Ocean tides found to influence climate

Scientists report ocean conditions are related to distinct changes in earth's climate

By Environmental News Network staff

March 27, 2000
Web posted at: 3:24 p.m. EST (2024 GMT)

Distinct changes in Earth's climate can be tracked in cycles of ocean conditions over thousands of years, report scientists from Scripps Institution of Oceanography at the University of California.

These cycles reveal that Earth is currently experiencing a natural rise in global temperatures and this, combined with warming from the greenhouse effect, will push the planet through an era of rapid global warming.

Strong oceanic tides are the engines behind this warming-cooling cycle that may help determine future climate change, according to Charles Keeling and Timothy Whorf of Scripps. Their research was published in the March 21 online edition of the Proceedings of the National Academy of Sciences.

"The greenhouse effect is probably the best known of those factors influencing climate, but up to now any role played by tidal mixing of the surface ocean has not been included in future scenarios," said Whorf. "The greenhouse effect may be the larger of these two effects, but it is important to know how much of future warming might be natural or manmade."

Their research is the first comprehensive study of the effects of tidal mixing on climate change spanning a 1,000-year period. The current phase in the cycle suggests that a natural warming trend began 100 years ago, picked up in the 1970s, and should continue over the next 500 years.

Strong oceanic tides drive changes in climate due to their ability to increase vertical mixing in the ocean and thereby transport cold ocean water to the surface, suggest the researchers. Strong tides elicit cool conditions on the sea surface, which in turn lowers temperatures in air and over land, resulting in cooler climates around the planet, often accompanied by drought conditions. Weak tides lead to less cold water mixing and result in warmer periods on Earth.

The position of the Earth in relationship to the moon impacts tidal forces

"The 1,800-year (cycle) arises when tidal forces become maximized near optimal alignment and closest approach of the sun and moon with the Earth. Slow changes in the orbital parameters which control how strong the tidal effect might be, bring about the 1,800-year cycle," said Whorf. "Such astronomical changes also occur on shorter time scales, such as 90 and 180 years, causing episodes of cooling and warming depending upon whether the tidal forcing is stronger or weaker."

"One such episode of cooling during the period 1940-1975, when tidal forcing was stronger, may have temporarily masked the appearance of the greenhouse effect in global temperatures, and contributed to the controversy of whether greenhouse warming was occurring at all," said Whorf.

"If that is true, then it becomes pretty clear that if today's natural warming trend is combined with the greenhouse effect, then we'll soon see the effect of combined warming all over the world," said Keeling.

In addition to offering clues to climate change, the research also shows a new mechanism for analyzing events in world history.

The paper reports on the near coincidence of major tidal fluctuations with worldwide phenomena, including the Little Ice Age of 1400-1700 A.D., major dust layers in Minnesota lake sediments spaced about 1,800 years apart and a major drought in the Amazon Basin around 2200 B.C. It also could explain a 2000 B.C. drought that may have contributed to the collapse of Akkadia, a Mesopotamian civilization regarded as the world's first empire.

The Vikings inhabited Greenland in temperate conditions in the 10th century near the end of a period of weak tidal activity, but perished or left Greenland when tides strengthened near the beginning of the Little Ice Age in the 13th century.

"One of the principle benefits of the tidal hypothesis is that researchers can compare the timing of specific historical events with predicted times of warming or cooling to see whether they coincide or not," said Whorf, a research associate in the Geosciences Research Division of Scripps. "If we are correct, then the 1,800-year tidal cycle will be important in understanding future climates as well as events of the past."

The National Science Foundation and the U.S. Department of Energy also supported the study.

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Scripp's Institution of Oceanography
The National Weather Service
The Evergreen Project
The relationship between the ocean and climate
The National Science Foundation
The U.S. Department of Energy

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