Why the Current Global Warming is Not Natural

Wallace: Yes, there have been dramatic climate swings in the past, but with very few exceptions, they have occurred gradually, on time scales of thousands of years.
Some of the most dramatic climate change in the past was the alternation between glacial epochs, or ice ages, like the one that ended 15,000-20,000 years ago, and non-glacial conditions, like we're experiencing now.

The cause of these swings was the subtle variations in the Earth's orbit around the sun due to the pull of the other planets. These orbital changes don't change the total incoming solar energy significantly, but they dramatically change the strength of the sunlight over high northern latitudes, like Northern Canada, during the summer season.

The conditions that favor weak summer radiation in these areas are:

1. A large tilt to the Earth's axis

2. A large departure of the Earth's orbit from a circular shape, so that the Earth is much closer to the sun in part of its orbit than in the other part.

3. The Earth is farthest from the sun during the Northern Hemisphere's summer.

When these three things occur, solar radiation is weak in the high northern latitudes in the summer. Under these conditions, snow and ice that accumulate during the winter don't melt during the summer, so the ice builds up and an ice age results.

The end of the ice age comes when the orbit arranges itself so that solar radiation during summer is very strong. These changes take place very slowly - over thousands of years. In contrast, greenhouse warming is taking place much more rapdily - on a time scale of a century.

Hence ecosystems have much less time to adjust to the changes.

Human civilizations have experienced climate change before, but nothing as dramatic worldwide as we're seeing now.

Why are we convinced the current warming is not due to natural processes, like these orbital changes? First: Because we know greenhouse gases are accumlating in the atmosphere in levels that haven't been experienced in the past few tens of millions of years.

Second: Because the pattern of the observed warming fits the pattern we would expect from warming caused by the buildup of greenhouse gases. (ie- almost all areas of the planet are warming; the Earth's surface and lower atmosphere are warming; the upper atmosphere is cooling; the temperature changes are greatest in the Arctic during winter.)

Third: The warming is much more rapid than most of the natural variations we've seen in the past.