Wednesday, May 29, 2019

Cycles of Rapid Climate Warming

Published in the Pacifca Tribune May 28, 2019

What’s Natural

Cycles of Rapid Climate Warming

The globally averaged temperature rose 1.5°F from 1880 to today. Various narratives suggest the rise since 1950 was driven by increasing concentrations of CO2. The rising temperature before 1950 was considered natural. Since 1990, Arctic temperatures rose 2 to 3 times faster than the global average. So, are rapidly rising Arctic temperatures evidence of an impending climate crisis? 

Astute students of climate history recall rapid Arctic warming has happened often and naturally. During the last Ice Age when COconcentrations were just half of today’s, 25 abrupt warming eventshappened. Arctic temperatures rose 9°F, and sometimes as much as 14°F in just 40 years. These rapid warming episodes are now called Dansgaard–Oeschgerevents (D-O events) in honor of the researchers who first detected them in Greenland’s ice cores. These D-O episodes affected global climate, changed ocean currents along California’s coast and altered the range of European forests.

What caused such abrupt warming? Basic physics dismisses changes in greenhouse gases or solar insolation because neither radiative effect induces such rapid warming. The most reasonable explanation suggests episodes of ventilating heat, that had accumulated in the Arctic Ocean, rapidly warmed the air. 

The notion of stored heat in a freezing Arctic Ocean seems unfathomable to many laypeople. But it is scientifically well documented.Tropical Atlantic waters experience intense heating and evaporation. This results in warm salty water that is relatively dense. The Gulf Stream and its branching currents then transport that warm water northward. Because the water is salty and dense it sinks below colder and fresher surface waters as it approaches the Arctic Ocean. As a result, there is a layer of warm Atlantic water stored at depths between 300 and 2,700 feet below the Arctic Ocean’s surface.Arctic researchers report, “the total quantity of heat is substantial, enough to melt the Arctic sea ice cover several times over.”

Warm Atlantic Water in the Arctic Ocean

Sea ice and a layer of cold fresh water normally inhibit subsurface heat from ventilating to the atmosphere. But as the incoming heat increases and accumulates, the warm Atlantic Waters can eventually melt the overlying ice cover. Other times, changes in the direction of Arctic winds will blow sea ice out into the Atlantic, as it did in the late 1990s. Either way, without insulating ice, a burst of heat ventilates from the ocean and warms the atmosphere. 

Recently, anthropologists studying past Arctic cultures found the pre-Dorsett culture periodically abandoned then recolonized the Arctic coast as changes in sea ice affected temperatures. When sea ice covered coastal waters for 2 months longer than today, temperatures became 3-7°F cooler. Arctic people then abandoned the coast and moved south. A few hundred years later they re-colonized the coast when periods of open water, lasting 4 months longer than today, allowed heat to ventilate and raise temperatures 10°F warmer than today.  Such changes alternated over several hundred years. And that raises the question, is the Arctic still experiencing similar cyclical warming?

Over the past several hundred years, melting Arctic sea ice corresponds with observed periods of increased intrusions of warm Atlantic waters. The dramatic Arctic warming during the 1920s and 1930s corresponded with increased intrusions of warm water accompanied by Atlantic fish species normally found further south. As the 1922 newspaper clipping reveals (see above), the warming of the Arctic was so dramatic it raised concerns the frigid Arctic would soon be converted to a warmer “temperate zone”. 

Arctic 1922

When warm water inflows began retreating around 1950 so did the Atlantic fish. Sea ice then increased.  Such cycles have been recorded infishery data for hundreds of years. The most recent cycle of melting Arctic sea ice likewise coincided with intruding warm Atlantic waters with patterns of invading fish very similar to the 1920s-1930s warming episode.

So, we are now in the midst of an instructive natural experiment. If the loss of Arctic sea ice and warmer temperatures are due to rising CO2concentrations, we should soon see a total loss of Arctic sea ice as predicted by some climate scientists. In contrast, if natural oscillations are controlling intrusions of warm Atlantic waters, Arctic sea ice will soon rebound. Indeed, a recent shift in ocean oscillations is now decreasing warm water intrusions. Temperatures should fall as less heat ventilates into the atmosphere. Based on earlier 20thcentury patterns when lost sea ice rebounded in the 1960s and 70s, Arctic sea ice should begin rebounding by the year 2030. But then again, warmer temperatures did last for 300 years during cycles a few thousand years ago. Either way, natural climate cycles predict Arctic temperatures will not experience further accelerated warming. We will soon see which theory is most accurate within the coming decade.

Jim Steele is retired director of the Sierra Nevada Field Campus, SFSU