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Global climate change
Evidence for climatic change over the last 20 000 years.
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Lots of evidence exists to suggest that climate has changed significantly over the course of Earth History.  In the past we have had hot house earth conditions with no ice on the planet (e.g. 56 million years ago after a surge in atmospheric carbon) and scientists have even hypothesized about a snow ball earth with average global temperatures as low as -50C (e.g.635million years ago was the lasted suggested event).  Climate change is defined as a significant and lasting change in the statistical distribution of weather patterns over different time periods ranging from decades to millions of years. This change may be a change in average weather conditions or the distribution of events around that average (e.g., more or fewer extreme weather events). Climate change also differs in scale, as it may be limited to a specific region or may occur across the whole Earth, and this is known as global climate change.

The Earth has gone through a series of warm and cold periods within its history and climate cannot be said to be static by any means.  Times when the temperature has dropped significantly are known as GLACIAL periods, and are marked by advances of the World's ice masses to lower latitudes and lower altitudes.

Times when temperatures are warmer for extended periods of time are known as INTERGLACIALS and these periods are marked by the retreat of ice to higher altitudes or Latitudes. 

It has been recognised that over the last 100 years, Earth's average surface temperature increased by about 0.8 C (1.4 F) and the rate of temperature increase sped up towards the end of that time frame. Scientists are more than 90% certain most of it is caused by human activities which have increased concentrations of greenhouse gases such as deforestation and burning fossil fuels. This is different from climate change (of which global warming is a part) because it has an anthropological or human origin.  This is human enhanced global warming.

Climate model projections by the Intergovernmental Panel on Climate Change (IPCC)  indicate that during the 21st century the global surface temperature is likely to rise a further 1.1 to 2.9 C (2 to 5.2 F) for their lowest emissions scenario and 2.4 to 6.4 C (4.3 to 11.5 F) for their highest.  The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations.

 

Climate variations
Over the past 2 million years it can be seen on the graph below that glacial periods have lasted longer that interglacial periods and that the temperatures of glacials have dropped lower than the rises of the interglacials.  It should also be noted that the world is nearing the end of our current warming pattern, and what happens next to global temperature is still the subject of much debate.    You should also note that the difference in temperature between the mean global air temperature and a glacial period is as little as 2C colder than the global mean, and that the difference from the mean to today's warm period is only 2-3C warmer.  It therefore takes only small changes in temperature to switch global climate from hothouse to ice house

The last Ice Age
The last ice age to occur in the Quaternary period (which began 2 million years ago) started around 30,000 years ago, peaked 18,000 years ago when ice in Britain reached as far south as Bristol and London and ended between 12,000 and 10,000 years ago.  The impact of this ice age was marked in the British landscape and evidence can be seen in the landforms that make up many of our much loved countryside areas. Since then we have seen one of the warmest periods in Earths history

Feedback mechanisms

A feedback mechanism is one in which something affected by climate change can make climate change happen more or less quickly.  For example, climate change is currently thinning and melting ice caps and ice sheets, such as in the Arctic.  Currently these ice caps reflect lots of sunlight back to space because of their high albedo, but this would be lost.  This means more absorption of insolation and so more warming.   More warming means more ice cap loss and so on.....

Another example under investigation is methane release.  A warming world has the impact of allowing more permafrost in our Northern Hemisphere high latitudes to melt.  This melting tundra allows decomposition of previously frozen dead vegetation.  This releases Methane (CH4), which is a far more potent greenhouse gas than CO2, which accelerates the warming and releases more methane.

 

The climate record

 

Evidence of climatic change over the last 20 000 years.

Actual measurements of climate cover a very narrow period of time from the nineteenth century onwards and for only a very narrow range of places.  The last 50 years give us a much more accurate picture of climate and we have a huge amount of data for this period.  However, to deduce what climate was like prior to the nineteenth century over the past 20,000 years (and in some cases longer) we have to use proxy data sources that allow us to INFER what the climate was like.  Using a range of these proxy sources (listed below) gives us a good picture of what climate was like in the past.  We can also use historical records such as diary extracts, crop yields for local registers and even paintings to recognize what climate was like in the past.  Paintings of ice fairs and markets on the River Thames during the Little Ice Age, 3 periods of colder temperatures from 1550 to 1850, are one of the proxy sources we have used to infer climate change. Archaeological evidence from cave paintings and fossil bones also gives us an indication of climate change

The evidence;

Changes to valley glaciers

The mass of glaciers is determined by their mass balance which is governed by accumulation and ablation.  It is for this reason that glaciers are a sensitive indicator of climate change.  As temperatures warm, glaciers tend to retreat unless snow precipitation increases to make up for the additional melt; the opposite is also true. We must take average change over long periods of time to use glaciers to infer changing climates as they can advance and retreat on a yearly basis naturally.

