9.4 Increases in the Greenhouse Gases

Anthropogenic means caused by humans. The rise in these gases since about 1750 is anthropogenic: CO₂ has climbed from about 280 parts per million (ppm) before industrialization to over 420 ppm today, a rise of about 50 percent. Methane has more than doubled, and nitrous oxide has risen by roughly a quarter.

• CO₂: burning coal, oil and natural gas for electricity, heat, transportation and industry is by far the largest source. Deforestation adds more, both by burning trees and by removing plants that would absorb CO₂. Cement production also releases CO₂.
• Methane (CH₄): cattle and other livestock (bacteria in their stomachs make methane), rice paddies, landfills, manure, and leaks from oil, gas and coal operations.
• Nitrous oxide (N₂O): mostly from nitrogen fertilizer and manure on farm soils, plus fuel combustion and some industrial processes.

In 1958 scientist Charles David Keeling began continuous measurements of atmospheric CO₂ at the Mauna Loa Observatory in Hawaii, high on a volcano and far from local pollution sources. The resulting graph, called the Keeling Curve, shows two patterns.

First, a steady long-term rise, from about 316 ppm in 1959 to over 420 ppm in the 2020s, and the yearly increase has itself sped up.

Second, a small zigzag every year. CO₂ falls during the Northern Hemisphere's spring and summer, when plants photosynthesize heavily and pull CO₂ from the air, and rises in fall and winter, when photosynthesis slows and decomposition keeps releasing CO₂. The Northern Hemisphere controls the pattern because it has much more land and vegetation than the Southern Hemisphere.

Global average sea level has risen about 20 cm (8 inches) since 1900, and the rate has more than doubled in recent decades. Two causes drive it. Thermal expansion: water expands as it warms, and the ocean has absorbed most of the extra heat. And melting land ice: water from glaciers and from the Greenland and Antarctic ice sheets flows into the ocean.

Melting sea ice, which already floats, does not raise sea level much, just as melting ice cubes don't make a full glass overflow. Rising seas cause coastal flooding, erosion, saltwater intrusion into freshwater aquifers and farmland, and loss of coastal wetlands. Low-lying island nations and delta regions, such as parts of Bangladesh, face the greatest risks, and some communities may have to relocate, becoming climate migrants (environmental refugees).

Warmer temperatures let disease vectors such as mosquitoes and ticks survive farther from the equator and higher up mountains. That can spread malaria, dengue, Zika and Lyme disease into regions where people have little experience or immunity (topic 8.15). Many plants and animals are also shifting their ranges toward the poles or uphill to stay within their temperature tolerance, while species that can't move fast enough decline.