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

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

Climate Sensitivity Lea (2004) – Comparing tropical SSTs to ice core records to isolate CO 2 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 CO 2 is first order, estimated climate forcing is calculated by: – With the effect of methane taken into account.

Slope: °C (W m -2 ) -1 Intercept: °C – Positive due to warmer than present conditions at some previous interglacials For 4 W m -2 = °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 °C (W m -2 ) -1 slope

Comparing Observations to Models For tropical SST found sensitivity to be °C 15 Climate models – 3.5°C (mean) with range of 2.0 – 5.1°C Why different? – Smaller effect that glacial CO 2 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. CO 2 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 – 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 CO 2 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, Otto-Bliesner, B.L. and co-authors (2006): Last Glacial Maximum and Holocene Climate in CCSM3. Journal of Climate, 19, Hansen and Sato 2011: