London needs a 5% increase in green space, the report said
An extra two million trees should be planted in London to combat rising temperatures, mayor Boris Johnson's environment plan has suggested.
Projections indicate that average summer temperatures could be 3.9C higher in the capital by 2080.
Increasing tree numbers by 2025, to help create 5% more green space in London, could help to thwart such rises in temperature, the report found.
But the Green Party said Mr Johnson's plan "lacks substance".
Isabel Dedring, Mr Johnson's environment adviser, said: "People really struggle to see how an old, Victorian built-up city like London can really be a low-carbon city, which if you look at the targets it needs to be.
"There are lots of things we can do collectively offsetting those rises, and we have to start doing that now."
She said that trees in London's streets improved air quality.
In his election campaign, Mr Johnson promised to fund 10,000 extra trees. Some 1,500 have been planted so far.
The report also called for more roofs and walls to be covered with vegetation to counter the "urban heat island effect", where buildings absorb and release heat.
But Green Party London Assembly member Jenny Jones was scathing about the findings.
"This is a huge let-down.
"The mayor has no coherent plan, no specific targets and too little money.
"This document lists a series of sound-bite projects, but where is the brave thinking, tough targets and bold initiatives?
"Planting more trees is important, but the mayor needs to take bold steps to prevent climate change, not just react to it."