The University of Arizona

Basic Concepts

By Zack Guido | The University of Arizona | September 14, 2008

The climate system is dynamic and complex. While weather details changes over short periods of time, like when anvil-shaped clouds burst in the late afternoon during monsoon season, climate describes average conditions over longer intervals. Understanding this distinction and other basic concepts such as radiative forcing and the enhanced greenhouse gas effect can clarify how the climate is changing and how humans have contributed to that change. The foundation of a comprehensive understanding of the climate system includes basic knowledge about:

Weather and climate

Do you trust your local weather forecaster to tell you the precise highs and lows for this day next year? Probably not. But you probably do expect next summer to be warmer than next winter. This expectation describes the difference between weather and climate.

Weather is driven by temperature differences in the atmosphere caused by the Sun’s uneven heating of the Earth’s surface. The heating gives rise to weather phenomena experienced as changes in rain, temperature, and winds that are complex and chaotic. Because of this chaotic nature, weather forecasts of more than a few weeks often are not accurate. Weather is also dynamic. It can change in minutes, like when an overcast sky gives way to thunderstorms. Weather also varies over the course of a day, as nighttime temperatures typically fall below daily values, and over seasons, when summer temperatures soar above those of winter. In essence, weather describes the evolution of the current state of the atmosphere and short-term fluctuations in variables like temperature and precipitation at a location over periods of minutes to several weeks.

Climate, on the other hand, is a measure of the typical, or average, weather for a particular place or time of year. Climate often describes the conditions over periods of decades to millennia. Whereas a particular thunderstorm is a weather event, the numerous storms of the monsoon season are a characteristic of the Southwest’s late summer climate. Climate is often represented by statistics, such as the mean and standard deviations, which describe the variability around the mean.

Natural variability and climate change

The climate varies naturally from one year to the next. Climate can also vary on much longer timescales, from decades to centuries. Climate change refers to the shift in the mean state of a particular climate parameter, such as temperature or precipitation.  In more mathematical language, climate change occurs when the statistical distribution of a climate variable changes.1

Natural climate variability refers to the variation in climate parameters caused by non-human forces. There are two types of natural variability: those external and internal to the climate system. Variations in the Sun, volcanic eruptions, and changes in the orbit of the Earth around the Sun exert an external control on climate variability. These processes are the driving force behind changes that occur over long time periods, such as oscillations between ice ages and interglacial periods. Natural variability is also influenced by processes internal to the climate system that arise, in part, from interactions between the atmosphere and ocean, such as those occurring in the tropical Pacific Ocean during an El Niño event. These changes occur over shorter time periods, from months to decades. In any given year, natural variability may cause the climate to be different than its long-term average.

The Intergovernmental Panel on Climate Change (IPCC) uses the phrase “climate change” to refer to any climate change that has occurred or will occur, whether from natural variability or human activity. In popular use, however, the phrase is often synonymous with the phrase “global warming,” which is used to describe the world’s ongoing temperature rise as it relates to society’s emissions of greenhouse gases.

Teleconnection

The climate system is interconnected and contains relationships in which an event in one geographic area causes changes in another area, often times thousands of miles away. These teleconnections are a natural consequence of the chaotic atmospheric system but have a recurring pattern that can be observed over periods of weeks to years. Although many teleconnections have been identified, combinations of only a small number of patterns can account for much of the interannual variability in the climate2. These natural cycles create variability in the weather and climate that at times may accentuate the warming caused by the enhanced greenhouse effect or may suppress, and even override, human-caused warming.3

El Niño-Southern Oscillation (ENSO) is the most important source of interannual teleconnections across the globe and causes large changes in Southwest climate. It is characterized by a temperature change in tropical Pacific sea surface temperatures that alters the strength and direction of atmospheric trade winds. This modification in turn causes additional atmospheric changes, including decreased winter precipitation in parts of the U.S. Southwest and increased winter precipitation in parts of the Pacific Northwest during La Niña events.  In addition to ENSO, other teleconnections that seem to impact the weather and climate in the Southwest include the Pacific Decadal Oscillation (PDO), Northern Annual Mode (NAM), and Atlantic Multidecadal Oscillation (AMO).

