because THE ATMOSPHERIC system is dynamic, variations in temperature and precipitation regimes have occurred throughout the Earth's history. Glaciations and deglaciations, for example, show how changeable climate can be. Detecting climate changes shows that climate differs significantly from some previous episode. Detection differs from attribution, which denotes the causes of those changes. While daily fluctuations in weather happen as a consequence of a chaotic atmosphere; climate fluctuations, based on smoothed aggregations of weather events, represent significant shifts in the longer-term averages. To discern these shifts, numerous techniques and methods are employed.
Climate changes, which occur at a variety of temporal scales, are assessed using both directly-observed and proxy data. Directly-observed information refers to meteorological and climate-related variables that have been directly evaluated, such as temperature measured by a thermometer, or cloud tops captured by satellite imagery. Directly-observed data have been used to chart recent increases in air temperature and carbon dioxide (CO2), concentration, as evidenced by Charles Keeling's famous graph of climbing CO2 concentrations at the Mauna Loa Observatory.
Proxy data are non-meteorological data, from which climate information is inferred; they serve as a satnd-in for such climate evidence when direct observations are not always available. Examples of proxy data include ice cores, lake sediments, and historical harvest records. Among the information gleaned from proxies are temperature, moisture, sea level, and chemical composition of the atmosphere. These facilitate the reconstruction of past climates.
Although directly-observed data may be preferable, the instrumental record is often too short for all but the most recent evaluations of climate changes. For example, the longest continuous temperature record shows monthly mean temperature only since 1659 (for central England); daily records for the same location extend only to 1772. Many other records are less than 100 years old. Temperature records also require filtering because of biases that may be present in the readings. The effects of urbanization, instrument upgrades, and location changes can result in false changes. Newer technologies, such as satellites, yield even shorter records. However, much climate change research assesses variability at temporal scales up to millions of years, rendering proxy evidence a necessity.
Proxy or substitute data extend climate change knowledge beyond the instrumental record, although there is a decreasing resolution and confidence with increasing time. Because climate variability has been identified at various temporal scales, the resolutions of the various proxies differ. Removing the climate signal and reconstructing past climates from them requires the proxy to be related to the climate. Therefore, some link between the two needs to be established; for example, by experimentation or construction of models.
Written historical records, such as harvest yields or military records, as well as diaries and phenological records, have been used to derive information. Temporal resolutions vary, from daily to annual. Farmers' diaries often discuss the weather. Grain harvests, for example, have been linked to precipitation. Dates of cherry blossomings in Japan can provide temperature trends. Historical records may be biased toward significant weather events (such as blizzards), so care must be taken when interpreting them.
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