The Weekend Wonk: Global Warming and Snow Cover

February 14, 2015

Above, I chatted with Jennifer Francis of Rutgers back in the spring of 2013, and the topic turned briefly to  snowcover in a warming world, and some interesting paradoxes.

To kick this off, here are some graphs from the Rutgers Snow Lab. Can you spot the global warming effects?

rutgerssnow3

rutgerssnow2

rutgersnow1

Listen to Francis, above, read this excerpt from Kevin Trenberth’s recent piece on this topic – and look again.

Kevin Trenberth in The Conversation:

There is a saying that it can be “too cold to snow”! Of course, this is a myth but it has a basis in fact because the atmosphere gets freeze dried when it is very cold. That’s because the amount of moisture the atmosphere can hold depends very strongly on temperature. Under cold conditions, the snow is likely to consist of very small crystals and sometimes is very light and fluffy and like “diamond dust”.

By contrast, the heaviest snowfalls occur with surface temperatures from about 28°F to 32°F – just below the freezing point. Of course, once it gets much above freezing point, the snow turns to rain. So there is a “Goldilocks” set of conditions that are just right to result in a super snow storm. And these conditions are becoming more likely in mid-winter because of human-induced climate change.

The physics behind this phenomenon is governed by a basic law that tells us the maximum amount of moisture in the atmosphere increases exponentially with temperature – that is, the warmer the atmosphere, the more moisture the air can hold and thus, the more potential for precipitation.

For most conditions at sea level, there’s a rule of thumb that says the atmosphere can hold 4% more moisture per one degree Fahrenheit increase in temperature. Some complications come in as the ice phase enters, but we set those aside for now. That translates into a big difference in moisture across temperature differences: At 50°F (10°C) the water-holding capacity of air is double that at 32°F (0°C) and at 14°F (-10°C) the value is only 24% that at 50°F.

In fact, this relationship is fundamental to why it rains (or snows).

When a parcel of air containing water vapor is lifted, it moves into lower pressure, expands and cools. At some point, it can no longer hold as much moisture and so the moisture condenses into a cloud and ultimately forms rain or snow. The lifting of air comes mostly from storms, especially in warm fronts, as warm air moves over cooler air, or cold fronts, as cold air pushes under warmer air.

In all storms, the main source of precipitation is the moisture already in the atmosphere at the start of the storm. This moisture, as water vapor, is gathered by the storm winds, brought into the storm, concentrated and precipitated out. Accordingly, if there is more moisture in the environment, it rains (or snows) harder.

How does this play out when temperatures are below freezing? Temperatures in the Goldilocks range of between about 28°F and 32°F, accompanied by moisture, mean more snow: indeed, the amount of snowfall at 32°F would be at least double that at 14°F. It could be much more because the warm moist buoyant air may also contribute to intensification of the storm itself.

So the memory of global warming is mainly in the oceans. On average the air above the oceans is warmer by more than 1°F and moister by 5% since the 1970s from global warming. In the North Atlantic, there has been additional warming and surface sea temperatures are over 2°F above a 1981-2010 average (which includes a global warming component) over a huge expanse extending more than 1000 miles from the coast of North America. (see graphic, above). Some of this extra warmth may have arisen from the absence of much hurricane activity in the Atlantic this past summer.

In February 5-6, 2010 a snow “bomb” occurred and led to what was referred to at the time as “Snowmaggedon,” which was used by several conservative Senators to mock global warming and Al Gore. Yet it was winter and there was plenty of cold continental air. There was a storm in the right place. And there were unusually high surface sea temperatures in the subtropical Atlantic Ocean – up to 3°F (1.5°C) above normal – which led to extraordinary amounts of moisture being fed into the storm. And it resulted in exceptional snow amounts in the Washington DC area.

precip_heavy

Earlier this week, between January 26-28, 2015, the area targeted by the latest winter storm, called Juno by some, was a bit further north. The developing storm was in just the right position to tap into the high moisture over the ocean and develop as it experienced the sharp contrast between the continent and the relatively warm ocean.

Over three feet of snow fell in some areas, blizzard conditions were experienced in New England, and heavy seas and erosion occurred in coastal regions in association with the higher sea levels associated with global warming.

Going forward, in mid winter, climate change means that snowfalls will increase because the atmosphere can hold 4% more moisture for every 1°F increase in temperature. So as long as it does not warm above freezing, the result is a greater dump of snow.

In contrast, at the beginning and end of winter, it warms enough that it is more likely to rain, so the total winter snowfall does not increase. Observations of snow cover for the northern hemisphere indeed show slight increases in mid-winter (December-February) but huge losses in the spring (see snow cover figure below) This is all part of a trend to much heavier precipitation in the United States (see figure below), especially in the northeast.

4 Responses to “The Weekend Wonk: Global Warming and Snow Cover”

  1. GarryRogers Says:

    Reblogged this on GarryRogers Nature Conservation and commented:
    GR: The video’s explanation of the changes producing more extreme weather is excellent.

  2. redskylite Says:

    Yes the graphs detail trends that are unmistakable and are a warning to us that the “times they are a changing”. NASA’s January 2015 stats are out today and guess what .. #2 warmest since records began and especially exaggerated in the Northern high latitudes, Jennifer Francis’s ideas make logical sense, as we are trying to make sense of relatively short changes in climate. We do not have the luxury to wait. All the trends are continuing the future is set.


  3. Thanks for this, it backs up what I thought. I tried to point this out to Euan Mearns, although perhaps not so eloquently, here: http://euanmearns.com/northern-hemisphere-snow-cover-2/ He didn’t like that and followed up with this: http://euanmearns.com/green-thinking-is-it-science/

    I’ll let your readers decide whether his rant was justified…

  4. dumboldguy Says:

    Great post with great graphs—-more nails in the coffin. (and some more info for Omno to puzzle over as he asks the great question “Why is there snow?”.)

    I lived through Snowmageddon, and it was worse out here to the west of DC in northern VA. Broke many big branches off my trees and busted up a 20 foot tall Leyland Cypress hedge so bad I had to take it down. Those folks in Boston and NE are going through hell.


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