Global air cargo volumes are expected to increase modestly—at an average annual rate of around 2.4 percent—to 2025.

That was the overall conclusion with respect to air cargo to be found in the latest edition of the “World Airport Traffic Forecasts (WATF) 2016–2040” released by the Airports Council International.

The forecast for air cargo contrasts markedly with the outlook for passenger traffic, where growth is expected to be more robust.

Besides a dull general trade picture, the report also sees a trend where a growing number of cargoes are switching from air to ocean.

“The weakened global economy and a sluggish global trade environment were definite deterrents to growth in air cargo volumes,” said Angela Gittens, director general of ACI World. “There also continues to be a structural substitution effect in the delivery of goods across modes of transport, even in the face of strong economic fundamentals. While the shipment of raw materials and perishables have been affected the most by a move away from air cargo services to ocean freight, the modal shift can also be seen in shipments of high-tech and machinery parts. The largest trade flow from Asia has experienced the weightiest shift away from air cargo.”

Passenger traffic, by contrast, which hit seven billion in 2015, is expected to double by 2029, for a projected growth rate of 5.2 percent per year. “Future growth in air transport demand will originate to a large extent from emerging markets,” said Gittens. “Large population bases and increases in per-capita incomes in these markets are major forces driving this demand.”

Also contributing to that growth are heightened competition in the aviation sector and the increased presence of no-frills low cost air fares, which will continue to stimulate demand. Geopolitical unrest, terrorism, and security threats in certain parts of the world represent the principal risks to aviation growth.