ecosystems than acidification.

Now we could, theoretically, try to solve both. Is focusing on plastic bags the way to do it?

The answer depends on whether you view the fight against plastic bags as something that will catalyze more people to care about the environment and take action — or an empty gesture that will allow people to pat themselves on the back, imagine they've done something to save the planet, and move on.

i think most people feel they're doing their part for the environment simply by bringing a reusable bag to the store

In some places, it might seem like the former is a possibility. California, in fact, has its own successful cap-and-trade program that limits carbon emissions, and many cities — including Chicago — are taking big steps to cut emissions within their borders.

But at a national level, we still aren't close to enacting a comprehensive plan that could make a significant dent in climate change. And as a cynic, I think most people feel they're doing their part for the environment simply by bringing a reusable bag to the store.

If we want to avoid catastrophic climate change, we need to reinvent our energy systems and make other big, structural changes. An environmentalism that focuses on personal choices and visible, superficial steps is not one that leads to it.

Banning plastic bags is a nice, small, mostly symbolic step towards helping the environment. Now is absolutely not the time for nice, small, symbolic steps.