L2 Flashcards
Define Climate sensitivity
Change in climate (temperature) for a given forcing
What is natural variability
Natural changes in climate such as El nino which change the climate naturally
What is the Ruddiman hypothesis
The theory that anthropogenic climate change did not start in the 18th century but 8000 years ago beginning with intensive farming, because this is the time greenhouse gases stopped following their natural patterns
when was the first global themometer measurments taken
1850
what is a climate proxy
- A recorded quantity from which some aspect of the climate at a particular time in history can be inferred.
what is the advantage of a climate proxy
very long record ( potentially millions of years)
what are the disadvangages of climate proxies
- responds to several climate variables in a complex way
- limited resolution ( space and time)
- must be preserved (record may be biased or overprinted)
what do wide and narrow rings in tree rings mean
wide ring = warm days and sufficient water
narrow ring = stressful conditions eg water shortages
what properties of a tree ring can be measured
width
density of early wood
density and width of wood grown late in season
isotope chemistry of wood –> teaches us composition of rainfall and the rate of photosynthesis
what is the oldest trees ever measured (dead)
50,000 years ago, chilean trees covered in mud that was then uncovered in an earthquake in 1960AD
What are the oldest living trees cored
9000 year old californian bristlecone pines
what is an isotope
a chemically identical form of an element with different atomic mass
how do we use oxygen isotopes
16 O is preferabbly evaporated meaning more 18 is left behind. 18 is preferentially rained out.
hotter
when temps are colder there is less 18O in ice sheets and more in the oceans
how are calcium carbonate shells and oxygen isotopes linked
the amount of 18o in shells can be measured to link the temperature (similar to a ice core)
what are the differences between antarctic and greenland ice core
antarctic ice cores have a large sense of time, greenland ice cores have a much higher resolution
what are written records like
go back 1000 years
quite unreliable
more about amounts of harvest etc
what was the little ice age
cold period in the 16th century
when was the little iceage
from 1550 for about 150 years
what and when was the medival warm period
warming from 600 ad to 1250 AD
Name four proxies or other ways in which we can reconstruct the climate before instrumental measurements.
Examples covered in the lecture included: tree ring width, oxygen isotopes, written records, pollen records in bogs, cave deposits, etc. There are others!
What are the difficulties of using tree rings to reconstruct climate?
- Tree rings respond to several different environmental factors (e.g. water availability, light, temperature, disease), which may vary with location and may not directly link to the variable being reconstructed, so it can be difficult to infer that variable (e.g. temperatures) from the width of the ring.
Roughly how far back in time can tree rings be used to reconstruct the climate?
Some studies have managed to go back to the last ice age, over 12 thousand years ago (ka) in a continuous record by connecting tree ring records of living and dead trees. The oldest trees in the world are ~ 9 thousand years old. There are studies of 50 thousand years ago (but these don’t extend to present).
Where can we find a record of oxygen isotope abundance to reconstruct climate, and how far back does it go?
In ice cores or in lake and marine sediments. Ice cores records have high temporal resolution and can go back several hundred thousand years (~800 thousand years max with EPICA Dome C ice core in Antarctica, Greenland is much less). Sediment cores can go back tens of millions of years, but the temporal resolution is often quite low because they are controlled by sedimentation rates.
Explain what enables oxygen isotopes to be used for climate reconstruction.
Oxygen in water (H2O) can be either abundant 16O (“normal” oxygen) or less abundant 18O (heavy oxygen). Water containing the heavier 18O is not evaporated as easily as 16O, and condenses to form clouds and rain more easily than 16O. At lower temperatures this effect is enhanced. So when temperatures are low, the 18O becomes depleted in high latitude ice caps because it is both depleted during evaporation at lower latitudes and then preferentially rained out from the atmosphere as the air is transported to the pole (i.e. before it reaches the ice cap). The signal is archived in ice (There is a similar record in cave deposits; speleothems).
Because the oxygen isotopes that falls as snow over ice is then locked away in the ice until it melts, the 18O:16O ratio of global seawater changes when the size of the ice sheets change. So 18O in seawater also records the global ice volume. Generally, 18O of global seawater is higher (enriched) when there is more terrestrial ice because snowfall over the polar ice caps is depleted in 18O (i.e. locking away relatively more 16O in the ice, and leaving more 18O for the oceans).
Calcifying marine biota form their shells from the seawater around them. Chemical fractionation of oxygen occurs during this process. When temperatures are cooler, the organisms enrich their shells in 18O (so 18O is higher). When temperatures are warmed, the shells are depleted in 18O (so 18O is lower).
So, how 18O records temperature change depends on the archive you are measuring (e.g. ice core versus fossilised marine biota)!
What is the Central England Temperature, how was it obtained, and over what period?
The CET is a reconstructed climate record for a region of central England based on written records. It goes back to about 900 CE, although more robust quantitative measurements only go back to 1659.
What is a climate anomaly?
An anomaly is a period of climate that is significantly different from the long-term average.
What is the Medieval Climate Anomaly (MCA)?
The MCA was a transient anomalously warm period of several hundred years from about 1100 to 1300 CE.
Did the MCA occur over the whole world?
No, the MCA was restricted to parts of the northern hemisphere; i.e. it was not hemispherically synchronous.