Researchers predict that global warming generally will make summers hotter and winters milder from Southern California to the northeastern United States to parts of Africa, Asia and Latin America. The impact will vary depending on different regions of the planet, according to a report published Wednesday by the National Oceanic and Atmospheric Administration, or NOAA. While the U.S., Canada and northern Europe are expected to see modest increases in mild to hot days, people living in the tropics should brace for significantly more heat, said authors of the study, which appears in the journal Climatic Change.