Atmospheric particles containing lead might have offset the Earth's warming in the 20th century.
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/did-lead-cause-global-cooling
[2] http://admin.indiaenvironmentportal.org.in/category/author/jeff-hecht
[3] http://admin.indiaenvironmentportal.org.in/category/journal/new-scientist
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lead
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/fuel-standards
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-change
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-science
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/global-warming
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/united-states-america-us