1  Draft Natural and Human Causes of Climate Change

Climate is driven largely by radiation from the sun. Incoming solar radiation may be reflected, absorbed by land surface and water bodies, transformed (as in photosynthesis), or emitted from the land surface as longwave radiation. Each of these processes influences climate through changes to temperature, winds, the water cycle, and more. The overall process is best understood by considering the Earth’s energy budget.

The Earth’s Energy Budget

The Earth’s climate is driven by the sun. The balance between incoming and outgoing radiation—Earth’s radiation or energy budget—determines the energy available for changes in temperature, precipitation, and winds and, hence, influences atmospheric chemistry and the hydrologic cycle. The Earth’s surface, atmosphere, and clouds absorb a portion of incoming solar radiation, thereby increasing temperatures. Energy as longwave radiation (heat) is re-emitted to the atmosphere, clouds, or space, thereby reducing temperatures at the source. If the absorbed solar radiation and emitted heat are in balance, the Earth’s temperature remains constant.

Figure 1.1: The Earth’s radiation balance is the main driver of our climate. Image courtesy of National Aeronautics and Space Administration (1)

Natural factors contributing to past climate change are well documented and include changes in atmospheric chemistry, ocean circulation patterns, solar radiation intensity, snow and ice cover, Earth’s orbital cycle around the sun, continental position, and volcanic eruptions. While these natural factors are linked to past climate change, they are also incorporated in the analysis of current climate change.

Since the Industrial Revolution, global climate has changed faster than at any other time in Earth’s history (Mann et al. 1999). This rapid rate of change—often referred to as human-caused climate change—has resulted from changes in atmospheric chemistry, specifically increases in greenhouse gases due to increased combustion of fossil fuels, land-use change (e.g., deforestation), and fertilizer production (Figure 2-1) (Forster et al. 2007). The primary greenhouse gases in the Earth’s atmosphere are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), water vapor (H2O), and ozone (O3).

Figure 1.2: Changes in important global atmospheric greenhouse gas concentrations from year 0 to 2005 AD (ppm, ppb = parts per million and parts per billion, respectively) (Forster et al. 2007).

Incoming solar radiation is either absorbed, reflected, or re-radiated from the Earth’s surface. Since greenhouse gas concentrations are greatest near the surface, a large fraction of this reflected and re-radiated energy is absorbed in the lower portions of the atmosphere (hence the increase in surface temperatures and the term “greenhouse effect”—see sidebar). For the total energy budget to balance, the energy (and temperature) at the top of the atmosphere must decrease to account for the increase of energy (and temperature) near the Earth’s surface.

At natural levels, greenhouse gases are crucial for life on Earth; they help keep average global temperatures above freezing and at levels that sustain plant and animal life. However, at the increased levels seen since the Industrial Revolution (roughly 275 ppm then, 400 ppm now; Figure 1.2), greenhouse gases are contributing to the rapid rise of our global average temperatures by trapping more heat, often referred to as human-caused climate change. In the following chapters, we will refer to the impacts and effects of climate change as a result of both natural variability and human-caused climate change.

The Greenhouse Effect

The Earth’s climate is driven by the sun. The high temperature of the sun results in the emission of high energy, shortwave radiation. About 31% of the shortwave radiation from the sun is reflected back to space by clouds, air molecules, dust, and lighter colored surfaces on the earth. Another 20% of the shortwave radiation is absorbed by ozone in the upper atmosphere and by clouds and water vapor in the lower atmosphere. The remaining 49% is transmitted through the atmosphere to the land surfaces and oceans and is absorbed. The Earth’s surface re-emits about 79% of the absorbed energy as longwave radiation. Unlike shortwave radiation, the Earth’s atmosphere absorbs approximately 90% of the longwave radiation emitted from objects on its surface. This results because of the presence of gases such as water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and various industrial products (e.g. chlorofluorocarbons; CFCs) that more effectively absorb longwave radiation. In turn, the energy absorbed by these gases is reradiated in all directions. The portion that is redirected back towards the surface contributes to warming and a phenomenon known as the greenhouse effect.

Figure 1.3: Climate change occurs when the Earth’s energy budget is not in balance. Such change generally takes place over centuries and millennia. Human-caused climate change has been occurring over the last 200 yr, largely because of the combustion of fossil fuels and subsequent increase of atmospheric CO2. Carbon dioxide, as well as CH4 and other gases, absorb and re-emit longwave radiation back to the earth’s surface that would otherwise radiate rapidly into outer space, thus warming the Earth. This increase in incoming longwave radiation is the greenhouse effect. Image courtesy the National Academies of Sciences (NAS undated).