Active Volcanoes on Mars?

A new article suggests that there may be at least three volcanoes on Mars that are dormant rather than extinct. Each of the suspected volcanoes are more than three times the size of the largest volcano on Earth, which is found in Hawaii. New studies of lava flows on Mars show that they may have been active within the last 2 million years. If an eruption is possible in the future - especially if all three were to become active again - I suspect this would have some substantial effects on the atmosphere of the planet due to the gases that would be emitted.