Chapter 11, Image 22

Caption

Earth's tilt goes up and down, ranging from about 21.8° to 24.4° and back over about 41,000 years (the range widens or narrows a bit with each cycle). The tilt is now just over 23.4° and on the decrease. When the tilt is most pronounced, it allows for stronger summer sun and weaker winter sun, especially at high latitudes. Ice ages often set in as the tilt decreases, because the progressively cooler summers can't melt the past winter's snow. At the other extreme, increasing tilt produces warmer summers that can help end an ice age.