Angela - rsmasclimate

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Transcript Angela - rsmasclimate

Paleoclimatology
Why is it important?
Angela Colbert
Climate Modeling Group
October 24, 2011
What is Paleoclimatology?
• The study of past climates.
– Use proxies to reconstruct temperature and other
records
– Use models to try to test theories to explain the
proxy records
• Proxy Examples
– Sediment cores – lakes and oceans, use forams
– Ice cores – very detailed for more “recent” history
– Tree rings – annual ring made, very detailed
Geological Time Scale
Holocene Began ~ 10,000 years ago
Last Glacial Maximum ~ 21,000 years ago
Pleistocene – Last Glacial Period
Pliocene – Last Non-Glacial Period
http://www.knewance.com/storage/post-images/geologic_time.jpg
What Can We Learn?
• The importance and impact of changing levels
of carbon dioxide
• Past changes in ocean/atmospheric
circulations
• What has occurred naturally in the past and
on what time scales
• Climate sensitivity
Interactions within Climate System
Why Is It Important?
• Rates of Climate Change
– Tectonic, Orbital, Abrupt
• Climate Sensitivity
– What are the triggers for climate change?
– How much natural variability is there?
• Adaptation of ecosystems
Rates of Climate Change
Orbital Climate Change
Abrupt Climate Change
Amy will discuss this more later!
Marine Isotope Stages
http://www.geo.arizona.edu/palynology/geos462/06ocenzscor.html
Climate Sensitivity
• Lea (2004) – Comparing tropical SSTs to ice
core records to isolate CO2 changes and
determine climate sensitivity.
• Hansen and Sato (2011) – A slightly more
urgent approach…
Lea (2004)
Methodology for Climate Sensitivity
• Linear Regression
– Can be achieved by interpolating both records
(SST and ice core) to a constant 2,000 yr sample
interval.
– Radiative effect of CO2 is first order, estimated
climate forcing is calculated by:
– With the effect of methane taken into account.
• Slope: 1.4 + 0.1°C (W
m-2)-1
• Intercept: 0.5 + 0.2°C
– Positive due to
warmer than present
conditions at some
previous interglacials
• For 4 W m-2 = 4.4 5.6°C warming for
doubling of CO2
Methodology for Climate Sensitivity
• Multivariate Regression
– Linear regression does not take into account other factors:
ice volume, atmospheric dust
– Derive relative weightings for each independent factor
that can control tropical SST
• Total greenhouse forcing, local insolation, NH insolation, two
different records of ice volume, Vostok ice core dust
– Find 1.3 + 0.1°C (W m-2)-1 slope
Comparing Observations to Models
• For tropical SST found sensitivity to be 4.4 5.6°C
• 15 Climate models – 3.5°C (mean) with range
of 2.0 – 5.1°C
• Why different?
– Smaller effect that glacial CO2 reduction has on
tropical ocean temps in models (1-2°C)
– Strong cooling effect on the Tropics from heat
transport of ice sheets (possible factor?)
The Paleoclimatic Approach
(according to Lea)
• Advantage
– Provides series of equilibrium climate sensitivity
experiments over a range of atmos. CO2 levels
– No model assumptions
• Disadvantage
– Other climatic influences (size and distribution of
glacial ice sheets, surface albedo, insolation changes,
concomitant changes in ocean and atmos. circulation)
could influence the past in a way that is not applicable
to the future
– Dust shows a comparable signal amplitude, but is
limited spatially by sources
Hansen and Sato (2011)
• The formation of ice sheets resulted in
global cooling.
• Global temperatures were much
warmer in the past than even today.
• Shows the influence of plate tectonics.
What They Argue
• Paleoclimate data on climate change and
climate sensitivity can yield accurate estimates
for “the dangerous level of global warming”
• The current agreement is the try to keep the
warming below 2°C relative to pre-industrial
times.
• They argue that this would be “a disaster
scenario for much of humanity”
• A global warming of 2°C would result in
heading back towards Pliocene-like
conditions.
Why They Argue That
• Can use interglacial periods to assess what the
threshold is before its dangerous
– Peak Holocene temp occurred around 8,000 BP
– MIS 5e and 11 were less than 1°C warmer than
the peak Holocene, thus also less than 1°C
warmer than in the year 2000.
– Early Pliocene with sea levels 25 m higher than
present also shows about a 1°C warming higher
than present
Climate Modeling
• Otto-Bliesner et al. (2006)
– Examined the IPCC CCSM3 Models
– Last Glacial Maximum
• Global cooling of 4.5°C with Tropical SST cooling of
1.7°C
• Half of the cooling is from reduced CO2 levels (~50% of
present-day)
• Increase in Antarctic currents and deep water
stratification and weaker North Atlantic
– Mid-Holocene
• Global cooling of less than 0.1°C
• Regional and seasonal variations are more significant
with weaker ENSO variability
The Paleoclimatic Approach
• Advantages
– Can test theories on the limits and sensitivities of
the climate system with boundary conditions that
can be validated to a certain extent.
– If models show something significant, it’s possible
to examine cores to try to find evidence
• Disadvantages
– If it happened in the past, it does not mean the
climate system will react the same way
– Although improvements have been made, there
are uncertainties for both models and the proxy
records
Resources
• Larry Peterson – MGG 676 Paleoclimatology
• Lea, D.W. (2004): The 100,000-Yr Cycle in
Tropical SST, Greenhouse Forcing, and Climate
Sensitivity. Journal of Climate, 17, 2170-2179.
• Otto-Bliesner, B.L. and co-authors (2006): Last
Glacial Maximum and Holocene Climate in
CCSM3. Journal of Climate, 19, 2526-2544.
• Hansen and Sato 2011:
http://pubs.giss.nasa.gov/abs/ha05510d.html