Understanding how tiny particles emitted by cars and factories affect Earth's climate requires accurate climate modeling and the ability to quantify the effects of these pollutant particles vs. particles naturally present in the atmosphere. One large uncertainty is what Earth was like before these industrial-era emissions began. In a paper just published in Nature, scientists collaborating on the GoAmazon study describe how they tracked particles in the largely pristine atmosphere over the Amazon rainforest, which has given them a way to effectively turn back the clock a few hundred years.