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The impacts of many human activities are experienced, at least initially, at the local or regional level. Urban smog, the degradation of watersheds, and the loss of local wildlife habitat are examples. But some human impacts affect the Earth on a much wider scale. This section looks at developments in the areas of climate change, stratospheric ozone depletion, and the global nitrogen cycle. These trends reflect some of our impacts on the global commons – those natural systems and cycles that underpin the functioning of ecosystems everywhere.
It is the scale of human activities, rather than the individual activities themselves, that poses the greatest risk to global resources. Greenhouse gas emissions from a single power plant or automobile hardly threaten the global atmosphere, but multiply these sources by several orders of magnitude and the combined effects become global in their reach. Today, carbon dioxide emissions from human sources – mostly from the burning of fossil fuels – average more than 7 billion metric tons of carbon per year and have begun to alter the dynamics of the world’s climate system.
Over the past 25 years, global energy use has risen some 70 percent and is expected to keep climbing. Options, such as renewable energy sources, energy efficiency improvements, and even traditional nuclear energy, are available to keep up with the world’s power needs without letting greenhouse gas emissions spiral out of control, but their development and use on a wider scale will require concerted action by the international community.
Other challenges also require the world’s attention. The struggle to repair damage to the stratospheric ozone layer is far from over. Acid rain has become a major problem in Asia; and the massive use of fertilizer and the burning of fossil fuels have disrupted the natural cycling of nitrogen compounds that form the basis of plant and animal nutrition everywhere. Again, the scale of these threats is very large, and no single nation’s action will suffice to reduce them.