**12. Planetary climate change: a health security threat**

Our planet is changing and evolving constantly, with continued climate transitions throughout the Earth's history becoming the scientifically accepted norm [234–236]. For the purposes of this discussion, we will refer to this complex set of phenomena simply as "planetary climate change" or PCC. Key areas affected by PCC include agriculture [237], forestry [238, 239], species migrations [240], vectorborne infectious diseases [241], urban air pollution [241], wind activity [75, 242], as well as changes in water availability [243]. Of importance, all of the above elements are closely interrelated, and it will be extremely difficult to elucidate any binary cause-and-effect relationships, making any debates around the topic of PCC both circumstantial and highly controversial. Beyond various direct and indirect effects of climate change on human health [244], it has been suggested that among the manifestations of the observed human response to PCC are armed conflict and widespread migration [245, 246].

Perhaps one of the most impactful aspects of the current PCC trends is the emergence of increasing temperature variability [247], which can, in turn, create local weather conditions that regions of the planet may simply not be prepared to handle effectively [248, 249]. One of the proposed models suggests the emergence of "severe and widespread droughts in the next 30-90 years" [235, 250], with clear implications to other topics discussed in this review such as food security, emerging infectious diseases, and human migrations [251].

Another important aspect of the overall management of PCC is the need to better understand any effects of solar (and even beyond that, cosmic) energetic inputs, with special focus on the relationship on such activity on the observed patterns of climate behavior [252, 253]. This includes the potential interplay between the Earth's magnetic field, solar output, and cosmic rays as climate modulators, including key determinants of various phenomena such as cloud formation [254–256]. Although far too vast—and inadequately understood—the topic of "planetary health" is by default the overarching determinant of human well-being and sustainability. In the United States, multiple organizations and agencies are involved in activities and actions revolving around PCC, including the Department of Agriculture, the US Agency for International Development, the National Science Foundation, the National Aeronautics and Space Administration, the Environmental Protection Agency, the Department of Transportation, and the Department of State, among others.
