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cycle dynamics - How are major carbon sinks and sources varying with ?

David Schimel Fortunat Joos National Ecological Observatory Network, Boulder, USA; [email protected] and Environmental Physics and Oeschger Centre for Research, University of Bern, Switzerland; [email protected] he entire Earth System, , the trapping of heat in the atmosphere, increasing atmospheric aleoclimate data are essential to gauge Atlantic heat transport and shifts in the rain explain the observations. For example, re- Toceans, and geology are leading to global climate change. As as temperatures warm. Pthe magnitude and the speed of change belt of the Inter Tropical Convergence Zone constructed and simulated spatio-temporal involved in the (Archer climate changes, it has consequences When humans burn fossil fuels, they in concentrations, (ITCZ). These ”Dansgaard-Oeschger” and re- evolution of C-cycle parameters during the 2010). Carbon dioxide is taken up by for the carbon cycle. The rates of many transform organic carbon compounds acidification, and climate. These parameters lated Southern Hemisphere climate swings last 11 ka (Fig. 1) provide evidence that the

the land through photosyn- biogeochemical processes, such as that last for millions of years into at- co-determine the anthropogenic impacts on demonstrate that the can millennial-scale CO2 variations during the Ho- thesis, and released through , respiration, nitrogen mospheric carbon dioxide. The uptake natural and socioeconomic systems and their switch into new states within decades – a po- locene are primarily governed by natural pro- and (Schimel et al. 2001). cycling and increase in warm- and stabilization of carbon dioxide into capabilities to adapt. tential for unpleasant future surprise. cesses (Menviel and Joos 2012), in contrast to The take up carbon through er conditions (Field et al. 2007). These long-lived carbon compounds – which Concentrations of the three major an- Surprisingly, variations in atmospheric previous claims of anthropogenic causes. Paired Perspectives on Global Change Perspectives Paired on Global Change Perspectives Paired closely coupled physical, chemical higher process rates may speed up defines the lifetime of carbon dioxide – thropogenic greenhouse gases were much CO2 remained quite small during Dansgaard- Paleoscience allows us to test hypoth-

and biological processes (Sarmiento biological activity, producing potential takes a lot of time. While photosynthe- smaller during at least the last 800 ka than Oeschger events. This indicates a certain esis. Can we mitigate the man-made CO2

and Gruber 2006). They absorb carbon positive and negative to the sis and air- gas exchange can draw modern and projected concentrations. The insensitivity of atmospheric CO2 to shifts in increase by stimulating marine productiv-

dioxide in regions where mixing and carbon cycle. In some models, the feed- CO2 concentrations down quickly, i.e. record reveals that the average rate of the ITCZ, changes in the ocean’s overturning ity and promoting an ocean by

biological activity maintain low ocean back effects of the carbon cycle influ- within years, actually transferring that increase in CO2 and the from circulation, and abrupt warming of the boreal artificial ? Paleodata (Röth-

carbon dioxide levels, allowing chemi- ence atmospheric CO2 by tens to even carbon into stable chemical forms, safe the combination of CO2, CH4, and N2O oc- zone. Related variations in N2O and CH4 are lisberger et al. 2004) and modeling suggest cal diffusion from these active regions hundreds of parts per million, corre- from recycling back to the atmosphere curred by more than an order of magnitude substantial (Schilt et al. 2010), but the result- that we can’t – past variations in aeolian iron of the sea. In other regions, carbon-rich sponding to significant effects on tem- requires ten to thirty thousand years faster during the Industrial Era than during ing radiative forcing is several times smaller input are not coupled to large atmospheric

water mixes to the surface and releases perature (Cox et al. 2000). Temperature (Archer 2010). any comparable period of at least the past 16 than current anthropogenic forcing. Green- CO2 changes. carbon to the atmosphere. These pro- effects may be largest in high latitude Because of the long lifetime of ka (Joos and Spahni 2008). This implies that house gas concentrations react to climate Warming might set carbon free from

cesses are natural and have always oc- regions, where enormous post-glacial changes to the carbon cycle, the pa- current global climate change and ocean change and amplify it, but the data may also permafrost and or CH4 currently caged curred. deposits of carbon in permafrost could leo-perspective is critical. For example, acidification is progressing at a speed that is indicate that such an amplification of man- in clathrates in , which would in

