Climate change

What is climate? What is the climate system?

Three layers of meaning of the word "climate"

I think that people who use the term "climate" have a broad agreement about the meaning of the word. But there is no standard definition (as far as I understand). Even the same person may use the term differently among contexts.

I have arrived upon an idea that the meaning of the word "climate" consists of the following three (3) layers. As scientific knowledge has developed, new meanings are added step-by-step, but older meanings are still in use as well.

[Note: Dessler (2015, Section 1.1) quotes the definition of "climate" in a publication of the American Meteorological Society. (It is from AMS's glossary. See .) The first sentence "The slowly varing aspects of the atmosphere-hydrosphere-land surface system" corresponds to my item "3", and the rest corresponds to my item "2".]

The meaning "1" of "climate" existed before the beginning of modern science. Activities of human beings have developed adapting to the limitation of the natural environment. Thus, the style of life in each region reflects the climate of the region.

Modern science began around the 17th Century in Europe. During the early days of modern science, concepts in physics such as "pressure" and "temperature" developed together with experience to measure air pressure and air temperature around the people. In meteorology, instrumental observation in terms of physical quantities started in the early days of the discipline (contrary to such sub-disciplines of earth science such as geology, which developed first as descriptive natural history separate from physics). Around the 19th Century, methods and time intervals of meteorogical observations are standardized, and collections of multi-year observation records are made. It became possible to discuss climate (of the region which has observing stations) by using statistics of meteorological observations.

Meteorological elements, such as air temperature, have obvious diurnal cycles and annual cycles. Thus, it was decided to make observations at several fixed times of day, and to make multi-year statistics sorted by months of solar (Gregorian) calendar. The most used statistics are called "climatological normals", which are 30-year avarages of meteorological observations. The World Meteorological Organization (WMO) renews their normals every 30-year, and their current normal is based on observations of the years 1961 - 1990. (As observations of the year 2020 is taken and quality-checked, they will replace with the normal based on 1991 - 2020.) The Japan Meteorological Agency (JMA, Kisyôtyô 気象庁) renews their normals every 10-year, and their current normal is based on observations of the years 1981 - 2010.

Climate, in the meaning "2", is such phenomena that can be described by statistics of meteorolgical elements. But statistics is not just averages. The spread of values (maybe expressed by variance or standard deviation), the extreme values, and correlation (maybe expressed by covariance or correlation coefficient) between multiple meteorological elements, are important as well.

Weather and climate

Variation of meteorological elements consists of variations in various time scales superposed together.

Variations in short time scales are considered to represent "weather". Variation in long time scales are considered to represent "climate".

The boundary is not well defined. Speaking in English, it is usual that phenomena with time scales larger than one month is considered to belong to the realm of "climate". In the USA, the National Oceanic Atmospheric Administration (NOAA) has "Climate Prediction Center", which is in charge of prediction of the condition of the atmosphere and the ocean several months ahead.

In Japanese, "weather" is "tenki (天気)", "climate" is "kikô (気候)", but there is another word "tenkô (天候)" in between. The boundary is not well defined either. In my personal usage, phenomena with time scales larger than 10 or 30 days are considered to be "tenkô", and those larger than 10 or 30 years are considered to be "kikô".

Climate change and climate variation

Suppose that the climatological normal of air temperature at a place of the years 1981-2020 is evidently different from the cimatological normal of the years 1951-1980. Then, we can say "climate has changed here". (What maked the difference "evident" is not always straightforward. Scientists usually make use of concepts of statistics such as hypothesis testing.)

Suppose, on the other hand, monthly mean air temperature of a place in August 1994 was evidently different from that in August 1993. In my terminology, we see "climate variation" here, but we do not see "climate change" here. In my terminology, "climate (kikô)" is such things that can be expressed by 30-year statistics. Different states of the atmosphere in August 1993 and in August 1994 are "tenkô", which are part of variability of the "kikô" of the period 1981-2010 (for example).

Anthropogenic climate change

Recently, the term "climate change" is used, not literally, but rather a shorthand of "anthropogenic climate change". Here, "anthropogenic" means "caused by human activities".

Climate may change without human activities. That is "natural climate change".

We (climate scientists) consider that human activities since the 20th Century force the climate system mainly by increasing the concentration of carbon dioxide in the atmosphere. It will cause such climate change that includes rise of temperature (near the surface) all over the world as its typical symptom. So, we also use the term "global warming", not just to mean rising temperature, but also to cover changes in various elements of climate, which are caused by increasing carbon dioxide. "Anthropogenic climate change" and "global warming" are practically synonyms. In Japanese, I prefer "tikyû ondanka (地球温暖化)" corresponding to "global warming".

Development of the idea of "climate system"

We want to understand causes of climate change. We also want to predict climate change. Then, it was understood (around 1970) that it is difficult to understand causes and to predict by pursuing the states of the atmosphere only, even if we consider that "climate" is state of the atmosphere, as in my meaning "2". The atmosphere does not seem to have "memory" (metaphorically speaking) for time scales larger than one year. It is the ocean, or snow and ice, or soil wetness, that can have "memory".

Thus, in the former half of the 1970s, scientists started thinking of the "climate system", in which, the atmosphere, the ocean, the cryosphere [a term that cover snow and ice on the earth together] and the land surface influence each other.

The idea was promoted by the scientists who study the atmosphere and the ocean with physics-based approach, mainly working in the United States. (Some of them were migrants from Japan.)

