Climate course

The Amazon rainforest, the African Savannah, the sands of the Sahara, the prairies of North America , the forests of Siberia, the ice sheet of Antarctica. It can be easy living in cities, as most of us do, to neglect how much land is on our planet. The continents and islands of our planet cover almost thirty percent of the globe, and most of that fraction is empty wilderness. What determines how these areas look is a combination of topography, and climate. We’ll now focus on the effect that climate has on these areas, and rank each climate zone by size, in this, the latest article in my Climate Casebook.

If you’ve viewed any of my other articles then you should be familiar with the Koppen climate system, which breaks down the world into zones of climate types based on temperature and rainfall patterns. If you haven’t, then I recommend you check out my Secrets of World Climate series, or for a quick overview, the summary article on the Koppen-Geigen Climate Classification.

There are 29 Koppen zones in total, but I’ve grouped most of these into the twelve more distinct categories that formed each chapter in my Secrets of World Climate series. So with that out the way, let’s get right into which climate zones are the smallest and the largest, in ascending order.

At number twelve the smallest of the world’s climate types is the Subtropical Highlands – located in the tropics, but experiencing mild temperatures year round thanks to the cooling effect of altitude. These areas are distributed relatively equally across Latin America, Africa and around the Himalayas in Asia. The monsoon variety of this climate, featuring wet and dry seasons dominates over the year-round variant, with the higher altitude cool summer variant extremely rare.

At number eleven, we have the next largest zone – that of the Oceanic – the mild, wet climate found on the west coasts of the continents at temperate latitudes. Most of the land area is found in North-West Europe, with a significant proportion in Oceania. The main variant of warm summers dominates, with the cool summer Subpolar variant restrict to rocky coasts in the far north and south.

At number ten, we have the Oceanic’s southern neighbour, the Mediterranean climate of dry summers along the west coasts of the temperate latitudes. The country with the largest single land area here is, interestingly the United States, whose long west coast is almost exclusively of this type. Australia and Chile have some areas here, but the vast majority is given over to the countries around the sea to which this climate owes its name. The relative land areas of the hot and warm variants of this climate are pretty even, with Csa being slightly more widespread.

The next largest climate type, at number nine, is the Tropical Rainforest, covering selected parts of the equatorial regions of our planet where rain is constant year round. Almost half of this area is within Brazil and Indonesia, with the remaining countries scattered across South America, Africa and SE Asia.

Moving onto number eight now, and we switch to the opposite end of the Earth and opposite temperatures, with the frigid wastes of the Tundra. Most of this land is in the Arctic, with over half of it in just two countries – Russia and Canada. You might be wondering what China’s contribution is all about here – well that’s the extensive Tibetan plateau, which, being at an altitude of over 20,000 feet is predominantly alpine tundra. The United States fraction is almost all from one state – Alaska.

Onto number seven in our climate land areas list, and we have the climate zone that has the largest population of any other, the Humid Subtropical of hot summers and cool winters mixed with plenty of rain, occurring on the eastern fringes of each continent. Almost half of the global land area of this type occurs in just two countries, the United States and China, with the rest scattered across other continents. The year-round rain variant is the larger of the two, with the smaller being the monsoon form with dry winters.

Into the final six now and we have the Continental climate types, which stretch across the vast plains of North America and Eurasia at the mid latitudes, characterised by warm to hot summers and cold winters, but with adequate rain to support large populations. This climate type forms the heartlands of three countries – Russia, the United States and Canada, which together make up over half of all global area for this climate. China’s densely populated north-eastern part features also – with the remainder being in Eastern Europe and NE Asia. There are many subtypes to this climate, varying by summer temperatures, and annual rainfall distribution. A single type dominates, that with year round precipitation – rain in summer, snow in winter, together with a warm summer.

The fifth largest climate zone is the Cool Desert, stretching across the rain-starved continental interiors, where large temperature ranges in combination make for a tough existence. Most of this area is covered by the countries of Central Asia – with most of western China in this category. Similarly, most of the western half of the United States is comprised of this climate zone. The remainder are scattered across the other continents. The true desert and semi-arid forms of this climate are equally matched in prevalence.

Onto number four, and no it has not been forgotten, it’s the most forbidding of all climate zones – the Icecap. Almost the entirety of the large continent of Antarctica is covered in ice year round, and this continent alone makes up the vast majority of the land area of this climate type, with the remainder being in Greenland, and a comparatively tiny scrap as glaciers across the highest mountains in the rest of Earth.

As we move to number three, and we have the climate zone with the next coldest winters, the vast boreal forests of the Subarctic that cover twelve percent of the entire land area of earth. Dominating the northern continental interiors of North America and Eurasia, it is no surprise that two countries make up the vast majority of this land area – Russia and Canada. The variant with cold winters and year-round precipitation dominates this climate type, with monsoon and very cold winter forms comparatively isolated.

