How do we know the climate is changing?
Using data from direct measurements and proxies, scientists continue to learn more about the Earth’s past climate. Studying the long-term climate record has given researchers many clues about the nature of climate change. Earth’s climate changes with some regularity, going through ice ages and other periods of natural flux. However, since the Industrial Revolution in the mid-1700s, Earth’s average temperature has risen at a much higher rate than any other time in the reconstructed climate record. This temperature increase is primarily caused by increased atmospheric carbon dioxide levels, which are now the highest they’ve been in three million years.
With this temperature increase, changes have been observed around the world and more are expected. Throughout the 20th Century, there has been a worldwide decrease in the size of glaciers, a decrease in snow cover for many regions in the Northern Hemisphere, a decrease in Arctic Sea ice thickness and extent, shorter ice seasons for lakes and rivers, rising permafrost temperatures, warmer ocean and surface temperatures, increases in extreme events (heat waves, storms, precipitation), and rising sea levels, among other effects.