Industrial real estate developer ProLogis estimates the total warehouse space in the United States at more than 5 billion square feet. About 1.25 billion square feet of that space is commercial warehouse space available for public use, according to estimates from logistics consulting firm Armstrong & Associates—that's the space of more than 20,000 football fields.

Much is simply public warehouse space rented at a list price for short-term use. But a portion of this commercial space is operated by third-party logistics (3PL) providers who offer long-term warehousing contracts along with transportation management and value-added services. And another portion is refrigerated space where the temperature is carefully controlled to keep food and other perishables from spoiling.

Here, we take a close look at the top companies running third-party logistics operations and public refrigerated warehouses. We rank these companies based on the amount of warehouse space they operate in North America, based on data compiled by Armstrong & Associates and the International Assoc. of Refrigerated Warehouses.

The ranking of third-party logistics providers shows DHL & Exel Supply Chain at the top of the list—just as it was last year—with more than twice the space of its nearest competitor. Americold Logistics also retains its No. 1 ranking on the refrigerated warehouse list, but business deals in the refrigerated market are threatening this long-held position.

Mergers and acquisitions were the big news when we reported on the third-party logistics market last year, and these business deals are continuing to shape the 3PL market.

The 26 million square feet operated by CEVA Logistics, for example, represents a combination of the space formerly operated by TNT Logistics (last year's No. 12) and EGL (last year's No. 15).

Last fall, a private equity firm bought TNT Logistics and renamed the company CEVA Logistics. The firm then bought EGL and merged it with CEVA this summer, creating a company large enough to tie for the fifth spot in our 3PL rankings.

The impressive growth at Jacobson Cos. is also the result of private equity investment. Jacobson was purchased by an equity firm in 2004, and the company quickly began making acquisitions.

Jacobson appeared on our list two years ago at the No. 20 spot and moved up to No. 11 last year. The company now sits at No. 7, thanks to recent acquisitions in such markets as Phoenix and Atlanta. Jacobson was sold to a second private equity firm this summer.

In our No. 12 spot this year is the combination of refrigerated warehouse space from Atlas Cold Storage and VersaCold. The two Canadian companies have been purchased by the Icelandic transportation company Eimskip. Eimskip is operating Atlas and VersaCold as separate businesses for now, but Armstrong & Associates chose to combine them for the purposes of their ranking.

Armstrong doesn't include Eimskip competitor AmeriCold in its rankings because AmeriCold doesn't make enough information public, according to Armstrong & Associates chairman Dick Armstrong.

Armstrong says he expects mergers and acquisitions to continue in the 3PL market, but he expects the nature of the deals to change.

The subprime mortgage fallout that dominated financial headlines earlier this fall, he says, has dried up many private equity deals. With “the money guys” out of the picture, he says, prices offered for 3PLs will become more reasonable, and companies not backed by private equity will once again be able to afford to strategically acquire their competitors.

When asked about other trends in 3PLs, Armstrong says he expects the phenomenal growth enjoyed by the North American 3PL market in recent years to begin slowing down along with the rest of the economy.

“We expect growth in 3PLs,” he says, “but we expect it to be single-digit rather than double-digit growth.”

Armstrong says he's seen a slowdown in growth of the storage and movement of goods, but that slowdown doesn't seem to be affecting operators of public refrigerated warehouses. In fact, Bill Hudson, president and CEO of the IARW, says the flow of refrigerated and frozen products continues to increase.

“We don't hear a lot of our members saying business is slow or that they're having trouble staying full,” Hudson says. “More and more manufacturers, distributors and processors are relying on third-party storage and third-party logistics.” This, he says, is because these companies prefer to focus on making and selling their cold and frozen products than on storing and shipping them.

Outsourcing non-core operations is, of course, a general business trend, but the incentive to outsource refrigerated warehousing is particularly strong: thanks to unique energy and maintenance needs, operating a refrigerated warehouse is much more expensive and complicated than operating a traditional one.

As public refrigerated warehousing grows, says Hudson, it's also becoming increasingly international. “What we're seeing is companies building global supply chains,” he says.

A perfect example, he says, is Eimskip. The company is putting together a worldwide network of refrigerated facilities. It currently operates offices and subsidiaries in 15 countries in Europe, North America and Asia, including the largest cold storage facility in China.

As we've already noted, Eimskip recently purchased two of North America's largest refrigerated warehouse companies: Atlas Cold Storage and Versacold. The companies appear as No. 2 and No. 3 on our list of refrigerated warehouse operators. Because the companies are operating separately for now, the IARW chose to list them separately this year.

The combination of Atlas and Versacold is the biggest news in the refrigerated market this year. Their combined space actually surpasses the public space of longtime market leader AmeriCold Logistics, creating a true competitor for AmeriCold for the first time.

Another notable change in the market is the growth of Conestoga Cold Storage, which did not appear on last year's Top 20 list, but now sits at No. 17. Hudson says Conestoga recently opened new facilities in Canada. Much of the company's success, he adds, is due to its investment in automation and technology.

Conestoga's president, Larry Laurin, says the company has developed a powerful proprietary software system. Conestoga also operates automated storage and retrieval systems in two of its warehouses and, he says, is developing an automated picking system.

Also experiencing impressive growth in the last year are Preferred Freezer Services (No. 5), a company with many clients in the seafood business, and Inland Cold Storage (No. 12, up from No. 15), which, according to Hudson, does a great deal of business with Wal-Mart.

Editor's note: In past years, Modern has included a ranking of the Top 20 private warehouses along with this ranking of 3PLs and public refrigerated warehouse operators. Accurate information on private warehouses has become increasingly difficult to find, so we excluded the private warehouse list this year.