This additional alert from the Alliance of World Scientists (AWS) examines the critical role that microorganisms play in the biological factors of climate change. The way in which microorganisms respond to changes in the climate is key to the impact of climate change on ecosystems and life on Earth. This article was published online in Nature Reviews on June 18, 2019. Its two lead authors are Ricardo Cavicchioli and William J. Ripple. Here is a link to the pdf file.