Climate Change Unit 3
Aims Is global warming a recent short term phenomenon or should it be seen as part of long term climate change? What evidence is there of long-, medium-, and short- term climate change? What are the human and natural causes of climate change? What is the evidence for and against the view that current climate change is largely as a result of human activity?
Climate is constantly changing Climate - the average condition of precipitation, temperature, pressure and wind over a 30 year period. Climate change - is any significant long term trend (of at least several decades) or shift in the average value for any climatic element (eg rainfall, drought, storminess)
Climate time scales Climate change can be assessed across long, medium and short timescales. These next slides show the evidence of climate change. As you consider the various time scales decide whether the causes are natural or anthropogenic (man-made)
Long term Climate Change (Geological timescale)
Long-term climate change Taking temperature as an indicator of climate, there have been huge changes over Earth s history Temperatures have been much higher at some points in the past than they are today Looking at more detailed data for the last 400,000 years (on the previous slide) you can see significant fluctuations Climate shifted between cold glacial periods that lasted around 100,000 years and warmer interglacial periods that lasted around 10,000 years. We are in an interglacial period now.
Medium/Long-term Climate Change
The last glacial period ended around 18,000 years ago. The warming of the climate after this was fast at times, but it wasn t constant about 13,000 years ago the climate switched back to cooler conditions for about 1000 years Around 5,000 years ago, temperatures were 1-2 C higher than today.
Medium-term Climate Change
Short-term Climate Change (recent) Global warming is the term used for the rapidly increasing global average temperature on earth over the last century there has been a sharp rise in temperature when you look at the last 1,000 years (see graph on next slide)
Short-term Climate Change
Short-term Climate Change The overall pattern over the last century shows a general increase in temperature, but the pattern has not been constant. Global temperature rose steadily from the early 20 th century until the 1940s (although they fluctuated annually), then dropped back down. Scientists thought there would be another glacial period, but temperatures have risen rapidly again since the 1970s (this is global warming)
Global warming You need to think about global warming changes in the context of all the other climatic changes that have occurred It is important to remember that climate change is not fully understood but the changes in temperature (and the speed at which they are occurring) over the last century are worrying because they are thought to be unprecedented
Evidence for Long-term Climate Change 1. Ice core analysis 2. Ocean core analysis 3. Pollen analysis Read P76-78 Philip Allan
Evidence for Long-term Climate Change 1. Ice Cores for Carbon and Oxygen Scientists drill deep into ice sheets to extract cores of ice in Greenland and Antarctica. Ice sheets are made up of layers of ice one layer is formed every snowfall and compressed by later falls. So the ice at the bottom of the core is really old They can analyse the gases trapped when the ice formed to tell what the temperature was each year. Then they can figure out how temperature has changed over time. A core has been extracted from the East Antarctic ice sheet to a depth of 3200 m and has been used to show temperature for the last 800,000 years.
a) Carbon analysis Technique Air bubbles trapped in the ice contain atmospheric CO2. Low concentration of carbon dioxide occur in cold glacial periods. High concentrations are found in interglacial periods. So this allows a time series of CO2 levels in the atmosphere to be reconstructed. Findings CO2 levels have varied between 180ppm and 280 ppm over the last 800,000 years. It is clear that atmospheric CO2 levels now are greater than at any time in the last half million years. In 2007 they were 383 ppm.
Carbon dioxide
Reliability? The number of CO2 sequences that correlate well with each other suggests this is quite reliable
b) Oxygen analysis Techniques The ice itself preserves a record of oxygen isotopes. The 18 O/ 16 O ratio provides a record of ancient water temperature. Water 10 to 15 C cooler than present represents glaciation. As colder temperatures spread toward the equator, water vapor rich in 18 O preferentially rains out at lower latitudes. Low 18 O in ice core water ice = cold glacial period High 18 O in ice core water ice = warm interglacial period
Findings The oxygen and carbon dioxide isotope records correlate well. When carbon dioxide levels were low so was the sea level. Around 20,000 years ago as the ice sheets reached their maximum extent in the glacial climate, sea level was 130m lower than today.
Reliability? As with carbon dioxide sequences the number of oxygen isotope records that broadly agree on sea level changes is high and the correlation between with carbon dioxide levels is good.
Evidence for Long-term Climate Change Pollen Analysis Pollen from plants is often preserved in sediment (taken as cores from peat bogs and lake beds). The preserved pollen can be identified and dated to show when it was released and the ecology of the past. Scientists know the conditions that plant lives in now. When they find preserved pollen from similar plants, it indicates that conditions were similar when that pollen was produced. Different plant species have characteristic pollen shapes that can be identified.
