By Michael Szabo Warsaw (Reuters) - Governments want to launch a platform at United Nations climate talks to help set common standards and accounting rules and tie together national and regional emissions trading schemes, but developing countries and green groups warned that talk of a global carbon market is premature. Almost 200 nations are in Poland for a November 11-22 meeting to plan a 2015 U.N. deal in Paris that would start to tackle climate change in 2021. Most developed countries see carbon markets as crucial under any new agreement because they seek out the cheapest emissions reductions, making climate change targets more achievable. More than 40 mainly developed countries, including New Zealand and members of the European Union, have, or are in the process of developing, markets to help cut their output of climate-warming emissions by putting a price on carbon dioxide.