When 16-year-old climate activist Greta Thunberg sailed across the Atlantic Ocean on a zero-emissions boat, she did so to make a point about the millions of tons of carbon dioxide emitted into the atmosphere every year as a result of air travel. Spend about 10 hours flying this holiday season and your travel could add as much as a metric ton, or 2,000 pounds, of carbon to the atmosphere.

If you feel guilty about those climate-warming carbon emissions, you might be tempted to purchase a carbon offset. In the past year, Google searches for “carbon offsets” have been on the rise, and those who sell them say their sales are up.

Carbon offset vendor Cool Effect says individual purchases of their carbon offsets have risen 700 percent since May. Gold Standard, an organization that certifies carbon offset programs, has seen individually purchased offsets quadruple over the past year.

The market-based approach to reducing emissions is not without criticism. Some say the focus on individual actions distracts from the more impactful improvements that happen when industry is regulated.

Currently, countries signed onto the Paris Climate Agreement are negotiating rules for an international carbon market to buy and sell carbon credits, though experts say those rules would impact large-scale emitters, not individuals purchasing low-cost offsets.

The offsets you might buy for a flight have no federal oversight, and not all have transparent business practices. If you’re looking to purchase an offset to feel a little less guilty about flying, here’s what you should know.

What are carbon offsets?

Marisa de Belloy, CEO of Cool Effect, thinks of climate issues like a math problem.

An offset purchased from their site, she says “is equal to one metric ton of carbon emissions that were not emitted. The term offset just means you’re using that ton to offset a ton you have put into the atmosphere.”

Their emissions-reducing programs vary from planting trees to providing communities with clean-burning cookstoves.

To more than offset that cross-country flight, a consumer could pay $8.50 to a program in Honduras that replaces open fire cooking pits with custom-built brick and mortar stoves that require less wood to cook meals and funnel smoke outdoors via a chimney. Cool Effect estimates one new stove could reduce three metric tons of carbon emissions per year.

Why not just donate directly to a charity that builds cookstoves?

“The real difference is when you buy an offset, you are verifiably reducing carbon emissions,” says de Belloy. “When you donate to a big environmental non-profit, you don’t know exactly what the impact has been.”

Voluntarily purchased carbon offsets are also distinct from the kinds of carbon credits used in a cap-and-trade system. Under that regulatory framework, a company that emits less than their legally-mandated limit can sell a “credit” to a company that exceeds a legal limit and therefore faces a potential fine.

What are some of the pitfalls?

“Consumers and companies should look first to reducing their emissions before looking to source offsets for those emissions reductions that are not possible or are not cost effective in the near term,” says Kelley Kizzier, an expert in carbon markets at the Environmental Defense Fund.

“There are a lot of questionable offsets out there,” she adds, “and it can be difficult to navigate the sometimes-murky world of offsets.”

To truly make a difference in carbon emissions, de Belloy says offset projects have to fulfill a concept carbon offset groups call “additionality” by providing an additional benefit that would not have occurred without money from the carbon offset group.

For example, if you were to pay someone to preserve a forest, it would count as an offset if that forest had originally been scheduled for development. A landowner in need of money from the timber would then be instead paid to keep their trees standing. If there was never a threat to the forest, your payment to the landowner wouldn’t count as an offset because your money provides no additional benefit—the forest would have remained regardless.

There’s also no guarantee for how long a project bought with a carbon offset will last. An investigation by ProPublica published earlier this year found numerous examples of carbon credit programs failing to protect tropical forests.

To protect a forest from deforestation, revenue from carbon offsets have to be more competitive than the oft-lucrative industries that lead to deforestation, like cattle ranching and soy production. Lands sustainably managed one year could fall under new political will or management the next.

How do you find a reputable one?

“When offsets first came onto the market in the 90s, it was more of a Wild West period. There was a lot of variability in programs,” says Peter Miller, an expert in carbon offsets at the National Resources Defense Council.

Offsets entered the political conversation after the Kyoto Protocol, an international climate treaty, was signed in 1992, but Miller says growing awareness about climate change has led to their current popularity.

While some regulation has emerged, it’s largely done by independent certification groups like Gold Standard and Green-e. In the United States, there’s no federal regulation over the carbon offsets purchased by individuals.

“If someone wants to sell an offset and they can find someone willing to buy it, there’s nothing to prevent that transaction,” says Miller.

He says consumers looking to purchase an offset should make sure that projects are vetted by third-party groups that actively monitor projects and ensure they’re providing a real reduction in carbon.

Do they make a difference?

“Offsets shouldn’t be seen as an alternative to regulatory requirements,” says Miller. He emphasizes that ambitious international agreements are where real reductions to carbon emissions will be made.

On a global scale, an individual flight is marginal. According to the U.S. Energy and Information Administration, electricity generated by coal produced more than a billion metric tons of carbon emissions in 2018 alone.

But in a person’s daily life, flying is by far one of the most carbon-intensive activities. One flight can produce as much carbon as driving for more than 2,000 miles.

When flying is unavoidable, purchasing offsets through a credible vendor can technically reduce the amount of carbon going into the atmosphere.

De Belloy expects Cool Effect to continue growing. She credits a growing discontent with government and private industry’s failure to deal with climate change for motivating individuals to take personal action.

“People are really beginning to understand that we’ve got to come together,” she says.