3.1 Climate processes; external and internal controls

Learning objectives

After reading this module, students should be able to

• define both "climate" and "weather" and explain how the two are related
• use the Celsius temperature scale to describe climate and weather
• discuss the role and mechanisms of the major controls on Earth's climate using the concepts of insolation, albedo and greenhouse gases
• identify and describe the mechanisms by which major external and internal changes to the climate (including solar output variation, volcanoes, biological processes, changes in glacial coverage, and meteorite impacts) operate
• know that the Earth's climate has changed greatly over its history as a result of changes in insolation, albedo, and atmospheric composition
• describe the processes that can lead to a "Snowball Earth" using the "positive feedback" concept, and be able to contrast the climate factors that influenced this period of Earth's history with others, including the dominant factors that operated during the Cretaceous
• state the major ways in which carbon dioxide is both added to and removed from the atmosphere, and be able to describe why levels of carbon dioxide and other greenhouse gases can be kept in balance

Introduction

The Earth's climate is continually changing. If we are to understand the current climate and predict the climate of the future, we need to be able to account for the processes that control the climate. One hundred million years ago, much of North America was arid and hot, with giant sand dunes common across the continent's interior. Six hundred and fifty million years ago it appears that the same land mass—along with the rest of the globe—was covered in a layer of snow and ice. What drives these enormous changes through Earth's history? If we understand these fundamental processes we can explain why the climate of today may also change.

In discussing climate in this chapter, we will be using degrees Celsius ( ^o C) as the unit of temperature measurement.

Weather describes the short term state of the atmosphere. This includes such conditions as wind, air pressure, precipitation, humidity and temperature. Climate describes the typical, or average, atmospheric conditions. Weather and climate are different as the short term state is always changing but the long-term average is not. On The 1 ^st of January, 2011, Chicago recorded a high temperature of 6 ^o C; this is a measure of the weather. Measurements of climate include the averages of the daily, monthly, and yearly weather patterns, the seasons, and even a description of how often extraordinary events, such as hurricanes, occur. So if we consider the average Chicago high temperature for the 1 ^st of January (a colder 0.5 ^o C) or the average high temperature for the entire year (a warmer 14.5 ^o C) we are comparing the city's weather with its climate. The climate    is the average of the weather.