President Obama signed an executive order Thursday directing the federal government to cut its greenhouse-gas emissions by 40 percent from 2008 levels over the next decade, and to increase the share of renewable energy in the government’s electricity supply to 30 percent over the same period.

Simultaneously, federal suppliers including IBM, General Electric, Honeywell and United Technologies are pledging to reduce their carbon emissions by 5 million metric tons over the next 10 years, compared with 2008 levels. Taken together, the government and private-sector proposals would cut overall U.S. emissions by 26 million tons by 2025, the equivalent of taking nearly 5.5 million cars off the road for a year.

Speaking to reporters Thursday at the Energy Department. Obama said, “America once again is going to be leading by example.” He also argued that climate and economic policy do not need to be in conflict.

“We’re proving that it is possible to grow our economy robustly while at the same time doing the right thing for our environment and tackling climate change in a serious way,” he said.

White House senior adviser Brian Deese estimated the new measures will save $18 billion; the federal government has already cut its overall emissions 17 percent since Obama took office, saving $1.8 billion — mainly through reduced energy use in federal buildings.

The move came on the same day the president met with Britain’s Prince Charles — a prominent environmentalist and an advocate of efforts to counter climate change — and his wife, the Duchess of Cornwall, in the Oval Office. Obama and Charles were slated to discuss the two nations’ efforts to address global warming, encourage corporate social responsibility, create opportunities for young people, and preserve historical and cultural ties.

The executive order detailed how the government will meet the new climate target. It will include reducing energy use in federal buildings by 2.5 percent per year between 2015 and 2025, instructing agencies to obtain 25 percent of their energy from carbon-free sources by 2025, and increasing the carbon-per-mile efficiency of federal fleets 30 percent from 2014 levels over the next decade while increasing the percentage of zero-emission and plug-in hybrid vehicles in federal fleets. It also requires formal climate training for some federal personnel.

After signing the executive order at the White House, Obama toured the Energy Department’s rooftop solar panels and attended a roundtable there where some federal suppliers discussed their new climate commitments. The companies will publicly disclose how closely they are meeting these greenhouse-gas-reduction pledges over time.

The federal government is the largest energy consumer in the United States — with the military accounting for roughly half of its total energy use — though its greenhouse-gas emissions were only 0.6 percent of the nation’s total output in 2013, according to federal data. Still, Christy Goldfuss, acting director of the White House Council on Environmental Quality, said federal actions can have a huge impact given the reach and the scope of the government’s supply chain.

“President Obama has made it clear that climate change is an all-hands-on-deck challenge,” Goldfuss said, noting the federal portfolio boasts 360,000 buildings, 650,000 fleet vehicles and $445 billion in spending on goods and services.

While the federal government met the climate target Obama set out in his first term relatively easily, it has faced challenges in reducing greenhouse-gas emissions in some areas. The 2007 Energy Independence and Security Act dictated that all new federal buildings and those undergoing major renovations use fossil-fuel-free energy by 2030. The Energy Department has been slow to implement the rule, and a bipartisan coalition in the Senate is seeking to roll back the measure.

In October, the Energy Department put out guidance that keeps the target in place but allows for exceptions, including an exemption if an agency determines that it is not practical to meet it.

Dave McCurdy, president and chief executive of the American Gas Association, said in a statement that while his group supports efforts to address climate change, the government shouldn’t make it harder for Americans to use natural gas.

“We have repeatedly reminded this administration about the benefits of direct use of clean natural gas in homes and businesses,” McCurdy said.

Republicans declined to comment on the new initiative, though Senate Majority Leader Mitch McConnell (R-Ky.) made it clear Thursday that he hopes to block a key part of the president’s climate agenda. McConnell sent a letter to all 50 governors urging them not to comply with the Environmental Protection Agency’s proposed carbon limits on existing power plants.

Meanwhile, the Center for American Progress, a liberal think tank, issued a report Thursday finding that greenhouse-gas emissions generated by gas, oil and coal extracted from federal lands and waters accounts for one-fifth of the nation’s total carbon output.

The group — which is usually supportive of the administration and employed both Deese and Goldfuss before they joined the White House — called the emissions “a blind spot in U.S. efforts to address climate change.”