Illustration of Earth's energy balance

Figure 1. Estimate of the Earth’s annual and global mean energy balance. Over the long term, the amount of incoming solar radiation absorbed by the Earth and atmosphere is balanced by the Earth and atmosphere releasing the same amount of outgoing longwave radiation.
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Credit: Intergovernmental Panel on Climate Change, 2007

Radiative forcing

Radiative forcing measures the influence that climate-altering factors have on the energy balance of the Earth. Examples of factors that can alter the Earth’s energy balance include atmospheric concentrations of greenhouse gases, aerosols from volcanoes and air pollution, and the amount of solar radiation delivered to the Earth by the Sun.

In a state of equilibrium, solar radiation entering the atmosphere equals the radiation that leaves the atmosphere. Figure 1 illustrates this energy balance. The radiative forcing of a particular factor, such as atmospheric concentrations of carbon dioxide (CO2), is the change in the energy balance that results from a change in the factor, such as an increase in CO2 concentrations. When the radiative forcing of a factor is positive, increases in the factor cause warming of the atmosphere. In contrast, for a negative radiative forcing, increases in the factor ultimately cause cooling.4

Illustration the greenhouse gas effect

Figure 2: An idealized model of the natural greenhouse gas effect.
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Credit: Intergovernmental Panel on Climate Change, 2007

Enhanced greenhouse gas effect

Greenhouse gases (GHGs) such as carbon dioxide, methane, and water vapor, are naturally part of the Earth’s atmosphere. These gases trap heat from the Sun’s radiation in the atmosphere and act as powerful amplifiers of temperature. The natural greenhouse gas effect (Figure 2) helps to elevate the average temperature in the lower atmosphere to a comfortable 60 degrees Fahrenheit. Without GHGs, average atmosphere temperatures would fall below the freezing point of water at approximately 32 degrees F.

The enhanced GHG effect occurs when gases such as carbon dioxide are emitted into the atmosphere in quantities that increase temperature. Usually, the implication is that the sources for these additional quantities stem from human activities, such as burning fossil fuels.

Figure 3. Using a normal distribution, or bell curve, to represent natural variation in temperature from an average, this figure illustrates how climate change will not only affect average temperatures, but also extremes. Since projections are for warmer temperatures, this implies more hot and extreme heat days than what has occurred in the past.
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Credit: Intergovernmental Panel on Climate Change, 2007

Extreme events

Extreme events, such as floods, heat waves, and hurricanes, are rare weather occurrences that often cause the greatest damage to society. In the yearly distribution of rainfall, for example, extreme precipitation events that cause flooding occur less frequently than the “average” rainfall events, but the economic toll of floods is many times greater.

Any climate variable can be characterized by a probability distribution, which conveys the likelihood that an event will occur. Often, these distributions have a shape similar to a bell-curve, with the average, and therefore most frequent condition, occurring in the middle and extreme events falling at the ends of the frequency distribution (Figure 3). In the case of an increase in the average temperature, the number of days with extreme temperatures will also increase, while the number of days with cold temperatures will decrease.

Although it is difficult to attribute the cause of any one event to global warming, there is solid theoretical justification for expecting the frequency and/or magnitude of some extreme events to increase in the future. It is also likely that while some events will become more common and powerful, others will become less frequent and intense, like in the temperature example mentioned above.

References

  1. Roesch, C., K. Miller, D. Yates and D.J. Stewart. 2006. Climate change and water resources: a primer for municipal water providers. AWWA Research Foundation, Denver, Colorado and University Corporation for Atmospheric Research (UCAR), Boulder, Colorado.
  2. Trenberth, K.E., et al. 2007. Observations: surface and atmospheric climate change. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  3. Keenlyside, N.S., et al. 2008. Advancing decadal-scale climate prediction in the North Atlantic sector. Nature, 453, 84–88.
  4. Forster, P., et al. 2007. Changes in atmospheric constituents and in radiative forcing. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.