Most climate scientists say human activities are the main cause behind observed climate changes and possible global warming.
Elizabeth Kolbert, a writer for the New Yorker magazine, told an interviewer:

It's true that the climate varies naturally, and some of the recent rise
in global temperatures may well be part of a natural cycle.
The point that's important to keep in mind is that the greenhouse gases
we are adding to the atmosphere are overwhelming the natural forces that cause climate variability.
In effect, we humans are becoming the drivers of the climate system,
and we are doing so without knowing where we are going.

To the left is the kind of image scientists use to analyze climate change. It was generated by the National Oceanic and Atmospheric Administration (NOAA) from measurements of visible and microwave radiation taken by satellites and sent back to Earth. You're looking down at the North Pole, and you can see the various political boundaries, lines of latitude and longitude, and the extent of snow (white) and sea ice (yellow) over the Northern Hemisphere on December 18, 2003. (What's the difference between snow - and sea ice that's covered with snow? Good question: Sea ice is frozen ocean, and snow is snow-covered land.) In this first "Snow and Ice" lesson, you'll be examining similar images of the globe to compare snow and ice cover over a period of three years. In "Snow and Ice: The Sequel," you'll compare images over a longer period of time...