ESC 211 Unit 3
Changes in atmospheric composition: gases
Greenhouse gas concentrations have fluctuated --Carbon dioxide and methane levels closely follow changes in glacial periods. This is related to changes in plants and animals, and the output/uptake by them of CO2. --The next slide shows how CO2 and temperature have varied over very long time periods. The introduction of agriculture by humans some 8,000 years ago is known to have altered local and small regional climate patterns. There's much debate on how (if at all) this affected global climate.
Coral Reefs
Grow in subtropical and tropical oceans; many records come from tropical Pacific Provide a complimentary record to ice cores and tree rings from higher latitudes and altitudes Improves spatial distribution of records to gain a more complete picture of climate in the recent past
ice cores and ocean sediments
Ice cores and ocean sediments indicate that compared to today, during the last ice age (>18,000 years ago): Temps 6o C colder CO2 levels 30% lower CH4 levels 50% lower H2O levels were lower These cores also indicate that: 130,000 years ago it was a bit warmer than today 50% change in CO2 associated with 8oC change in temperature 6-8oC decrease in temperature produced incredibly different climate: Ice Age
Principles of fossil pollen analysis (palynology)
Plants produce pollen (or spores) in large numbers Preserved pollen in soil serves as proxy for plant species Total assemblage of pollen indicates vegetation assemblage Vegetation assemblage indicates climate
Palynology
Pollen and spores from plants have accumulated over time in sediment on land These provide a record of the past vegetation of an area Changes in the vegetation of an area may be due to changes of climate Interpreting past vegetation through pollen analysis may provide information about past climatic conditions Sediment cores that have pollen mixed in with the soil are drilled by researchers. Usually these are taken from the bottom of lakes
Medieval Climate Anomaly
The Medieval Climate Anomaly (Medieval Warm Period) shown on the previous slide was identified in 1965 by British climatologist Hubert Lamb. --He published this temperature reconstruction in 1965 based on historical documentation --One of the first temperature reconstructions Good weather and abundant harvests were common in the Middle Ages in Europe Recent evidence suggests it may have been limited to specific regions in the northern hemisphere. Cooling is now being identified for the same time in southern hemisphere and the Pacific Ocean region. A good explanation for this climate period has not yet been found. There are many untested hypotheses.
explain Ocean Sediments: Oxygen Isotopes from marine life
These are extracted from deep sediments on the ocean floor Drills on ships are used to core into the ocean floor Oxygen isotopes from dead microscopic marine life is used as a proxy for climate. These are the skeletons that are compacted into layers, called "carbonate microfossils"
give details about ice cores
They measure the amount of different gases, dust and isotopes in the air bubbles trapped in the ice Ice cores provide information gases, oxygen isotopes, and dust that can be measured. Each year when it snows, the previous year's snow is compacted into ice. This forms layers of ice that can be distinguished. Small bubbles form in the ice, trapping gasses and dust. Gasses, such as carbon dioxide and methane can be measured. Oxygen isotopes can also be measured. All of this can be dated and used to get a very accurate estimate of past temperatures and precipitation. Dust trapped in the ice can also be measured and used to date volcanic eruptions
how do scientist get ice cores?
drill long tubes deep into the ice extract these "cores" bring them back to their labs on board ships (in large freezers)
What Caused Past Climate Changes?
earth's surface changes: Continental changes (extremely long term- millions of years) Astronomical/orbital changes: Changes in earth's orbit/axis (very long to long term - hundreds of thousands of years to thousands) Solar changes: Changes in sun output, sunspots (centuries) atmospheric composition changes: CO2 changes related to orbital changes//Volcanic activity (years)//Pre-industrial human activity (centuries, decades)
Atmospheric Composition Changes: Volcanic Activity
After a major volcanic eruption, temperatures tend to cool due to more particles reflecting away sunlight. This cooling, however, usually lasts only a few months to a year. Some major eruptions have caused global cooling for longer. The longest volcanic related cooling period in recorded history lasted 2-3 years when Mt. Tambora erupted in 1815.
Astronomical Changes
Anything that alters the amount or distribution of solar energy intercepted by the Earth (solar variations, orbital variations). This helps to explain the past ice ages and long term changes over hundreds of thousands of years. Milankovich Theory of Ice Ages ---Named for a Russian scientist who first proposed the theory. ---Relates orbital variations to past climate changes.
Earth Surface Changes and Climate Change
Anything that alters the flow of energy at the Earth's surface or changes its distribution. Continental drift (plate tectonics) is a major type of earth surface change. ---Helps to explain extremely long term climate change. ----Millions of years. Not much is known about climate that far back
Atmospheric Composition Changes
Anything that changes the radiative properties of the atmosphere. These include volcanic particles/dust (called "aerosols") and gases, and of course, greenhouse gases such as carbon dioxide. Helps to explain climate changes on the short term to longer term scales
Milankovitch Theory of Ice Ages
Attempts to explain ice ages by variations in orbital parameters Three cycles: --Eccentricity (100,000 yrs) -- Tilt (41,000 yrs) -- Precession (23,000 yrs) Changes the latitudinal and seasonal distributions of solar radiation
Dendroclimatic reconstruction
Collect (sample) data from a set of trees (within a tree population) which have been selected on the basis that climate (e.g. temperature, moisture) should be a limiting factor Build up a network of site chronologies for a region Use these relationships to reconstruct climatic information from the earlier period covered by the tree-ring data
How do we know how warm it was millions of years ago? Most common methods are...
