Global Warming Sure Can Be Cold: Cold Snaps and Climate Reversals

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Record cold weather on the East Coast this winter emboldened a few climate change skeptics to make some pretty interesting remarks.  Rush Limbaugh went on a rant, complaining that the media “just created” the concept of the polar vortex to explain the recent cold snap.  (Never mind that the concept has been around for over a half century.)  On the same day, Oklahoma Senator James Inhofe took to the Senate floor to catalog events that he says were intended to promote action on climate change but took place during, or were canceled because of, snow or other cold weather.  Inhofe’s list of anecdotal evidence stretches back from 2004.

This may surprise some in the Midwest and the East Coast, but average temperatures in the United States for January were just one-tenth of a degree below normal for the country as a whole because while the East Coast was unusually cold, the West was unusually hot and dry.  And globally, January 2014 was the fourth warmest January on record.  The week after Inhofe spoke on the senate floor, the Department of Agriculture designated twenty counties in Inhofe’s Oklahoma as disaster areas due to drought.

In a recent piece in the New York Times, Justin Gillis emphasizes why a regional cold snap does nothing to negate the reality of global warming – though human beings are wired to believe otherwise.  Gillis points out that people in a hot room are more likely to agree that global warming is a reality than people in a cold room.

We go through this exercise – trying to wrap our minds around regional cold weather even as the global climate warms – on a regular basis.  In fact, instead of writing a new piece, Gillis might have gotten away with simply revising the date on his January 2011 article, “Cold Jumps Arctic ‘Fence,’ Stoking Winter’s Fury.”

Things get even more interesting when one takes a longer-term perspective.  I have been reading Thomas Cronin’s Paleoclimates: Understanding Climate Change Past and Present.  “Paleoclimates” are Earth’s ancient climates.  Scientists study paleoclimates to better understand what may happen in the future.  Since we cannot run controlled experiments on Earth’s climate system, looking to its past for information is critical.  The forecasting ability of a climate model, for example, is judged largely based on how well the model can recreate past events – how else could you test it?

Studying more recent changes – and in this context, recent means changes occurring since the last ice age – may be the most enlightening because more information is available and because, well, Earth hasn’t changed as much in the last 25,000 years as it has in the last 25 million years.  Cronin surmises that our climate’s emergence from the last glacial maximum into the Holocene interglacial may be the most studied period in paleoclimatology.

Our planet emerged from the last ice age during the period from about 22,000 years ago through around 11,500 years ago.  Average temperatures increased globally by about 5˚C, with a greater increase (perhaps over 10˚C) near the poles and a more muted increase in the tropics.  Ice sheets melted.  Oceans warmed.  Sea levels rose about 120 meters.  And, yes, all of this from an average temperate swing of just 5˚C (Cronin).

But here’s the rub: the warming did not happen at a constant rate across the globe.  Near the beginning of the Bølling-Allerød warm period, which began around 14,650 years ago, temperatures over Greenland rose about 9˚C in just a few decades (Cronin).  And the warming was interrupted, at least in the northern hemisphere, by several periods of cooling known as “climate reversals.”

During the Younger Dryas, a cooler period occurring from around 12,900 years ago (following the Bølling-Allerød) until around 11,500 years ago, the northern hemisphere cooled, with temperatures dropping 2˚C to 10˚C in various regions (Cronin).  (Keep in mind, a 1˚C change is a 1.8˚F change, so 2˚C to 10˚C equates to 3.6˚F to 18˚F.)  The Younger Dryas then ended abruptly, with temperatures in Greenland likely jumping 5˚C to 10˚C in a century, possibly as much as 10˚C in a decade.

How could this be?  How could Earth be gaining and losing so much heat over such short periods of time?  How could there be significant cooling within a general warming trend?  The answer, most likely, is that these big swings were more a matter of the distribution of heat around the globe than a matter of more (or less) total heat being added to the climate system.

Over long periods of time the amount of energy entering our planet’s climate system varies, primarily as a result of changes in Earth’s tilt (our planet “wobbles” a bit) and the shape of its orbit (which becomes more or less elliptical at times).  Orbital climate theory explains how these factors gradually increase or decrease the amount of the sun’s energy entering the climate system.  But these changes occur gradually – over periods of tens or hundreds of thousands of years.  Other climate mechanisms explain how those very gradual changes can result in major climatic shifts over much shorter time periods.  And usually some sort of tipping point provides the explanation.

In the case of the Younger Dryas, the cool down was likely triggered when the melt water from retreating North American glaciers began flowing into the North Atlantic, instead of draining through the Mississippi to the Gulf of Mexico (Cronin).  Fresh water is less dense than salt water and as this fresh water mixed into the North Atlantic, the ocean’s surface water became less dense.  That changed things.

To simplify, winds drive surface water from the tropics north in the North Atlantic.  This warm water radiates heat (bringing some warmth to Europe) and then, as it cools, it becomes denser and sinks.  This colder water flows back south along the bottom of the ocean.  (A quick Google search of “ocean conveyor belt” or “thermohaline circulation” will turn up explanatory graphics and some good reading.)

Thus, as the water became less dense (due to the fresh water from the retreating glaciers), the sinking slowed.  The ocean’s overall circulation slowed.  The transfer of heat from south to north slowed.  And the northern hemisphere got colder, for around 600 years or so, illustrating how a gradual warming of the climate can translate into an abrupt regional cooling.

I am not suggesting that a few cold winters amount to the beginning of a climate reversal.  Let’s leave it to real climate scientists to sort such things out.  But climate reversals offer us a valuable perspective on understanding regional cold weather amid a global warming trend and they help us to better understand the inherent instability of Earth’s climate system.