Planet Earth is continually undergoing climate change, a slow shifting of climate patterns caused by the general cooling or warming of the atmosphere. The current trend of global warming—which refers to the observed warming of the Earth’s climate as atmospheric levels of greenhouse gases increase—is extraordinary because it is happening more quickly than climate changes in the past and appears to be linked primarily to human agency. Greenhouse gases (GHG; carbon dioxide, methane, water vapor, and other gases) are essential to keeping the Earth’s incoming and outgoing radiation balanced in such a way that the Earth’s surface is maintained in a temperature range hospitable to life. Similar to the glass panes of a greenhouse, these gases allow solar radiation to pass through the atmosphere and strike the Earth’s surface; the gases also allow much of this radiation to bounce back into space as surface radiation. But some radiation is re-reflected back to the Earth, keeping its surface (like the interior of a greenhouse) warmer than it would be if incoming and outgoing radiation were in balance. Figure 1.13 shows this system of incoming and outgoing radiation and the role of greenhouse gases in trapping some of the outgoing radiation and sending it back to warm the Earth. Now, as greenhouse gases are being released at accelerating rates by the burning of fossil fuels and by the effects of deforestation (see Figure 1.7A, C, E), the evidence indicates that the Earth is warming at a very fast rate.

Most scientists now agree that there is an urgent need to reduce greenhouse gas emissions to avoid catastrophic climate change in coming years. Climatologists, biogeographers, and other scientists are documenting long-term global warming and cooling trends by examining evidence in tree rings, fossilized pollen and marine creatures, and glacial ice. These data indicate that the twentieth century was the warmest century in 600 years, and that the current decade is the hottest on record. Evidence is mounting that these are not normal fluctuations. Very long-term climate-change patterns indicate that we should be heading into a cooling pattern, but instead it is estimated that, at present rates of warming, by 2100 average global temperatures could rise between 2.5°F and 10°F (about 2°C to 5°C).

However, a key problem is that those most responsible for global warming (the world’s wealthiest and most industrialized countries) have the least incentive to reduce emissions because they are the least vulnerable to the changes global warming causes in the physical environment. Meanwhile, those most vulnerable to these changes (poor countries with low human development and with large slums in low-lying coastal wetlands) are the least responsible for the growth in greenhouse gases, and have the least power in the global geopolitical sphere; hence they have the least ability to affect the level of emissions (Figure 1.14).

Greenhouse gases exist naturally in the atmosphere. It is their heat-trapping ability that makes the Earth warm enough for life to exist. Increase their levels, as humans are doing now, and the Earth becomes warmer still.

Electricity generation, vehicles, industrial processes, and the heating of homes and businesses all burn large amounts of CO₂-producing fossil fuels such as coal, natural gas, and oil. Even the large-scale raising of grazing animals contributes methane through the animals’ flatulence. Unusually large quantities of greenhouse gases from these sources are accumulating in the Earth’s atmosphere, and their presence has already led to significant warming of the planet’s climate.

Widespread deforestation worsens the situation. Living forests take in CO₂ from the atmosphere, release the oxygen, and store the carbon in their biomass. As more trees are cut down and their wood is used for fuel, more carbon enters the atmosphere, less is taken out, and less is stored. The loss of trees and other forest organisms produces as much as 30 percent of the buildup of CO₂ in the atmosphere. The use of fossil fuels accounts for the remaining 70 percent.

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In percentages, the largest producers of total greenhouse gas emissions in 2010 were the industrialized countries and large, rapidly developing countries. The caption of Figure 1.14 lists the countries that are the most responsible for GHG emissions, total (A) and per capita (B). Note that the United States is among the leaders in both categories. For the period of 1859 to 1995, developed countries produced roughly 80 percent of the greenhouse gases from all types of industrial, home, and transportation sources, and developing countries produced 20 percent. But by 2007, the developing countries were catching up, accounting for nearly 30 percent of total emissions. As developing countries industrialize over the next century and continue to cut down their forests, they will release more and more greenhouse gases every year. If current patterns hold, greenhouse gas contributions by the developing countries will exceed those of the developed world by 2040.

