This past year illustrated perhaps even more intensely than ever the ways in which population health is influenced by upstream factors. Countries all over the world continue to grapple with the health consequences of sedentary lifestyles and unhealthy diets linked to the ways in which we have organized transportation, work, and the production and distribution of food. At the same time a mind blowing number of people all over the globe lack something as simple as access to clean water. The critical role of environmental factors was starkly illustrated by the unprecedented levels of air pollution experienced by many cities in China just a few weeks ago. Average world temperatures continue to increase at an alarming pace (just here in Philadelphia last week over 60 degrees in December!). Over half of the world's population lives in cities, and urban areas are likely to be among the most affected by heat waves and other weather-related correlates of global warming such as floods and storms. Discrimination, operating through mechanisms like racial segregation and police brutality as well as the more subtle effects of daily experiences of being slighted, stereotyped or treated unfairly because of skin color, sexual orientation, or national origin continue to affect the health of many people all over the world. Inequalities in health by race, ethnicity or social class remain unacceptably large in many countries including the United States.

But these challenges also present opportunities for public health. They suggest ways in which we can work with government, non-governmental organizations, and communities to act to improve population health both globally and locally, through large (even world wide) policy initiatives, like recent international efforts to curtail global warming, to smaller, but equally important local actions such as community partnerships to improve housing and create welcoming public spaces right here in our West Philly neighborhoods. To be successful we will need a solid evidence base as well as partnerships with a broad range of disciplines and sectors. We no longer have the luxury of saying that we will improve population health through medical care or through the work of traditional public heath agencies alone, important as they may be. But public health is not only about advocacy. Like medicine it is also about diagnosing, understanding causes, and identifying the best treatment. But on a much larger scale than medicine of course. This is what makes public health such a vibrant, challenging, and socially relevant field. As we end 2015 take a minute to read about some of the ways we at the Dornsife School of Public Health are working to improve population health through evidence and action, here and abroad. And as we start a new year, let us redouble our commitment to create the policies and environments necessary for the health of all of us.