Climate

• 1. Introduction

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  Climate is what you expect, weather is what you get. That popular phrase captures a distinction people often blur. A thunderstorm is weather. The fact that you carry an umbrella for the season is climate. More precisely, climate is the long-term weather pattern in a region, typically averaged over 30 years. But why 30 years, and not 10 or 100? Who decided that figure, and in what room? This documentary follows the science of reading a region's character across time. It asks how scholars classify the world's climates into tidy bands. It asks how anyone can know the weather of an age long before thermometers existed. And it asks what happens when the baseline itself begins to move.

• 2. Reading The Average

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  30 years is the standard averaging period, though other spans may be used depending on the purpose. The figure is not arbitrary. A 30-year window is long enough to filter out short bursts like El Nino-Southern Oscillation. It is short enough to still reveal a longer trend underneath. The World Meteorological Organization calls these reference points climate normals. A climate normal is the arithmetic average of an element, such as temperature, across that 30-year span.

  The Intergovernmental Panel on Climate Change, in its 2001 glossary, framed climate narrowly as average weather. More rigorously, it called climate the statistical description of the mean and variability of relevant quantities over months to millions of years. The classical period of 30 years there traces back to the World Meteorological Organization. Climate in a wider sense is the state of the whole climate system.

  Climate is more than the average alone. It also includes the magnitudes of day-to-day and year-to-year variation. The variables measured are familiar ones. Temperature, humidity, atmospheric pressure, wind, and precipitation are the common ones. In a broader sense, climate is the state of the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere, and the interactions between them. A location's climate answers to its latitude, longitude, terrain, altitude, land use, and nearby waters.

• 3. The Thirty-Year Baseline

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  The 1934 Wiesbaden meeting set the first reference frame. There a technical commission designated the period from 1901 to 1930 as the standard for climatological normals. The body behind it was the WMO, which grew out of the International Meteorological Organization. That older organization had created a technical commission for climatology in 1929.

  In 1982 the WMO agreed to update its climate normals. The new figures were built on data running from the 1st of January 1961 to the 31st of December 1990. The 1961-1990 climate normals serve as the baseline reference period. The next set the WMO planned to publish covers 1991 to 2020.

  Air temperature, pressure, precipitation, and wind are the most common atmospheric variables collected. Others fill in the picture of changing conditions. Humidity, visibility, cloud amount, solar radiation, soil temperature, pan evaporation rate, days with thunder, and days with hail are gathered too. These many readings feed the systems that try to sort the world's climates into types.

• 4. Drawing The Zones

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  Ancient Greece is where the idea began, with climes defined to describe weather according to a location's latitude. Modern classification splits into two broad approaches. Genetic methods focus on the causes of climate. Empiric methods focus on its effects.

  The Koppen climate classification, first developed in 1899, is one of the most used schemes. It was originally designed to identify the climates tied to certain biomes. A climate classification can correlate closely with a biome classification, since climate strongly shapes life in a region.

  The Thornthwaite system has been in use since 1948. It folds evapotranspiration in with temperature and precipitation, and it serves the study of biological diversity and how climate change affects it. Its major classes are microthermal, mesothermal, and megathermal. The Bergeron and Spatial Synoptic Classification systems take a different angle, focusing on the origin of the air masses that define a region's climate. Every such scheme shares one flaw. They draw sharp boundaries between zones, when nature more often offers gradual transitions.

• 5. The Memory Of Ice

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  Paleoclimatology is the study of ancient climates across the great stretch of Earth's history. Its practitioners seek to explain climate variation for all parts of the planet during any geologic period, beginning with the time of Earth's formation. The problem is evidence. Very few direct observations of climate existed before the 19th century.

  Proxy variables fill the gap. Non-biotic evidence includes sediments found in lake beds and ice cores. Biotic evidence includes tree rings and coral. Pollen and rocks join the record too. Together these proxies span time scales from decades to millennia. They reveal periods of stability and periods of change, and can show whether change follows regular cycles.

  Thermometers, barometers, and anemometers built the modern record across the past few centuries. That record carries a bias. Long-term modern measurements skew toward population centres and affluent countries. Since the 1960s, satellite launches have let records be gathered on a global scale. That now includes places with little or no human presence, such as the Arctic region and the oceans.

• 6. When The Baseline Moves

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  In the 1960s, the words climate change meant something different than they do now. Back then the phrase described what we now call climate variability: inconsistencies and anomalies. Today climate change implies change that is both long-term and of human causation. Climate variability itself covers variation in the mean state on all spatial and temporal scales beyond individual weather events.

  Earth has passed through four major ice ages. Each holds glacial periods, colder than normal, separated by warmer interglacials. During a glacial period, accumulating snow and ice raise the surface albedo. That reflects more of the Sun's energy into space and keeps the atmosphere cooler. A rise in greenhouse gases, such as from volcanic activity, can warm the globe and produce an interglacial. Suggested causes of ice ages include the positions of the continents, variations in Earth's orbit, changes in solar output, and volcanism.

  Those natural shifts unfold far more slowly than the present rate of change, which comes from human greenhouse gas emissions. According to the EU's Copernicus Climate Change Service, average global air temperature passed 1.5C of warming over the period from February 2023 to January 2024. Scientists have named Earth's Energy Imbalance a fundamental metric of the status of global change.

• 7. Life On The Move

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  A 3 degree Celsius change in mean annual temperature, climate scientist Lesley Ann Hughes has written, corresponds to a shift in isotherms of roughly 300 to 400 km in latitude in the temperate zone, or 500 m in elevation. The consequence follows directly. Species are expected to move upward in elevation or toward the poles in latitude as climate zones shift. Recent warming, discussed in terms of global warming, redistributes biota in just this way.

  The ocean does its own redistributing. The thermohaline circulation warms the northern Atlantic Ocean by about 5 degrees Celsius compared with other ocean basins. Other currents move heat between land and water on a more regional scale. Vegetation matters too. The density and type of plant cover affects solar heat absorption, water retention, and rainfall across a region.

  Some climate determinants are nearly constant over historical time. Latitude, altitude, the proportion of land to water, and proximity to oceans and mountains change only over millions of years, through processes such as plate tectonics. More dynamic forces sit on top of them. Earth's climate oscillations correlate with astronomical factors, including solar variation, cosmic ray flux, and Milankovic cycles.

• 8. Modelling The Whole System

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  Physics equations are the heart of a climate model. These quantitative tools simulate the transfer of radiative energy between the atmosphere, oceans, land surface, and ice. Every model balances, or nearly balances, incoming short wave radiation against outgoing long wave infrared radiation. Any imbalance changes the average temperature of the Earth.

  Resolution sets the limits. Models range from coarser than 100 km down to 1 km. High resolution in a global model demands heavy computation, so only a few global datasets exist. Global models can be downscaled, dynamically or statistically, to regional models that examine local impacts. ICON is one example, and CHELSA, short for Climatologies at high resolution for the earth's land surface areas, is another.

  Complexity spans a wide range. Simple radiant heat transfer models treat the Earth as a single point with averaged outgoing energy. The most elaborate are coupled atmosphere-ocean-sea ice global models that solve the full equations for mass and energy transfer. In recent years these models have most often been put to inferring the consequences of rising greenhouse gases, chiefly carbon dioxide. They project an upward trend in global mean surface temperature, with the most rapid rise at the higher latitudes of the Northern Hemisphere.