Finding the right mix of green energy technologies for generating electricity will be crucial in reducing the global impact of pollution for the next generation, according to a United Nations report co-written by a Yale professor.

Without such efforts by policy-makers worldwide, the report warns, greenhouse gas emissions may double by the year 2050. The report is being released as leaders from nearly 200 countries gather in Paris to discuss a possible agreement on limiting carbon emissions.

"Green Energy Choices: The Benefits, Risks, and Trade-Offs of Low-Carbon Technologies for Electricity Production," is a comprehensive comparison of the greenhouse gas mitigation potential for a number of alternative energy methods - including wind, solar, geothermal, and hydro. The International Resource Panel produced the report for the United Nations Environment Programme (UNEP).

"Renewables come out strong in terms of reducing pollution and offer us a way to keep pollution at bay from rising electricity demand," said Edgar Hertwich, director of Yale's Center for Industrial Ecology, professor of industrial sustainability, and member of the International Resource Panel. "If we continue with fossil fuel systems we will see pollution rise."

Electric power generated by renewable energy sources causes substantially less pollution than energy generated from fossil fuels, the report says. Renewable electricity produces just 5% to 6% of the greenhouse gas emissions created by coal-fired energy plants, and 8% to 10% of those generated from gas-fired plants.

The report also investigates damage from other types of pollution, such as particulate matter and toxic metals. Damage by such pollutants to the environment from renewables is 3 to 10 times lower than damage from fossil fuel based systems, the report says. As for health implications, the human health impacts from renewables are 10%-30% of those from state-of-the-art fossil fuel power.

The report points out strengths and weaknesses for all methods of producing electricity. Offshore wind farms, for example, can produce energy for a long period of time, but they come with higher installation and maintenance costs than land-based wind farms. There also are concerns about bird and bat fatalities with wind technology, although there may be radar systems that can slow wind turbines as birds approach.

The report also provides a closer look at the environmental impact of building roads and bringing in construction equipment to develop hydro power in Africa and South America; the land-use advantages of solar technology; and the costs of large-scale energy storage.

"There are many surprises in this data, even for someone who has worked in this field for a while," said Hertwich. "I was surprised to see the toxic emissions data from coal mines, the information about mine runoff, and the long-term emissions to soil and water from coal mines."

Similar emissions are caused by the iron, aluminum, and copper mines needed to produce material for renewable systems, Hertwich noted, but the massive scale of coal mining makes its emissions much more significant by comparison.

Choosing the best technology to generate power - and picking the best sites for those projects - will have a dramatic impact on the global environment, according to the report. The report also urged leaders in government and the private sector to act with urgency.

"The transition of energy systems takes 100 years," Hertwich said. "It's not something we can do by snapping our fingers. Renewables have been tremendously successful, and they're coming online a lot faster than people might have predicted."