Waiting for a package? The holiday shipping countdown has begun: Online retailers are issuing regular reminders of how many days are left to order for delivery by Christmas, while shippers such as UPS and FedEx are scrambling to distribute record volumes of parcels from all of that e-commerce. Dubbing this “Peak Week,” UPS said it would deliver 29 million packages Tuesday, a single-day company record; FedEx saw its peak day, also a company record, on December 2 with 22 million shipments.

Many of those shipments are traveling by air. This week, UPS is sending up 2,388 cargo flights per day, 465 more flights than a typical day in November. It’s clear from this view on FlightAware that FedEx also has its fleet of more than 630 cargo planes working overtime to meet shipment demand.

People, too, are traveling more and farther by plane than ever. Last year, 2.9 billion traveled on airplanes, three times the number of air travelers in 1986, according to a new analysis by the Worldwatch Institute. The average distance of a flight is double what it was in 1950, and has hovered around 1,800 kilometers (1,118 miles) for the last decade.

Much of that travel originates in the United States, which by a large margin has the most airports and the largest fleet of aircraft in the world, the Worldwatch report said. That adds up to a lot of jet fuel: U.S. air carriers have consumed just over 16 billion gallons per year, on average, for the last five years.

Airplane fuel efficiency has improved markedly over the past 50 years—an average international flight gets 121 percent more seat-miles per gallon. But the Worldwatch report notes that progress on efficiency over the last two decades has slowed. “Relative to all sources of [greenhouse gas] emissions, the [aviation] sector is responsible for about 4 percent of climate change, and its role is rapidly rising,” Worldwatch researcher Michael Renner wrote, saying there are “questions about whether the continued growth of the airline industry can be compatible with climate stabilization goals.” (See related quiz: “What You Don’t Know About Flights and Fuel.”)

In recent years, higher fuel prices have prompted action from U.S. airlines, including operational tweaks (flight speed, routes, capacity) and switching to two-engine jets for domestic flights. Given that fuel costs account for more than a quarter of an airline’s operating costs, the incentive to save is significant. One airline, Delta, went so far as to purchase a refinery in 2012; the Pennsylvania facility turned its first profit this year.

Manufacturers are working on an array of innovations aimed at making planes more fuel efficient, from different wing designs to better, lighter materials. (See related story: “Reshaping Flight for Fuel Efficiency: Five Technologies on the Runway.”) And aviation biofuel, certified for commercial passenger flights since 2011, continues to make strides. Still, biofuel has not yet become cost competitive, and no one expects it to replace conventional fuel anytime soon. The airline industry’s trade group, IATA, believes second generation biofuels could have a share of 3 to 6 percent of aviation fuel by 2020; the European Union’s target is 4 percent. (See related post, “First Commercial U.S. Biofuel Flight Takes Off,” and story, “As Jet Fuel Prices Soar, a Green Option Nears the Runway.“)

Worldwatch’s Renner wrote that “fuel prices alone are an insufficient and unreliable driver [of fuel efficiency]. Government policy needs to provide a push for the development and use of more energy-efficient technologies.” Specifically, Renner points to the need for a global emissions standard for new aircraft, which is being developed by the United Nations’ aviation body, ICAO (International Civil Aviation Organization). But ICAO’s efforts are mired in controversy over an EU policy that would require airlines to pay for emissions from international flights over European airspace. The policy was suspended last year amid an outcry from the airline industry in the United States and elsewhere, but could be reinstated in April if a new compromise is not reached.