1. 1SOES 6006: Climate Dynamics Climate Dynamics Lectures 13: Climate and the wider Earth system Lecture given by Kevin Oliver

2. 2SOES 6006: Climate Dynamics Lecture outline This course has focused on the physics of the climate system, in the atmosphere, the ocean, and in the coupled system, also accounting for some simple ice interactions. Today we explore the interactions between climate and the wider Earth system, including biogeochemical processes. We will use glacial climate cycles as an illustrative example.

3. Climate-related Earth System Processes & Timescales A rough estimate of timescales in the Earth system: From: University of Southampton

4. 4SOES 6006: Climate Dynamics Lecture outcomes By the lecture's end, you should be able to: (1) Summarise key feedbacks between climate, ice sheets, the terrestrial and marine biosphere, and atmospheric CO 2. (1) Outline some of the mechanisms hypothesised to contribute to glacial climate cycles, and propose how these hypotheses could be explored in an Earth system model.

5. Components of the Earth's system outside of core climate modelling ice sheets terrestrial biosphere including soils lithosphere (e.g. weathering) marine sediments marine biosphere atmospheric chemistry

6. Ice-sheet dynamics Arguably the least well understood, and hardest to model, physical component of the climate system. Mass balance: F in due to snow accumulation. F out due to flow of ice and iceberg calving, sublimation and subglacial melting. © Hugo Ahlenius, UNEP/GRID-ArendalUNEP/GRID-Arendal

7. The carbon cycle Courtesy of: NASA Earth Science Enterprise

8. The natural carbon cycle: land Terrestrial vegetation/soils hold ~3 times as much carbon as the atmosphere. This can vary naturally: Different evironments have very different carbon contents: High: rainforest (in vegetation), peatland (in soil) Medium: most types of grassland Low: desert (both hot and cold deserts) Regions with very cold, and very hot dry climates, tend to hold less carbon.

9. The natural carbon cycle: ocean The ocean holds ~50 times as much carbon as the atmosphere, and this ratio can vary naturally: Air / surface ocean balance: CO 2 is more soluble in seawater at lower temperatures. Surface / deep ocean balance: Sinking organic matter sets up a vertical gradient. This gradient is reduced by vertical exchange, but only where that exchange doesn't simply drive more productivity and sinking of organic matter....

10. Carbon in the ocean: advection/diffusion vs. sinking organic matter ("unitilised" nutrients = "preformed" nutrients) From: Kevin Oliver, University of Southampton

11. What sets the ocean vs. atmospheric balance of carbon? At equilibrium, if air-sea carbon exchange is fast compared with surface ocean/deep ocean carbon exchange, and there is no change in chemistry: (1) The mean temperature of the global ocean (2) The fraction of nutrients in the ocean that reached the deep ocean (since last being at the surface) by advection/mixing rather than in sinking organic matter. Increasing (1) or (2) increases atmospheric CO 2.

12. Sediments and carbonate chemistry Increasing the carbon content of the deep ocean dissolves marine sediments. Counter-intuitively, this is a positive feedback, reducing atmospheric CO 2 despite transferring carbon from sediments to seawater. The explanation for this is the weathering equation: CO 2 (aq) + CaCO 3 (s) + H 2 O(l)  Ca 2+ (aq) + 2HCO 3 2- (aq) (This is a first order explanation only. There are further adjustments due to changes in pH.)

13. 13SOES 6006: Climate Dynamics Glacial cycles in.... Carbon Methane Antarctic Temp Oxygen Benthic Forams, Proxy For global ice volumes And therefore sea level Figure sourced from the Intergovermental Panel of Climate Change (IPCC), United Nations

14. 14 SOES 6006: Climate Dynamics Milankovitch cycles Precession: 20,000 yrs Obliquity (tilt): 40,000 yrs Eccentricity modulates precession: 100,000 yrs

15. 15SOES 6006: Climate Dynamics Milankovitch cycles Negligible influence on globally and annually averaged top-of-atmosphere energy input. Precession (~20,000 yr) has a bipolar component (currently warms south and cools north). Precession and obliquity (~40,000 yr) mainly control seasonality. High obliquity causes high seasonality in both hemispheres. Seasonality due to precession is out of phase in north and south. The eccentricity (~100,000 yr) cycle is a small modulation on the precessional cycle.

16. 16SOES 6006: Climate Dynamics Discuss in groups... In understanding glacial cycles, how might the following processes interact? (1) Solar insolation, "mass balance" of ice sheets, & global mean air temperature. (2) Air temperature and carbon storage in the terrestrial biosphere. (3) Global mean atmospheric temperature, carbon storage in the ocean, and carbon in marine sediments. (4) Atmospheric meridional temperature gradients, winds & ocean circulation.

17. 17SOES 6006: Climate Dynamics Glacial cycles in.... Carbon Methane Antarctic Temp Oxygen Benthic Forams, Proxy For global ice volumes And therefore sea level Summer Insolation (65 o N) Figure sourced from the Intergovermental Panel of Climate Change (IPCC), United Nations Insolation figure from: Robert A. Rohde, submitted to wikipediawikipedia

18. 18SOES 6006: Climate Dynamics A 100 kyr timescale in ice-sheets? - Albedo provides the major positive feedback in ice sheet mass balance, and depends on horizontal extent. - The flux out of the ice sheet increases as the ice sheet gets higher, due to basal melting & increased slopes, a negative feedback. - Ice sheets grow quickly horizontally, but very slowly vertically due to limited accumulation. - Positive feedback on short timescales, negative feedback on long timescales? See e.g. Bintanja & van der Wal (2008, Nature) for more on a poorly understood topic.

19. 19SOES 6006: Climate Dynamics If there were no Milankovitch cycles? Milankovitch cycles probably pace glacial-interglacial cycles, not cause them. Positive and negative feedbacks, acting on different timescales, in the Earth system cause glacial cycles. The exact nature of these feedbacks is not well known. grey lines - observations blue and red lines - observations Reproduced by permission of American Geophysical Union: Tziperman, E., Raymo, M.E., Huybers, P., Wunsch., Consequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing, Paleoceanography, v. 21, PA4206, 10 November 2006. Copyright [2006] American Geophysical Union.

20. 20SOES 6006: Climate Dynamics Glacial/interglacial CO 2 changes Present 20 ka Sea level SST Terrestrial biosphere Figure sourced from the Intergovermental Panel of Climate Change (IPCC), United Nations

21. 21SOES 6006: Climate Dynamics Glacial/interglacial CO 2 changes: large uncertainties Progress has been made, but uncertainties are still large. Reproduced by permission of American Geophysical Union: Kohfeld, K. E., Ridgwell, A., Glacial-interglacial variability inatmospheric CO2, in Surface Ocean--Lower Atmospheres Processes, Eds.Le Quéré, C., Saltzman, E.S., AGU Geophysical Monograph Series, v. 187, 350 pp. 28 January 010. Copyright [2010] American Geophysical Union.

22. 22SOES 6006: Climate Dynamics Summary Climate interacts with the remainder of the Earth system on a range of timescales. There is evidence for many of these interactions in glacial cycles. In the geoengineering part of your project, you have the option of proposing to investigate deliberate perturbation to the marine carbon cycle. Should our limited understanding of the natural carbon cycle be a concern?

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