Obama mentioned climate change during his evening speech organized by a club for leaders from the internet and communications industries was on the theme, "Fear Less, Innovate More".
North American cities commit to tackle climate change in spite of the Trump administration.
"We sought to set ambitious climate targets globally", the former president told the summit.
Chicago officials are billing the summit as the first of its kind in the city and Mayor Rahm Emanuel says he's honoured Obama will join.
As he discussed the Paris agreement to limit carbon emissions, which Trump pulled the USA out of, Obama got in another dig, saying "we have a little bit of a pause in American leadership".
Former President Barack Obama was expected to address the crowd in the afternoon, following sessions on transportation, energy and waste management for mayors from cities including Paris, Mexico City, San Francisco and Phoenix.
Trump further argued if the Paris agreement were completely implemented by all countries, "14 days of carbon emissions from China alone would wipe out the gains from America". Or, in other words, much like with Sanctuary Cities, an effort to circumvent the USA government under Trump to advance the liberal agenda.
Paris Mayor Anne Hidalgo said city residents will be victims as a result.
Chicago Chief Sustainability Officer Chris Wheat says the idea is to "fill the void" the Trump administration has left.