Back in 2015, I got pretty serious about reducing or offsetting my carbon footprint. I don’t have kids, I don’t own a car and I don’t eat meat, so I already had three of the biggies covered. To make up for my electricity use, I started buying credits from a nonprofit that funds wind turbines and other renewable energy projects in New England. Then it was time to examine my habit of boarding kerosene-fueled jet aircraft.

An online calculator showed that the flights I take every year put a yikes-inducing 15 metric tons of carbon into the atmosphere—equivalent to the overall annual carbon emissions of three average earthlings. So I signed up with a company called terrapass to buy offsets for 12 tons of carbon a year, at about $10 per month. Terrapass uses that money to do commendable things such as capturing methane from landfills, building wind farms and preserving carbon-sequestering forests.

I’m not under the illusion that these projects cleanse my sins as an air traveler. At best, they simply prevent the release of an equal quantity of greenhouse gases down the road. The offsets do help me and other consumers feel less guilty about flying—which is probably why airlines such as United and Delta now offer them as part of the booking process. And on a larger scale, there’s evidence that offsets function as a kind of self-imposed carbon tax, encouraging people who buy them to keep their own energy use in check. But the reality is that voluntary offsets will never come close to matching aviation emissions, which account for 2 percent of overall human-induced carbon emissions.

For one thing, any benefit from offsets is likely to be overwhelmed by growing demand for air travel. According to a recent report from Airbus, about 40 percent of the world population is now middle class, and by 2037 this group will have mushroomed to more than 50 percent, or some five billion people—“all in the pool of regular or potential new” passengers.

And buying an offset isn’t a guarantee that your flight emissions will actually be, you know, offset, since it’s difficult to prove that carbon-avoidance projects wouldn’t have happened anyway or that the neutralized carbon will never be released in the future. And critics say offsets can be an excuse for inaction. Australian engineer and author Sharon Beder has called them “a greenwashing mechanism that enables individuals to buy themselves green credentials without actually changing their consumption habits.”

Regardless of their relation to consumer trends, offsets aren’t a solution to the underlying physics problem in aviation, which is that today’s long-haul passenger jets can’t take off without burning a high-energy-content fuel such as kerosene. That’s why OPEC is confident that worldwide demand for jet fuel will reach nine million barrels a day by 2040, up from 6.3 million in 2017.

Short of drastic rationing of air travel, the only long-term solution for aviation’s carbon woes is electrification. Biofuels from feedstocks such as sugarcane, algae and household garbage, which burn more cleanly than fossil fuels, could help in the short run—United has been mixing them into traditional jet fuel since 2016. But the real hope lies in projects such as E-Fan X, a test plane from Airbus, Rolls-Royce and Siemens in which one of the four gas-powered turbofans is replaced by an electric motor. The partners see the project as a step toward meeting the European Union’s ambitious “Flightpath 2050” goal of reducing aviation’s carbon dioxide emissions by 75 percent by 2050.

Start-ups are getting into the game, too: Seattle’s Zunum Aero, backed by Boeing and JetBlue, is designing a regional jet with batteries in the wings and fans powered with a “hybrid to electric” power train. To be light enough for flight, aviation batteries will need a specific energy—a measure of how much power a battery contains for its weight—far beyond that of today’s lithium-ion battery packs. So, for the time being, Zunum’s power train will run partly on jet fuel.

The improvements in batteries and motors needed to fully electrify the skies could take “the next few decades,” Zunum co-founder B. Matthew Knapp acknowledged in a recent Nature Sustainability op-ed. Meanwhile buying offsets is a substitute that both feels good and does good. Just don’t assume that it will keep our climate-spoiling travel habits aloft forever.”