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Note: links are provided to summaries of journal articles (abstracts) or to the full pdf version.
My main research interest lies in understanding the variability of the climate, both its internal variability and the response to external forcing. In particular, how changes related to varying forcings relate to variations due to intrinsic (unforced) climate variability such as oscillations in the ocean's deep thermohaline circulation that affect ocean heat transports. I mainly use large-scale general circulation models for the atmosphere, and fully coupled ocean-atmosphere models to investigate these questions,
The evidence of long term paleo-climate variability exists primarily in the form of proxy data recorded in deep sea cores, ice cores, tree rings and other proxies such as the the skeletal remains of corals. Recently, my work has focussed on ways to reconcile the paleo-data with models. The main difficulty is that the proxy data are records of multiple processes and hence, it is difficult to unambiguously ascribe a climatic cause to any particular recorded event.
Currently, I help develop the GISS ocean and coupled GCMs to improve the representation of the present day climate while investigating their response to external forcing. Our new GCM is called ModelE and is being used for the GISS modelling contribution to the upcoming IPCC 4th Assessment Report.
The source code, documentation and external datasets for running the model (in a number of different configurations) are available at the ModelE website . As model runs and IPCC simulations are completed, the output will be made available as well. Feel free to do your own runs! (The basic version of the model will run on almost any platform PC(Linux), Mac or Unix - see the system requirements in the documentation for details).
The validation for the atmospheric component is now published (Schmidt et al (2006)), while a rather technical discussion on how ocean-ice-atmospheric boundary conditions should be handled in such models appeared in Ocean Modelling Schmidt et al. (2004). Papers describing the coupled model climatologies, results and the sensitivity to many different forcings are also available: Hansen et al (2005a), Hansen et al (2005b) and Hansen et al (submitted).
The principle proxy used for inferring information about past ocean conditions is the oxygen-18 ratio measured in carbonate found in deep sea sediments (and corals). This ratio is a function of two effects: the background isotopic ratio in the seawater and the local temperature as the carbonate is secreted. Understanding the variability of the seawater oxygen-18 signal and it's relationship to changes in climate is thus essential to interpreting the carbonate record through time. (There are two related Science briefs: Cold Climates, Warm Climates: How Can We Tell Past Temperatures? and Tracing the Water Cycle, Isotopically which explain this in a simple way). Water isotopes (including deuterium as well as oxygen-18) are also measured in ice cores from Greenland, Antarctica and mountain glaciers, as well as in lake sediments, speleothems (cave deposits) and tree cellulose. Our latest results for the atmospheric model are reported in Schmidt et al (2005).
Work is also being done with coupled models which include oxygen-18 and deuterium as independent tracers (abs) These models incorporate the complex dependence on ice volume, moist convection, clouds, evaporation, precipitation and regional ocean and sea ice processes of the global oxygen-18 signal. The surface oxygen-18 ratio from a model simulation can be seen in this plot.
One application of these results concerns the relationship of oxygen-18 to salinity in the oceans. This relationship is mostly linear though modelling studies can indicate where this assumption may break down. This has important implications for paleosalinity calculations.
In order to validate the ocean simulations, I and my collaborators have amasssed a collection of well over 20,000 data points of oceanic measurments. The Global Seawater Oxygen-18 database is now available on-line and accessible by other interested observers. Any additional contributions to this database are most welcome.
A further way to combine the models and the data is to forward model the signal that would be recorded in the sediments or corals given a modelled climatic event.
When the oxygen-18 ratio in the seawater is combined with simple ecologic models of foraminifera or coral growth it can provide a mapping of a particular modelled climatic event (meltwater pulses, changes in atmospheric forcing etc.) to the isotopic signal that would be recorded in carbonate sediments. In addition, using a range of plausible assumptions in the biological models (i.e. seasonal succession, depth variability), the extent to which our uncertainty about the ecology limits the accuracy of the derived climate records can be investigated. This method is a promising approach to tackling the inverse problem: what do observed changes in carbonate proxy data imply about past climate changes?
