The United Nations has appointed Patricia Espinosa, a Mexican diplomat, as the top U.N. official leading the fight against global warming. Espinosa said in a statement Thursday that she is "grateful and honored" to be named executive secretary of the U.N. Framework Convention on Climate Change. The appointment was announced by U.N. Secretary-General Ban Ki-moon. [divider style="solid" top="20" bottom="20"]Mexico joins 174 other countries in histo…