A world glacier inventory has been compiled since the 1970s, initially based mainly on aerial photographs and maps but now relying more on satellites. More than 100,000 glaciers have been tracked covering a total area of approximately 240,000 km2, and preliminary estimates indicate that the remaining ice cover is around 445,000 km2. The World Glacier Monitoring Service collects data annually on glacier retreat and glacier mass balance. From this data, glaciers worldwide have been found to be shrinking significantly, with strong glacier retreats in the 1940s, stable or growing conditions during the 1920s and 1970s, and again retreating from the mid 1980s to present.

Longer term Ice cores from Antarctica

Analysis of ice in a core drilled from the Antarctic ice sheet can be used to show CO2 concentrations and inferred temperatures.  The air trapped in bubbles in the ice reveals the CO2 variations of the atmosphere from the distant past, well before modern environmental influences. The ice core record now stretches back 650,000 years, and reveals that the Earth is normally cooler than it is now, that ice ages are common and that current CO2 and temperature levels are above what has been experienced in the past 650,000 years.  The Earth has been hotter than it is now in the past though, outside of the ice core records time frame.

 

Evidence from radiocarbon dating

Carbon has different isotopes and one of these, C-14, can be used as an accurate technique to date any preserved organic matter.  Plants take in Carbon during the carbon cycle as they grow.  Carbon 14 decays at a known rate over time, and can be compared to Carbon-12 which does not.  This means that we can accurately date sediments and plants for an age of up to 50,000 years.  This is useful for climate change because using our current knowledge of plants we can determine which plants were alive at which time in the past and what climatic conditions would have been needed for those plants to survive.

 

Glacial deposits and landscapes in current non glacial areas

The British Isles has a host of glaciated landscapes that have no ice in them at the present time.  Hanging valleys and U shaped troughs in the Lake District, erratics and Crag and Tails in Scotland all indicate Britains much colder glaciated past, more evidence of climate change.

 

Pollen grain evidence

Pollen grains are found in many sediments and can be used to infer which plants existed at a certain time and their geographical distribution.  Since plants types vary under different climate conditions, this distribution of pollen can be used to infer the climate type for that location at that time. Different plants have pollen with distinctive shapes and surface textures, and since the outer surface of pollen is composed of a very resilient material, they resist decay. Changes in the type of pollen found in different layers of sediment (which can be dated using the principle of stratigraphy or radio carbon dated) in lakes, bogs, or river deltas indicate changes in plant communities. These changes are often a sign of a changing climate.

 

Dendrochronology

Trees add a ring of new material every year.  Dendrochronology is the analysis of this tree ring growth to infer past climates.  When rings are wide and thick it indicate a fertile, well-watered growing period, whilst thin, narrow rings indicate a time of lower rainfall and less-than-ideal growing conditions. This record of climate change is obviously limited to the age of the tree.

 

Evidence of changing sea levels

Global sea level change can be determined using recent altimeter measurements and accurately determined satellite orbits. To measure sea levels prior to instrumental measurements, scientists have dated coral reefs that grow near the surface of the ocean, coastal sediments, marine terraces, and near shore archaeological remains. These remains can also be radiocarbon dated.  Relic coastlines and drowned coastlines also indicate changing sea levels Rias (drowned valleys) in Cornwall, Fjords (drowned glacial valleys) in Western Norway and raised beaches and caves (e.g. Kings Cave on the Isle of Arran in Scotland) all point to changing sea levels in recent times.  Changing sea levels infer changing climates.

Arctic sea ice loss

The decline in Arctic sea ice, both in extent and thickness, over the last several decades is further evidence for rapid climate change. Sea ice is frozen seawater that floats on the ocean surface. It covers millions of square miles in the Polar Regions, varying with the seasons. In the Arctic, some sea ice remains year after year, whereas almost all Southern Ocean or Antarctic sea ice melts away and reforms annually. Satellite observations show that Arctic sea ice is now declining at a rate of 11.5 percent per decade, relative to the 1979 to 2000 average. Al Gore revealed that the US navy has recorded thinning sea ice in the Arctic from its submarines over the past 40 years (they need a certain thickness of ice before they can surface through the ice).

 

The Oxygen isotope record from sea bed sediments

Oxygen has isotopes, and 2 of particular interest to us are 18O/16O.   18O is two neutrons heavier than 16O and causes the water molecule in which it occurs to be heavier by that amount.  The 18O/16O ratio provides a record of ancient water temperature. This is because 16O is lighter and more easily evaporated from oceans, especially during cold phases, this is then deposited in higher concentrations in glacial ice stored on land and in ice sheets.  However, this means that 18O is left behind in high concentrations in the oceans.  In warmer periods, 16O is released back into oceans, and ice records reflect these changes.  In addition, sea floor sediments can be analysed for their 18O/16O ratio.  If the 18O is higher, it indicates a cooler period, if the 16O is raised it indicates warmer temperatures.