In 1970s, an international scientific research program called GARP (Global Atmospheric Research Program) was ongoing. It is promoted by scientists of various countries gathered under the ICSU (International Council of Scientific Unions, now International Council for Science), national meteorogical services gathered under the WMO (World Meteorological Organization), and space agencies such as US NASA that operate artificial satellites. The main target of GARP was to understand weather and to improve its prediction. But people who organize GARP thought that they will study climate next. (Accordingly, WCRP (World Climate Research Program) started in 1980 and continues today. Its web site is .)

As a schematic diagram of the climate system, the one that was published in 1975 in the report of the U.S. Commitee for GARP "Understanding Climatic Change" is very well known. But, its revised version is published in the same year 1975 in the report of the ICSU-WMO Joint Organizing Commitee, that was responsible for promotion of GARP at the international level. The title of the book is "Physical Basis of Climate and Climate Modelling", and the diagram appears as its Figure 3.1 . See the page in the following link.

The climate system consists of subsystems: "atmosphere", "ocean", "cryosphere", "land surface".

There are interactions between the subsystems:

"System" in what sense?

The term "system" is used in many contexts. The meaning is various. I have used the term "climate system" since 1980s, but I became able to explain it is a system in what sense just since 2010s. I will discuss it in more detail in later weeks. Here I describe it very briefly. I think that the sense of "system" for the "climate system" is the following two concepts combined.

(In Japanese, "change" may be translated as "henka (変化)" or as "hendô (変動)". "Hendô" may also correspond to "variation". So, I think that a better term for "climate change" should be "kikô henka". However, some governmental documents have used "kikô hendô" as the equivalent of "climate change", and it became an official terminology. I try to avoid "kikô hendô" when I feel it ambiguous.)

Climate system, global biogeochemical cycles, earth (surface) system

The concept of the climate system (explained above) is built by physics-based idea (not chemistry- or biology-based idea). Therefore, chemical components of the atmosphere and of the ocean is not explicitly handled. It cannot be ignored, however, that water exists in the atmosphere as water vapor (and a little as clouds which consists of liquid droplets and solid crystals), and its quantity changes by exchange with the ocean and with the land surface. So, it is customary that air is handled as if it consists of two components of water vapor and "dry air", and that sea water is handled as if it consists of two components of water and "salt substance". Concentration of other substances that change relatively slowly (e.g. carbon dioxide in the atmosphere) is handled as conditions given from outside the climate system. Spatial distribution of physical quantities such as temperature and velocity is handled explicitly.

On the other hand, if, for example, we want to understand how the concentration of carbon dioxide in the atmosphere changes, we need to think about solution equilibrium between the atmosphere and the ocean of carbon dioxide, bicarbonate ions (HCO3-) and carbonate ions (CO32-), also about photosynthesis by terrestrial plants and decomposition of organic matter, and also about circulation of carbon atoms around the atmosphere, ocean and land. When we want to understand how the carbon cycle is regulated, we also need to think about cycles of other elements such as nitrogen and phosphorus. Thus, in 1980s, a concept of the "global biogeochemical cycles" is built mainly by scientists who study geochemistry (study of the earth by chemistry-based approach) and ecology (a subdiscipline of biology).

The climate system and the global geochemical cycles share the same physical space, and they have interactions, of course. In 1980s, a movement to study these together grew. A well known diagrams is published in 1986 and again in 1988 in a report "Earth System Science" by an advisory commitee of NASA. The diagram is sometimes called "Bretherton diagram", taking the name of the chairperson of the commitee.

I believe that the report is in the public domain, but I do not find it on the Internet. I include a temporary link to a version of the diagram. (I will change the target of the link if I find a better one.)

The upper half of the figure corresponds to "climate system", and the lower half corresponds to "global biogeochemical cycles". Such separation of components is certainly not representation of structure in the real world, but representation of the current division-of-labor of scientific investigation, which happens to be limited by the ability of human beings to handle various types of knowledge together.

Human activities occur in the same physical space, but they are often handled as forcing from the outside of the system.

Since the time around publication of this diagram, the term "earth system" may refer to such system as the climate system and the global geochemical system combined. But, the term "earth system" may mean something different, for example, the system of the interior of the earth.

"Earth system" in the context of global environmental issues

SHIMAZU Yasuo (島津 康男, 1926-2019) was a scientist, originally studying the interior of the earth by phisics-based approach. He turned to environmental issues in the 1970s. In a book titled "Sizen no Sûri (自然の数理, Mathematics of Nature)" co-authored with GAMBO Kanzaburo (岸保 勘三郎) & TAKANO Kenzo (高野 健三) publshed in 1975, he introduced a concept called "the total system of the earth". It was not the system of the earth as a whole in the physical sense. It was the system of the interaction of human activity and the natural environment, which occur in the part of the earth near the surface. In literal translation of Shimazu's words, the "total system of the earth" consists of 3 subsystems "nature", "organisms", and "humans". I want to rephrase them as "climate system", "ecosystem", and "society". The following link leads to my representation of the concept.

Using this schema, I conceptualize "global environmental issues" as follows.

Climate change as a scientific fact vs. climate change as an issue of policy-making

The meaning of the term "climate change" may be different according to context.

When the context is that of natural science, climate change is a group of scientific facts. Usually we include both natural climate change and anthropogenic climate change.

When the context is that of social science or real politics, "climate change" is usually an issue which demands policy-making. Usually we discuss anthropogenic climate change without explicitly mentioning "anthropogenic".

I made a rough schematic diagram to explain the difference of concepts.

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