Apple announced Thursday that 21 manufacturers in its supply chain have recently vowed to obtain all their electricity from renewable sources, bringing to more than 5 gigawatts the total amount of renewable energy that will be used by the company and its suppliers by 2020.

That would bring the use of renewable energy to more than 40 percent for Apple and its manufacturing supply chain, the company said, and the total number of participating suppliers to 44.

Last year Apple said it had purchased enough renewable energy to cover all its own operations and much of its suppliers'.

Yet in a testimony to the complexity of reducing the single corporation’s contributions to rising global temperatures, Apple acknowledged that nearly three-fourths of its carbon footprint comes from manufacturing, almost all of which Apple outsources.

Lisa Jackson, vice president of environment, policy and social initiatives at Apple, said a majority of the suppliers were based in China.

“We’re super excited about the fact that this program has taken off the way it has,” she said. “In doing all of this, we based it on our belief that companies would join us if we could show that it was economically feasible and doable.”

The announcement from Apple, whose market capitalization for a time last year crossed the $1 trillion threshold, comes at a time when shareholders and customers are increasingly asking companies to step into the leadership void left by the U.S. government after President Trump’s commitment to withdraw from the Paris climate accord.

Jackson, who before joining Apple served as President Barack Obama’s Environmental Protection Agency administrator, mentioned that retreat from the international agreement that her old boss brokered as a motivation for the computer giant.

“We felt it was really important for U.S. businesses to step up and make clear that we’re still in and understand our responsibility and commitment to address climate change,” Jackson said.

Apple is among a handful of deep-pocketed Silicon Valley firms to take steps toward getting more of the power for manufacturing and data processing from carbon-free sources.

Sometimes their technological prowess means they can help in ways other firms cannot. Google, for example, has launched an initiative to help local political leaders identify sources of carbon pollution. Apple, too, has an online platform to help suppliers find renewable energy sources from which to draw power.

Apple said it has also tapped $2.5 billion in “green bonds” it raised for use in promoting renewable energy and climate-related projects. The projects funded have covered 66 percent of Apple’s own electricity needs. The company contributed to 40 environmental projects, which ranged from solar rooftops in Japan to water conservation in an Oregon aquifer.

At the same time, environmentalists have pointed to the incessant cycle of product releases as contributing to the mountains of “e-waste” as consumers abandon old devices for new ones. Apple’s decision to frequently change the design of charging ports on its cellphones and laptops, for example, leads to a lot of abandoned power cords.

While Jackson said Apple is “absolutely working on” the charger question, it has identified higher priorities regarding a goal it set two years ago to get all of the material in its iPhones, MacBooks and other devices from recycled sources to reduce the impact of mining.

Last October, Apple took a step with a relatively easy metal to recycle: aluminum. The company committed to getting all of the aluminum for its MacBook Air and Mac mini lines of computers from recycled sources.

“I have to be honest, when we start looking at materials to prioritize, we prioritize those materials that we think are either conflict materials — or that can be problematic to source, I’ll say that — or that are huge energy users,” Jackson said, noting that aluminum smelting remains a large source of carbon emissions.