Climate
The World Meteorological Organization established a technical commission for climatology in 1929. At its 1934 Wiesbaden meeting, the group designated the thirty-year period from 1901 to 1930 as the reference time frame for climatological standard normals. This decision created a baseline for comparing current weather patterns against historical averages. In 1982, the organization agreed to update these climate normals using data collected between the 1st of January 1961 and the 31st of December 1990. The next set of climate normals published by the WMO covers the years 1991 through 2020. A 30-year period filters out interannual variations like El Niño events while still showing longer climatic trends. Scientists measure air temperature, pressure, precipitation, wind, humidity, visibility, cloud amount, solar radiation, soil temperature, pan evaporation rate, days with thunder, and days with hail. These variables help track changes in global climate conditions over decades.
One of the most used classification schemes is the Köppen climate classification first developed in 1899. Modern methods divide into genetic approaches that focus on causes and empiric methods that focus on effects. The Thornthwaite system has been in use since 1948 and incorporates evapotranspiration alongside temperature and precipitation information. Major classifications within the Thornthwaite framework include microthermal, mesothermal, and megathermal zones. The Bergeron and Spatial Synoptic Classification systems focus on the origin of air masses that define regional climates. Originally climes were defined in Ancient Greece to describe weather based on a location's latitude. Climate zones correlate closely with biome classifications because climate influences life in a region. A common shortcoming of these schemes is that they produce distinct boundaries rather than gradual transitions found in nature. Plant hardiness zones and evapotranspiration levels often determine empirical classifications for specific biomes.
Paleoclimatologists seek to explain climate variations for all parts of Earth during any given geologic period. They begin their work from the time of Earth's formation until the present day. Since very few direct observations existed before the 19th century, scientists infer past climates from proxy variables. Non-biotic evidence includes sediments found in lake beds and ice cores drilled from glaciers. Biotic evidence comes from tree rings and coral samples collected from ocean floors. These records demonstrate periods of stability and periods of change across thousands or millions of years. Some changes follow patterns such as regular cycles while others appear random. Scientists use data spanning decades to millennia to reconstruct the state of ancient atmospheres. This approach allows researchers to understand how natural forces shaped environments long before human industrial activity began.
Details of the modern climate record come from measurements taken by thermometers, barometers, and anemometers over the past few centuries. The instruments used to study weather have changed significantly regarding observation frequency and known error margins. Long-term modern climate records skew toward population centers and affluent countries where equipment is most common. Satellites launched since the 1960s allow global scale data gathering including areas with little to no human presence. Arctic regions and oceans now receive consistent monitoring through orbital sensors rather than ground stations alone. Changes in immediate environment and exposure must be considered when studying climate history from previous centuries. Modern networks track atmospheric pressure, temperature, precipitation, wind speed, humidity levels, cloud cover, solar radiation, soil temperature, evaporation rates, thunder days, and hail days. These tools provide a comprehensive view of Earth's changing atmosphere compared to earlier limited datasets.
Earth has undergone periodic climate shifts throughout its history including four major ice ages. Glacial periods feature colder conditions separated by interglacial periods where temperatures rise naturally. Accumulation of snow and ice during glacial periods increases surface albedo reflecting more Sun energy into space. Increases in greenhouse gases like those produced by volcanic activity can raise global temperatures creating interglacial periods. Suggested causes for ice age periods include continental positions variations in Earth orbit changes in solar output and volcanism. Natural forces add variability but occur on much slower time scales than current rapid changes driven by human emissions. Climate oscillations correlate closely with astronomical factors such as barycenter changes solar variation cosmic ray flux and Milankovitch cycles. Heat distribution modes between the ocean-atmosphere system also influence these natural patterns significantly over millennia.
The primary driver for increased global temperatures in the industrial era is human activity according to scientific consensus. Recent warming results in redistributions of biota across different regions of the planet. As climate scientist Lesley Ann Hughes wrote: a 3°C change in mean annual temperature corresponds to a shift in isotherms of approximately latitude or elevation. Species are expected to move upwards in elevation or towards poles in response to shifting climate zones. Average global air temperature passed 1.5C of warming during the period from February 2023 to January 2024. The accumulation of greenhouse gases like carbon dioxide and methane determines how much solar energy remains trapped by the planet. This process leads to either global warming or global cooling depending on atmospheric composition. Scientists have identified Earth's Energy Imbalance as a fundamental metric of the status of global change today.
Climate models use quantitative methods to simulate interactions and transfer of radiative energy between atmosphere oceans land surface and ice. All climate models balance incoming short wave electromagnetic radiation with outgoing long wave infrared electromagnetic radiation from Earth. Any imbalance results in a change in the average temperature of the planet. Models range from simple radiant heat transfer treatments to complex coupled atmosphere-ocean-sea ice systems. High resolutions require significant computational resources so only a few global datasets exist at fine scales. Global climate models can be dynamically downscaled to regional models to analyze local impacts of climate change. Examples include ICON or mechanistically downscaled data such as CHELSA for earth's land surface areas. These simulations predict an upward trend in global mean surface temperature with rapid increases projected for higher latitudes of the Northern Hemisphere.
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Common questions
When did the World Meteorological Organization establish a technical commission for climatology?
The World Meteorological Organization established a technical commission for climatology in 1929. This body later designated the thirty-year period from 1901 to 1930 as the reference time frame for climatological standard normals at its 1934 Wiesbaden meeting.
What are the specific years covered by the current WMO climate normals published after 1982?
The next set of climate normals published by the WMO covers the years 1991 through 2020. The organization agreed to update these climate normals using data collected between the 1st of January 1961 and the 31st of December 1990 in 1982.
Which classification scheme was first developed in 1899 and remains one of the most used today?
The Köppen climate classification is the scheme that was first developed in 1899 and remains one of the most used methods globally. Modern methods divide into genetic approaches that focus on causes and empiric methods that focus on effects.
How do paleoclimatologists infer past climates before direct observations existed in the 19th century?
Scientists infer past climates from proxy variables because very few direct observations existed before the 19th century. Non-biotic evidence includes sediments found in lake beds and ice cores drilled from glaciers while biotic evidence comes from tree rings and coral samples collected from ocean floors.
What caused Earth to undergo four major ice ages throughout its history including glacial periods?
Suggested causes for ice age periods include continental positions variations in Earth orbit changes in solar output and volcanism. Climate oscillations correlate closely with astronomical factors such as barycenter changes solar variation cosmic ray flux and Milankovitch cycles.