Coal plant in China
Human activities probably started to affect the world's climate as early as about 10,000 BC, when people started farming and cut down a whole lot of trees to create their fields. Trees take in carbon dioxide and convert it to oxygen, and without them, carbon dioxide levels rose, and that trapped more sunshine and made the world warmer. When people began to use iron about 1000 BC, they again cut down a lot of trees to burn in the process of making tools and cooking pots.
But the third way that people are causing global warming is much bigger than these first two ways. When we ran short of wood from trees, about 1800 AD, we began to burn coal and oil, which are the remains of trees and other plants that died millions of years ago, in the time of the dinosaurs. That releases a whole lot more carbon dioxide into the atmosphere, and it is causing much faster and more serious climate change.
Port Jefferson after Hurricane Sandy
We are already feeling the effects. Warmer ocean water causes more frequent and more serious hurricanes like Hurricane Katrina in New Orleans and Hurricane Sandy in New York City, and in Bangladesh and the Philippines. Melting glaciers mean snowier winters and more downpours all over northern parts of America, Europe, and Asia. The drought in California is driving up prices for fruit and vegetables in the United States, and there are more and bigger wildfires all across the American West.
All of these problems are expected to get much worse within the next fifty years. Rising sea levels due to ice melting will mean that many cities near oceans will have to be abandoned or moved. Even if we change to solar and wind power now, many of these changes will still happen, because there is already so much carbon dioxide in the air. And we're not changing now, anyway.