Climate

Climate is the average of the dynamic processes in the earth's atmosphere, determined by meteorological methods, related to small-scale localities (meso- or regional climate) or to continental dimensions (macroclimate), including all fluctuations in the course of the year and based on a large number of climate elements. Climatic conditions are not only controlled by solar radiation and the physical and chemical processes within the atmosphere, but also by the influences and interactions of the other four Earth spheres (hydrosphere, cryosphere, biosphere and lithosphere). In order to represent, in addition to all other weather processes, the temperature course in a statistically relevant time frame with sufficient accuracy, the World Meteorological Organization (WMO) recommends the use of reference periods (also called normal periods or CLINO periods), in which the monthly mean values are summarized as a time series over 30 years in a data set. Up to and including 2020, the reference period of the years 1961 to 1990 was the valid and generally used standard of comparison. This was replaced by the new normal period 1991 to 2020 at the beginning of 2021.

The laws of climate, its components, processes and influencing factors as well as its possible future development are the subject of research in climatology. As an interdisciplinary science, climatology cooperates with disciplines such as physics, meteorology, geography, geology and oceanography and partly uses their methods or detection procedures.

Paleoclimatology is an important branch of both climatology and historical geology. Its task is to reconstruct climatic conditions over historical or geological periods in the form of a climate history using isotope studies and data series from climate archives and indirect climate indicators (proxies), and to decipher the mechanisms of past climate change events, such as the influence of periodically changing solar radiation due to Milanković cycles on the Earth system.

Term

Definition

Depending on the stage of development and the focus of climate research, there were and are different definitions. The Intergovernmental Panel on Climate Change (IPCC) works on the basis of a broad definition:

This IPCC definition encompasses a deep time perspective and takes into account other subsystems (Earth spheres) in addition to the atmosphere. It reflects the development since the second half of the 20th century, in which interdisciplinary research into climate dynamics, including its causes, became possible and moved to the forefront of interest. As a result, the temporal dimension gained in importance over the regional dimension.

The German Weather Service (DWD) defines climate more narrowly, with spatial reference and on a time scale of decades:

In geographical climatology, climate was defined by Joachim Blüthgen in his General Climatic Geography as follows:

In meteorological climatology, climate is defined as follows according to Manfred Hendl:

The definition fundamental to climatology came from the Viennese meteorologist Julius von Hann (1839-1921), who understood the term as "the totality of all meteorological phenomena that characterize the mean state of the atmosphere at any point on the earth's surface." (Handbuch der Klimatologie, 1883) Von Hann thus founded "mean climatology". In his definition, he drew on Alexander von Humboldt's definition, which was influential in the 19th century and aimed at the human experience of a place; Humboldt understood climate as "all changes in the atmosphere that noticeably affect our organs" (Kosmos Vol. I).

Etymology

The word Klima (plural: Klimate or, closer to the Greek, Klimata; rarely (Germanized) also Klimas) is an adoption of the ancient Greek word κλίμα klíma, whose first meaning (around 500 BC) in this context was 'bend/inclination [of the sun's position]' and belongs to the verb κλίνειν klínein, 'to incline', 'to bend', 'to curve', 'to lean'. Via the Late Latin clima (verb: clinare, 'to bend', 'to curve', 'to incline') the term finally came into German.

Climate does not refer to the ecliptic, i.e. to the fact that the earth's axis currently has an angle of inclination of approx. 23.5 degrees to the plane of the earth's orbit, but to the spherical shape of the earth. This corresponds to the experience that only by moving in a north-south direction is it possible to observe other parts of the sky. The corresponding Germanization is the compound noun "Himmelsstrich", which, however, only designates the geographical area and no longer the associated weather.

In the 20th century, the conceptual understanding of weather developed from the weather totality (E. E. Fedorov 1927) to the synthesis of weather (WMO 1979).

Time dimension

In contrast to the weather occurring in a certain area (time frame: hours to a few days) and to the weather (time frame: up to about a week, more rarely a month or a season), in climatology fixed periods of time are statistically evaluated, predominantly in relation to the 19th to 21st centuries. The starting point is always the weather event including the meteorologically recorded data and measured values.

In this context, the WorldMeteorologicalOrganization (WMO) recommends so-called climate normal periods with a duration of 30 years. The previous standard was the 1961-1990 annual series, which was valid until 2020 in accordance with the usual regulation and has now been replaced by 1991-2020. For practical reasons, alternative normal periods are also used. In order to have an interval as close in time as possible, the Austrian Central Institute for Meteorology and Geodynamics (ZAMG) often used the period 1971-2000, also with regard to the glacier inventories which are important for the Alpine region. In addition, the WMO recommends to its member organisations the reference period 1981-2010, which is used in parallel to the respective valid reference period, among others for MeteoSwiss.

In addition, larger time periods are also evaluated, such as the centennial secular period 1851-1950, in order to present climatic anomalies and trends in a larger temporal context in this way. This principle is applied on a local as well as on a country-wide or global scale. The internationally recognized Goddard Institute for Space Studies (GISS) and NASA index includes global temperature anomalies from 1880 onward based on the 1951-1980 reference period.

In climate reconstructions, which cover geological periods and thus periods of millions of years, weather influences naturally no longer play a role. Instead, by evaluating sediments, animal and plant fossils, and isotope investigations, attempts are made to establish a climate characteristic of the respective epochs, including brief cooling or warming phases. Due to the rapid progress of the various analytical techniques, increasingly precise results are being obtained in this sector, also in terms of temporal resolution.

