Natural Climate Change

Climate scientists agree that the current rise in average global temperature is being caused by human activities. But climate change has occurred in the past, before people started changing the world, and indeed before humans existed. These shifts were caused by natural cycles that affect Earth's orbit around the Sun, by changes in solar radiation levels, and by catastrophic natural events such as massive volcanic eruptions. Some of these changes seem to have triggered positive feedbacks that dramatically increased their effect. It is possible that this might happen again.

Woolly mammoth

Woolly mammoth

ice ages

Earth has passed through several ice ages that were caused at least partly by orbital cycles. We are now living in a warm phase of an ice age that peaked about 20,000 years ago. Ice sheets covered vast areas of the north, fringed by snowy tundra that was the home of cold-adapted animals like this woolly mammoth.

Sunspot

Sunspot

sunspots and plages

The Sun has bursts of activity that cause dark patches, called sunspots, to appear on its surface within lighter areas called plages. The more sunspots and plages there are, the more energy the Sun is radiating. There are more of them now than in the early 1800s, but the variations in solar energy that they cause are quite small, and they do not match the current pattern of climate change.

Sun's rays axis drift

Another cycle alters the orientation of Earth's axis, so it is aligned with different points in space. Currently the North Pole points to the pole star, Polaris, but over time it drifts away to point at other stars, returning after 25,800 years. This changes the dates of the seasons, so halfway through the cycle, the dates of winter and summer are reversed.

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