Aircrafts' contrails function similarly to clouds by spreading out and trapping heat in the atmosphere. "High-altitude clouds like cirrus warm the planet by trapping heat. Contrail 'cirrus' does the same thing, but the question is: how much? We know that contrails trap some extra energy in the atmosphere: their radiative forcing trapped 10 milliwatts per square metre (mW/m2) in 2005, according to an estimate by the Intergovernmental Panel on Climate Change. That compares with 28 mW/m2 trapped by all of the CO2 released by aircraft engines since the start of aviation."