The anthroposphere, the sphere of be released as CO2 and as paleorecords tell us that while atmo- unprecedented at least since the agricultural made climate change may remain moderate turn amplify global warming and ocean acidi-

human activities, transforms this cycle these thaw (Koven et al. 2011). spheric carbon dioxide concentrations period. compared to anthropogenic emissions. This fication. However, , atmospheric CO2 and in two ways. First, humans burn carbon Precipitation also changes as the can increase quite rapidly, it can take Ice, terrestrial and oceanic records reveal is in line with results from Earth System Mod- carbon isotope data suggest a carbon sink, from geological reservoirs that would warming atmosphere causes more much, much longer for it to be locked huge changes in climate during glacial peri- els and probabilistic analyses of the last mil- and not a source, in peatlands during peri-

normally be stable over millions of evaporation. This intensification of into long-lasting forms and provides a ods on the decadal time scale (Jansen et al. lennium temperature and CO2 records (Frank ods of past warming (Yu 2010). Likewise, ice

years and introduce this additional car- the hydrological cycle will cause some timescale for these dynamics. During 2007). These are linked to major reorganiza- et al. 2010). core data show no extraordinarily large CH4 bon into the more active atmosphere- regions to become wetter, but arid re- one well-documented event 55 million tions of the oceanic and atmospheric circula- Theories and Earth system models must variations nor supporting isotopic signatures ocean-land system, increasing the total gions may become drier. Drier condi- years ago, atmospheric carbon dioxide tions, including a stop and go of the poleward undergo the reality check to quantitatively (Bock et al. 2010) for thermodynamic condi-

amount of carbon circulating in the tions in the tropics will decrease plant nearly doubled over about 20,000 years, tions potentially favoring CH4 release from Earth System (Archer 2010). Second, growth and reduce carbon storage in with an accompanying increase in tem- clathrates, i.e. during periods of rapid warm- human converts biologically these regions (Fung et al. 2005). Warm- perature of perhaps 6°C. While atmo- ing and . stored carbon in soils and plants to car- er and wetter conditions in mid-lati- spheric carbon dioxide increased very Yet, it is not always clear to which extent bon dioxide in the atmosphere, chang- tude regions may actually increase car- rapidly, it took nearly 7-8 times longer a comparison of the past with man-made cli- ing the balance of stored and circulat- bon storage. Most evidence suggests to return to previous levels (Doney and mate change is viable. We need to improve ing carbon (Schimel 1995). the tropical effects are larger, leading Schimel 2007). our mechanistic understanding of the under- These human activities increase at- to overall release of carbon Carbon scientists are finding cre- lying processes to better assess the risk of on- mospheric carbon dioxide and thereby stores. This creates a global ative ways to combine paleodata, pro- going greenhouse gas release and associated cess studies and modern-day observa- climate amplification and feedbacks in or- tions to model the ”fast out-slow in” der to better guide emission mitigation and processes that determine the longevity climate adaptation efforts. Paleoresearch, and severity of climate change. The pa- offering unique access to time scales and leorecord provides an essential obser- complex climate variations neither covered vational basis for assessing the reality in the instrumental records nor accessible by of model-based scenarios of the long- laboratory studies, is the key to reach this pol- term future. icy-relevant goal, but its potential has barely been exploited. Selected references Full reference list online under: Selected references http://www.pages-igbp.org/products/newsletters/ref2012_1.pdf Full reference list online under: http://www.pages-igbp.org/products/newsletters/ref2012_1.pdf Archer D (2010) The Global Carbon Cycle (Princeton Primers in Climate), Princeton University Press, 224 pp Frank DC et al. (2010) Nature 463: 527-530 Doney SC and Schimel DS (2007) Annual Review of Environment and Menviel L and Joos F (2012) Paleoceanography 27(1), doi: Resources 32: 31-66 10.1029/2011PA002224 Cox PM et al. (2000) Nature 408: 184-187 Röthlisberger R et al. (2004) Geophysical Research Letters 31, doi: Fung IY, Doney SC, Lindsay K and John J (2005) PNAS 102: 11201- 10.1029/2004GL020338 Figure 1: Human perturbation of the global carbon budget over the period 1850-2012 AD. The values on the right 11206 Figure 1: Evolution of atmospheric CO and δ13C of atmospheric CO (lower panel) and changes in deep ocean Schilt A et al. (2010) Quaternary Science Reviews 29(1-2): 182-192 represent the amount of C emitted and absorbed during the last decade (2000-2010 AD). Figure modified from 2 2 Koven CD et al. (2011) PNAS 108: 14769-14774 ion concentration (μmol kg-1) over the last 10 ka (upper panel). data are shown by circles and (2011), updated from Le Quéré et al. (2009) and Canadell et al. (2007). Yu Z (2010) The Holocene 21(5): 761-774 crosses, and results from the Bern3D model by blue lines and color contours (after Menviel and Joos 2012). 12 13

PAGES news • Vol 20 • No 1 • February 2012 PAGES news • Vol 20 • No 1 • February 2012