The summit will bring together Britain, Canada, France, Germany, Italy,
Japan, Russia and the United States in the mountain resort of Toyako in northern Japan.
On Tuesday Ban Ki-moon, the UN chief, appealed to G8 leaders to lead the campaign on the "triple crisis" of climate change, poverty and rising food prices.
"They have the capacity, they have the resources and I hope the leaders will demonstrate their political will," Ban said in Tokyo on the first leg of an Asian tour.
Over the weekend he said: "We have resources. We have technologies. But largely lacking is the political will".
The 2005 summit in Scotland called for a doubling of rich nations' foreign aid by 2010 to $50bn, half of it to Africa.
Ban urged G8 leaders to implement the 2005 commitment, saying the world's poor had no access to decent healthcare and education.
"Therefore it is absolutely necessary that the leaders of the G8 should lead this campaign," he added.
Last year global warming dominated the G8 summit in Heiligendamm, Germany, following blunt warnings by UN scientists.
According to the Japanese daily Nikkei, a draft statement will seek an agreement on setting country-based goals on reducing emissions beyond 2013.
The area is already covered by UN talks on a pact to succeed the provisions of the Kyoto Protocol beyond 2012.
Referring to the upcoming G8 summit, Phil Clapp of the Pew Environment Group, said: "What you are likely to see is a large rhetorical statement, saying that everyone is committed to reduce their emissions."