INTRODUCTION

The focus of the previous chapter was on factors that shaped and directed changes to the Earth's environment over the four and a half billion years before the start of the Industrial Revolution in the eighteenth century. But what effects have humans had since global industrialization began? How are the changes to the global environment caused by humans since then influencing plants, in particular, today?

Answers to these and other key questions are by no means complete. Even where answers have been provided, the data on which they are based and their interpretation are often contradictory or contentious. This is not surprising but is confusing even for those who have the broadest and deepest knowledge. Not only is human influence growing and changing constantly but the environment itself is composed of a complex array of components which interact with one another often in ways about which there is inadequate knowledge or none at all.

THE IMPACT OF HUMAN ACTIVITIES ON THE ATMOSPHERE

Central to any understanding of human influence on the global environment is our effect on the atmosphere. The composition of the Earth's atmosphere determines inward transmission of the Sun's energy, its distribution across the globe, and its radiation back into space. The difference between the incoming and outgoing energy from the Sun determines the surface temperature of the Earth, important to organisms because all have a temperature range to which they are adapted.