These days, it’s common to hear a politician, an economist, or even an oil company profess that pricing carbon is the most efficient way to combat climate change. But real-life climate policy is often far from efficient; we’re left settling for second-best (or third- or fourth-best) solutions. In the run-up to the Paris climate talks at the end of this year, a fair question then is whether or not we can expect any kind of global carbon pricing mechanism to emerge from the negotiations.

Spoiler alert: probably not, but not for want of trying.

Over at The Christian Science Monitor, Cristina Maza takes a deep dive into the logic behind — and viability of — carbon pricing at the national and international levels. While the global approach has been piecemeal so far, she writes, a handful of countries have given the concept a shot in one way or another:

Currently, about 40 national and over 20 sub-national jurisdictions have implemented or scheduled carbon-pricing systems, according to a report by the World Bank and Ecofys, a renewable-energy consultancy. That represents nearly a doubling of such systems since 2012. All together, global carbon taxes and trading systems are estimated to value just under $50 billion, according to the World Bank and Ecofys.

But not all carbon-pricing systems are created equal. Critics of cap-and-trade systems, for example, often tout trading mechanisms as inequitable. If a polluting plant can still pay to pollute, the argument goes, the poorer communities where such plants are often located will continue to bear the brunt of poor air quality. Environmental justice groups often advocate on behalf of a flat carbon tax or, more simply, mandatory emissions cuts (and more recently, “revenue-neutral” policies like fee and dividend). Under the World Bank’s definition, all of the above (except mandatory emissions cuts) count as carbon pricing.