Spatial dimension

The term climate is often associated with the world climate or the global climate. However, the global temperature trend is not representative of individual regions, which can even show a contrary trend over a certain period of time. An example of this is a stable cold blob in the subpolar Atlantic south of Greenland, which has apparently developed over decades and may owe its existence to extensive meltwater input from the Greenland Ice Sheet. Conversely, a local record summer can "disappear" in globally determined data series.

With regard to spatial dimensions, a three-level classification has proven useful:

• The microclimate covers a few metres to a few kilometres, such as a terrace, an agricultural area or a road.
• The mesoclimate refers to areas of land (for example a mountain range) up to a few hundred kilometres in extent.
• The macroclimate describes continental and global interrelationships.

While in weather there is a close relationship between the spatial dimension and the event duration, this aspect is less relevant for climatological analyses.

Microclimate (or microclimate)

Microclimate refers to the climate in the area of the air layers near the ground up to a height of about two metres or the climate that forms in a small, clearly defined area (for example on slopes or in an urban environment).

The microclimate is decisively shaped by the surface structure and the ground friction of the wind that occurs there. In this environment there are weaker air currents but greater temperature differences. The diversity of soils, terrain and plant communities can cause great climatic contrasts in a small area. The microclimate is particularly important for low-growing plants, as they go through their most climate-sensitive life stage in the air layer near the ground, and plays an important role, for example, in the characteristics of a vineyard site in quality viticulture.

Humans are also directly exposed to the microclimate. Particularly in the living space of a city, the microclimate often deviates from the natural conditions due to different building materials, architectural design, solar radiation or shading and can change rapidly and permanently through interventions in the respective building fabric or its surroundings.

Mesoclimate

Mesoclimates consist of different individual climates, which have an extension of between a few hundred metres and a few hundred kilometres, but usually cover areas in the lower kilometre range. Due to this broad but local spectrum, many aspects of applied meteorology and climatology play a major role here, for example the urban climate or the rainforest climate. In general, all local climates and terrain climates are counted as mesoclimates, as are the local climates of ecosystems, although the transition to microclimates is fluid.

Regional climate

The regional climate is the climate of a spatial unit on the mesoscale. Accordingly, it has many similarities with the mesoclimate. The regional climate is characterised by the fact that it depends primarily on regional conditions such as land use. In addition, the regional terrain is an important influencing factor.

Since the regional climate is particularly important for forestry, agricultural and infrastructural processes, regional climatic maps are used for this purpose. Normally, regional climates are investigated in relation to natural, administrative or landscape territorial units.

Macroclimate (or large-scale climate)

Macroclimates include large-scale atmospheric circulation patterns, ocean currents, or climate zones greater than 500 kilometers in extent. These include the current combination of the Thermohaline Circulation, which unites four of the five oceans into one water cycle, and the periodically occurring effects of the Atlantic Multidecadal Oscillation. The various wind systems of the Planetary Circulation, for example the monsoon, the trade winds, or the oceanic and atmospheric Rossby waves, are also assigned to the macroclimate, as are large regional climates such as the Amazon rainforest. All macroclimates influence each other and in their entirety form the global climate system.

Meteorological observatory on the Hoher Peißenberg (Upper Bavaria), 977 metres above sea level

Climate zones and climate classification

Areas with the same climatic conditions are divided into climatic zones and thus classified. The best-known classification was made by the geoscientist Vladimir Köppen (1846-1940). His work Geographical System of Climates, published in 1936, is considered the first objective climate classification (see figure on the right). It became widespread mainly through Köppen's collaboration with the climatologist Rudolf Geiger and is still of great importance today.

The extent, structure and location of climate zones depend on the state and fluctuations of the global climate over different periods of time. According to several studies, there has been a clear trend towards the formation of warmer and drier climates since the middle of the 20th century. If this trend continues, it is very likely that existing climatic zones will shift and new ones will be established.

In science, it is generally assumed that with further increasing warming, considerable consequences for flora and fauna of all climate zones are to be expected. By 2100, for example, almost 40 per cent of the world's land area could be affected by the incipient transformation of existing climates, with the risk of extensive species loss and large-scale deforestation. Sub-tropical and tropical areas would be particularly vulnerable to this change, since according to palaeobiological analyses they have been subject to only marginal fluctuations in recent millennia and therefore have a low capacity for adaptation. Among the most lasting effects of the warming process would be on the Arctic regions if the current trend of polar amplification continues in this region. Temperature changes have a considerable impact on the biotopes that exist there. If anthropogenic emissions continue to increase, the Mediterranean region and parts of Chile and California will also be highly affected by this development, with the risk of regional desertification.

In addition to the emerging shift in climate zones, there are also changes in the distribution of vegetation in mountain ranges located in the tropical belt. For the 6263 metre high Chimborazo in Ecuador, for example, a comparison with earlier records showed that over the last 200 years, due to glacial melt and increasing global warming, the plant cover has spread about 500 metres further upwards.

Major climates of the Earth (effective climate classification according to Köppen-Geiger, simplified representation): Tropical rainforest climate Savannah climate Steppe climate Desert climate Etesia Climate Humid climate Sino climate Humid continental climate Trans-Siberian climate Summer dry cold climate Tundra Climate Ice Climate