Air travel is predicted to increase in the coming years, the exhaust from planes filling more of our skies with clouds known as contrails. These clouds are largely unaccounted for in climate policy and could triple the climate impact of air travel from 2006 levels by 2050, according to new research.

In a study published in Atmospheric Chemistry and Physics on Thursday , researchers from the German Aerospace Center's Institute for Atmospheric Physics detail how they created four simulations to test the potential effects on the formation of clouds by airplane contrails. They accounted for increased air traffic, climate change, improvements in fuel efficiency, and decreases in soot emissions.

“When you look at the sky and see those thin stripes, you don’t think that contrails can have an impact,” co-author Ulrike Burkhardt said. “But they do.”

By 2050, air traffic is predicted to increase four times from 2006 levels. This could, according to the study, triple contrail cloud formation in that same time frame. Contrails, which are made of condensed water and freeze into ice clouds, trap heat in our atmosphere like a “light blanket,” as NASA describes them. This heat-trapping is called “radiative forcing.”

According to the study authors, how these contrails will affect surface temperature is still unknown, but they have an immediate impact on the temperature higher in the atmosphere, in a layer called the troposphere. Contrails may also disrupt precipitation cycles, the researchers say.

Air traffic already plays a big role in our effect on the climate—in 2005, it accounted for about 5 percent of anthropogenic radiative forcing, with contrail clouds making up the largest chunk of this impact. Contrail clouds have had greater climate impact than CO2 emissions from air travel since the beginning of aviation, the researchers say. New technology will also push flights to slightly higher elevations, the authors say, increasing contrail clouds formation above places that were previously too warm, such as the tropics.

Carbon dioxide emissions are easier to reduce and offset through policy, as you can simply measure how much you emit. Contrails interact with the environment in more complicated, non-linear ways, researchers say, and we need to keep building better models to fuller understand their impact.

This need for more research is largely what has held back efforts to properly address contrails in climate or emissions policies.

William Raillant-Clark, spokesperson for the International Civil Aviation Organization (ICAO), a United Nations agency, said that any emissions mitigation policy depends on scientific consensus, which he said doesn’t yet exist for the impact of contrail clouds.

“There is a high level of scientific uncertainty regarding the exact level of the non-CO2 impact,” Raillant-Clark told Motherboard in an email. Because of this uncertainty, Raillaint-Clark said, the ICAO focuses on reducing and offsetting CO2 emissions.

Burkhardt said that offsetting carbon dioxide emissions is a great start, but that non-CO2 emissions such as contrails need to be considered even without a perfect handle on the problem. Using cleaner fuel sources, however, can reduce contrail cloud formation and carbon dioxide emissions simultaneously.