Weather and Climate

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Weather and Climate

Introduction

Weather is the behavior of the atmosphere—wind, temperature, humidity, air pressure, precipitation—at any given point on a planet’s surface at a given time. Winds, calms, storms, clouds, droughts, and rains—all are weather events. Weather events last anywhere from seconds to months. Climate is weather averaged over some area (a single spot to the whole globe) for at least a few years. Often, the climate of a region is viewed as its long-term average pattern of seasonal weather. Any change inan average seasonal pattern is a climate change, even if several such changes happen to cancel each other when the seasons are averaged together. For example, if over a period of some years New England were to get less snow in winter and more rain in summer, its climate would have changed even if the total average amount of yearly precipitation remained the same.

Meteorologists (scientists who study weather) and climatologists (scientists who study climate) stress that climate sets the background conditions of weather. As climate changes, the overall chances or probabilities of various weather events also change. Droughts may become more frequent, or heavy rains; cold snaps less frequent, and heat waves more prevalent during periods of climate change. However, no single weather event is a climate event, or proves or disproves the existence of changes in climate. No particular hurricane or a droughtcan be specifically attributed to climate change, and noparticular snowstorm or cold snap contradicts the realityof climate change. It takes many storms, many droughts, many weather events to add up to a climate, or to a change in climate.

The energy driving all weather and thus, all climate, is derived from the sun. It arrives in the form of light and is stored, moved, transformed, radiated, absorbed, and re-radiated inside Earth’s complex climate machine. At this time, Earth radiates to space slightly less energy than it receives from the sun.

Historical Background and Scientific Foundations

For tens of thousands of years, human societies feared weather, welcomed it, sought to predict it, and, if only through prayer and sacrifice, to control it. The development of modern physics in the 1600s and 1700s gave scientists the tools they needed to begin deciphering the mechanisms of weather, but scientific recording of largearea weather patterns did not begin until the mid-nineteenth century. In the 1960s and 1970s, with the advent of affordable, powerful computers, scientists began to produce mathematical models of weather, that is, systems of equations describing the various physical forces that produce it. Today, governments around the world spend billions of dollars studying weather and predicting it days in advance. Satellites ring Earth, observing its weather, and as of 2008 space probes were constantly observing the weather on Mars and Saturn from orbits around those planets. Some of the largest supercomputers in existence are devoted to weather modeling. Weather prediction is of high economic value. Agriculture, shipping, emergency response, and planning of outdoor events from picnics to battles all rely on weather forecasting

Despite all the effort expended on predicting weather, however, weather forecasts are not very reliable for more than about a week into the future. The reason is fundamental: The weather system is chaotic. That is, a very small change in conditions today will produce large changes in the exact weather pattern tomorrow, larger changes the day after that, and so on. Any slight uncertainty in today’s conditions (and precise knowledge of all the relevant conditions is impossible) leads to large and

WORDS TO KNOW

CLIMATE MODEL: A quantitative method of simulating the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

GLOBAL WARMING: Warming of Earth’s atmosphere that results from an increase in the concentration of gases that store heat, such as carbon dioxide (CO2).

growing uncertainty about weather conditions at increasingly future times. Weather cannot be predicted very far in advance.

Climate

The idea that scientists can predict climate changes decades into the future is often countered with the argument, “If they can’t predict next week’s weather, how can they predict climate 10 years from now?” This argument is mistaken because climate, unlike weather, is not a chaotic process. The climate system is complex but relatively well-behaved: Small changes to the climate system’s inputs are unlikely to have effects that get larger and larger over time, whereas large changes are likely to have large effects. In particular, greatly increasing the amounts of greenhouse gases in the atmosphere (carbon dioxide, methane, nitrous oxide, chlorofluorocarbons), as human beings have been doing for several centuries, is highly likely to have effects on climate that can be predicted, though with some range of uncertainty around the predictions. Computer climate modeling is therefore a different game from weather modeling. Today’s climate models have a good, and steadily improving, ability to hindcast climate changes—that is, knowing the climate record up to a certain point in the past, they can predict with reasonable accuracy climate changes from that time forward, which are already on record and so can be compared with the predictions.

Impacts and Issues

Climate forecasting, like weather forecasting, is a major concern of earth scientists today. The stakes are high. Since climate is not chaotic, the long-range future is not a blank. All but a few climatologists are now agreed that the global climate is warming because of human activities, especially deforestation and the burning of fossil fuels, and that it is going to continue to warm. Global warming does not mean that every place on Earth is becoming hotter and drier: The same mathematical models of global climate change that predict overall warming also predict cooling in central Antarctica, which is being observed. Some places will get more precipitation as climate changes, others less. Earth’s climate system is complex, its changes are complex, and many of its features are still poorly understood.

As the non-chaotic future of climate can be predicted at least in outline, and since climatologists are now confident that human activities are shaping that future, human beings face choices about climate. We cannot control specific weather events, but we can, at least in theory, choose a specific climate future, within a certain range of real-world possibilities. How much fuel we burn, how we choose to alter or not alter Earth’s ecosystems, what lifestyles we adopt (that is, those of us lucky enough to have such choices)—all these have affected climate, are affecting it, and will affect it. Climate, in turn, sets the background for all of the billions of particular weather events and conditions, day by day, that determine the livability of the planet.

BIBLIOGRAPHY

Books

Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment. Belmont, CA: Brooks Cole, 2006.

Solomon, S., et al, eds. Climate Change 2007: Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Web Sites

National Aeronautic and Space Administration (NASA). “Weather.” http://www.nasa.gov/worldbook/weather_worldbook.html (accessed May 12, 2008).

National Oceanic and Atmospheric Administration (NOAA). “National Weather Service.” http://www.nws.noaa.gov/ (accessed May 12, 2008).

Larry Gilman

Environmental Science: In Context