It’s Not Just Subtropical Cornwall — Climate Zones Everywhere are on the March Poleward

A few weeks ago, the University of Exeter found that parts of Cornwall, England had become subtropical. The study stated that since average temperatures had risen to above 10 degrees Celsius (50 Fahrenheit) for periods of time longer than seven months, this part of England situated on a latitude line north of most of Newfoundland has become part of a climate zone that during the early 20th century extended as far south as the southern tip of Florida.

Seemingly oblivious to the new oddity and possible peril implied by such a significant climate shift, the study went on to cheerfully observe that:

Parts of Cornwall have become subtropical since 2000 and this could create opportunities to grow new, unusual plants. Sunflowers, maize, grapevines and tea are already grown in the Duchy.

The study also pointed out that the added heat might present a problem or two, instances that might be sorted out simply by getting scientists to work together with farmers:

While sub-tropical conditions may create opportunities to grow exotic crops, the lower frequency of frosts is also making Cornwall more susceptible to invasive species. As the temperatures continue to warm, we need to ensure we manage the risks carefully as well as capitalising on the opportunities. This will require scientists to continue to work hand-in-hand with the horticultural sector.

Subtropical Cornwall — Sahara Desert Marches North as Coastal Zones Flood

Unmentioned, however, was how bizarre the notion of a subtropical Cornwall actually is and what such a major climate shift may mean for the globe, Europe, and Cornwall itself — a shift that may put the odd notion of ‘capitalising on the opportunities’ practically out of reach.

Unfortunately, big climate-zone movements of the kind shown in the Exeter study risk an amazing range of ecological and geophysical damage. A short and incomplete list includes melting glaciers, shrinking sea ice, rising sea levels (a very real worry for the numerous coastal communities near or within Cornwall, including places like Falmouth, Penzance, St Ives, and Exeter itself), stratifying and increasingly anoxic oceans, mass coral bleaching, and habitat loss for species on a global scale risking mass extinction. Any such mention of this appropriately worrying context was notably and oddly absent from the Exeter study.

Subtropical climates

(The subtropical climate zones of the world as of the late 20th century. Image source: Commons.)

Of particular interest to Cornwall and regions nearby is that the northward expansion of the subtropical regions of the world has long been a concern among climate scientists due to its ability to enhance desertification in highly-populated regions like Europe. The issue is that as the tropics and subtropics drift poleward, they bring hot, dry weather along with them. Because the subtropical regions support conditions conducive for arid climates, there is a high likelihood that deserts will march northward into more densely-inhabited regions.

A 2010 study by UCAR found that under continued fossil-fuel burning, the Sahara Desert essentially leaps across the Mediterranean and fully expands into southern and central Europe. In other words, though Cornwall may be able to support subtropical plants now, the northward movement of the arid zone related to the incoming warmth would make growing such plants an increasingly difficult prospect as time moves forward.

Drought Zones Expand under global warming

(A 2010 UCAR study found that drought zones dramatically expand as the tropical and subtropical climate zones march northward due to human-forced warming. Image source: Drought Under Global Warming.)

It’s Not Just Cornwall — Climate Zones are Moving Everywhere

As we consider what the onset of intensified warming may mean for Cornwall — with deserts expanding and seas stratifying as they rise — it’s also worth thinking a little bit about the related warming-forced movement of the world’s isotherms. Isotherm is a meteorological term for a line connecting regions of the same temperature. Averaged over the coldest or warmest months of a year, these lines of temperature also mark the boundaries between climate zones.

The tropics, for example, are bounded in the Northern and Southern Hemispheres by a line of 18 C (64.4 F) average coldest-month temperatures. The edges of the two polar zones are bounded by a line of 10 degrees Celsius (50 F) average temperatures during the hottest months of the year (July in the Boreal region, January in the Austral region).



(NOAA global isotherm map. Image source: NOAA Climate Zones.)

Tucked between these two zones are the temperate and subtropical climates. Over the past 40 years, rapid warming has shoved these isotherms poleward. This shift has created a new climate reality for Cornwall and pretty much everywhere else.

At an average decadal march away from the Equator of about 35 miles, these respective climate boundaries have moved by around 140 miles over the last four decades, expanding the total reach of the tropic zone by 280 miles northward and southward. In the Northern Hemisphere, the polar climate zone has shrunken toward the continental edges by about 14o miles, while in the Southern Hemisphere, the polar zone is shrinking off the tip of South America and across the Southern Ocean toward Antarctica by a similar distance.

These changes in the isotherms are rough measures, of course. In some places, like Cornwall, climate zones are moving toward the poles at an even faster rate. And the Arctic climate zone is notably shrinking at a more rapid rate than the Southern Hemisphere polar zone.

Rapid Climate Zone Movement Risks Species Extinction

Movement of these climate zones is driven by a rate of global warming in the range 0.15 to 0.20 degrees C every ten years, a pace of warming about 30 times faster than the warming at the end of the last ice age. As a result, plants and animals are finding that the habitats to which they’ve adapted are swiftly and dramatically changing. Plant and animal ranges have moved behind these climate zones at a rate of only about four miles every decade. Basically, habitats are moving too quickly for the creatures they support to catch up.


(As the globe warms due to fossil-fuel burning, the tropical Hadley cell expands even as the top of the polar and mid-latitude cells rise. This combination results in severe climate shifts and more persistent weather patterns due to a weakened polar jet stream. Such change in climate results in loss of animal habitat and increasingly difficult and extreme weather conditions for human beings. Image source: NASA/Commons.)

Back in 2008, climate scientist James Hansen noted:

If emissions of greenhouse gases continue to increase at the current rate—“business as usual”—then the rate of isotherm movement will double in this century to at least 70 miles per decade. If we continue on this path, a large fraction of the species on Earth, as many as 50 percent or more, may become extinct. The species most at risk are those in polar climates and the biologically diverse slopes of alpine regions. These animals, in effect, will be pushed off the planet—though some like the polar bear may be “rescued” and allowed to survive in zoos.

It is important to also consider that loss of plant habitat due to warming and drying is a primary driver of the mass tree death and increase in wildfire rates we have seen across the globe in recent years. Along with the mass exodus of fish from warming Equatorial oceans and the flipping-on of the global coral bleaching switch during 2014-2016, we can see these species-threatening impacts in heart-wrenching and terrifying effect today.

Given that the rapid poleward progression of the isotherms has continued since Hansen’s 2008 writing, and given that the pace of global warming appears to have accelerated through the 2014 to 2016 period, it appears that his statement is all the more salient today — especially now that we are starting to see some of the mass loss of animal and plant life due to warming that Hansen mentioned. And especially since parts of Cornwall in England are now considered subtropical.


Subtropical Cornwall Climate Could Mean Exotic New Crops (among other things)


Drought Under Global Warming

NOAA Climate Zones

Scientist James Hansen, “The 800 Pound Gorilla”

66 Million Trees Dead in California

This Global Coral Bleaching Event Just Won’t End

Tropical Fish Moving Toward Poles


Hat tip to Cate

Hat tip to DT Lange

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