Next time you're in an airplane racing down the runway for takeoff, make a mental note of that moment when the plane leaves the ground. In a warmer world, that moment of liftoff will come later.
Airplanes take off when the air passing over their wings creates enough lift — lift greater than the aircraft's weight. That moment is determined by, among other things, the air's density. Air is denser at sea level where there's greater atmospheric pressure. It's also denser in colder conditions; a cold molecule takes up less space than a warmer one.
Somewhat counter-intuitively, humidity decreases air density as well. A water molecule occupies as much space as other molecules in the atmosphere — chiefly oxygen and nitrogen — but its molecular weight, one oxygen atom and two hydrogen, is much lower. (Nitrogen and oxygen molecules contain two identical atoms; O2 is nearly double the mass of H2O, while N2 is almost half again as heavy.) You may be sweating and feeling suffocated, but all that humidity actually makes the air thinner.
By now, you've probably put two and two together: Temperatures are rising, and they're predicted to rise more this century. Warmer air holds more humidity. Perhaps more importantly, days of extreme highs have — and will likely continue to — become more frequent. All of which changes that moment when a plane speeding down the runway generates enough lift to take off.
Everything else being equal, in warmer conditions, planes take off later. Few, and perhaps none, of the nation's airports were built with a warmer future in mind. The question: In a warmer world, will planes have enough runway for liftoff?
The US Department of Transportation noted [PDF] global warming's impact on aviation as far back as 2002 :
Higher or more frequent extreme temperatures associated with climate change may, in conjunction with aircraft type (rated cargo and passenger capacity, engine size and efficiency), runway length, destination elevation and location (requirements for additional fuel storage) and other factors, reduce aircraft cargo carrying capacities.
A follow-up report last year elaborates [PDF]:
[H]igh temperature, combined with moisture and field elevation is used to calculate “density altitude”, used to quantify engine combustion efficiency and the needed runway length for take-off and landing at specified aircraft loads. On hot summer days at high altitude airports, such as at Denver International, aircraft may have to burn fuel, unloading weight, in order to have a safe take-off roll. Airport runways are, of course, designed to the climatological conditions of temperature, moisture, wind velocity, and visibility; therefore, for example, higher altitude airports have longer runways. Still, with more days of higher temperatures, the number of days of limited operations at high altitude airports will increase, essentially at airports in the intermountain west (Arizona, Colorado, Idaho, Montana, New Mexico, Nevada, Utah and Wyoming). Moist air, being less dense than dry air, also contributes to higher density altitude, but temperature is a more important factor in the calculation.
Then comes the kicker: "For aircraft that use up most of the pavement on even the longest runways, even a 1 or 2 percent increase in density altitude from increased moisture may put those aircraft out of commission for daytime operations on certain days. With more days of higher temperatures, the number of days of limited operations at high altitude airports would be expected to increase."
Recent hot summers have seen flights cancelled due to heat, especially in high altitude locations. Economic losses are expected at affected airports. A recent illustrative analysis projects a 17 percent reduction in freight carrying capacity for a single Boeing 747 at the Denver airport by 2030 and a 9 percent reduction at the Phoenix airport due to increased temperature and water vapor.
Climate change will hit especially hard at high-altitude airports where the air is already thin, particularly in the Southwest, where temperatures have risen faster than elsewhere — by about 1.5 degrees F. compared to temperatures 40 years ago. (Alaska, where runways and highways are built on now-melting permafrost, is another story.)
But that doesn't mean adaptation isn't possible. In a phone conversation, Thomas Peterson, a climatologist at NOAA's National Climatic Data Center, and coauthor on the two later reports referenced above, ticks off a list of possible adaptations: Shift most flights to early morning or evening when the air is cool, for example. Or make runways longer.
But such retrofitting isn't always easy. For some airports, extending is difficult; there may be something in the way. In many coastal cities — New York, Boston, San Francisco — runways are at water's edge.
At first glance, that seems easy to solve: Truck dirt in, dump it, create dry ground, extend runway. But what about sea-level rise? Thermal expansion has already raised seas globally by about 8 inches in the past century, and it's predicted to raise seas by between 0.6 feet and 2 feet more this century.
That estimate doesn't include glacial melt, and the more scientists learn about how glaciers melt, the more uneasy they become about the prospect of Greenland and Antarctica melting. Both are melting faster than anticipated.
But let's say a few feet of sea-level rise is manageable. It's really storm surges that present the problem. In a warmer world, storms are predicted to become more intense. The storm that previously occurred every 100 years could arrive as frequently as every decade by century's end.
Still, no problem: Build a dike around the runway to keep it dry. Unfortunately, as mentioned in the second report referenced above, a wall at the end of your runway greatly limits its use:
Dikes can reduce the effective length of a runway. Assuming a typical descent angle of 3 degrees and that the runway stretches all the way to the dike, a one foot rise in sea level effectively shortens the runway by 20 feet.
In New York City, a Category 3 hurricane, such as the "Long Island Express" hurricane that roared through the area in 1938, could bring a 20-foot storm surge. (Here's a storm surge map for Long Island. And here's an article about New York City's efforts to prepare for more intense storms.)
According to the numbers above, a dike able to hold a 20-foot storm surge at bay would also render over 400 feet of runway useless. And that seems to negate the rationale for extending it in the first place.