Is there a plausible scenario where humans can continue traveling in airplanes? I have heard that airplanes create a tremendous amount of greenhouse gas emissions, the global number of air miles continues to increase and that there is no technology on the horizon to create a carbon neutral airplane. Is it feasible to capture all the carbon emitted by airplanes or to really stop climate change are we going to have to stop flying?
Another option that could be explored (and indeed is, to a limited extent) is the increased use of airships. Being neutrally buoyant, they need much less energy per trip, and the propulsion could be provided by solar power (they also have a large area on top of the envelope to hold solar cells). The downside, of course, is the need for large amounts of lighter-than-air gases for lift, and the slower pace compared with a jet.
Air travel sustainability problems, even if you limit the category to climate change impacts, are not limited to greenhouse gas emissions. Atmospheric transport of emissions is difficult and, since jet fuel isn't a pure (or even homogeneous) substance, it's not always possible to know what is coming out of the aircraft in what concentrations.
Biofuels were discussed in other answers. 'Biofuels' is a pretty broad term; there are some quite sustainable algal fuels on the make. University research and R&D groups are busy altering the genes of algaes to have them produce hydrocarbons directly, rather than having to press oil from algae cells and use that for diesel production. If we created a reasonable fuel from this, which would be light but energy-dense enough to suit air travel, it could replace current jet fuel (which generally has things other than just hydrocarbons in it). The purer the fuel, at least the easier it will be to model and determine how to mitigate damages.
NOx emissions from air travel create O3 which-- in the right quantities and at the right altitudes-- protect the earth from radiation (which is good). But when the plane is taking off and landing, O3 is created on the ground. This causes warming and affects human health. A high quality biofuel could reduce these; a low quality biofuel might actually increase NOx emissions.
Thermal emissions are a localized problem affecting O3 formation and degradation, as well as how emissions interact with water vapor in the atmosphere. I wish I could hear these discussed more, but for engineering purposes the atmosphere is treated as a limitless thermal sink. :S It doesn't really lend itself to being quantified.
Altering atmospheric chemistry is very tricky. We hardly understand the way the atmosphere works, so predicting the long-term results of our changes is even more difficult. The best we can do is minimize the changes as much as possible. To that purpose, I'm very excited to hear about the solar airplane. Ideally, we would be able to develop light enough solar and energy storage, such that we could replace freight flights with solar machines. Public trust would need to be built over time, as it was for the original passenger airplane.
For all-electric passenger jets, battery capacity must increase to hold four times more energy per unit mass
Currently available batteries have poor specific energy compared to jet fuel:
Jet fuel: 11.9 kWh/kg
Lithium metal batteries (e.g. LiPo): 0.5
Lithium-ion batteries: 0.24
This is an issue because as an aircraft increases in weight, it needs more fuel to achieve lift, which adds to the weight, which requires more fuel, ad infinitum.
As a result, while all-electric aircraft have been flying since at least the 1970s (see the Wikipedia article for a brief and interesting history), these have all been unpiloted or ultralight aircraft with limited payload, range, and speed.
A study earlier this month from Progress in Aerospace Sciences developed an "all-electric aircraft design and optimization program." Using this program, the authors modeled "optimized 180-passenger aircraft based on the Airbus A320neo configuration."
They concluded that
A factor of four increase in battery pack specific energy from current values of 200 Wh/kg to 800 Wh/kg enables 500 nmi flights.
(According to this study in Nature Energy around 50% of passenger flights are 500 nautical miles (900 km) or less).
All-electric jets reduce global warming impacts even if the grid isn't 100% renewable
The same study analyzed the energy and environmental implications if such aircraft were to be used for all 500 nmi flights globally:
The entire energy conversion chain, including charging, transport, and discharging of electrical energy, is considered. Despite the higher total energy use, narrow-body all-electric aircraft have the potential for lower equivalent CO2 emissions than conventionally-power aircraft if the electrical grid transitions toward renewable energy. This is largely enabled by the complete elimination of all high-altitude emissions, which would remove associated non-CO2 warming.
This study from the IPCC shows that high-altitude NOx and contrails both have greater global warming impacts than the CO2 in jet exhaust:
The 'solution' to catastrophic climate change is just as vast and varied as the problem. Air travel is only one part of the problem, but the 'solution' to it is spectrum wide. But I'm sure we will not stop travelling by air until the catastrophic part catches up with us, so we better look at the different aspects of the solution.
First off, everybody knows that plants absorb carbon. We need more plants, we need more trees, we need more greenhouse gas sequestration. There are some solutions that are moving forward - Urban greening and AirCarbon, just to give two examples.
More specifically about air travel, currently is using fossil fuels. Switching between fossil fuels isn't going to make much of a difference, but biofuels might, they still emit but they are emitting carbon that was recently sequestered. We could also put some solar panels on planes, or in some near future have airplane batteries that we powered by some sustainable means.
I believe I have heard some talk about low-orbit trajectories being a possible energy savings for long trips, we could also have micro-air travel, everybody in their little ultralite plane. There could be some other future game changer (High Velocity Tubes!) but for now we need to lower per mile emissions, fit more people in per flight, and sequester those gases.
Carbon capture (CC) to offset or create synthetic fuels seems poised to replace fossil fuels that are otherwise difficult to replace. In the plans I've read in the last few months, that seems to be the general idea for air travel.
CC from the air is being developed. E.g. Climeworks (Zurich, Switzerland), Carbon Engineering (BC, Canada). Extracting it from the air can offset what is otherwise generated. This the idea behind cap and trade, where the cost of the CC is included in the air travel ticket price. That would be offsets.
The captured CO2 can be turned into synthetic fuels. Essentially the same molecular composition of natural fossil fuels, but the carbon comes from the captured carbon. (The ammonia in fertilizer is essentially made the same way.) The jet fuel could then be refined from this synthetic base.
Both would be carbon neutral (vs. carbon negative), and synthetic fuels would never be anything but neutral, but it's a reasonable alternative. Plus you could make it locally (no shipping costs or tankers running into shoals), and there would be no exploration or drilling costs. Costs for synthesising are still a significant concern, but probably because it's nascent technology.
In the short term it's a good use for the carbon captured as sequestration requires certain local geological opportunities, which is one of the key limitors of CCS. It probably doesn't thrill many because it's only carbon-neutral not carbon-negative, but it's a practical alternative to some of the difficult areas for reducing C02, of which air travel is cited as a common example.
This NY Times article talks about some of the current technology being developed, as well as some of the economic difficulties.
Airplanes are producing perhaps 2% of the carbon dioxide emissions produced by the human race.
In contrast, managed forests have the power to sequester 10 GtC / year of carbon, by burying the wood underground. In comparison, carbon dioxide emissions are 8 GtC / year. Thus, 125% of annual carbon dioxide emissions can be buried underground in the form of wood every year.
As the other answers have noted, there is no substitute for jet fuel as used in aircraft.
I'd say for now, the best strategy is to bury some wood underground and pump some oil from underground, to be refined into jet fuel and burned in aircraft. If the same amount of carbon that is released by the oil refining and jet fuel is buried underground in the form of the wood, it's sustainable until we run out of oil.
Later, after all oil reserves will have been depleted and no new reserves can be found, Fischer-Tropsch can be used to turn forests directly into jet fuel.
Making air travel sustainable thus is trivial. The answer to sustainability is jet fuel, as is used currently. 2% is less than 125%.
The big issue is electricity generation (answers: nuclear, wind, grid energy storage), passenger cars (answer: plug-in hybrid electric vehicles or fully electric vehicles, perhaps hydrogen) and heating in cold climates (answer: geothermal heat).