Earth Affects Humans
Human interactions with the Earth system are two-way interactions. Most certainly, the environment of the planet affects humans, beginning with the basic difference between body temperature and air temperature that provides human chills or sweat. A less obvious reaction of humans to the Earth environment is the reaction to increased ultraviolet radiation that has passed through the stratosphere in recent decades. Increased UV radiation may cause skin cancers, or possibly lowered immunity to certain diseases, that otherwise would not occur. Another example might be the longer-term effect on human investments in, say, agricultural or forestry if these businesses are adversely affected by global warming. Indeed, there is a very long list of impacts that humans have on the Earth environment and vice versa.
Humans Affect Earth
A second type of human interaction with the Earth system is the effect of human behavior and decisions on the Earth environment. The past century has witnessed a remarkable increase in population, which has driven the conversion of grassy or wooded lands into cities and suburban areas. The removal of trees and forests due to the building of urban areas has played a role in modifying the carbon dioxide concentration in the atmosphere, leading to systematic modification of greenhouse gas concentrations. Asphalt parking lots absorb more incoming solar radiation than natural grassy or tree covered surfaces. Streets also change the hydrologic cycle: rainfall is directed into storm sewers rather than being absorbed by the land and subsequently evaporating. Thus human management of rainwater significantly reduces natural surface cooling by evaporation. Furthermore, buildings towering over the landscape present an artificially roughened "Earth's surface" that alters the wind flow, and changes its role in the surface energy balance. Finally, the construction of buildings presents a totally different "surface" to incoming solar radiation, changing the way radiational heating and cooling occurs. So-called "urban heat islands" may sometimes be a few degrees warmer than the countryside that the cities replaced. These are exemplary, local human effects on the environment that stimulate curiosity about what effects humans have on the global environment.
A Lesson Learned: Sometimes You Get Back What You Gave
It is of interest to note that the systematic decrease in stratospheric ozone that leads to serious modification of human health scenarios, was largely due to the creation of totally unnatural chemicals by humans that were slowly mixed up into the stratosphere. Indeed, human interaction with the Earth system is a key focus of Earth system science and ESSE. The knowledge of what those interactions are and how they come about is crucial for informing the decision-making process. Intelligent decision-making might lead to the conclusion that human development was a positive, not a negative, effect on the Earth system.
Lester R. Brown, President of the Earth Policy Institute, Washington, DC., has written an excellent, on-line article, “Rescuing a Planet Under Stress”
delving into the relations between human activities and Earth system science.
Brown, L.R., 2006, Rescuing a Planet Under Stress, The Futurist, 3 July 2006