People have been making impacts on the natural environment of the Earth on a steadily increasing scale for thousands of years. For most of that period, those impacts have been localised or temporary. As we neared the end of the twentieth century, however, we realised that we were entering an era in which our releases to atmosphere have the potential to change the whole environment of the planet. Although we have coped with air quality problems such as combustion smogs, photochemical smogs and depletion of the ozone layer by putting preventive measures in place when the symptoms got bad enough, climate change is in a different league. If you are young enough while you are reading this book to expect to live well into the second half of the twenty-first century, then you will probably experience the greatest level of environmental change, and the greatest efforts by humans to deal with it, that we have ever experienced. In this chapter and the one that follows, we will look at the physical causes, and at some of the possible consequences, of these changes.