Subtropical ⊊ warm temperate

In this post, I will give an introduction into the main climate zone concepts, which are too often confused in non-climatic literature.

Many authors in palaeobotany that interpret their floristic data towards past climatic or general-environmental conditions use ‘warm temperate’ in different context, often without providing the actual context. It is frequently used in opposition to ‘subtropical’, which contrasts the original concept of the ‘warm temperate climates’ (in German: ‘warm-gemäßigte Klimate’) introduced by Wladimir Köppen. For instance, in the recently published paper of Na et al. on the floristic component of the Jurassic Daohugou Biota, a paper co-authored by a renown and very experienced palaeobotanist, the following line can be found in the discussion: “In general, the Cycadopsida are more diverse in floras in tropical to subtropical zones than those in warm temperate zone. In cold temperate zone, they are rare.” Correctly, the sentence should have read: “Cycadopsida are more diverse in tropical and subtropical floras, with few lineages extending into the temperate zone.”, because “warm temperate” includes subtropical areas, whereas “cold temperate” refers to climates with short growing periods and strong winters such as today found in central Canada, the northern half of Scandinavia, and Siberia (modern cycads cannot stand frost or long snow covers).

The Köppen system—a generic climate classification

The most widely used climate classification system is the one proposed by Wladimir Köppen [brittanica][wikipedia]. Köppen was not only a brilliant scientist, but also one that travelled a lot. [Trivia: he also was a libertarian communist and father-in-law of Alfred Wegener, with whom he discussed heretic ideas. One could call Köppen the ‘grand-father’ of plate-tectonics. A landmark joint publication of Köppen and Wegener was Die Klimate der geologischen Vorzeit (The Climates of the Geological Past), published in 1924, nine years after Wegener introduced his continental drift theory.] 
On his travels, he recognised that there is a relationship between the vegetation and the climate. For instance, he noticed that oaks grow farther north (or higher up in the mountains) than beech trees. So, he included in his early concepts a ‘beech tree climate’, the climate zone in which beech trees can still grow, and an ‘oak climate’, a climate too harsh for beech trees, but still allowing certain deciduous oaks to grow. He finally came up with a generic classification and some quite arbitrary – on the first sight – cut-offs. The final Köppen-Geiger system (Köppen & Geiger 1928; Köppen 1936) recognised five major climate zones:
  • A, the equatorial (or tropical) climate zone, no month colder (in average) than 18 °C;
  • B, the dry climates; climates with too low precipitation compared to temperature levels;
  • C, the warm temperate climates (in German: warm-gemäßigte Klimate); climates without permanent snow cover in winter and rare frosts, no month colder than –3 °C. 
  • D, the snow climates (Schneeklimate or kalt-gemäßigte Klimate [cold temperate]); with permanent snow cover in winter and frequent frost; and  
  • E, the polar climates, no month with an average temperature > 10 °C.
Subdivisions dealt with the annual distribution of precipitation (rain) and heat coded in form of a second (or third) letter:
  • f = fully humid with little seasonality, and seasonal: s = summer-dry, w = winter-dry, m = monsoonal climate (high seasonality); for A-, C-, and D-climates.
  • a = hot summer, b = warm summer, c = cool summer, d = cold summer for C- and D-climates.
  • S = steppe climate (German: Steppe), W = desert climate (Wüste), h = hot (heiß), k = cold (kalt) for the dry (B) climates;
  • T = tundra climate (Tundra), and F = ice climate (Frostwüste).
Most of Köppen’s generic cut-off values are used until today, such as the mentioned isotherms – lines of same temperature. 11 years ago, Kottek et al. (2006) provided a fully formulised update of the Köppen-Geiger system and an updated global climate map. The results of this and later papers by this Vienna-based group have been made publicly available (including free Köppen-Geiger maps, different resolution, as GoogleEarth overlays) at

Note that some potential errors in the Kottek’s et al. 2006 map, particularly when determining the borders between Cs- and Cw-climates, have been corrected by Peel, Finlayson & McMahon (2007).

