www.ipcc.ch/

The Intergovernmental Panel on Climate Change (IPCC) website includes the history of  paneland its relationship to the United Nations alongside assessment reports on the impact of climate change.

http://webarchive.nationalarchives.gov.uk/20080910140413/http://www.hm-treasury.gov.uk/independent_reviews/stern_review_economics_climate_change/sternreview_index.cfm

The full report of the 2006 Stern Review on the Economics of Climate Change, including a range of contemporary material such as press releases and comment.  

unfccc.int/2860.php

The United Nations Framework Convention on Climate Change (UNFCCC) website includes the latest information on the Paris agreement and the Kyoto Protocol, including ratifications and the full text. 

https://www.cfr.org/energy-and-environment

This link contains content from the Council on Foreign Relations on energy and the environment, with  numerous analyses and interviews on US environmental policy.