On the grounds of the University of California, San Diego Health System, cacti and succulents thrive where grass once lived, flooding the area. Patches of bare ground await replanting or a blanket of mulch. Recycled “gray” water flows through pale purple pipes. But the real action is in hospitals, where another set of pipes carries nitrous oxide. It's a common anesthetic, also known as laughing gas, and it spews out greenhouse gases that linger in the atmosphere for about 114 years. The pipes leak, a lot. Up to 80 percent of the gas can leak. (JOANNE KENEN)