Houghton Lectures

“Biogeochemical Cycles and Climate”

Prof. Nicolas Gruber

Department of Atmospheric Sciences & IGPP
University of California, Los Angeles

You can view Niki's overhead presentations on the web
 by clicking here, going to 'Gruber' and 
scrolling down to 'Talks'.

Lecture 1: Monday, October 7, 2002 from 3:00 p.m. - 5:00 p.m.
Biogeochemical/physical-climate interactions and the carbon cycle

A brief introduction into the nature of biogeochemical cycles and how they can interact with the physical climate system. The two systems are interacting through a multitude of processes: Through modification of the atmospheric composition (greenhouse gases, aerosols, etc), changes in the land/ocean surface properties (albedo, roughness), or through changes in the hydrological cycle (evapo-transpiration, etc). The focus of the lecture series will be on the influence of earth's biogeochemical cycles on the atmospheric composition, with a strong emphasis on the carbon cycle.

Introduction of the global carbon cycle. Discussion of reservoir sizes and exchange rates, including the human perturbation. What controls the distribution of carbon between the three major reservoirs, and what is the time-scale of exchange between them? Highlight the role of the ocean in having a strong influence on the atmospheric CO2 concentration on time-scales longer than a decade. Furthermore, biological processes in the ocean in interaction with the large-scale oceanic circulation lead to a depletion of the surface ocean carbon content, causing a substantially lower atmospheric CO2 content than if they ocean was dead. What controls this "biological pump" in the ocean?

Lecture 2: Tuesday, October 8, 2002 from 2:00 p.m. - 4:00 p.m.
The fate of anthropogenic CO2

Only about half of CO2 that is being emitted by the burning of fossil fuel is accumulating in the atmosphere, the other half is being taken up by the ocean and land biosphere. Review mechanisms of ocean and land biosphere uptake. In the ocean, mechanisms are relatively well understood. Transfer across the air-sea interface and transport of the anthropogenic CO2 into the abyss by the "solubility pump". Review and discussions of constraints on this uptake. Oxygen and Carbon-13 based methods, direct determination of anthropogenic CO2 in the ocean. Comparison with Model estimates. On land, mechanisms are very poorly understood. Leading candidate processes are CO2 and nitrogen fertilization, land use history and fire suppression. Brief discussion of current understanding of mechanisms. Approaches.

Recommended reading: Sarmiento, J.L. and N. Gruber. Sinks for anthropogenic carbon, Physics Today, 55(8), 30-36, 2002. available from http://www.atmos.ucla.edu/~gruber/publ_fs.htm

Lecture 3: Thursday, October 10, 2002 from 2:00 p.m. - 4:00 p.m.
Future carbon-cycle/climate feedbacks

Climate change will very likely impact the ability of the land biosphere and the ocean to absorb anthropogenic CO2 from the atmosphere, leading to feedbacks between future climate change and the global carbon cycle. On the ocean side, the feedbacks involve both the physical uptake of anthropogenic CO2 across the air-sea interface and the transport of this CO2 into the abyss (the solubility pump), as well as interactions with the ocean's biological pump. How are these two pumps going to respond to climate change? Most models simulations indicate that the solubility pump acts as a positive feedback (enhancing future warming), whereas the ocean's biological pump overall acts as a negative feedback. Discussion about robustness of these results, and other effects. On the land side, no clear answer has emerged so far from modeling studies. Results depend crucially on the nature of the assumed nature of the current land carbon sink. What have people learned from manipulation studies? Should we engage in similar studies in the ocean?

Lecture 4: Tuesday, October 15, 2002 from 2:00 p.m. - 4:00 p.m.
Insights from the past: What caused the glacial/interglacial
CO2 transitions? Part I

Earth has experienced four major glaciation cycles over the last 420,000 years with large implications (and interactions) with the global carbon cycle. We know from the analysis of air bubbles trapped in Antarctic ice-cores that each of these glaciations was accompanied by a 80 to 100 ppm drop in the atmospheric CO2 concentration. Comparison between these atmospheric CO2 records and reconstructed temperatures for Antarctica show that there exists a tight linkage between the two at nearly all frequencies longer than five thousand years. This remarkable coupling between earth's physical climate and the global carbon cycle is not understood and one of the outstanding puzzles of carbon cycle research. These atmospheric CO2 variations are controlled by the ocean, so we must seek an answer in changes in the ocean carbon cycle.

Review the components that are relatively well constrained, such as the global ocean temperature change and the salinity change associated with the loss of freshwater. Discuss large-scale (ocean and

atmosphere) circulation during glacial times. Provide overview of proposed hypotheses, stratified according to mechanisms that mostly rely on changes in ocean circulation and those that mostly rely on changes in ocean biology. Start discussion with mostly physically based hypotheses, such as the sea-ice mechanism, etc. Highlight role of the Southern Ocean and of the CaCO3 cycle.

Lecture 5: Thursday, October 17, 2002 from 2:00 p.m. - 4:00 p.m.
What caused the glacial/interglacial CO2 transitions? Part II

Discussions continue with mostly biologically driven mechanisms with a focus on the iron cycle and its multi-faceted imprints on the ocean's nitrogen and silicon cycle. Provide a short introduction to these two cycles. Nitrogen Fixation and denitrification. Opal production and dissolution. Review and discuss N2-fixation/denitrification hypothesis, assumptions, limits, paleo evidence. Review and discuss the linked iron/diatom/CaCO3 hypothesis, assumptions, limits, and paleo evidence. Conclude with discussion about approaches to move forward; do we need a much more integrated approach, i.e. investigate the linkage between land, ocean and atmosphere?