We all know that weather varies from day to day, that it changes with the seasons, and that no two years are ever exactly alike. One way to describe the distinction between weather and climate is that “climate is what you expect, and weather is what you get.” In other words, weather describes the evolution of the current state of the atmosphere, while climate is a measure of the typical weather for a particular place, hour of day, and time of year. Climate, as a statistical concept, measures not only expected average conditions, but also the characteristic range of variability of those conditions. Climate change will alter the likelihood of various types of weather events.
The climate system, as shown in Figure 1, includes the atmosphere, oceans, ice, land, vegetation, and surface water. The Sun and human activities act as important external influences on the climate system. Interactions among all of these components determine the geographical and seasonal distribution of climates across the surface of the globe. A change in any of these elements will cause changes in global and regional climates. The process may involve a sequence of adjustments and feedbacks in other components of the system. For example, there is a positive feedback between temperature and ice cover. Suppose that a period of increased solar input causes air temperatures to warm. That would tend to reduce the total area covered by ice and snow. Because snow and ice are very bright, they reflect sunlight back into space. With less ice and snow, the surface of the planet would be darker and would absorb more solar radiation. That, in turn, would cause further warming in the affected areas.
Figure 1. An idealized graphic of the climate system. Source: Bureau of Meteorology 2003. The Greenhouse Effect and Climate Change, Bureau of Meteorology, copyright Commonwealth of Australia reproduced by permission.
The Sun is the source of energy that drives the climate system. Solar radiation heats the atmosphere and the surface of the Earth. To balance the amount of energy coming in from the Sun, the Earth must radiate the same amount of energy back to space − in the form of infrared radiation. Greenhouse gases, which include water vapor, carbon dioxide, methane, nitrous oxide and a variety of human-made chemical compounds, trap some of the outgoing infrared radiation. When the energy balance is upset, for example by increases in the amount of greenhouse gases in the atmosphere, then the Earth will warm until a new balance is established, centuries later.
Figure 2 provides a globally averaged view of the Earth’s energy budget. The term “greenhouse effect” refers to the fact that the atmosphere absorbs most of the infrared radiation leaving the surface of the Earth and re-emits part of that energy back toward the earth’s surface. Increased concentrations of greenhouse gases in the atmosphere act to increase the “back radiation” term on the right-hand side of the figure. That, in turn, would warm the Earth’s surface. Increased loss of energy through infrared radiation, and the release of latent heat through increased evapotranspiration and precipitation are among the processes that act to restore the energy balance.
Figure 2. Global heat flows (Kiehl and Trenberth 1997)
The global water cycle plays an important role in the global energy balance because evaporation and cloud formation help to regulate both incoming and outgoing radiation. Water vapor is itself one of the most important greenhouse gases, and because a warmer atmosphere can hold more water vapor, it provides a powerful positive feedback to other sources of warming. Clouds, in particular, play a complicated role in the energy balance. They act as a blanket − warming the Earth’s surface by absorbing and emitting thermal radiation. On the other hand, they also act to cool the surface of the Earth by reflecting incoming sunlight back into space. These opposing effects almost cancel each other out, but in our current climate, clouds appear to have a slight net cooling effect.
Because the Earth is a sphere, the Sun’s heating is uneven (Figure 3). There is an energy surplus near the equator and a deficit near the poles. The circulation of the atmosphere and oceans transports heat from the tropics toward the poles, making the Earth’s tropical regions cooler, and its polar regions warmer, than they would be if the Earth had no atmosphere or oceans.
Figure 3. Heating dynamics of the Earth. Source: Courtesy of Kevin Trenberth revised from Trenberth et al. 1996
The atmosphere and oceans are constantly in motion. That motion displays some stable patterns, which define contrasting climatic zones. For example, the Intertropical Convergence Zone (ITCZ) is a broad band that girdles the equator, characterized by rising air, frequent convective storms, and high annual precipitation. Just north and south of the ITCZ, centered at latitudes around 30 degrees north and south, are bands of hot, dry, descending air that create deserts in the world’s subtropical regions. In the temperate regions, storms are steered by broad wind bands, called jet streams, that flow from west to east. The position of each jet stream migrates with the changing seasons, and planetary waves of high- and low-pressure regions develop within the jet streams. These vary over time, but they are anchored, to some extent, on underlying geographical features such as mountains and boundaries between oceans and land. That anchoring results in semi-permanent predominant storm tracks that help to define the characteristics of regional climates. During each hemisphere’s winter season, there is a greater imbalance between the energy deficit at that pole and the energy surplus in the tropics. This contributes to the formation of storm fronts, as the poleward transport of heat intensifies.