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While it is not clear exactly what the impacts of rising global temperatures will be, it is clear that they will not be uniform across the globe. One prediction is that the glaciers and polar ice caps will melt, causing a corresponding rise in sea level. In fact, this phenomenon has been observable for several years. Satellite imagery analyzed by scientists at the National Aeronautics and Space Administration (NASA) shows that between 1979 and 2005—just 26 years—the polar ice caps shrank by about 23 percent. The amount of polar ice cap shrinkage wavers from year to year, with the ice caps regaining ice to some extent during winter months, but the overall trend in recent years is 12 percent shrinkage per decade. The polar ice caps normally reflect solar heat back into the atmosphere, but as the ice melts, the dark, open ocean absorbs solar heat. This warming of the oceans not only hastens ice cap melting, it is changing ocean circulation, the engine that drives weather and climate globally.

The melting of the ice caps also has several other effects. Already, trillions of gallons of meltwater have been released into the oceans. If this trend continues, at least 60 million people in coastal areas and on low-lying islands could be displaced by rising sea levels. Another issue is the melting of high mountain glaciers, which are a major source of water for many of the world’s large rivers, such as the Ganga, the Indus, the Brahmaputra, the Huang He (Yellow), and the Chang Jiang (Yangtze). Similar melting effects on major rivers are expected in South America. Scientists have monitored mountain glaciers across the globe for more than 30 years; while some are growing, the majority are melting rapidly. Over the short term, melting mountain glaciers could result in the flooding of many rivers, but eventually river flows will decrease as mountain glaciers shrink or disappear entirely.

Over time, higher temperatures will shift northward in the Northern Hemisphere and southward in the Southern Hemisphere, bringing warmer climate zones to these regions. Such climate shifts could lead to the displacement of large numbers of people because the zones where specific crops can grow are likely to change. Animal and plant species that cannot adapt rapidly to the changes will disappear. Higher temperatures also will lead to stronger tornados and hurricanes because these storms are powered by warm, rising air (Figure 1.15). One example is Hurricane Sandy, the unusually large and powerful hurricane that struck the Atlantic Coast of North America in the autumn of 2012. In some areas, drought and water scarcity may also become more common since higher temperatures increase water evaporation rates from soils, vegetation, and bodies of water. Another effect of global warming is likely to be a shift in ocean currents. The result would be more chaotic and severe or dry weather, especially for places where climates are strongly influenced by ocean currents, such as western Europe (see “Vegetation and Climate” in Chapter 4).

The vulnerability a place has to climate change can be thought of as the amount of risk its human or natural systems have of being damaged by such impacts as sea level rise, drought, flooding, or increased storm intensity. Many of these vulnerabilities are water related; others are not. Scientists who study climate change agree that while we can take measures to minimize temperature increases, we can’t stop them entirely, much less reverse those that have already occurred. We are going to have to live with and adapt to the impacts of climate change for quite some time. The first step in doing this is to understand how and where humans and ecosystems are especially vulnerable to climate change.

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In 1997, an agreement known as the Kyoto Protocol was adopted. The protocol called for scheduled reductions in CO₂ emissions by the industrialized countries of North America, Europe, East Asia, and Oceania. The agreement also encouraged, though it did not require, developing countries to curtail their emissions. One hundred eighty-three countries had signed the agreement by 2009 (Canada withdrew in 2011). The only developed country that had not signed was the United States, which was then and still remains one of the world’s largest per capita producers of CO₂.

In December 2009 in Copenhagen, 181 countries worked on a plan to halt the rising concentrations of CO₂ in the atmosphere by 2020. However, wealthy nations did not offer to curb emissions sufficiently enough to make a difference. The only progress that came out of Copenhagen was an agreement to help developing countries create clean-energy economies and otherwise adapt to climate change; but adequate funds were not allocated even for this.

In December 2011, the UN Framework Convention on Climate Change (UNFCCC) convened, this time in Durban, South Africa, again with the goal of controlling emissions. After grueling negotiations, all 190 countries, including the top three emitters—China, the United States, and India—agreed to a plan to cut emissions significantly by 2020. But was this sufficient progress? According to climate scientists, 2020 is too late to begin major changes. Rather, to avoid a temperature rise of more than 2°C, carbon emissions would need to be in decline well before 2020. To achieve this, the agreements would need to be far more binding. Current research into CO₂ emissions suggests that no significant reductions have been made.