The separation of the subtropical zone

Particularly in the U.S., biogeographers and climatologists were unhappy with the broad-definition of Köppen’s warm temperate climate zone. They sought for alternatives to separate and define the subtropical zone – non-tropical climates without pronounced winter (little to no frost) – from the temperate climate zone(s) – the moderate climates with pronounced winter, and a restricted growing phase.
[There may have been some political angle, too: it was after World War II and the McCarthy era just took off. And Köppen had not only been German and Russian, and published most of his papers in these languages, but also a communist.]
Several alternatives were proposed for the separation of the subtropical zone from the temperate zone. Genetic systems emerged that were stratified latitudinally and mainly relied on isotherms using rounded values (e.g. 0 °C, 10 °C, 20 °C monthly averages). Areas classified by Köppen as dry (B-)climates were included in the tropical, subtropical or temperate zone.  The alternative system that finally prevailed was, however, that of Glen Trewartha. Trewartha, an American geographer, possibly first sketched it in the 3rd edition of his book, An Introduction to Climate (Trewartha 1954), and finalised it 26 years later in the 5th edition (Trewartha & Horn 1980).
Trewartha’s aim was a system that is less generic and better reflects vegetation patterns in the U.S. He accepted Köppen’s dry climates, but eventually divided Köppen’s warm temperate (C-)climates into the subtropical [C] climates and the temperate [D] climates using the number of months with an average temperate > 10 °C. Köppen’s snow (D-)climates with pronounced summer were included in the temperate zone, and the remainder (three or less months with > 10 °C) separated as the boreal [E] climates. Originally, “highland” (H-climates; high-montane settings) were recognized, but finally considered to be superfluous by Trewartha. In literature, Köppen’s system with no subtropical or boreal zone but a clearly defined warm temperate zone, also known as ‘Köppen-Geiger’, and Trewartha’s modification, known as ‘Köppen-Trewartha’, distinguishing a subtropical, temperate, and boreal zone, coexist. Note that the latter terms are also used to address and classify vegetation in various contexts and without reference to a particular climate classification. Which may explain the confusion by Na et al. and many other authors.
[Köppen-Trewartha is naturally more dominant in the U.S. literature, but also used in China because it differentiates between the northern Cwa and southern Cwa climate in China proper. Köppen-Geiger is predominant in European literature and more common throughout the rest of the world, and probably the widest used climate classification system. Measured in Wikipedia entries, Köppen cleary had (and has) more global impact. Trewartha only has a short English and a Chinese page, but Köppen’s pages have duplicates in many western Eurasian (including Esperanto, he apparently published several articles in this artificial lingua franca) and some Asian languages.]

Generalised comparison between different climate classifications, illustrating the evolution of Trewartha's modification of Köppen's system.

Translating Köppen’s climates into subtropical, temperate, and boreal

When talking about climate and vegetation, particularly in a palaeo-context, we may want to use terms such as ‘subtropical’ and ‘boreal’ as descriptors for our vegetation, but do so in reference to Köppen’s climate system (which I personally find more intuitive, better structured; and well-accessible thanks to the Vienna group). So, here’s the deal:
  • The subtropical climates as defined in the final Köppen-Trewartha climate classification include Köppen’s Csa climate and the larger part of his Cfa and Cwa climates (see e.g. Belda et al. 2014 for a 1:1 comparison), that is warm temperate climates with hot summers.
  • Analogously, the boreal climate of Köppen-Trewartha essentially covers Köppen’s snow climates with cool and cold summers (Dfc, Dfd, Dwc, Dwd, Dsc).
  • Anything in-between (mostly the remaining warm temperate and all snow climates with hot/warm summers) can then be addressed as temperate.
With respect to vegetation, Trewartha’s definition of a boreal climate may be too narrow. Climate types such as warm temperate climates with cold summers (mainly Cfc, which typically has more than three months above 10 °C) and the mid-montane variant of ‘Mediterranean’, summer-dry climates, i.e. Dsb (snow climate with warm summer) maybe better addressed as boreal climates than as temperate ones. Trewartha himself pointed out that the microthermal (boreal) climates are characterised by birch and conifer forests in contrast to the mesothermal (temperate) climates with their broadleaved deciduous forests.