Findings Pollen sequences show that the ecosystems have changed in the past in response to climate change. In the UK tundra was present in the glacial periods and forest gradually colonised areas in the interglacial conditions.
Reliability? Accurate pollen reconstructions rely on good preservation of the pollen. Long pollen sequences are rare. Vegetation change may lag behind the climate change.
Evidence for Long-term Climate Change Sea Level Change Sea level is affected by things like the volume of water stored as ice Past sea levels are shown by raised beaches (formed when sea levels were higher). Raised beaches can be dated. They can indicate that less water was stored as ice (i.e. It was warmer)
Evidence for Medium-term Climate Change Historical Records Historical records can indirectly indicate different conditions in the past They are called Proxy records. They include artistic and photographic sources, time series data (such as grape harvest dates), and written descriptions, diaries and records (such as the Greenland sagas).
Findings They seem to clearly indicate two different climatic periods in the past: The Medieval Warm Period (sometimes called the Medieval Climate Optimum) dating from around 800-1300AD. The end of this period coincides with the Black Death. There was an unusually warm North Atlantic climate. The Little Ice Age from about 1400-1850, but with colder spells around 1770 and 1850.
The Thames Frost fair
Burgundy grape harvest dates
Reliability? These sources did not set out to record the climate They must be used with care They are usually local Long time series data are rare It is difficult to use them to generalise Often extremes are recorded rather than the norm Much of the data has been lost Grape harvest could be influenced by nonclimatic factors like disease and conflict
Evidence for Medium-term Climate Change Tree Rings A new tree ring is formed each year as a tree grows. Trees are sensitive to changes in temperature, rainfall and sunlight. If conditions that year were good, the tree ring produced will be thick. Narrow rings are produced in years of climatic stress. Scientists can take cores and count the rings to find the age of a tree. They then look at the thickness of each ring to see what the climate was like each year. Tree rings can reliably show and date climate change up to 10,000 years ago. Living trees, such as the Bristlecone pines in W USA are as old as 4500 years.
Reliability? The accuracy of the tree ring records is good but they are very localised. It is difficult to determine the relative importance of temperature, precipitation, sunlight and wind.
Evidence for Medium-term Climate Change Retreating Glaciers Valley glaciers in places like the Alps grow and shrink in response to the climate. Changes in a glacier can be tracked by examining secondary records such as old paintings, photographs and maps. Scientists can also tell how big a glacier was and how far it extended by looking at the position of rocks (snout and morraine) deposited by it. These rocks can be dated to show when they were deposited. The distance of the rocks from the current glacier indicates climate change E.g. If the front of the glacier is now miles away from the rocks it indicates that temperatures have increased over that period of time.
Pasterze glacier, Austrian Alps 1875 and 2004
Findings Evidence suggests that the majority of glaciers reached their recent maximum extent in 1850. This correlates well with the Little Ice Age and colder temperatures in the 17 th and 18 th centuries. Most glaciers have retreated since 1850.
Reliability? Reliable results date back to about 1880. Before this time the record is patchy and relies on more proxy historical records.
Evidence for Short-term Climate Change Weather Records Details of weather conditions have been constantly collected since 1861. These can be used to show detailed climate changes over the short time period they have been collected.
IPCC 2007 Global mean temperatures are rising faster with time Warmest 12 years: 1998,2005,2003,2002,2004,2006, 2001,1997,1995,1999,1990,2000 Period Rate 50 0.128 0.026 100 0.074 0.018 Years /decade
Direct Observations of Recent Climate Change Global mean temperature Global average sea level Northern hemisphere Snow cover IPCC 2007
Findings They show that the near surface air temperature rose by 0.74 degrees C between 1900 and 2000 The warming trend has been almost constant since 1960. 11 of the world s hottest 12 years happened between 1995-2006 Oceans have warmed to depth of 300om Warmer oceans cause problems for temperature sensitive species like corals There are fears about ocean acidificiation ph of the ocean has decreased from 8.25-8.14 since 1750 (likely due to increased CO2 dissolved)
Instrumental records show that global sea level has increased. Between 1961 and 2003 it rose by 1.8mm per year. By 1993-2003 it was rising 3.1mm per year. Most is thought to be associated with thermal expansion. Thermal expansion is the increased volume of the oceans due to their higher temperatures. It accounted for about 60% of sea level rise in the late 20 th century. The ice melt from glaciers and ice caps so far is thought to have a lesser effect on sea levels.
Evidence for Short-term Climate Change Ice Response Ice is found in glaciers, as ice caps on mountain ranges, as ice sheets in Greenland and Antarctica and as sea ice in the Winter in high Northern latitudes. In a warming world, ice might be expected to melt.
Recent changes in global ice cover