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CLIMATE CHANGE | The KoppenTrewartha Climate Classification System

Basics of the Koppen Trewartha Climate Classification System

Wladimir Koppen developed the first quantitative classification system for the Earth’s climate in his first model in 1900 (Kottek et al., 2006). Since Koppen, many different climate classification models have been developed, but the Koppen climate classification system (Koppen, 1923, 1931, 1936) with various modifications to its original form remains the dominant climate classification model.

In contemporary times, the most highly utilized models are the Koppen-Geiger climate classification model (Koppen, 1936, Geiger, 1954) and the Koppen-Trewartha climate classification model (Trewartha, 1968, Trewartha & Horn, 1980) modifications to the Koppen climate classification system.

The Koppen Climate Classification System

Climate classification can be based on the cumulative climactic measurements in individual stations or they can be based on climactic characteristics such as precipitation, cloud cover, etc. Each climate variable such as temperature and air pressure can be analyzed separately or the data on climactic variables can be aggregated. In aggregating climactic variables to form climate classification models, several climactic variables can be integrated or certain climactic variables can be selected as distinctive and as having an overall effect across climactic regions.

The Koppen-Geiger climate classification model and the Koppen-Trewartha climate classification model use vegetation, heat and precipitation as the distinctive and significant climactic variables determining the climactic features for most if not all climactic regions in unison.

The Koppen-Trewartha Climate Classification Model

Glenn Thomas Trewartha proposed his modifications to the Koppen climate classification model (Trewartha, 1968, Trewartha & Horn, 1980) for adjusting the original criteria for temperature and the thresholds that separate dry and wet climatic regions. The resulting model is called the Koppen-Trewartha climate classification system. The most widely applied form of this model was finalized in the Trewartha and Horn climate classification model (1980), which makes various adjustments to Koppen’s original model with particular attention to making the proposed climactic regions better correspond to the observed boundaries of natural landmasses.

Fig: Global Climactic Regions in the Koppen-Trewartha Climate Classification Model
Source: International Journal of Climatology

The Koppen-Trewartha climate classification model (Trewartha & Horn, 1980) describes 6 principal climactic groups. Five of them, including zones denoted as A, C, D, E and F have heat as the determining factor, whereas B is the dry climactic zone in a similar manner to the original Koppen-Geiger climate classification model. Similar to the Koppen-Geiger climate classification model, surface air temperature and amounts of precipitation are the determining factors of climate classification. The individual climate types are –

A – Tropical Humid Climates

Like in the Koppen-Geiger climate classification model, the mean air temperature for this zone in the coldest month and for all months should be above 18oC. Also, the subtypes for this group are determined according to the cycle of precipitation occurring over these regions. The two principal subtypes are Ar, or tropical wet, which is also called the tropical rainforest climate, and Aw, or tropical wet and dry, which is also called the savanna climate. The subtype As is quite rare. The distinction in the Koppen-Trewartha climate classification model with the Koppen-Geiger climate classification model is in the amount of precipitation used to differentiate between a wet and a dry month, with a threshold of 6 cm in the Koppen-Trewartha climate classification model instead of 5.5 cm.

B – Dry Climates

The definition of dry climates poses a great difference between the two models. Out of the various subtypes that exist for this category, they are differentiated according to mean precipitation by equations. For Koppen, the equations were –

R = 2T + 14 for evenly distributed rainfall

R = 2T for rainfall concentrated in winter

R = 2T + 28 for rainfall concentrated in summer

Here, R represents annual mean precipitation threshold in cm, and T the annual mean temperature in oC. The BS subtype, the semi-arid or steppe climate, occurs if the annual mean precipitation is less than 1 R but higher than 0.5 R. If the value is less than 0.5 R, the Koppen-Geiger climate classification model classifies the region as under BW, or arid climate. Trewartha and Horn (1980) argued that these measurements were not accurate, and followed Patton’s (1962) modification which goes as follows –

R = 0.5 T – 12 for rainfall evenly distributed

R = 0.5 T – 17 for rainfall concentrated in winter

R = 0.5 T – 6 for rainfall concentrated in summer

Here, R has been modified to annual mean precipitation threshold in inches, and T is modified to annual mean temperature in oF. The modification ultimately serves to change the boundaries for these zones in the Koppen-Trewartha climate classification model as opposed to the Koppen-Geiger climate classification model.

