The politically defined threshold of dangerous climate change is an increase of 2 degrees Celsius in the mean global temperature.
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/greenhouse-gas-emission-targets-limiting-global-warming-2-c
[2] http://admin.indiaenvironmentportal.org.in/category/author/malte-meinshausen
[3] http://admin.indiaenvironmentportal.org.in/category/author/nicolai-meinshausen
[4] http://admin.indiaenvironmentportal.org.in/category/author/william-hare-et-al
[5] http://admin.indiaenvironmentportal.org.in/category/journal/nature
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-change
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-science
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/green-house-gases
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/global-warming
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/emission-targets