The big picture: Over the last two decades, Walmart and Amazon have carved out their own territories. Walmart reigns over redder, more rural and lower-income America, while Amazon commands the larger, liberal metros. Along the way, both have trod on the other's turf. But now they are stepping up their bloody fight for market share.

• As we've reported, Amazon — along with other retailers — is attempting to loosen Walmart's viselike grip on low-income shoppers with discounted Prime memberships and pilot programs that accept SNAP benefits for grocery delivery.
• But at the same time, Walmart has been encroaching on Amazon's territory — younger, wealthier, largely urban shoppers.
• 82% of households making more than $112,000 per year subscribe to Prime, according to investment bank Piper Jaffray.
• Only 23% of Prime members live in rural areas, and the most active buyers are 22–39 years old, per market research firm Gartner L2.

Neither Walmart nor Amazon responded to emails seeking comment. But Walmart sees the affluent market as essential to its growth: The potential for Walmart in discount retail is beginning to disappear as the field gets more crowded and its consumer base suffers from the one-two punch of stagnating wages and likely tariffs on common consumer goods, says Nelson Lichtenstein, editor of "Walmart: The Face of Twenty-First-Century Capitalism."

So, over the past few years Walmart has acquired Jet.com to bolster its e-commerce business, plus fashion brands like Bonobos and ModCloth that have gained popularity among the same consumers who are loyal to Amazon.

• It modernized its website and plastered the New York City subway with well-designed ads for Jet that more closely resemble marketing products from hip, millennial brands like Away and Glossier than Walmart's classic look.
• And just last month, it debuted JetBlack, a new membership service. At $50 a month — nearly four times Prime's monthly $12.99 cost — you get access to a personal shopper whom you can text to order from Walmart, Jet, Saks, Pottery Barn and other retailers.
• The investments appear to have paid off. Walmart's e-commerce business grew by 40% in 2018, per earnings reports.

But, but, but: This is a tough battle, and it's not clear who is winning. Neither Walmart nor Amazon break down their sales in a way that reveals this market.

But in one suggestive peek behind the curtain, Walmart e-commerce boss Marc Lore said two-thirds of Jetblack members spend $1,500 monthly. He did not reveal the size of the user base.

In another clue, Walmart’s curbside grocery pickup business is booming and will account for 33% of the giant's digital revenue by 2020, Cowen and Company projects.

• "Most grocery pickup shoppers, in general, are affluent, busy, young professionals often with children," writes Grocery Dive's Krishna Thacker, citing market research firm Numerator.
• "By acquiring new brands and by using its website to showcase more premium products, Walmart has started to gain more traction with wealthier shoppers," says Neil Saunders of GlobalData Retail.

The big picture: Consumers have gotten hooked on speed — and the efficiencies that e-commerce injected into retail are getting erased because now there are more deliveries of smaller numbers of packages.

With this trend, emissions have grown:

• The annual sustainability report from UPS, one of the biggest enablers of the e-commerce boom, says it emitted 13.8 million metric tons of CO2 while delivering 5.1 billion packages in 2017, by ground and air.
• Emissions from FedEx, the other major shipper, were 15.1 million metric tons in 2017. The U.S. Postal Service emitted about 4.3 million metric tons of CO2 in 2016. (Numbers from both include all mail, including e-commerce and personal packages and letters.)

Together, that's equivalent to the annual greenhouse gas emissions of just over 7 million cars, per an EPA calculator. It's almost the combined total number of cars in the states of Illinois and Tennessee. It's also about 0.5% of the total 6 billion metric tons of U.S. CO2 emissions per year. That's "not huge, but it's big. And it's growing," says Costa Samaras of Carnegie Mellon University.

On top of UPS, USPS and FedEx, many other players in parcel delivery — including Amazon itself — are adding to the total impact.

"Nobody is looking at the environmental footprint of being consumers with all of this convenience"

The backdrop: In theory, e-commerce is good for the environment, says Don Mackenzie, who leads the University of Washington's Sustainable Transportation Lab. Instead of a neighborhood worth of people driving to stores in their personal cars to shop, one truck can deliver everything. But that calculus is changing.

"There are climate benefits to e-commerce, but those disappear as delivery gets faster and faster," says Miguel Jaller, a professor at UC Davis. "It goes against everything they have been achieving in terms of efficiency."

• Now, flashy memberships that offer free, fast shipping regardless of the size of a cart have eliminated shoppers' incentive and the shippers' ability to bundle goods. They're instead ordering a steady stream of packages to their doorsteps, pushing e-commerce and logistics companies to keep up by adding trucks, jets and even air hubs.

Amazon started it with Prime, which offers free shipping on 100 million products, whether you order a cartful of things or just one box of tissues. Amazon's retail rivals, Target and Walmart, have done the same:

• Walmart has come out with free next-day delivery for orders of $35 or more.
• Target has long had a $99 membership program that offers free same-day delivery, and it has just announced same-day for non-members who are willing to pay a flat fee of $9.99.

The increasing warehouse space required to support the barrage of orders also has an impact.

• E-commerce companies are building more and more warehouses, particularly on the outskirts of cities, so they can cut delivery times to a few hours. About 255 million square feet of warehouse space is under construction in the U.S., per a new CBRE report — and all of it needs light, heat and air conditioning, notes CMU's Samaras.

And there's more: The packing material that goes into delivery boxes is a major driver of the global plastics crisis, says Axios energy columnist Amy Harder.

But, but, but: There are ways to curb e-commerce's hit to the environment.

• Drone delivery uses less energy than vehicles, Samaras tells Axios. "They're super-light and charged by electricity, and electricity is getting cleaner." Both Amazon and Walmart have filed a slew of drone patents.
• Electrifying truck fleets can also make a dent, and many shippers, like Amazon and UPS, are adding EVs to cut emissions, but drones are still the cleanest delivery method, Samaras says.

In a statement to Axios, Amazon said it is committed to bring down its contributions to climate change:

• "We’ve eliminated more than 244,000 tons of packaging materials and avoided 500 million shipping boxes, and with anticipated and continued progress in electric vehicles, aviation bio fuels, and renewable energy we have set an ambitious goal to reach 50% of all Amazon shipments with net zero carbon by 2030."

Target said it aims to reduce its carbon footprint by 30% by 2030.

Walmart did not respond to an email. Its 2018 report on sustainability set a goal to reduce emissions 18% from 2015 to 2025.

"The market for drone logistics can be tremendous," says Sertac Karaman, a professor at MIT.

What's happening: Amazon stole the headlines last week when it debuted a brand new package delivery drone at its annual re:MARS conference. But the e-commerce titan has a worthy opponent in Walmart, which has actually outpaced Amazon in the drone patent race in 2018 and 2019, per the FT.

• Amazon envisions using drones to make thousands of super-fast deliveries from warehouses that are on the outskirts of cities to doorsteps, skipping all the traffic jams.
• Walmart, with its 5000-store footprint, could use drones to challenge Amazon's delivery prowess and transport millions of bags of groceries in minutes.

And the two could force other smaller retailers to use drones in order to compete.

There are already companies hoping to capitalize on this need, Karaman says. Top Flight Technologies in Boston makes drones and the software that powers them. Another called Flirtey is offering drone delivery as a service.

Worth noting: Chinese companies are far ahead of their American counterparts when it comes to delivery by air. Chinese e-commerce giants JD.com and Alibaba have used drones to transport packages to China's hinterlands since 2016.