The Earth's climate changes. It is vastly different now from what it was 100 million years ago when dinosaurs dominated the planet and tropical plants thrive in high latitudes. It is different from what it was even 18,000 years ago when ice sheets covered much more of the northern hemisphere. In the future, it will surely continue to evolve. In part, the evolution will be driven by natural causes, such as slow changes in the Earth's orbit over many thousands of years. But further climatic change, unlike that of the past, will probably have another important source as well as human activities.