01/13/2011 12:51 pm ET Updated May 25, 2011

Why the Wild Winter Weather?

Winter weather has been especially harsh this year, as it was last year. Scientists have long predicted that weather across the Northern Hemisphere could get colder with global warming. How might this work?

Some commentators have invoked changes in atmospheric circulation over Siberia to explain the severe winter weather besieging Europe and parts of the U.S. But there is another part of this puzzle lying closer to our own shores. Warming may be leading to cooling via melting Arctic ice.

For the past 16 months a climate phenomenon known as the North Atlantic Oscillation (NAO) has been locked in one mode; a negative phase, an unusually long stretch with unusually high atmospheric pressures over the Arctic, and low pressure to the south.

Highs persist over cold areas, for cold air is heavy and sinks. Lows, meanwhile, form over warm regions, for hot air rises. And winds and weather fronts flow "downhill," as it were, from highs to lows.

So why might a High pressure system be locked in over the North Atlantic?

During the past decade Arctic ice (where the North Pole sits) has melted and thinned much faster than models predicted. (The floating North Polar cap is only several feet thick.) In addition, many Greenland outlet glaciers are melting from above, calving from below (as they meet warmer seas), and sliding and slipping along their bases into the sea. This melting ice spreads cold, fresh water across the North Atlantic Ocean surface, contributing to a North Atlantic High.

To the south in the Atlantic, the Azores bask in warm sea surface temperatures, as does most of the globe, for the world ocean is where heat has built up over the last century. Indeed, since the late 1950s, the world ocean has accumulated 22 times as much heat as has the atmosphere, and the warmed ocean is the engine for changing weather patterns globally.

Meanwhile, in the Middle East, unusually hot and dry conditions have become the new norm (ask our soldiers and the residents), creating persistent Lows in that region. It is the large gradient between the Atlantic High and the Middle East Low that pushes and pulls the bitter winds and weather fronts now whipping off the North Atlantic across Europe.

For the U.S, the North Atlantic High is a blocking high, deflecting fingers of the Jet Stream deep down into the south, bringing lots of continental polar air to usually temperate and sub-tropical states. And with warming seas and atmosphere overall, there's plenty of water vapor evaporating to generate large amounts of snow, sleet and rain.

The details of this argument are not critical. What is important is that global warming (of the atmosphere and oceans) is causing climate change -- meaning changing temperatures and weather patterns. Weather has become more extreme -- drier and hotter in some areas and wetter and cooler in others -- and precipitation is coming in heavier downpours across the globe. And the warming world ocean and shrinking ice cover are playing leading roles in driving the weather extremes.

Changes in winds and weather patterns with global warming, we have begun to discover, are not limited to the warm seasons. Winter weather anomalies are an emerging feature of climate change and result in orthopedic injuries and travel accidents, deaths, crop failures, power outages, business interruptions, and insured and uninsured losses. Harsh winter weather may, in the future, play an even more disruptive role to society than the heatwaves and spread of infectious disease more commonly considered.

Climate instability -- with greater volatility and wider swings to extremes -- raises the risks of major anomalies that threaten our health and economies. "God does not play dice," Albert Einstein famously said (as he grappled with the uncertainty of quantum mechanics). With regard to weather it seems we have stepped into a high stakes global casino.

But there is hope. An unstable system can re-stabilize and establish a new equilibrium. But we must back off the forces changing Earth's climate -- burning fossil fuels and felling forests -- and fast. We can hope that growing recognition of the mounting risks of climate change help spur a new level of commitment, domestically and internationally. Bold measures are needed, backed by global funds, to stabilize Earth's climate and provide new impetus for a healthy global economy.

Paul R. Epstein, M.D., M.P.H. is associate director of the Center for Health and the Global Environment at Harvard Medical School. He and science writer Dan Ferber are co-authors of the forthcoming book, Changing Planet, Changing Health, to be published this spring by the University of California Press.