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Concentration of CO2 in the Atmosphere

A Slowing Gulf Stream

Flow of the Gulf Stream. NASA image.

Flow of the Gulf Stream. NASA image.

By George Harvey

There are some things nature brings us that we feel we can count on, such as the warmth of summer and winter snow. We depend without thought on rain to grow our crops.

Perhaps most of us have never heard of the Year without a Summer. (bit.ly/wikipedia-year-without-summer). That year, 1816, global temperatures were 0.7°F to 1.3°F below normal, probably because of volcanic activity. That might not sound like much, but it left the weather so unstable that there were damaging frosts every month in much of the United States. In some places, even hay could not be harvested. It was a worldwide phenomenon, and millions of people starved. It was a small change with big ramifications.

Scientists have known for a long time that the Gulf Stream was not moving quite as fast as it had in the past. That was not necessarily anything to worry about, because there are always variations in such things. In the last couple of weeks, however, many scientists seem suddenly to have become very worried.

The Gulf Stream is part of the Atlantic Meridional Overturning Circulation (AMOC). Because of a combination of the warmth of the sun, the cold of space, the spin of the Earth, and other factors, warm water from the Gulf of Mexico flows north and east, eventually passing north of Europe. Any cold weather from the Arctic has to pass over the warm water, and this modifies the temperatures in Europe. That is why the French grow grapes for wine instead of apples and blueberries, though Paris is considerably farther north than Montreal.

Two recently published scientific papers agree that the AMOC has slowed down much more than had been believed. In fact, it has slowed to about 85% of the slowest point measured or estimated in the past 1600 years. The scientists say that at least part of the slowdown must be linked to human causes.

The models for the AMOC are very complicated, and this may explain why their projections were wrong. The water melting from the Greenland ice is fresh, so it spreads across the top of the ocean water, potentially covering warmer salt water with a colder surface. Water contracts as it cools, but only until it gets to 39° F, below which it expands as it cools. So the salty ocean water goes to the bottom at that temperature. Even so, it is worrisome that the models do not even hint at such a slowdown. That fact suggests that we are in territory that is not merely uncharted; it was not even imagined.

Scientists believe that the AMOC was disrupted during the last Ice Age badly enough that temperatures fell 5°C to 10°C (9°F to 18°F) over the course of only two to three years. That would mean that temperatures in Paris might drop to those of Bismark, North Dakota in a short time. Compare that change with the Year without a Summer, when the difference was less than 2°F.

I hope they are wrong. But we must be prepared for their being right.

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