Climate: Does solar activity drive North Atlantic currents?

A new study found that Greenland temperatures fell from the 1970s through the early 1990s while temperatures across much of the rest of the Northern Hemisphere rose. This map shows the average difference in surface temperatures between 1920-1940 and 1975-1995. Grey areas indicate regions where not enough data was available to calculate long-term temperature changes. Credit: Takuro Kobashi
Greenland temperatures fell from the 1970s through the early 1990s while temperatures across much of the rest of the Northern Hemisphere rose, and solar activity may be an important factor.
Credit: Takuro Kobashi

Low solar activity could speed Greenland Ice Sheet melting in coming years

Staff Report

FRISCO — Solar activity could be an important factor in determining how fast the Greenland Ice Sheet melts, scientists concluded in a new study after analyzing ice cores and historical temperature records.

Based on their analysis, the researchers found that High solar activity starting in the 1950s and continuing through the 1980s played a role in slowing down ocean circulation between the South Atlantic and the North Atlantic oceans.

That could help explain why Greenland cooled during the 1970s through the early 1990s while most of the Northern Hemisphere warmed under a thickening blanket of greenhouse gases. Other research shows that an influx of fresh water from melting glaciers is also part of the Greenland climate equation.

The new study has been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

The current phase of low solar activity could play out over the next few decades by stimulating that same ocean circulation, which would increase the amount of warm water and air flowing to Greenland.

Starting around 2025, temperatures in Greenland could increase more than anticipated and the island’s ice sheet could melt faster than projected, according to Takuro Kobashi, a climate scientist with the Department of Climate and Environmental Physics at the University of Bern in Switzerland and lead author of the new study.

This unexpected ice loss would compound projected sea-level rise expected to occur as a result of climate change, Kobashi said. The melting Greenland ice sheet accounted for one-third of the 3.2 millimeters (0.13 inches) rise in global sea level every year from 1992 to 2011.

“We need to really consider how solar activity will change in the future,” said Kobashi. “If solar activity becomes really low, as scientists expect, the Greenland ice sheet will melt faster than we expected from the climate model with just greenhouse gas [warming].”

The research team studied ice cores drilled from the Greenland ice sheet to reconstruct snow temperatures for the past 2,100 years. With a new technique to measure trapped gases, they were able to reconstruct a finely scaled temperature record showing that changes in Greenland temperatures have generally followed any temperature shifts occurring in the Northern Hemisphere, with vacillations coinciding with changes in the sun’s energy output.

When the sun’s energy output increased, there was a bigger drop in Greenland’s temperature compared to the change in average temperature across the Northern Hemisphere. When the sun’s energy output decreased, there was a larger increase in Greenland’s temperature compared to the change in average temperature that occurred across the Northern Hemisphere.

The key current is called the Atlantic Meridional Overturning Circulation, which moves warm water from the South Atlantic toward the North Atlantic, transferring heat toward Greenland. As the water cools, it sinks to the ocean floor and travels south toward the tropics, completing the circular pattern.

During a period of high solar activity, more energy from the sun reaches Earth and is transferred to tropical waters. When this warmer-than-usual water reaches the North Atlantic, it is not dense enough to sink. With nowhere to go, the water causes a traffic jam and the water circulation pattern slows down.

Changes in solar activity can also alter the atmospheric circulation pattern over the Atlantic, which in turn affects ocean circulation, but how this process works is still unknown, said Kobashi.

The slow-down in the Atlantic Meridional Overturning Circulation caused the air above Greenland to cool and temperatures there to drop, according to the new study. Because the oceans take a long time to heat up or cool down, the temperature changes in Greenland lagged 10 to 40 years behind the high solar activity, showing up from the 1970s through the early 1990s.

Based on their findings, the scientists said Greenland temperatures could warm dramatically in the next few decades because when  there is less solar energy reaching the Earth, water reaching Greenland easily sinks and returns to the tropics along the ocean floor. The water circulation pattern speeds up, quickly funneling heat toward Greenland and warming the island.

In a press release, the AGU included a response from climate scientist Michael Mann, who has also been studying how climate change will affect ocean currents and the Greenland ice sheet.

Mann said the new study makes a good case that the solar maximum in the 1950s through the 1980s may have played a role in cooling Greenland during the 20th century. Both studies suggest buoyant meltwater from melting glaciers would have interrupted the sinking of the AMOC and its return to the tropics along the bottom of the ocean.

But the new research suggests solar activity is the main driver behind the changes to the ocean circulation pattern.

“I’m open-minded that the real answer is more complicated, and it may be a combination of the two hypotheses,” said Mann. “This article paves the way for a more in-depth look at what is going on. The challenge now will be teasing apart the two effects and trying to assess the relative importance of both of them.”

Kobashi contends that solar activity explains the change in ocean circulation and Greenland warming since 1995, which he says cannot be explained by increasing greenhouse gases alone.

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