Some countries have already adopted such a tax and discussions are ongoing in others. There are proponents of a global carbon tax too. Yet, governments are often keener to adopt measures other than a tax to contain carbon emissions. And amongst countries that have adopted a carbon tax, the levels vary substantially, and other measures exist alongside it. It is natural to ask why a carbon tax is not used universally in preference to other policies to limit emissions and why, even if there is such a tax, other policies are needed too