In addition to these broad global climate patterns, the nature of local climates depends on such things as proximity to large bodies of water and the location of mountain ranges. For example, the windward side of a mountain range generally receives considerably more precipitation than nearby locations on the downwind side.
What one calls a climate change depends on the time period being considered. Climate varies naturally from one year to the next, and over decades and centuries as well, so the distinction between climate variability and climate change is somewhat fuzzy. Any trend or persistent change in the statistical distribution of climate variables (temperature, precipitation, humidity, wind speed, etc.) constitutes a climate change. Regional climate changes may result from persistent changes in the details of oceanic and atmospheric circulation. For example, the El Niño-Southern Oscillation (ENSO) phenomenon causes changes in the distribution of heat within the and the surrounding atmosphere. That, in turn, leads to changes in predominant storm tracks. The effects on local climates can be striking, with some areas receiving much heavier than normal precipitation, while other areas experience severe drought.
Figure 4 demonstrates that the effects of El Niño episodes (warm events) and the cool events, known as La Niña, occur across the entire globe.
Figure 4. Expected seasonal effects of El Niño (warm episodes) across the globe during December−February (top) and expected seasonal effects of La Niña (cold episodes) during the same time period (bottom) (from Climate Diagnostics Center, NOAA)
There are also longer-term changes in ocean-atmosphere circulation − marked by shifts in the location and/or intensity of the semi-permanent high- and low-pressure cells. These changes can persist for several decades. For example, temperature and circulation patterns in the North Pacific appear to get “stuck” in one of two modes for long periods. Various indices provide measures of this tendency, but they all strongly depend on the intensity and position of the winter Aleutian low-pressure system.
Figure 5 displays one such index: the Pacific Decadal Oscillation (PDO) Index. When the PDO is in its positive coastal warm phase, as it was for most of the period from 1977 through the mid-1990s, sea surface temperatures along the west coast of North America are unusually warm, the winter Aleutian low intensifies, and the is unusually stormy.
Figure 5. Pacific Decadal Oscillation. Upper panel: sea surface temperature and wind stress anomalies. Lower panel: Monthly values of PDO Index. Red is coastal warm phase; blue is coastal cool phase (courtesy of Dr. Nathan Mantua, JISAO, and Stephen Hare, International Pacific Halibut Commission).
The slowly evolving state of the ocean, as measured by the PDO, interacts with the more rapid ENSO-related changes to influence storm tracks and, thus, the likelihood of unusually heavy or light seasonal precipitation. For example, a positive PDO appears to reinforce the effects of an El Niño, making wet winter conditions in the southwestern and dry conditions in the more likely than would be the case if the PDO were in the negative (coastal cool) phase.
A similar pattern of multi-year variability occurs in the Atlantic basin as well. The North Atlantic Oscillation (NAO) measures swings in the relative intensity of the winter low-pressure cell centered over , and the high-pressure cell centered over the . The NAO is in a positive phase when that pressure difference is larger than normal. A positive NAO pattern drives strong, westerly winds over northern Europe, bringing warm stormy winter weather, while southern Europe, the Mediterranean and experience unusually cool and dry conditions (Figure 6a). Also in the positive phase, northeastern is more likely to experience unusually cold winter conditions. In the negative phase, the pressure differential is smaller than average and winter conditions are unusually cold over northern Europe and milder than normal over Greenland, northeastern , and the . There have been long periods during which the NAO has tended to be either unusually low or unusually high. In particular, it was generally low throughout the 1950s and 1960s, and then abruptly switched to a positive state for most of the period from 1970 to the present (Figure 6b).
Figure 6a. Schematic of the positive index phase of the North Atlantic Oscillation (NAO) during the Northern Hemisphere winter (courtesy of Dr. James Hurrell, CGD/NCAR).
Figure 6b. NAO Index 1864–2003 (courtesy of Dr. James Hurrell, CGD/NCAR)
ENSO, the PDO, and the NAO are all natural modes of climate variability, but any change in global climate is also likely to affect these processes. At the global scale, climate changes depend on changes in the Earth’s energy budget. In particular, increased concentrations of greenhouse gases, such as carbon dioxide, in the atmosphere are likely to cause warmer global average surface temperatures.