Airline travel will be in full force for the holidays. For many, a long flight will make up a big chunk of their greenhouse emissions for the year. For instance, a round trip from Tampa to Los Angeles is equivalent to about 6 months of driving or powering your home.

If you are so inclined, you can offset those emissions by purchasing carbon offsets. But how do they work and are they effective?

Carbon offsets allow you to feel better about the carbon you give off by paying a project to reduce the equivalent amount, thereby canceling out your carbon contribution.

Companies are increasingly using this method to limit their carbon footprint, especially for operations where it’s hard to reduce their emissions directly.

Let’s take for example an airline Jet fuel which produces lots of carbon emissions. So far, the energy source is the only way to power large commercial jets. If you’re an airline looking to reduce emissions you have no choice but to buy offsets.

In a press release released Tuesday, Jet Blue said they view offsets as a “bridge solution” until lower carbon options are more available and they continue to see value in high-quality projects that truly remove or avoid CO2.

But therein lies the concern. It is very difficult to evaluate the quality of projects.

For example, growing or preserving forests is the most common type of offset because trees take up carbon dioxide. But a warming climate means an increase in wildfires and as these trees go up in smoke, so do the carbon credits.

Paying farmers to sequester carbon in their soil is also growing in popularity but a recent study suggests that soil may not be as good as once thought.

Another option is directing carbon offset money to fund renewable energy projects. This can help accelerate the transition to clean energy but again it’s hard to know just how much emission reduction these projects will produce.

The voluntary carbon market is trying to address this uncertainty, with new organizations popping up to set standards and certify the quality of projects.

But critics argue that offsets are a distraction, that the funds would be better spent by directly reducing emissions and that companies can use offsets to greenwash – pretend to be greener than they are – and get a free pass to pollute more.

Despite the concerns, the industry is expected to grow by leaps and bounds. Today the voluntary carbon market is worth $2 billion and by 2030 it is expected to balloon to $50 billion.