Callery pear trees in Pittsburgh. The smell of the invasive trees has been compared to rotting fish and other stinky things. Luke H. Gordon/Flickr hide caption

itoggle caption Luke H. Gordon/Flickr

Volunteers search for needles and other drug paraphernalia along Church Street in Austin, Ind., in April. The region has recorded 142 new HIV cases since December, according to the state, in an outbreak tied to injected-opioid use. Seth Herald/Nurphoto/Corbis hide caption

itoggle caption Seth Herald/Nurphoto/Corbis

A giant cluster of about 3,000 stars called Westerlund 2. The cluster resides in a raucous stellar breeding ground known as Gum 29, located 20,000 light-years away in the constellation Carina. NASA, ESA, STScI/AURA hide caption

itoggle caption NASA, ESA, STScI/AURA

Work partners Jon Chintanaroad (left) and Mike Prestano are all smiles now, but founding a tech startup together threatened their friendship — and their business. April Dembosky/KQED hide caption

itoggle caption April Dembosky/KQED

A cherry tree and its blossoms are covered with snow in an orchard near Traverse City, Mich. Three years ago, almost every fruit crop in Michigan was frozen out when cold temperatures followed some 80 degree days in March. John L. Russell/AP hide caption

itoggle caption John L. Russell/AP

"Part of the reason we're here is because climate change is threatening this treasure and the communities that depend on it," Obama said Wednesday of his visit to Everglades National Park in Florida. "If we don't act, there may not be an Everglades as we know it." Mandel Ngan/AFP/Getty Images hide caption

itoggle caption Mandel Ngan/AFP/Getty Images