A number of papers have addressed this issue, Schmidt (1999), Schmidt and Mulitza (2002). (Note that colour versions of the black and white figures are available here as well), and more recently this technique was applied to the 8.2 kyr event (see Science Brief) in LeGrande et al (2006).
This topic is a main focus of work at GISS, and recently, I have been working with Drew Shindell and others in examining whether the some specific circulation changes in recent Northern Hemisphere climate (the Arctic Oscillation trend) can be reproduced in models. Interestingly, it appears that only models that include a well resolved stratosphere capture this part of climate change realisticly. Two papers discuss this in more detail: a 1999 Nature article and a more thorough explanation in more recent paper. There is also a popular science report that explain this more clearly. A comparison of all the models in the IPCC AR4 project has recently also been completed (Miller et al, 2006).
One region where the combination of dynamic and radiative impacts appears to be clear is in the Southern Hemisphere around Antarctica. There the combination of stratospheric ozone depletion and greenhouse gas changes is discussed in Shindell and Schmidt (2004).
I am also interested in looking at past climates. We recently wrote a paper describing modelling approaches for understanding the climate of the Holocene (Schmidt et al, 2004, QSR). One focus I (along with colleagues Drew Shindell and David Rind) have is the role of natural forcing mechanisms, such as solar or volcanic variability, over the last few hundred years. In particular we looked at the so-called Little Ice Age, or more precisely the Maunder Minimum (at the end of the 17th century). We found that by reducing the solar forcing in line with estimates (Lean et al, 1997), we can get substantial regional changes in surface temperatures (particularly over the NH continents) even though the global change is relatively small. This then may be a plausible solution for discrepencies between Little Ice Age glacial advances, and minimal evidence for a large global cooling. The impact of volcanism over this same period cannot be neglected, and we found that both effects are likely to have been important, but each has a specific regional expression. A number of papers have now appeared that look at this issue from the data and modelling standpoints: Shindell et al. ( 2001; 2003; 2004).
The Paleocene/Eocene Thremal Maximum (PETM) is another interesting candidate for seeing whether or not GCMs can replicate the sensitivity of the climate to forcings that happened (in this case) 55 million years ago. This global warming event is hypothesised to have been forced by massive releases of methane gas from froxen methane hydrate deposits on the sea floor. The methane and its oxidation product CO2 are both powerful greenhouse gases, but the relative importance of methane and CO2 is controlled by atmospheric chemistry. Squaring the circle of forcing, modelling and outcome for this event is the subject of Schmidt and Shindell (2003). There is a simple box model incorporating our atmospheric chemistry results that can downloaded as supplementary material.
The PETM is but one example of the influence of atmospheric methane on climate (and vice versa). I recently wrote a article for La Recherche (a French popular science magazine) that discusses how methane went from obscurity 30 years ago to one of the most important subjects in climate science today. (The article is originally in French, but there is an English translation as well). We have also worked on understanding the long-term variations of methane seen in the ice core record Schmidt et al (2004, GRL).
A very interesting period for modellers and paleo-oceangraphers is the so-called 8.2 kyr event. This is the last abrupt climate change that occured in the North Atlantic, and appears to have been co-incident with a sudden outburst event from Lake Agassiz - then the largest freshwater lake on the planet. This event is proving to be useful for understanding the sensitivity of the ocean circulation and its sensitivity to freshwater additions. Some explanation of why this may be so is explained in a recent editorial at QSR (Schmidt and LeGrande, 2005) and some of our own modelling results can be found in LeGrande et al (2006).
We are continuing to work on other climate periods, such as the Last Glacial Maximum or the Younger Dryas (i.e. Rind
et al,
2001a,
2001b), and hopefully we will have some results soon!
Please feel free to contact me for further information or reprints.
NASA Goddard Institute for Space Studies
and
Center for Climate Systems Research, Columbia University
2880 Broadway, New York, NY 10025 USA
Email: gschmidt@giss.nasa.gov
Tel: (212) 678 5627
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