The distinguishing factors between weather and climate are (1) the time interval in which they are taking place (such as a day versus a season) and (2) the scale of the area they are taking place over (such as a village versus a country). Weather is what the atmosphere is currently doing-snowing, raining, or clear skies. Climate is how the atmosphere behaves over relatively long time intervals-such as hot summers, cool winters, or wet springs.
Climate also refers to the average condition of a region, measured in characteristics such as temperature, amount of rainfall or snowfall, how much snow and ice cover there is on the ground, and the characteristics of the predominant winds; the long-term trends of an area. As an illustration, Hawaii may be described as having a tropical climate, Britain a maritime climate, and Saudi Arabia an arid climate. Climate applies to long-term changes (months, years, and longer); weather the shorter fluctuations that last hours, days, or weeks. The weather may be snowing, raining, clear and sunny, hot, or windy. Weather conditions generally apply only to limited geographical regions and can change rapidly.
When scientists say that climate has changed, they are referring to the fact that the averages of the daily weather have changed over time—such as a region is drier, receiving half as much rain, than it was 50 years ago. There can also be short-term climatic variations, such as those associated with El Niño, La Niña, or volcanic eruptions. These will be looked at in greater detail in chapter 5.
Climate affects everyone on Earth in some way, just as global warming will affect everyone on Earth. The weather plays some part in everything humans do, whether it is in transportation, growing food, producing an adequate water supply, or producing and manufacturing goods. Global warming will have an impact on every human on the globe, in varying intensities. Scientists have determined that rising global temperatures will change precipitation patterns, melt ice caps, raise sea levels, affect water supplies, damage the world's forests, spread disease, cause both floods and droughts, and encourage hazardous weather events.
All of the Earth's ecosystems will be affected. Because crop yields, food systems, and water supplies are being jeopardized as the Earth's atmosphere heats up at an accelerated rate, it is critical that scientists study, understand, and unlock the mysteries of global warming now before more damage is done. Technology has advanced to the point that climatologists (scientists who study the climate) have developed several tools that allow them to not only study current climate, but piece together evidence of past climates, and build models to predict future climates.
It is important to understand that even without global warming, the Earth's climate is always fluctuating. Change is natural. The Earth's climate has changed throughout geologic time as the Earth's continents have shifted positions, as the Earth's orbit and the tilt of its axis have changed, and even as the chemical composition of the atmosphere has evolved.
The Earth's climate system consists of air, land, water, ice, and vegetation. Climatologists study these components in terms of their cause and effect—also called forcing and response. The term forcing describes the things that cause the change; the responses are the changes that occur. Figure 1 (on page 4) illustrates the overall climate system along with the interaction of the components within the system. Sometimes climatologists find that the forcings can be cyclic in nature. For instance, various changes in the Earth's orbit are cyclic, and this pattern is universally reflected in climate data.
In order to fully understand climate, scientists must study climatic records that go back thousands to millions of years. Unfortunately, historical weather records (such as rainfall, sea level, and temperature) have only been kept for the past 100 years or so. In order to infer climate change of the distant past, a proxy (an object whose physical properties represent climatic conditions of the past, such as tree rings, ice cores, lake sediments, and corals) must be used.
Earth's climate system is composed of many components that all interact. A change in any one component causes changes in others.
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