OTTAWA -- Prime Minister Justin Trudeau is back from an appearance at an international climate change conference with a simple question hanging over his head: What now?

A 90-day countdown is underway before Trudeau meets again with Canada's premiers to hammer out a climate-change framework that would guide federal and provincial efforts in meeting the ambitious talk from the Paris conference.

The provinces will be looking to the federal government for money to help pay for their climate-change commitments. But the Liberals have a slew of green spending promises of their own to keep, totalling more than $13 billion in new, green spending over the next four years.

And missing from all of it are the details of what the money will buy and where it will be spent -- details that observers say would spell out whether the government will be able to meet its emission reduction ambitions.

"It starts putting us on the trajectory towards achieving the kinds of deployment of clean technology and the associated emission reductions that we're going to need," said Dan Woynillowicz, policy director at Clean Energy Canada.

"Is it enough? I can't answer that until we actually see a much more comprehensive plan that lays out when, where and how those dollars are going to be invested."

Those details are expected to come out over the next 90 days in the leadup to Trudeau's next meeting with premiers and beyond that as the Liberals get closer to releasing their first budget in 2016.

The first cash infusion came Monday when Trudeau followed through on a pledge to spend $300 million for research and development in the renewable energy sector. Erin Flanagan, federal policy director with the Pembina Institute, said there's no simple way to show how much each R&D dollar is worth in terms of emissions reductions, putting the onus on the Liberals to prove their spending is working.

"If the government wants to have public confidence on these expenditures, they will have to bring us into that story," Flanagan said.

"They will have to prove that there is some benefit to Canada, to Canadians and also to this global process of emissions reductions."

The story is the same for the approximately $11.3 billion in extra spending on investments in green infrastructure and public transit, with $2 billion more set aside for emission-reducing projects the Liberals have promised over the next four years.

"The use of public transit is a huge factor in all this," Transportation Minister Marc Garneau said last week in an interview with The Canadian Press.

"If more people are leaving their cars at home and getting into modes of transportation that are public ... that also contributes to reducing greenhouse gas emissions."

About 23 per cent of Canada's greenhouse gas emissions come from the transportation sector, just behind the oil and gas sector, which tops the list.

Reducing emissions in the transportation sector is going to require some strategic spending, Flanagan said. Depending on how the government does its math, it could end up paying for subways in Toronto or more buses in parts of B.C. to meet its emission goals, she said.

Some of that money could go towards renewable energy sources and upgrades to the electricity grid. A report from a group working with the United Nations Sustainable Development Solutions Network estimated would need an extra $13.5 billion in annual investment spending to help Canada get closer to its emission targets.

Louise Comeau, executive director of the Climate Action Network, said that investment would go further to reducing emissions in the next five years than spending in any other area, including public transit.

Public transit investments will take more time and money than other investments the government is proposing and doesn't reduce emissions much in the short-term, she said. The funding, she said, is an "important long-term play."

"That's not to say it (public transit) shouldn't be there. It's necessary for the long-term reductions, but in the short term to get us to our goals for 2020 and 2025, the focus should be on electricity and on vehicles and buildings," Comeau said from Paris, where she is attending the international climate conference. "That is where you drive down demand, and where you shift the source of supply."