The constant cycle of phone upgrades — in which consumers buy phones once a new model comes out every two or so years — is having serious effects on the environment, according to a new study.

The study, published this week in the International Journal of Life Cycle Assessment, reviewed scientific literature on how the manufacture, use, and disposal of smartphones impact the environment. It found that many phone companies don’t have incentives in place for consumers to return their old phones for recycling purposes, and that when phones are discarded or simply kept by their owners instead of being recycled, the metals that go into cell phone manufacture — including gold, copper, and zinc — also remain unused.

And, according to the study, making new cell phones to replace these old ones is carbon-intensive. James Suckling, research fellow at the University of Surrey in England, said in a statement that across the U.K., there are about 85 million phones sitting unused. In total, these phones contain about four metric tons of gold, an amount that would take 84,000 metric tons of carbon dioxide to replace (i.e. if 85 million new phones were put together to replace these unused ones). And, if these phones end up in landfills — instead of sitting unused in a customer’s drawer — they can leach hazardous amounts of lead and other metals into the environment.

Suckling told ThinkProgress that the biggest problem with this scenario is that few phone companies have incentives in place for customers to return their old phones when they’re done with them. That means that too often, old cell phones sit in people’s homes until they’ve lost most of their value, at which point they’re often discarded, rather than recycled. Suckling said this is what concerns him the most.

“The issue is not so much the greenhouse gases emitted producing metals within the phones, but more the valuable resources which are languishing in consumers’ homes and may never make it back to manufacturing,” he said. “I am more disturbed by the millions of phones in that situation. From an environmental perspective, in general it is better to recycle metals than extract them from the ground and if we allow those phones to go to landfill, those metals can only be replaced by further mining.”

The report recommends that phone companies move to a cloud-based business model, in which all the phone’s major memory storage and processing would be moved to a remote server. That way, phones themselves would be simpler, and would require fewer precious metals for memory chips and other hardware.

“The proposed cloud based business model would allow the consumer access to communications and as part of that they are loaned a handset in order to do so,” Suckling said. “This means that the incentive to return the phone is one of the business, not the consumer. The business can then explore how best to extract the value from the phone, either through reuse or recycling.”

Suckling said he isn’t yet sure what will need to happen in order for phone companies to consider such a system. As far as recycling old phones, he said that Europe’s Waste Electronic and Electrical Equipment directive is a start, because it sets targets for the recycling of e-waste. But there still aren’t enough economic incentives out there for recycling.

In the U.S., the Environmental Protection Agency encourages electronics recycling, but it doesn’t have set targets for it — however, according to the agency, 25 states have passed legislation mandating recycling of electronics. The EPA lists multiple businesses that recycle phones, including Staples and Best Buy. There are also a few nonprofits that deal in recycling cell phones, including Cell Phones for Soldiers, which sells donated phones to recyclers and refurbishers and uses the proceeds to buy calling cards for troops deployed overseas.

Still, even when a piece of electronics is recycled, there’s a chance some parts could end up being exported to a developing country like China and Ghana, where people — even children — pick through piles of electronics for valuable metals and burn the rest. Cell phones contribute to this overall problem of e-waste, which is a huge environmental and health problem in developing countries. According to a United Nations report from earlier this year, 90 percent of the world’s e-waste is illegally traded or dumped each year. One EPA-endorsed company, E-Stewards, does make it easier to find responsible recyclers by setting a standard for companies and organizations that participate in its certification program, forbidding the recyclers from shipping e-waste overseas.

A September 2014 study found that 44 percent of the 1.8 billion new cell phones that were likely bought in 2014 will end up in buyers’ drawers, 4 percent will end up in landfills, and just 3 percent will end up being recycled. And it found that of the 40 elements in a cell phone, just 17 are are recovered completely, even in high-tech electronics recycling plants.

Suckling also said that as much as a new system for phone companies is important, a shift in the mindsets of consumers away from an upgrade every year or two culture towards a culture of keeping phones if there’s nothing wrong with them is also key.

“A cultural shift to understanding the wider impacts of what we do is essential to making take back schemes work,” he said. “I cannot speak for all companies, but many realize that resources are becoming more scarce and simply staying in business in the middle future will need different ways of working. But at the moment, the economics of selling as many phones as possible outweighs the environmental implications.”