Global health. The current scenario and future perspectives

119 climate system to understand its basic concepts. It then analyzes the relationship between anthropogenic climate change and population health, as well as the main actions taken at the international, regional and national levels. It concludes with some climate action strategies and the role of different actors in society. Climate System and Climate Change: Basic Concepts To understand the relationship between climate change and population health, it is necessary to know some basic concepts related to the climate system. Life on Earth is possible due to complex interactions between the components of the climate system: the atmosphere, hydrosphere, cryosphere, lithosphere and biosphere. Briefly, the hydrosphere comprises all water bodies on Earth, including oceans, fresh water in rivers and lakes, groundwater and water in the atmosphere. The cryosphere is limited to the portion of solid or frozen water. The lithosphere refers to the solid layers of the Earth. The biosphere encompasses all living organisms in the atmosphere, soil and oceans. Finally, the atmosphere contains different gases, with the most abundant being Nitrogen (N), Oxygen (O), Argon (Ar) and Carbon Dioxide (CO 2 ). These gases include the greenhouse gases (GHGs), which trap heat and are responsible for maintaining a habitable planet at an average global temperature of 15°C. The greenhouse effect occurs when the Sun emits solar radiation that reaches the atmosphere and heats the Earth. Approximately two-thirds of this radiation is reflected back into space, but the remaining third passes through the atmosphere, where some of it is absorbed by water vapor and other GHGs, and the rest by the Earth’s surface. This absorbed energy heats the Earth, but some of this energy is re-emitted from the Earth into the atmosphere as heat, where GHGs trap it to keep the atmosphere warm (Figure 1, left diagram).