C – Subtropical Climates

In the subtropical climate types of climate classification, there should be 8 to 12 months with a monthly mean temperature of more than 10oC, with the temperature of the coldest month less than 18oC. Like in A, the subtypes are based on the annual cycle of precipitation, with the letters s, w and f having the same meaning as in the Koppen-Geiger climate classification model. However, the difference lies in precipitation totals being calculated for the wettest and driest months of the season. Like in the Koppen-Geiger climate classification model, Cs stands for subtropical dry-summer climate, also called Mediterranean, Cf for subtropical humid climate, and Cw for subtropical dry-winter climate.

D – Temperate Climates

This type of climatic zone includes regions having between 4 and 7 months of mean air temperature of above 10oC. This is sub-divided into the subtypes continental Dc and oceanic Do, with the 0oC threshold mean air temperature in the coldest month dividing these subtypes, marking a difference from the Koppen-Geiger climate classification model. This in entirety is somewhat differently defined in the Koppen-Geiger climate classification model.

E – Boreal Climates

In these climatic zones, it is necessary to have between 1 and 3 months with monthly mean air temperature above 10oC. Trewartha and Horn (1980) placed no subtypes for this group. This represents an addition to the Koppen-Geiger climate classification model.

F – Polar Climates

In these climatic zones, all months must have a monthly mean air temperature below 10oC. The subtypes are Ft, or tundra, with the warmest month’s air temperature above 0oC and Fi, or ice cap, where air temperature for all months of the year remains below 0oC. This exists in a similar definition as in the Koppen-Geiger climate classification model (M. Belda et al., 2014).

The Koppen-Trewartha Climate Classification and Climate Change

A paper by S. Feng et al. (2011) titled ‘Evaluating observed and projected future climate changes for the Arctic using the Koppen-Trewartha climate classification’ talked about climactic patterns under the influence of climate change with a focus on the Arctic, thought by many to be among the most vulnerable regions to the effects of climate change. With the scheme provided in the Koppen-Trewartha climate classification, modelled simulations were modelled for the period between 1900 and 2099. Based on this, the Intergovernmental Panel on Climate Change (IPCC) in its 4th Assessment projected 16 global climate models continuing into the near future. In this assessment, the average annual mean surface temperature over the Arctic landmasses is expected to witness increases varying between 3.1, 4.6 and 5.3oC (S. Feng et al., 2011).

Using the Koppen-Trewartha climate classification model, it was found that global area under tundra had been significantly decreasing throughout the 20th Century. Wang and Overland (2004) reported with the help of an updated dataset that since 1990 this decrease in global tundra has been rapidly advancing. In S. Feng et al.’s (2011) analysis, comprehensive reviews of vegetation patterns present and prospective were applied to the Arctic. Multiple climate model outputs were applied in impact assessment to reduce the inherent bias in individual models. Simulations were also applied in modelling projections for the period between 1900-2099.

The advantage that the Koppen-Trewartha climate classification model offered the researchers was its simple format, based only on heat and precipitation along with better definitions of locations. This simplicity allowed it to be juxtaposed to the variable of vegetation as impacted by climate change based on multiple model outputs and scenarios.

However, the simplistic model applied by the Koppen-Trewartha climate classification model cannot for example, explain the impacts of CO2 fertilization and impacts from other non-climatic factors such as soil type, permafrost changes, human land use dynamics, etc.

Most climate projection models using the Koppen-Trewartha climate classification model projected shrinkage of tundra coverage. A northward shift of boreal forest on a scale varying between 11 and 50 per cent as dependent on location and model used is expected to occur according to their research in the event that concentrations of CO2 in the atmosphere are doubled.

According to their results, a 33 to 44.2 per cent retreat in tundra cover is expected by the end of the century as per their dynamic vegetation models. This is in keeping, the authors argue, with the rate of change of vegetation cover in the Arctic over the preceding time and changes in climate (S. Feng et al., 2011).

A great ease is possible in providing the initial inference for prospective climate change by the Koppen-Geiger and the Koppen-Trewartha climate classification models because of them utilizing temperature and precipitation as principal determinants. These two variables are expected to become the two key climatic variables in the course of climate change. Some regions overall could remain relatively stable with the inset of climate change although incredible regional variances can be expected especially in the case of precipitation and the resulting change in vegetation and other macro and micro impacts.

The boundaries and transition zones are expected to be the first to show signs of palpable change, and this makes the benefit of climactic boundaries defined under the Koppen-Trewartha climate classification model more important, as this could conclusively establish that the Earth’s climate is indeed changing palpably. Instead of relying on individual incidents, a change in climactic patterns as indicated by changes in climactic zones as defined in widely accepted systems such as the Koppen-Geiger and Koppen-Trewartha climate classification models could suggest that climate change is indeed forcing changes in the Earth’s climatic patterns.

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