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Lesson1 climate and change GCSE Edexcel B GeographyPresentation Transcript
2.1a How and why has climate changed in the past? To recognise that Earth’s climate has changed over time To be able to explain the natural causes of Climate Change. Climate Change- the way the Earth has constantly evolved and changed temperature throughout history Specification Statement- Climate has changed in the past through natural causes, on timescales ranging from millions to hundreds of years
The sceptic argument... Earth's climate has changed long before we were pouring CO2 into the atmosphere. Europe was far warmer in the Middle Ages. During the 17th and 18th century, it was much colder, prompting the ‘The Little Ice Age’, when the Thames was frozen over months at a time. Further back, there were times when the Earth was several degrees hotter than current temperatures. Warming of several degrees often took only centuries or decades.
The 16th century is the first period for which we have a reliable history of climate and weather.
Private diaries, ships' logs, accounts of military campaigns, and similar sources give descriptions of wind direction, wind speed, cloud formations, and other weather indicators.
Precisely dated annals, chronicles, audited accounts, agricultural records, tax ledgers, and other archival material provide indirect information, particularly on extreme weather events, such as droughts, floods, or unusual cold.
Additional evidence is available from glacial moraines, lake and ocean sediments, pollen strata, deposits of insects, tree rings, coral structure, radiometric analysis of ice cores, archaeological sites, and many other sources.
All this information can be combined to reconstruct past climates
It's a well established fact that climate changes naturally and sometimes dramatically. The pertinent question isn't "has climate changed in the past?" (of course it has) but "what is causing global warming now?" To begin to answer that, it's helpful to look at the major causes of natural climate change in the past.
http://www.youtube.com/watch?v=S9ob9WdbXx0&feature=related a look at the debate between natural and human warming- shows graphical interpretations of differences
Solar variations have been the major driver of climate change over the past 10,000 years. When sunspot activity was low during the Maunder Minimum in the 1600's or the Dalton Minimum in the 1800's, the earth went through 'Little Ice Ages'. Similarly, solar activity was higher during the Medieval Warm Period.
However, the correlation between solar activity and global temperatures ended around 1975.
At that point, temperatures started rising while solar activity stayed level. This led a Finnish and German scientists to conclude "during these last 30 years the solar total irradiance, solar UV irradiance and cosmic ray flux has not shown any significant secular trend, so that at least this most recent warming episode must have another source
Caption for Image Four: The Sun shows signs of variability, such as its eleven-year sunspot cycle. In that time, it goes from a minimum (seen here in 1996) to a maximum (2000) period of activity that affects us everyday. When particularly active, solar storms can spew tons of radiation to Earth in the form of Coronal Mass Ejections (CMEs) that can affect power grids, spacecraft, and communication systems. SUPER: NASA / ESA
The Earth does not have some natural temperature to which it always returns. If it cools, then it must be receiving less heat from the Sun or radiating more into space, or both. If it warms, it must be receiving more heat or retaining more heat.
Earth's climate undergoes 120,000 year cycles of ice ages broken by short warm periods called interglacials.
The cycle is driven by Milankovitch cycles. Long term changes in the Earth's orbit trigger an initial warming which warms the oceans and melts ice sheets - this releases CO2.
The extra CO2 in the atmosphere causes further warming leading to interglacials ending the ice ages.
For the past 12,000 years, we've been in an interglacial.
The current trend of the Milankovitch cycle is a gradual cooling down towards an ice age.
Volcanic eruptions spew sulphate aerosols into the atmosphere which has a cooling effect on global temperatures.
These aerosols reflect incoming sunlight, causing a 'global dimming' effect. Usually, the cooling effect lasts several years until the aerosols are washed out of the atmosphere.
In the case of large eruptions or a succession of eruptions such as in the early 1800's, the cooling effect can last several decades. Strong volcanic activity exacerbated the Little Ice Age in the 1800's.
Observational and modelling studies (e.g. Kelly & Sear, 1984; Sear et al ., 1987) of the likely effect of recent volcanic eruptions suggest that an individual eruption may cause a global cooling of up to 0.3°C, with the effects lasting 1 to 2 years.
Such a cooling event has been observed in the global temperature record in the aftermath of the eruption of Mount Pinatubo in June 1991. The climate forcing associated with individual eruptions is, however, relatively short-lived compared to the time needed to influence the heat storage of the oceans (Henderson-Sellers & Robinson, 1986).
The temperature anomaly due to a single volcanic event is thus unlikely to persist or lead, through feedback effects, to significant long-term climatic changes.
One of the earliest and most comprehensive series is the Dust Veil Index (DVI) of Lamb (1970), which includes eruptions from 1500 to 1900.
When combined with series of acidity measurements in ice cores (due to the presence of sulphuric acid aerosols), they can provide valuable indicators of past eruptions.
Using these indicators, a statistical association between volcanic activity and global temperatures during the past millennia has been found (Hammer et al ., 1980).
Episodes of relatively high volcanic activity (1250 to 1500 and 1550 to 1700) occur within the period known as the Little Ice Age, whilst the Medieval Warm Period (1100 to 1250) can be linked with a period of lower activity.
Oceans store an immense amount of heat energy, and consequently play a crucial role in the regulation of the global climate system.
At present, north maritime Europe (us) warmed by poleward heat from the Gulf Stream. When the warm water meets cold polar air in the North Atlantic, heat is released to the atmosphere and the water cools and sinks. This is assisted by the increases in salinity (and therefore density) that occur when sea ice forms in the Arctic regions
The bottom water so formed, called the North Atlantic Deep Water (NADW), flows southward through the western Atlantic, round Southern Africa and Australia, and then northwards into the Pacific Ocean. The North Atlantic is warmer than the North Pacific.
It has been suggested that during a glacial period, the formation of the NADW is much reduced or even totally shut down. At these times, the Arctic ice sheets extends much further south into the North Atlantic, pushing the position of the polar front southwards. Cooler sea surface temperatures reduce evaporation and therefore salinity, further precluding the initiation of a thermohaline circulation.
The absence of the Gulf Stream could result in northern Europe being 6 to 8°C colder than during interglacial times (i.e. at present) (Broecker, 1987). The causes of the changes between the glacial and interglacial patterns of thermohaline circulation would then be seen as internal climate forcing mechanisms.