"We hear the term “climate” every day, so it is worth thinking about what we actually mean by it. Obviously, “climate” is not the same as weather. The climate is one of Earth’s fundamental life support systems, one that determines whether or not we humans, and millions of other species, are able to live on this planet. It is generated by four components: the atmosphere (the air we breathe); the hydrosphere (the planet’s water); the cryosphere (the ice sheet and glaciers); the biosphere (the planet’s plants and animals). By now, our activities had started to modify every one of these components. Our emissions of CO2 had started to modify our atmosphere. Our increasing water use had started to modify our hydrosphere. Rising atmospheric and sea-surface temperatures had started to modify the cryosphere, most notably in the unexpected shrinking of the Arctic and Greenland ice sheets. Our increasing use of land, for agriculture, cities, roads, mining—as well as all the pollution we were creating—had started to modify our biosphere. Or, to put it another way: We had started to change our climate."