Robertson was the only Canadian representative in the delegation of municipal leaders. In an interview with CBC News Tuesday, Robertson was critical of Canada's environmental track record.
"It's certainly a big concern for mayors globally when the Canadian government is such a laggard, and is fighting against the tide of commitments to fighting climate change," Robertson said.
"It's really a tipping point about climate change and it impacts on the most vulnerable people globally."
Urged bold steps
The municipal leaders urged their national leaders take bold steps at the Paris climate talks later this year, saying that may be the last chance to keep the warming of the Earth at levels safe for humanity.
Robertson and other delegates were invited because they back the Pope's environmental message.
Francis last month released an environmental encyclical that denounced what he calls a fossil fuel-based world economy that exploits the poor and destroys the Earth.
Experts have long said that cities are key to reducing global warming since urban areas account for nearly three-quarters of human emissions.
In a written statement, Robertson's office said the meeting was the most significant mayoral gathering ever held at the Vatican.
Robertson is a panellist at the conference, where he is speaking about the importance of sustainable economic development in cities and highlighting Vancouver's leadership in supporting green and low-carbon technology, the statement said.
Other mayors attending hail from Boston, Oslo, Norway, San Francisco and Boulder, Colo.
In addition to the climate declaration, mayors will be asked to sign a statement against human trafficking that was first penned by religious leaders at the Vatican last year.