Carbon is for ever (almost): Regulation of CO2 and climate on geological time-scales
Professor Andy Ridgwell, University of California Riverside, USA
Regulation of atmospheric CO2 (pCO2) and global temperature on million year time-scales is widely assumed to be driven primarily by global rates of rock weathering. If this were not the case and no long-term negative feedback on pCO2 existed, even relatively small changes in volcanic out-gassing would drive unbounded swings in pCO2 and climate – a situation that is not observed in the geological record. Yet proxy evidence for the link between pCO2, climate, and weathering is somewhat ambiguous. In this talk I’ll start by outlining the cascade of processes that act to remove excess CO2 in the atmosphere, focusing on the ultimate geological regulator of atmospheric CO2 – silicate weathering. I’ll introduce an interval of prominent and progressive warming that occurred during the early Cenozoic (ca. 58 to 49 Ma) alongside what existing evidence we have for how the global carbon cycle responded. I’ll then present a new global sediment dataset that on face value appears to suggest no change in the burial and geologic removal of the products of weathering and hence argues against a link between climate change and a weathering-mediated regulation of pCO2. I’ll finish by resolving the apparent contradiction between increases in pCO2 and weathering versus its sedimentary expression, through the use of a numerical representation of the global carbon cycle and climate – a tool for picking apart how often non-linear and opposing influences can combine to produce the phenomena we observe.
Permafrost thaw and its role as a carbon cycle feedback to global warming
Dr Charles Koven, Lawrence Berkeley National Lab, USA
The northern high latitudes contain the largest quantity of decomposable carbon in the terrestrial system, and are uniquely vulnerable to climate feedbacks because of arctic amplification and the abrupt nature of the freeze/thaw transition on physical and biogeochemical processes. These permafrost carbon stocks have built up over long periods of time due to a set of complex ecosystem and soil processes, and are likely to change dramatically with warming. This talk will focus on approaches to quantifying the magnitude and timing of the carbon cycle feedback arising from permafrost soils. We will begin by describing the observed distributions of carbon in permafrost soils and where the greatest uncertainties in the amount of permafrost carbon stocks lie. Next the talk will examine climate model projections of physical permafrost thaw, and discuss approaches to using these projections of changing soil temperatures in combination with observations of soil carbon distributions and laboratory-measured soil carbon losses upon thaw to estimate the likely carbon cycle responses to this warming. Building on these approaches, the talk will discuss projections of the amount and timing of permafrost C emissions as predicted by terrestrial carbon cycle models that include a more complex set of interactions, such as increased vegetation productivity due to both warming and the release of nutrients that are bound up in permafrost soils. Together, these approaches point to a potentially large feedback from permafrost on a centennial timescale, which must be accounted for in long-term carbon planning.
PMIP3: paleo view on feedbacks
Dr Pascale Braconnot, Laboratoire des Sciences du Climat et de l'Environnement, France
The climate response to anthropogenic activities involves physical processes, feedbacks and mechanisms that are the same as the ones that contributed to modulate the Earth’s climate in the past in response to external perturbations induced by solar variations, volcanic eruptions or tectonics. Past climates therefore offer a large set of natural experiences that can be used to better understand the relative role of different climate feedbacks arising from changes in the Earth’s global energetics and Earth’s hydrological cycle or from the coupling between climate and biogeochemical cycles. In addition, the numerous climate reconstructions from different and independent ice, marine and terrestrial climate archives allow us to test how climate models reproduce past changes, and to assess their credibility when used for future climate projections. The Paleoclimate Intercomparison Projects (https://pmip3.lsce.ipsl.fr/) was settled in 1991 with these objectives in mind. Using the results of simulations of the mid-Holocene and of the Last Glacial maximum, I will discuss the evolution in the vision these simulations provide on climate sensitivity, cryosphere feedbacks, and the role of ocean and vegetation feedbacks in monsoon regions. These two periods represent key references for model evaluation in PMIP and were included as part of the large CMIP5 set of simulations (http://cmip-pcmdi.llnl.gov/cmip5/) used as reference in last IPCC assessment (IPCC AR5, 2013). I will also highlight some of the current difficulties in the analyses resulting from model biases, and point out new possibilities resulting from the online simulation of the carbon cycle in climate models or from the comparison with other climatic periods considered in PMIP3.
Carbon cycle feedbacks and future climate change
Professor Pierre Friedlingstein, University of Exeter, UK
Climate and the carbon cycle are interacting on every timescale. On short, inter-annual timescales, there are numerous observational evidences revealing the strong response of the carbon cycle to climate variability. During warm El Niño years, atmospheric CO2 shows larger than average growth rate, indicating reduced storage in land and/or oceans; the opposite being observed during La Niña years. Multiple lines of evidence point towards tropical land ecosystems as main drivers of this variability. On such short time scales, the ocean shows much lower variability in its carbon exchange with the atmosphere.
On multi-millennial timescales, such as over glacial-interglacial cycles, ice core data clearly shows a strong correlation between climate and atmospheric CO2, with again, warm/cold climate being associated to higher/lower atmospheric CO2, i.e. lower/larger storage in ocean and land. Here, the ocean, probably the Southern ocean, is the main culprit for these changes.
On the centennial timescale of interest for the anthropogenic perturbation, there are indications of similar behaviour during warm/cold periods over the last millennium but no direct observations over the historical period. This is primarily due to the unprecedented input of CO2 in the atmosphere due to fossil fuel burning and land use change that dwarves any natural response of the land and ocean to the current warming. However, modelling studies unanimously show, again, a reduction of carbon storage both on land and ocean due to global warming. This induces, as during glacial cycles, a positive feedback in the climate system, a warmer world leading to larger atmospheric CO2 concentration.
The talk will review the observational and modelling evidence of a positive feedback between the climate system and the global carbon cycle, highlighting the implications for 21st century warming and cumulative emissions compatible with a given climate target.