Onto number two in our climate land areas list, and we are back in the tropics with the wet and dry seasonal climates – the Tropical Monsoon and Savannah. These lands dominate the tropics, making up almost 75% of land area in this region of earth, with the Tropical Rainforest making up most of the remainder. Although the vast savannah of Brazil makes up almost a quarter of this climate’s land area, the remainder is distributed across very large number of countries across Central and South America, Africa and SE Asia. The Savannah variant of more pronounced dry seasons dominates over the wetter Monsoon form.

And so we come to the world’s largest climate zone. By a process of elimination you might have guessed what it is. And it’s a fact that might bring you some concern, with all the talk of climate change. Because the most extensive climate regions on Earth are the Hot Deserts. Making up a full fifth of all continental areas, the Sahara itself being half of it, the scorching arid lands of the subtropical regions of our planet can appear deceptively smaller than the subarctic regions on a map because of the errors in projection that cause more polar regions to appear larger. When looking at the distribution of these lands, it’s not surprising to find the countries adjoining the Sahara dominating, although the single largest contributor is the land down under, whose continental bulk is made up predominantly of desert. The true hot desert dominates over its slightly wetter semi-arid form as this chart shows.

So, with our list complete, let’s now make a comparison of our twelve grouped zones, in our final graphic. There are two notable patterns here. The first is that almost half of the land on earth is practically unusable by humans, in the form of arid desert, ice-sheets or tundra. The second is that the three smallest zones combined are barely bigger than the next largest, and yet these zones contribute enormously to global agriculture, from the fruits of the Mediterranean and the cattle of the Oceanic, to the corn of the highlands of Central and South America. So if you’re from one of these regions, like I am, don’t feel bad. Remember, small is beautiful!

In two of the previous climate casebook chapters, we looked at how global population was distributed across each climate zone, along with the relative sizes of these zones. Armed with this information, we can now reveal perhaps the one of the most interesting metrics in human geography in relation to climate – the relationship between climate and population density – the key index of our planet’s habitability.

Following the chapter on population distributions across climate zones, there were a number of requests to reveal perhaps the most telling of statistics, how climate might determine the number of people that can be supported in a given unit area. This would reveal which climate zones are favourable to human settlement, and which ones are adverse. In short, the respective habitability of each climate zone.

So without further ado, I now present the list of least and most densely populated climate zones on Earth.

At number twelve, it is no surprise to find the Icecap as the least densely populated climate on our planet. With temperatures constantly below freezing, a continuous covering of ice and snow prevents any plant growth, with penguins being the only wildlife present on the desolate continent of Antarctica. With a “population” in the low thousands being all within the scientific community, these people are concentrated within isolated research stations, leaving the overwhelming majority of the ice sheet areas of Greenland and Antarctica being completely uninhabited.

At number eleven, and we have the next warmest zone, that of the frigid tundra wastes, where only a brief cool summer acts as a respite against otherwise perpetually frozen conditions. Naturally such an inhospitable climate supports a very low population, comprising mostly of Inuit in the north of Canada and Alaska, the Sami of northern Norway and Finland, and the Nenets of northern Russia. The other significant population is split between Chile and Argentina in Tierra del Fuego at the southernmost tip of South America in what is really a colder form of the Oceanic climate, without the frigid winters of the Arctic tundra. Spread over such a vast area, this naturally produces a very low population density, but still orders of magnitude up from the Icecap. Note that the values of this and the Icecap are inverse – showing the number of square kilometres per person, and not the usual persons per square kilometre.

At number ten, and, once again we progress to the next warmer zone, that of the Subarctic. Spread over a vast area of North America and Eurasia, its population is in the millions, so the density is orders of magnitude higher than the Tundra. Although this figure is deceptive, as the vast majority of these millions live within the few cities in this zone, leaving most of the endless wilderness of the boreal forests just that – empty.

At number nine, with just over 20 people per square km, and we increase our population density again by at least an order of magnitude, with the inhabitants of the Cool Desert regions deep within the northern continents and on the dry sides of mountains. With arid conditions combined with freezing winters, it’s no surprise that this is least hospitable climate zone outside of the polar regions of our planet as measured by population density. Still, the total population does number a third of a billion, and so these vast regions are home to a significant proportion of Earth’s population.

At number eight and we have the other form of deserts on our planet, those with searing summer temperatures, but without the freezing winters – the largest of all climate zones – the Hot Deserts. Remarkably, almost a billion people are spread out over the hottest and driest parts of the world, a testament to the ingenuity of our species. However, as always, average figures can be deceptive, as most of this population is confined to densely packed areas around rivers such as the Nile, Indus and Tigris/Euphrates, leaving the vast majority of the desert areas devoid of habitation.

