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Introduction to Climate

This section provides a brief overview of the earth's climate. The general concepts found in this section include the following:

  • The general distinctions between weather and climate.

  • Daily weather measurements are highly variable compared to long-term climate data making it difficult to detect long-term trends based on limited data.

  • Long-term climate averages are the result of significant annual climate variability. Random climate variability makes detecting climate change more difficult.

  • The earth's climate has changed over time and will continue to change. Paleoclimatologists study the earth's climate over millions of years using a variety of methods. Data for past climate changes can be gathered from sources beyond long-term weather observations.

  • Plant pollen is physically distinct in size and shape in different plant species and may survive for long periods of time in sediment deposits, depositing a record of plant species in the area. The study of plant pollen trapped in lake sediments and rocks is called palynology.

  • Dendrochronology is the study of past climate through tree rings and allows scientists to observe the direct impact of climate on annual tree growth patterns.

This section includes four classroom activities.


The earth's climate is generally defined as the average weather over a long period of time. A place or region's climate is determined by both natural and anthropogenic (human-made) factors. The natural elements include the atmosphere, geosphere, hydrosphere, and biosphere, while the human factors can include land and resource uses. Changes in any of these factors can cause local, regional, or even global changes in the climate.

How does climate differ from weather?

Weather is the current atmospheric conditions, including temperature, rainfall, wind, and humidity at a given place. If you stand outside, you can see that it's raining or windy, or sunny or cloudy. You can tell how hot it is by taking a temperature reading. Weather is what's happening right now or is likely to happen tomorrow or in the very near future.

Climate, on the other hand, is the general weather conditions over a long period of time. For example, on any given day in January, we expect it to be rainy in Portland, Oregon and sunny and mild in Phoenix, Arizona. And in Buffalo, New York, we're not surprised to see January newscasts about sub-zero temperatures and huge snow drifts.

Some meteorologists say that "climate is what you expect and weather is what you get." According to one middle school student, "climate tells you what clothes to buy, but weather tells you what clothes to wear."

Climate is sometimes referred to as "average" weather for a given area. The National Weather Service uses data such as temperature highs and lows and precipitation rates for the past thirty years to compile an area's "average" weather. However, some atmospheric scientists think that you need more than "average" weather to accurately portray an area's climatic character - variations, patterns, and extremes must also be included. Thus, climate is the sum of all statistical weather information that helps describe a place or region. The term also applies to large-scale weather patterns in time or space such as an 'Ice Age' climate or a 'tropical' climate.

Climate Variability

Although an area's climate is always changing, the changes do not usually occur on a time scale that's immediately obvious to us. While we know how the weather changes from day to day, subtle climate changes are not as readily detectable. Weather patterns and climate types take similar elements into account, the most important of which are:

Although weather and climate are different, they are very much interrelated. A change in one weather element often produces changes in the others - and in the region's climate. For example, if the average temperature over a region increases significantly, it can affect the amount of cloudiness as well as the type and amount of precipitation that occur. If these changes occur over long periods of time, the average climate values for these elements will also be affected.

Paleoclimates (Ancient Climates)

Good weather records extend back only about 130 years. In that time, the earth's global average temperature has increased by approximately 0.5 degrees centigrade or about 1 degree Farenheit.

Scientists have studied the earth's past climate extending back millions of years. To examine these long time scales, known as geologic time, scientists have had to gather clues from geologic and plant fossil records.

Evidence in the fossil record indicates that the earth's climate in the distant past was very different from today's. However, the climate has fluctuated substantially within the last several centuries - too recent to be reflected in the fossil record. Studying past climates and climate changes help us to better understand our current climate and what may happen in the future.

The earth's atmosphere has evolved over the course of its long history (approximately 4.5 billion years), resulting in significant changes to global, regional, and local climates. Some of the fossil record suggests that these changes are somewhat cyclical, with periods of global warming followed by ice ages. The most recent global ice age ended about 18,000 years ago with a gradual warming since then despite some intermediate periods of cooling.

Several techniques are used for measuring past climates, including

All of these methods provide clues to past temperature, precipitation, and wind patterns as well as the chemical make-up of the atmosphere.

Studying how the atmosphere has changed over the history of the earth helps scientists understand how our current climate may change as human activity continues to alter the concentration of some key atmospheric gases. For example, the addition of increases the atmosphere's ability to trap heat. By studying past atmospheric concentrations and correlating it to past climate regimes, we get an idea of what types of changes to expect as a result of increasing concentrations.

Concluding Thoughts

The earth's climate has always changed and always will. It is very difficult to look at shorter-term weather trends for any given area and make valid statements about long-term climate change. For students to truly understand the differences between weather and climate and to get a better sense of how scientists study past climates in hopes of understanding future climates, students should be able to answer the following questions:

  1. What is the difference between weather and climate?

  2. What is climate variability?

  3. What methods do scientists use to study past climates?

  4. How will a better understanding of past and present climates help us predict future climates?


The following activities will help your students better understand the concepts covered in this section.

To proceed, either click on Activities in the menu at the top or click on another unit to switch units.