With travel by train getting very expensive in some places, and not an option in others, vacations are being organized ever more over hectic, crowded flights. Although it's practically cheaper than going out for dinner, such short hops on planes are very costly for the planet.

Aviation worldwide accounts for at least 2 percent of global carbon dioxide (CO2) emissions, about the same of Germany's total emissions, not to mention harmful further impacts from emissions. But the biggest problem is expected growth of the aviation sector in the decades to come.

To limit global warming to well below 2 degrees Celsius (3.6 degrees Fahrenheit), as planned through the Paris climate accord, each person on the planet would be allowed to emit some 2 tons of CO2 per year — that's what's known as a personal carbon budget.

But a single return flight from Frankfurt to Singapore emits around 6 tons of CO2 per passenger, the German environmental group Atmosfair estimates.

The most eco-friendly solution would be to avoid flying altogether. But with low-cost flights and a growing middle class, air passengers could double by 2036 to 7.8 billion per year. 

Carbon offsets seem to be a convenient solution — passengers can voluntarily pay to help financing clean energy and forestry projects in developing countries. One carbon offset represents absorption of 1 ton of CO2, and offsets are offered in units to compensate the emissions from a trip.

But is this just an easy way to cleanse our conscience and keep polluting? How can we be sure that our money ends up in the right place? 

Distraction, or real climate action?

Kelley Hamrick, program manager of the Ecosystem Marketplace initiative with Forest Trends, an environmental group focusing on finance, told DW that "if you need to fly, offsets are a really great way to try to reduce your environmental impact."

Many consumers remain skeptical. On the one hand, carbon offsets create a scenario like an open bar, where it doesn't matter how much you consume and the effects of this, as long as you pay up later.

However, Forest Trends found in 2016 that companies making greater emissions reduction efforts were also the ones to offset the most. "Companies that offset were not buying their way out of the problem — they were actually doing more than their peers to reduce emissions reductions, in addition to buying offsets," Hamrick said.

A matter of trust

Still, only few passengers choose to offset. In 2016, just 1 percent of Lufthansa passengers — Germany's largest airline — did so, Thomas Jachnow, a Lufthansa spokesperson, told DW. 

That option isn't part of the booking process and translates into extra effort from passengers, Jachnow said. But beyond that, mistrust and a lack of knowledge are also reasons passengers avoid offsetting.

Certification methods are key. Voluntary certification bodies such as the Gold Standard and the Verified Carbon Standard monitor carbon offsets, ensuring they are only used once, that money is allocated properly, and that environmental or social impacts are in order, among other things.

The impact of such projects must be visible and measurable, and contribute to actions that otherwise would not have happened, the standard goes. Easy to say, but a great challenge nonetheless.

Offsetting benefits are difficult to quantify, especially for reforestation projects, points out Dustin Benton, policy director of the environmental group Green Alliance.

"If you pay to plant trees, how much carbon dioxide exactly will they remove from the atmosphere during their lifetime — enough to justify your long-haul flights? You have no way of knowing," he told DW.

By the time the trees grow, we may already have exceeded 1.5 or 2 degrees Celsius. That is, if the trees don't die off first, for instance in a forest fire, which would release even more carbon dioxide into the atmosphere.

The price of polluting

Flight emissions calculators are often run by environmental groups — Lufthansa partners with MyClimate, Brussels Airlines with CO2logic — ensuring companies don't profit from voluntary donations, and guaranteeing a fair emissions calculation.

The CO2 amounts per flight vary widely depending on plane efficiency, cargo weight and number of seats. The more space a passenger uses on the plane, the more tons to compensate, Jachnow explained.

For a roundtrip flight between New York City and Frankfurt, Lufthansa allocates 0.925 CO2 tons to economy passengers, but 2.9 tons for first-class. In a similar flight with Brussels Airlines, economy passengers would have to compensate about 2 tons of CO2.

The price of a carbon offset also differs since this is the amount of money a project needs to save a ton of CO2. With Lufthansa, each ton of CO2 costs €22.50 ($26), with Brussels Airlines €12. Atmosfair, where you can voluntarily offset flight emissions, sets a price of €23 per ton.

But such variations don't mean that certain projects are worth less than others. It's just like coffee, Hamrick explains: You can choose from Brazilian or Colombian, and though they have different prices, the quality could be equivalent. 

In the end, it's all about information — about being sure where the money is going, and who is certifiying it. Irregularities can always happen, but certification bodies do work hard to avoid them. 

A final hint: You won't find detailed information on the Ryanair website. 

DW asked Ryanair how its carbon offsets work, what projects it supports and who verifies them. Robin Kiely, Ryanair head of communications, has confirmed they offer a voluntary carbon offset system since January, but didn't provide any further details.

"Customers can now offset the CO2 emissions of their flight at the end of the booking process with a voluntary donation to a climate foundation. This option is already available to our customers and we will announce our selected partners in the coming weeks," Kiely told DW.

Well, you can be sure that DW will check it again later.