Living on the edge

So edgy

Welcome back to my continuing series on how to think about plausible, practical climate scenarios that are relevant to individuals and communities.

The path started from a place of certainty: the accumulation of heat and increase in temperature.

With this simple beginning, there are forks in the trail influenced by whether water is available (or not) and if temperatures are above or below freezing. In the last entry, I emphasized the importance of observing the surface around you, because those details reveal much about what climate change will actually mean to you.

In this entry I want to explore edges.

We have some intuition that edges are places of transition. Where a road cuts through a forest, whole new ecosystems emerge between that road and the surrounding landscape. It is a place of changing and often unusual behavior.

Life’s a beach

Coastlines are a particularly important edge because we place high value on them and build cities there. Coasts are important to trade and commerce, and we have developed whole economies that rely on stable shorelines. Now, however, sea level is rising, which is one of the most certain consequences that we face of a warming climate.

Sea-level rise is not like the water rising in your bathtub, uniform and methodical. It varies from place to place. Perhaps more important, in the next few decades, the rise will be felt because of storm surge as hurricanes and winter storms shove water and waves further inland. At the same time, heavy rain associated with these storms is dammed up; it can’t drain into the ocean. The ground saturates and ground water rises closer to the surface. We have these flooding events with several sources — compound flooding — and it is hard to imagine how we might protect our coastal built environments and persist there.

Fire up

An overhead view of a road cutting through a forest.

Where a road cuts through a forest, whole new ecosystems emerge between that road and the surrounding landscape. It is a place of changing and often unusual behavior. (Image: iStock.)

Another edge I have mentioned, at least implicitly, is the wildland-urban interface, which is important to fires and damage from fire. In this case, we have leverage at the edge to manage the landscape, which offers us straightforward ways to counter increasing fire risk from droughts.

There are other edges to consider regarding fire. As it gets warmer, forests and grasslands that historically have had adequate moisture to resist fire will, in times of drought, be more water-stressed. That means fire zones can creep into new places, including up mountainsides. A warming climate increases aridity at the forest edge; we’ve observed this situation in western North and South America.

There is a season

Now, let’s shift to more subtle edges, spring and fall, the edges between our cold and hot seasons.

Though we are tempted to think of spring and fall as progressing, moderate transitions following the Sun, they are, in fact, dynamic and associated with severe weather. In the eastern U.S., spring is the height of tornado season. In the Great Lakes region, we have the famous Gales of November. These storms make for a ragged edge, with punches of hot and cold air crossing over.

The ragged-edge seasons, spring and fall, are natural, but they are a good place to look for a changing climate.

Intuitively, as the planet accumulates heat and warms, the temperatures of the summer season become more common. They push spring and fall toward winter, confining winter more and more to January and early February. The observations of spring coming earlier and heat extending into fall are some of the most convincing metrics of the accumulation of heat.

There is another aspect of climate change that we can see in spring and fall. Storms, such as tornadoes, are more likely to occur in months and places that previously did not support tornadic conditions. For example, tornado alley has shifted to the east, an edge that continues to move.

In fact, the appearance of storms in unusual places and at unfamiliar times is a migration of edges. This migration is a consequence of climate change that bundles up the effects of accumulating heat. I consider this a far better measurement of climate change than the behavior of an individual storm, which I wrote about in an early Climate Blue column on Hurricane Dorian. Other consequences of shifting edges are longer fire seasons and, for example, malaria migrating to new regions.

Freeze frame

A graphic of a U.S. map showing a shifting freezing line.

The schematic represents an average freezing line. The thermometers represent increasing sea surface temperature, which is a reservoir of heat that fuels winter warming as weather systems transport heat to the land. As the oceans warm, the higher temperatures advance from the south and colder temperatures retreat to the north. The freezing line migrates northward toward the interior of the continent.
Many of the symptoms of warming winter syndrome occur around the freezing line because of changes to the types of precipitation, i.e., snow, freezing rain, and rain. At the freezing line, the climate is changing dramatically. Once an area is well south the freezing line, frozen precipitation becomes very rare. (Image courtesy of Ricky Rood.)

The final edge I will write about here is the freezing line, the edge between liquid water, ice, and snow. I drew a schematic that shows a notional average freezing line in winter in the eastern U.S. You can imagine that far north of the freezing line, historically, we have experienced snow environments – far south of the line, rain.

The figure is drawn to convey that as the climate warms, especially the oceans, the freezing line is moving further into the interior of the continent, mostly northward, and on the Atlantic coast, westward.

This migration of the freezing line is central to thinking about the messy weather we are seeing in winter. I call this “warming winter syndrome.” Near the freezing line, we see ice, snow, and rain. As the freezing line moves north, ahead of the line, we expect to see an increase in, for example, freezing rain. A couple of hundred kilometers behind the line, frozen precipitation becomes rarer, and it will eventually disappear. This edge, moving with the warming climate, is one of the most consequential effects of a changing, non-stationary climate. It is like a wand, which, when waved, changes the weather.

As we observe real-world consequences of what is happening at the edges, science must be cognizant about how to interpret and communicate these changes. Often when there are public arguments about how things are changing, people average everything together. They see little or no change. An important question should be: What are the edges, and what is happening at the edges? With that focus, the consistent symptoms of a warming climate become more clear.

In my next column, I will discuss storms and how to consider them along climate pathways.

(Lead image: A road collapses into the sea due to sea-level rise on the Holderness coast, an area of the East Riding of Yorkshire, on the northeast coast of England. Source: iStock.)


  1. Robert Williams - 1963

    A beautiful description of evidence of climate change!
    More importantly, is determining whether the climate change is due in large part to human behavior.
    Humans had no part in previous periods of warming and cooling of the earth .
    There is little evidence that future behavior will modify significant temperature changes in the future.
    We can hope but that would be ignoring the bigger picture.


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