These proxies seemed to indicate that oxygen concentrations in the oceans rose in several steps, approaching today's sea - surface concentrations at the start of the Cambrian, around 541 million years ago — just before more - modern animals suddenly appeared and diversified. (scientificamerican.com)
Statistical analysis of observed daily cloud cover and sulfate surface concentrations in Europe and North America indicates a significant negative correlation between clouds and sulfate. (pubs.giss.nasa.gov)
Figure 1, above: Surface concentration of black carbon for average of AeroCom models (left) and observations (right). (giss.nasa.gov)