The invaluable work of Walter and Schroeder

Somewhat related to Köppen’s system, and equally generic being based mainly on observation and equally useful, are the vegetation zone concepts of Heinrich Walter, a geobotanist (Walter 1973; Walter & Breckle 1983–1991) and Fred-Günter Schroeder (1998), a botanist (dendrologist). Walter’s ‘tropical’, ‘meridional’, ‘nemoral’, ‘boreal’ and ‘arctic-alpine’ zones, adopted and further refined by Schroeder, can be regarded to a large degree as vegetation equivalents to Köppen’s climate zones.

Comparison between Köppen's climate system and generalised vegetation zones. See Denk et al. (2013, table 4) for details.

These works comprise an enormous wealth of information regarding correlation between climate and vegetation and are possibly unparalleled in Anglosaxon literature. Unfortunately, Walter’s and Schroeder’s works have never (or only partly) made available in English; and appear to be largely unknown to researchers (including many German-speaking) that try to reconstruct past climates and vegetation using plant fossils. Schroeder is apparently still living, and maybe some publisher could take up the task to translate his book into English so it can get well-deserved global attention.

Cited papers and related palaeobotanical papers
Belda M, Holtanová E, Halenka T, Kalvová J. 2014. Climate classification revisited: from Köppen to Trewartha. Climate Research 59:1–13.
Denk T, Grimm GW, Grímsson F, Zetter R. 2013. Evidence from "Köppen signatures" of fossil plant assemblages for effective heat transport of Gulf Stream to subarctic North Atlantic during Miocene cooling. Biogeosciences 10:7927–7942. introducing Köppen signatures to classify plant taxa and providing a first interpretation of Köppen signatures towards vegetation (‘meridio-nemoral’, ‘nemoral’, ‘boreal’, ‘artic-alpine’, and ‘generalist’ taxa).
Grímsson F, Grimm GW, Meller B, Bouchal JM, Zetter R. 2016. Combined LM and SEM study of the Middle Miocene (Sarmatian) palynoflora from the Lavanttal Basin, Austria: Part IV. Magnoliophyta 2 – Fagales to Rosales. Grana 55:101–163. — Some more Köppen signatures and their interpretation towards vegetation (‘semihumid-meridional’, ‘eurytropical’, ‘tropical’, and ‘tropical-meridional’ elements)
Köppen W. 1936. Das geographische System der Klimate. In: Köppen W, and Geiger R, eds. Handbuch der Klimatologie, Band 1, Teil C. Berlin: Gebr. Borntraeger, p. 1–44.
Köppen W, Geiger R. 1928. Klimakarte der Erde [wall map 150 cm x 200 cm]. Gotha: Verlag Justus Perthes.
Kottek M, Grieser J, Beck C, Rudolf B, Rubel F. 2006. World map of the Köppen-Geiger climate classification updated. Meteorologische Zeitschrift 15:259–263.
Peel MC, Finlayson BL, McMahon TA. 2007. Updated world map of the Köppen-Geiger climate classification. Hydrology and Earth System Sciences 11:1633–1644.
Schroeder G-F. 1998. Lehrbuch der Pflanzengeographie. Wiesbaden: Quelle & Meyer.
Trewartha GT. 1954. An Introduction into Climate. 3rd edition. New York: McGraw-Hill.
Trewartha GT, Horn LH. 1980. An Introduction to Climate. 5th edition. New York: McGraw-Hill.
Walter H. 1973. Vegetation of the Earth in relation to the climate and the eco-physiological conditions. New York, Heidelberg, Berlin: Springer Verlag.
Walter H, Breckle S-W. 1983–1991. Ökologie der Erde. Stuttgart: Eugen Ulmer Verlag.

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