Carbon and Climate:

Understanding the carbon cycle

Carbon is the fourth most common element on earth, existing in a brilliant diversity of organic compounds and serving as the chemical basis for all life. The economic activities of our cities, towns, farms, and suburbs have grown because of carbon’s stored abundance in fossil fuels—mainly oil, natural gas, and coal. When fossil fuels are burned to produce energy, the carbon stored in them is emitted almost entirely as carbon dioxide.

Carbon dioxide is also transferred throughout ecosystems among a number of natural carbon reservoirs—a process known as the carbon cycle.

Plants remove carbon dioxide from the atmosphere and use it to build their leaves, stems, and roots—their biomass. This carbon, when converted to plant tissue, especially wood, can remain separate from the atmosphere for years until the vegetation dies and decomposes, releasing the carbon dioxide back to the atmosphere through respiration. Fossil fuels are the remains of plants and animals that became concentrated and isolated through hundreds of millions of years of geologic processes. When we burn fossil fuels, we rapidly decompose them through combustion releasing the carbon dioxide. We also cause release of stored carbon when we disturb soils, or convert forests and wetlands to other land uses such as houses or commercial development. This land disturbance allows stored carbon to decompose, releasing carbon dioxide to the atmosphere.

Humans and Climate Change

Humans have released millions of tons of carbon that had previously been removed from the atmosphere. These perturbations by human activities are the reason scientists have observed significant increases in atmospheric carbon dioxide in recent centuries.

As fossil fuel use increases globally and more of the Earth’s vegetated surface is disturbed or converted to human uses, the atmospheric concentration of carbon dioxide will continue to increase. Humans today emit some 28 billion tons of carbon dioxide into the atmosphere annually and the United States is responsible for roughly 20 percent of the world’s greenhouse gas emissions. These inputs could lead to widespread changes in regional temperatures and precipitation in only a few decades and may already be having an effect (Figure 3 from Translation document).

Human sources of carbon dioxide are growing in magnitude as industrial and commercial activities increase globally and deforestation and changing land uses diminish the Earth’s capacity to sequester carbon. Scientists now agree with a high degree of certainty that if these trends are not reversed the amount of carbon dioxide in the Earth’s atmosphere will continue to increase, causing widespread changes to climate, rising sea levels, and disruptions to the economic activities that depend on a stable climate and environment.

Scientists are often called upon to make predictions based on current and past data. Although it is impossible to know exactly what the future climate will be, scientists have used computer models that show a range of possibilities, or scenarios, based on projected uses of fossil fuels and changes in land use. Most scientists now believe that even under scenarios of relatively low emissions of carbon dioxide, the Earth’s climate will change in the coming decades—a prediction that has spurred an international coalition of scientists, government agencies, nonprofit organizations, businesses, and universities to call for a coordinated plan to reduce our net carbon dioxide emissions.

To learn more about how climate may change in the northeastern United States under various scenarios, view Confronting Climate Change in the Northeastern U.S.