Next on our list, and we go from among the driest to the wettest of all climates, with the Tropical Rainforest. Only a fraction of the size of the hot deserts, the rainforests support also a fraction of the population, and so are only marginally higher in terms of population density. The steamy jungles of the equatorial regions is not a biome that is suitable for human habitation, being filled with disease, biting critters and a thick undergrowth that only understands the blade of a machete knife.

Onto number six, now, and we are into our final half, where we find the most densely populated zones on earth. The first of these is the Continental climate of the interior of the northern continents, characterised by cold winters but warm to hot summers. The heartlands of the United States, Canada and Russia are within this large zone, which sports a large population also, providing a population density double that of the Tropical Rainforest. Unlike the preceding zones, here the population is not just confined to cities, but also spread out across the vast plains in the form of farms and market towns as this zone is the first in our list to be dominated by agriculture.

As we move to number five, we increase the population density again by half, with the Tropical Wet and Dry Climates – the monsoon and savannah. This is both the second largest and second most populated of all zones, and so is among the most important planet wide, covering a huge number of countries across the tropics. Fertile conditions for growing crops such as rice and corn are, however, mitigated by the presence of tropical diseases, which push this zone down in the list of population density as a result.

Onto number four, now, and the next three zones are the three smallest, starting with the Oceanic of the maritime continental west coasts. Cool winters and warm summers together with year-round rain yield the perfect conditions for a rich array of agriculture, but also free from tropical diseases. Most of the people of this zone are packed within one of the most densely populated regions on the planet – North-West Europe.

With a slightly denser population, at number three, and we have the Oceanic’s hotter and drier neighbour, the Mediterranean climate zone. With its unique dry summer conditions, agriculture is more of a challenge than the Oceanic, but with the right irrigation flourishes under the sunnier days. Perhaps this more desirable climate has offset any agricultural challenges in giving this zone a more than equal population density compared to its wetter northern neighbour.

At number two and we have the last in the trio of small climate zones, that of the Subtropical Highlands. Nestled in the uplands of the tropics, enjoying year round mild temperatures and being free from much of the diseases of the tropics, it’s no wonder that this zone sports a population density four times that of the Tropical Rainforest and fifty percent more than the Tropical Wet and Dry climates. Major cities such as Mexico City, Bogotá, Quito and Addis Ababa are surrounded by fertile farmland where the majority of the world’s coffee and tea are grown.

And so we come to the most densely populated climate zone of all, and it’s really in a league of its own. It’s the zone with the largest population, but having a land area only half that of the Tropical Wet and Dry. It is a climate type that most of you new to this channel would not have heard of but which has now achieved somewhat of a celebrity status here – it’s the Humid Subtropical of hot summers, cool winters and plenty of rain. Occupying every eastern continental margin just above the Tropics, it’s no surprise that this zone, having fifty percent higher density than the next highest zone, tops out this list, with the enormous populations of China and India packed into relatively small areas along the Yangtze, Pearl and Ganges Rivers. In these regions the most important of all world crops, rice, can grow easily. But unlike in other zones where this crop thrives, the cool winters kill off much of the disease that plagues its more tropical neighbours, improving life expectancy, and so boosting population density.

So with all our zones now calculated, let’s perform an overall comparison in our final graphic, which is probably the most interesting and telling in all of the articles I have yet written. Because in it we can see a clear relationship of how human habitability is affected by climate. The most densely populated zone, the Humid Subtropical is two million times greater in density than that of the least, the Icecap. That’s an enormous leap in orders of magnitude. Most of these orders are consumed within the polar regions of our planet, however, leaving most climate zones in the same order of magnitude, from the Cool Desert up.

It is clear that a lack of temperature and water are the enemies of human habitation, with the deserts and polar regions all being at the bottom. However the effect of cold temperatures is much, much greater than that of aridity, which is an interesting finding in and of itself, perhaps because technology is able to arrange for the channelling of water much more easily than the rather impossible task of warming the air and ground around you.

The top four most habitable zones are all temperate, featuring mild temperatures and sufficient rainfall, escaping the diseases of the tropics, supporting rich agricultural systems, and generally being pleasant to human experience. On that very last statement, I know some of you have already made plain your complaints of the “pleasantness” of the Humid Subtropical, but I bet that if we did a survey, three out of the top four climate zones listed here would all be in the top of your lists. You’re all free to share these in the comments, by the way – it’ll be an interesting exercise in human perception of climate.

A series of individual topics on particular places around the world or more advanced concepts relating to climate.

>>> The Koppen-Geiger Climate Classification System