Ice cores: bubbles in glacier ice contain samples of the atmosphere that existed when the ice formed. (ancient pCO2) Stable isotopes: oxygen isotopes in carbonate (microfossils of dead marine life) sediments from the deep ocean preserve a record of temperature. The records indicate that glaciations advanced and retreated and that they did so frequently and in regular cycles
Why model climates?
If we want to understand more about the past, and then make predictions about possible future climate, we need to create climate models. These use past data obtained from instruments and proxies and current instrumental data. Climate models are very different from weather forecasting models!
why is it important to learn about past greenhouse gas levels
It is important to learn about past greenhouse gas levels and climates in order to better predict the future. What the climate was like 100, 500, 1,000 years ago, even 500,000 years ago, can give us an idea of what the future holds. This information is also used to help us model the future and place current and future climates into context
Sunspots
Overall, there does not appear to be much, if any, correlation between sunspot activity and global temperature. However, the Maunder Minimum, a period of exceptionally low activity, does correlate with the middle of the Little Ice Age
what is paleoclimatology
Paleoclimatology is the study of ancient climates, prior to the widespread availability of instrumental records.
Solar variability
Sunspot cycles affect output of radiation This changes how much energy reaches earth. This suggests sunspots would have an effect on temperatures, and thus climate
Eccentricity (e): shape of earth's orbit
The eccentricity (shape) of the earth's orbit varies from a circle (e=0) to an ellipse (e=0.06) with a period of about 100,000 to 400,000 years. This is due to the gravitational pull of the other planets.
Obliquity: tilt of earth's axis
The tilt of the Earth's axis varies between about 22 and 25 degrees over about 41,000 years. This tilt is called "obliquity". Currently the tilt is at 23 ½
Tree rings in the southern hemisphere show...
These show that recent warmer temperatures "unprecedented" in last 1,000 years
Precession: which direction the earth's axis tilts
the direction of the earth's spin axis also precesses. This means that the earth's spin axis traces out a circle. Like obiquity, precession occurs on very long time scales. It takes about 20,000 years for the spin axis to trace out a circle once.
explain proxy data
the way we get information about the past. We will focus on ice cores and deep ocean sediments
Coral Reefs: Tropical Pacific and Indian Ocean
• Sea surface temperatures (SST) measured from ocean corals show a general warming trend. SSTs vary on a yearly basis depending on season (winter vs. summer) • As ocean temperatures are warming, coral reefs are dying. Thus corals need to be studied now or we will loose this opportunity to learn about ocean temperature trends!
where do scientists get ice cores?
- Alpine (mountain) glaciers - Greenland - Antarctica
explain the study of paleoclimatology
- Is the study of climate prior to the widespread availability of records of temperature, precipitation and other instrumental data. Useful in establishing the range of natural climatic variability in a period prior to global-scale human influence. We are particularly interested in the last few thousand years because this is the best dated and most sampled part of the ancient climatic record. Examine climate change going back hundreds and thousands of years using paleoclimatic records derived from environmental and climatic proxies
Dendroclimatology
- Study of tree rings. - Offers a high resolution (annual) form of paleoclimate reconstruction for most of the Holocene (8,000 to 12,000 years). - Chemical or isotopic variables such as Oxygen and Hydrogen Isotopes (e.g. Epstein et al., 1976)
what are some types of proxies
- Tree rings & corals (1,000+ years before present BP) - Trapped pollen (10,000+ years BP) - Glacial ice cores (100,000+ years BP) - Ocean sediment cores (1 Million+ years BP) - Geology (1 Billion+ years BP)
paleoclimatology is an important field of study because it:
- puts the current warming into context - provides insight into climate changes we may encounter in the future - helps to understand "abrupt" climate change (i.e. changes that occur within decades) - allows us to understand how humans have influenced the climate system
What is a good proxy?
-The proxy is sensitive to changes in environmental conditions temperature, precipitation, productivity, or other). -A good proxy can be calibrated. That means: establish studies that provide calibration of the proxy in contemporary settings or across environmental gradients. -A good proxy "records or finger prints" climatic or biological information and preserves it for long periods of time microbial life, ice, minerals, organic matter
how many years does paleoclimatology study
100-200
Milankovich Theory
>>The theory is based on changes in timing when earth is closes/farthest from sun, changes in the shape of the earth's orbit, changes in tilt of the axis, and changes in direction the axis tilts towards. >>Orbital changes account for almost 95% of the periodicity (repeat pattern) of cold periods - the ice ages.
