The gathering includes the heads of states of the U.S., Japan, Britain, Germany, France, Italy, Canada and Russia.
Regine Guenther, director of the World Wildlife Fund Climate Change Program in Germany, told reporters in the German capital that G8 leaders should commit to reducing emissions in their countries 40 percent by 2020 and 80 percent by 2050.
"If we don't achieve that, the world's climate will change in ways that we can't even imagine today," Guenther said.
The scorecard ranked Britain as the developed nation that has done the most to reduce carbon dioxide emissions and reach targets set by the Kyoto Protocol. France and Germany are close behind. Germany was praised for its investment in renewable energy.
"But all three countries are at best half as far along the road as they should be," a statement announcing the study said.
The scorecard was compiled by Ecofys, a Dutch consulting company, and commissioned by the World Wildlife Fund and insurer Allianz SE.
Joachim Faber, an Allianz board member who helped compile the scorecards, said a global emissions trading market is important to fighting climate change, and that the EU should lead its development.
"The EU-specific trading system we have at the moment must serve as model system for one that we can found outside the EU, for the world economy," he said.
The study criticized low energy efficiency in the U.S., but said there was hope in legislation under consideration by Congress and initiatives led by non-governmental groups.
The study also analyzed -- but did not rank -- five of the world's fastest growing economies: Brazil, China, India, Mexico and South Africa. "These countries cannot be measured with the same ruler as industrialized countries," the statement said.