The electricity sector is the largest source of greenhouse gases (GHG) emissions in the world, and reducing these emissions can often be costly. However, because electricity markets remain integrated at a shallow level (with different pricing regulations), many gains from deeper integration (adoption of marginal cost pricing everywhere) are yet to be realized. This paper assesses the benefits of deep integration between a "hydro" jurisdiction and a "thermal" one. It also underscores the inefficiency of trade when pricing rules differ. Our detailed hourly model, calibrated with real data from the provinces of Ontario and Quebec, Canada, estimates price, consumption, emissions and welfare changes associated with fully integrating electricity markets, under transmission constraints. A negative abatement cost of $37/tonne of CO2 was found (for more than 1 million tonnes), clearly illustrating the untapped potential of wealth creation in carbon reduction initiatives. Furthermore, given the inefficiency of shallow integration between markets, we found that removing interconnections between markets offers a relatively affordable CO2-reduction opportunity, at $21.5/tonne.