The legalization of cannabis and controlling greenhouse gases are the main topics of discussion at joint cabinet meetings between the governments of Quebec and Ontario.

Ministers from both provinces said Friday they will aim to co-ordinate the rules regarding marijuana and how it is sold and distributed.

Ontario has already decided that up to 150 government-run stores will control cannabis sales, similar to the LCBO which sells wine and liquor, and to impose a minimum age of 19.

Quebec has yet to come up with a plan, but cabinet ministers have repeatedly said they would prefer to have private industry sell cannabis, although with controlled pricing, marketing, and advertising.

Public Health Minister Lucie Charlebois plans to table legislation in the next few weeks.

When it comes to greenhouse gases, Ontario is going to join California and Quebec's cap-and-trade system on carbon.

On Friday Quebec Premier Philippe Couillard, Ontario Premier Kathleen Wynne, and California Governor Jerry Brown formally signed the agreement about the cap-and-trade system.

Ontario has held three auctions this year on the amount of pollution companies can emit, while Quebec and California's joint auction has been running them since 2014.

"It's an extremely important step, the two largest provinces, the biggest state [by population], working together and I think we cannot really overstate how important it is that we, as these large jurisdictions, that we play a leadership role, and that as we move forward we work to bring other jurisdictions with us," said Wynne.

Nova Scotia plans to introduce legislation for a cap-and-trade system this fall, and other provinces are weighing their options as the federal government has said they must set up a cap-and-trade system or impose a price on carbon.

Critics in Ontario have slammed both the cost to consumers -- an extra 4.3 cents per litre to the price of gasoline and about $80 a year to natural gas home heating costs, as well as indirect costs -- and the potential cost to the economy.

The auditor general has said that when Ontario links its market with Quebec and California, an estimated $466 million will leave the Ontario economy over three years, because it will be cheaper to buy allowances from those jurisdictions. Both the environmental commissioner and the auditor have said that means greenhouse gas emissions won't actually be cut in Ontario.

But Wynne has staunchly defended the plan, saying it is far more cost effective and effective at reducing emissions than a carbon tax, which the Opposition Progressive Conservative leader has touted.

"We have joined this carbon market because we believe that this is the best plan, that it is the cheapest way to reduce greenhouse gas emissions and that it is the best way to do that," she said Friday.

"In fact, doing it this way, joining this market, cap and trade, allows us to grow our industry, to support industry, to reinvest in the further reduction of greenhouse gas emissions and improve technology and by doing that we actually see economic growth."

Ontario's proceeds from cap-and-trade auctions -- about $1.5 billion this year, with one more auction to go -- are going to green programs, such as energy efficient retrofits for home and business, and providing smart thermostats to homeowners free of charge.

Wynne's two colleagues were more blunt in addressing critics of cap and trade, and climate change deniers.

"I think we are close to the tipping point, with 60 per cent of the world GDP soon covered by a carbon market," said Quebec Premier Philippe Couillard. "Who would have said that only five years ago? Of course, we always face resistance, by ignorance sometimes or lack of knowledge."

California Gov. Jerry Brown noted that the administration of U.S. President Donald Trump may not have an agenda of fighting climate change, but that is why it is important for subnational governments to move forward.

"Currently we are not following a climate agenda in Washington and at the highest levels in America, but at the grassroots, at the state level there is a very powerful movement growing by the day to take the steps necessary to make sure that our planet, that our organized civilization endures, and not only endures, but prospers because we align ourselves with nature," he said.

"The fact that some deny doesn't mean that others are not taking action."

New York has said it is looking into it, Brown said. China, Japan and Mexico have also talked about launching carbon markets.

"We're on the side of science," Brown said. "We're on the side of reality and because of that, others will catch on."