A voluntary club of 56 nations headed by King Charles, the Commonwealth evolved out of the British empire to cover 2.5 billion people, with 11 Pacific nations among its members.
"This is not a threat about extinction tomorrow, many of them are thinking, it is extinction today," Patricia Scotland said of the climate change risks that make small Pacific nations particularly vulnerable.
"If we look at what's happened in countries like Vanuatu and Nauru and all of them, they see this rise in sea level and the rise in temperature as an imminent threat," she said, speaking from the Samoan capital of Apia.
Samoa will be the first Pacific small island state to host the annual Commonwealth Heads of Government meeting, putting the spotlight on climate change.
The grouping wanted to put climate finance advisers in member nations to boost fund-raising efforts to tackle the fallout of climate change, Scotland said.
Other topics for discussion are artificial intelligence and digitalisation of the global economy.
"If the small and developing states are left behind, then their opportunity to take advantage of that huge potential increase in development and opportunity is going to be gone," she added.
A Commonwealth spokesman said plans for the Samoa summit factor in the attendance of King Charles, set to give his first speech as the head of the grouping after missingthe annual Commonwealth Day event this month to recover from cancer treatment. The Commonwealth celebrates its 75th anniversary this year.
