Two years after it launched an ambitious plan to nearly halve its greenhouse gas emissions over the next 15 years, the University is on track to meet its goal several years early, Yale officials said this week.

On Monday in Copenhagen, Denmark, Yale President Richard Levin announced in a widely anticipated speech that the University had already reduced carbon emissions by 17 percent since launching the initiative in the fall of 2005, an achievement that he said “has given us confidence that we are going to achieve our reduction well before our 2020 deadline.”

In the speech, Levin — who holds the Frederick William Beinecke Professorship in Economics — called for a carbon tax or emissions quota to be backed by the United States, Europe, India and China and adopted worldwide, saying that without either measure, the voluntary actions of organizations like Yale will not be enough to stem rising temperatures worldwide.

“The magnitude of the problem highlights one important fact: the solution must be global,” Levin said in his address, the first in a series of lectures on climate change sponsored by the University of Copenhagen in advance of a United Nations’ climate summit to be held in Denmark next year.

“If any one of these four economic powers refuses to participate in an international program to reduce carbon, we cannot succeed in stabilizing global temperatures,” he continued. “Any one holdout pursuing a business-as-usual strategy will make the cost of adequate global reduction prohibitive.”

At Yale, however, the cost has been far from prohibitive. Overall, the University expects to reach its goal for decreasing carbon emissions without spending more than the equivalent of 1 percent of the University’s annual operating budget, which stands at $2.4 billion this year.

In the first two years of its initiative, the University realized a 43,000 metric ton decrease in carbon emissions, officials said.

“So far, so good,” Tom Downing, the University’s senior energy engineer, told the News this fall. “We have lowered emissions while increasing the size of campus,” Downing added. “That’s been tough since they are seemingly divergent paths.”

The University was able to achieve that result through a number of factors, Levin said, including the installation of more efficient heating and cooling systems in dozens of campus buildings, the replacement of windows and efforts by Yale students to cut energy consumption in the residential colleges.

Yale undergraduates living on campus used 10 percent less energy last year compared to the year before, according to a report released in August by the Office of Sustainability. Energy use among students is expected to fall another 5 percent this year, according to the report.

“Nearly all of these projects require up-front investment, but the good news is that most of the actions we have taken to date have brought sufficient energy savings to yield a positive economic return,” Levin said in his speech. “Based on our experience, I am convinced that just about every large organization that carefully examines its energy sources and consumption will find many investments that have an economic payoff.”

Levin’s speech marked the beginning of an extended foreign visit for the Yale president, who is now in Davos, Switzerland, to attend the World Economic Forum. He visited Denmark because the University of Copenhagen is one of ten members of the International Alliance of Research Universities, of which Yale is a founding member.

The two universities announced last month that a permanent representative from Copenhagen would soon open an office at Yale, joining professors and officials from Australian National University and the University of Tokyo. Both of those universities, which are also IARU members, have already opened offices in New Haven through a one-of-a-kind partnership sponsored by Yale that is designed to promote collaboration among IARU members.

Even after delivering the speech, Levin’s foreign travels are not yet over.

On Saturday, he and the leaders of more than 20 major international universities will meet at the Global University Leaders Forum to discuss the issue of reducing greenhouse gas emissions, Yale Spokeswoman Helaine Klasky said. Next week, at the request of the Chinese Ministry of Education, Levin will travel to Shanghai to lead a workshop on environmental issues for administrators from dozens of China’s top universities, Klasky said.

Yale’s goal of cutting its greenhouse gas emissions by nearly half from its 2005 level would do the equivalent of cutting emissions by about 10 percent below the University’s 1990 level, or about 58 percent over what would be otherwise predicted for 2020, given the planned growth of Yale over the next 15 years, officials said.

That cut would be roughly on par with what would need to happen globally to heed off any worldwide temperature increase of more than 2 degrees centigrade over that time period, a jump that scientists predict would engender serious environmental consequences.

Levin, Downing and Julie Newman, the University’s sustainability director, could not be reached Wednesday for comment.