Long Term Climate Change (geological)
This refers to the cycle of ice ages, which began around 450,000 years ago. Ice ages and the warmer periods between them (interglacials) occured at almost regular intervals through the geological period called the Quarternary period.
Evidence: Ice Cores
Best evidence: ice cores from the Greenland and Antarctic ice sheets. Air can be sampled from bubbles trapped in the ice that was laid down over thousands of years and CO2 levels in the former atmosphere can be reconstructed. During the ice ages the CO2 levels are very low (180ppm ) but in the warm interglacials they rise to 280ppm. Present day (also known as the Holocene interglacial period) levels are around 384ppm, which is higher than at any time over half a million years. The cores can be removed from ice sheets revealing layers going down through the ice. Each layer records a season of snowfall, buried and compressed by later falls.
The 3,200m East Antarctic core records the climate of the last 800,000 years. The trapped air bubbles contain atmospheric carbon dioxide and the ice preserves a record of oxygen isotopes. The ratio of oxygen 16 to oxygen 18 is a good indicator of past sea levels. During glacial times, oxygen 16 was evaporated mor easily from the oceans, which became enriched with oxygen 18. Ice from glacial periods is enriched with oxygen 16. The oxygen isotope records and carbon dioxide records correlate well. When CO2 levels were low, so was the sea level. ~20,000 years ago, as ice sheets reached their maximum extent in the glacial climate, sea level was 130m lower than today. Landforms such as raised beaches also record past sea levels.
The number of carbon dioxide sequences and oxygen isotope sequences correlate well with each other, which suggests that these are reliable records.
Evidence: Pollen Sequences
Further evidence is provided by pollen extracted from sediment cores from peat bogs and lake beds, which records the ecology of the past. Pollen grains are preserved in waterlogged peat/sediments. Different-shaped pollen grains signify different trees/plants, some of which live in Arctic conditions (e.g. birch) and others in warmer conditions. With pollen, the evidence is incomplete and long sequences are rare. Accurate pollen reconstructions rely on good preservation of pollen. Moreover, vegetation change may 'lag' behind climate change.
Pollen sequences show that ecosystems have changed in the past in response to climate change. In the UK, tundra ecosystems were present in past glacial periods, whereas forest gradually colonised ares as interglacial conditions developed.
The world's oceans play a role in climate regulation. They act as carbon dioxide sinks, removing CO2 from the atmosphere. Evaporation from oceans is vital for cloud formation and maintaining precipitation levels. Ocean currents transfer warm water from the equator towards the poles, which 'evens out' temperature extremes between the poles and the equator. Surface currents are driven by wings, as part of the global circulation. Deep ocean currents form the thermohaline circulation*, which flows between the oceans.
* Thermohaline circulation: is a global system of surface and depp-water ocean currents, driven by differences in temperature (thermo-) and salinity (-haline) between areas of the oceans. An alternative name is the ocean conveyor.
Medium Term Climate Change (historical)
Proxy records are used to reconstruct climate before the start of instrumental records, as between AD1000 and AD1850, there are no direct climate records. These include paintings, poems, record books, diaries and journals which record the weather at the time. Proxy records need to be used with care, as their purpose may not have been to record the weather. It has been suggested that there was a 'Little Ice Age' from ~AD1500-1800 during which the Thames froze over regularly, following the Medieaval Warm Period from ~AD1000-1500.
Evidence: Historical Records
This technique works by analysing paintings, photographs and sequences such as the grape harvest data from Burgungy, France. Written accounts, such as the Greenland saga, are also useful. These records may indicate past climates. Evidence points to both a colder period (the Little Ice Age) and a warmer period (the Medieval Warm Period) in the historical past. However, these sources did not set out to record climate, and must be used with care. They are usually local, and it is difficult to use them to generalise.
Evidence: Tree Rings
Many trees are sensitive to annual changes in temperature, sunlight and precipitation. The thickness of annual growth rings records climatic conditions. Wide rings reflect good growing conditions, narrow rings periods of climate stress. Long-term sequences of tree rings can be obtained from living trees, e.g. the Bristlecone Pines of the western USA (some specimens are up to 4,500 years old). The accuracy of the tree ring record is good, but it is localised and it is also difficult to determine the relative importance of temperature, precipiation, sunlight and wind on the growth of the tree.
Evidence: Retreating Glaciers
Valley glaciers, e.g. in the Alps, grow and shrink in response to changes in the climate. These changes can be tracked by examining old paintings, photographs and maps, and taking direct mesurements of snout or moraine positions. Evidence suggests that the majority of glaciers reached their most recent maximum extent in 1850. This correlates well with the Little Ice Age, and the colder temperatures in the 17th and 18th centuries. Most glaciers have retreated since 1850. Reliable measurements extend to ~1880, before this records and patchy and rely more on proxy historical records.
Short Term Climate Change (recent)
This is more straightforward to analyse and quality instrumental records have existed for the last 100 years, as have detailed records of the response of ice sheets and glaciers. The ice sheets of Greenland, the Canadian Arctic and Antarctica are all aerially surveyed and monitored continuously. Air and ocean temperatures are recorded and ecosystem changes are monitored because their disappearance would have an impact on world systems.
Evidence: Instrumental Records
Instrumental records from weather stations show that near-surface air temperatures rose by 0.74C between 1900 and 2000. The warming trend has been almost constant since 1960, and 11 of the world's hottest 12 years since 1850 occured in the decade 1995-2006.
The oceans have warmed to depths of 3000m. Warmer oceans cause problems for temperature-sensitive organisms such as coral. There is a growing concern about ocean acidification. Measurements suggest that the pH of the oceans has decreased from 8.25 to 8.14 since 1750. The most likely cause is increased levels of dissolved carbon dioxide.
The instrumental record also demonstates that global sea level has risen. Between 1961 and 2003 it rose by 1.8mm per year, rising to 3.1mm per year between 1993 and 2003. Most of this rise is attributed to thermal expansion, with water from melting glaciers and ice caps, so far, having a lesser impact on sea level.
Evidence: Ice Response
Ice is a key indicator for climate scientists. In a warming world, ice might be expected to melt.
Melting of the Greenland ice sheet has increased by 16% since 1979. Many glaciers, which drain the ice sheet, have doubled their speed of flow since the late 1990s.
Evidence of melting ice in Antarctica is less clear than for the Arctic. So far, some small ice shelves have collapsed in the Antarctic Peninsula. In 2005, the British Antarctic Survey found that 85% of the Antarctic Peninsula glaciers had retreated by an average of 600m since 1953.
Floating Arctic sea ice has declined by 8.5% per decade according to NASA satellite data. If the trends since 1979 continue, the Arctic Oceanc could be ice free in summer by 2060.
In 2007 the World Glacier Monitoring Service reported that 30 valley glaciers were melting 3x faster than in the 1990s in 9 mountain ranges. Many valley glaciers are also 50% smaller than in 1850. Since 2000, the Alpine glaciers have thinned by an average of 1m per year. Similar rates were also observed in the Andes, the Patagonia, the Cascades range and the Himalayas.