Another metric is emissions per passenger per kilometer traveled, which for a recent two-onslaught journey to the ISS is around 700kg/km (I've assumed 400km each way with no fuel burnt on the return). In comparison, domestic planes emit 0.133 kg/km and vehicle travel emits 177 kg/km [ii].
The number of passengers traveling by plane in 2016 was estimated to be nearly 3 billion people worldwide, and it's expected that this will increase to almost 4 billion by 2025. The total number of vehicles in use worldwide is now around 8 billion, and this will increase to about 9 billion by 2025. Space travel is likely to become more common as well, with humanity hoping to establish colonies on other planets.
Since all aircraft are powered by jet engines, they emit greenhouse gases such as CO2 and NOx. The main source of pollution from airplanes is nitrogen oxides, which result from the reaction of nitrogen in the air with oxygen from the atmosphere or water vapor in the engine exhaust. These pollutants lead to ground-level ozone, which can cause respiratory problems for those who are exposed to it regularly. However, aircraft also release small amounts of water vapor and CO2 into the atmosphere when they fly over land.
Space ships would need to be built with energy conservation in mind. They could only travel between certain locations, so they would have to be built for the trip they intend to take.
Air travel is responsible for 2.5 percent of worldwide carbon emissions. In the United States, flying accounted for 9% of transportation emissions but just 3% of overall carbon emissions. When compared to other industries in the United States, it is a drop in the bucket: Transportation: 29% greenhouse gas emissions; Energy: 21%; Agriculture: 16%.
The main source of carbon emissions from air travel are jet fuel and the manufacturing process for the aircraft themselves. Flights emit carbon dioxide, water vapor, nitrogen oxides, and ozone-forming chlorine compounds. The aviation industry has undertaken efforts to reduce emissions but they still contribute to climate change.
The exact amount of carbon emitted by an individual flight depends on how far you fly and how much electricity you use while driving. Short flights (under 900 miles) using modern planes with low-emission engines tend to have lower emissions than longer flights on older models with higher-powered engines. A study conducted by UC Davis found that flights under 500 miles have about one-third the carbon footprint of flights over 500 miles.
The best way to minimize your impact on the environment is to fly less often. If you must fly, choose a green-friendly airline or look for ways to drive or take public transit instead. You can also try reducing the number of stops you make while traveling long distances by finding a cheaper option such as a cheap flight plus bus ticket or train ticket.
It is anticipated that global aviation—both passenger and freight—emitted 1.04 billion tonnes of CO2 in 2018. In 2018, this accounted for 2.5 percent of total CO2 emissions. Since the mid-1980s, aviation emissions have more than doubled. In 1988, they amounted to about 600 million tonnes of CO2.
Aviation emissions can be divided into two main groups: direct and indirect. Direct emissions are caused by the operation of engines and include water vapor, NOx (nitrogen oxides), SOx (sulfur oxides), and unburned fuel. Indirect emissions result from the need for jet fuel to be refined from crude oil and include carbon dioxide, methane, and other hydrocarbons.
The environmental impact of aviation depends on how it is driven: either by jet fuel or by oxygen. Oxygen drives most small aircraft such as gliders and model airplanes because they use only enough fuel to create thrust and not enough to lift them off the ground. Jet fuel drives all large commercial aircraft including jumbo jets and superjumbos. They use so much fuel that it causes climate change even before they enter into service. An average-sized plane using JD engines would account for 15% of the total emissions. A second source comes from other industries which process the raw materials used by airlines for fuel production.