Modelling the amount of climate change we expect requires estimates of the total emissions, which have historically increased with total GDP. Tradeoffs between climate mitigation and adaptation also depend strongly on how developed we expect different countries to be. By forecasting GDP we can ensure our climate models and climate change policies place most emphasis on worlds that have a high probability of existing. China, USA and India are currently the three most populous and most polluting countries in the world, collectively responsible for over half the CO2 emissions and around 40% of the world’s population.
The World Bank reports country-by country GDP adjusted for purchasing power on a yearly basis. A search tool is available here.