Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeBox 3.3. Paleoclimatic Data/Model ComparisonsTwo general approaches and information sources for studying past climates have been developed. Paleoclimatic observations(also known as proxy data) consist of paleoecological, geological, and geochemical data, that when assigned ages by variousmeans, can be interpreted in climatic terms. Paleoclimatic data provide the basic documentation of what has happened inthe past, and can be synthesized to reconstruct the patterns history of paleoclimatic variations. Paleoclimatic simulationsare created by identifying the configuration of large-scale controls of climate (i.e., solar radiation, and its latitudinal andseasonal distribution, or the concentration of greenhouse gases in the atmosphere) at a particular time in the past, andthen supplying these to a global or regional climate model to generate sequences of simulated meteorological data, in afashion similar to the use of a numerical weather forecasting model today. (See CCSP SAP 3.1 for a discussion on climatemodels.) Both approaches are necessary for understanding past climatic variations—the paleoclimatic observations documentpast climatic variations but cannot explain them without some kind of model, and the models that could providesuch explanations must first be tested and shown to be capable of simulating the patterns in the data.The two approaches are combined in paleoclimatic data/model comparison studies, in which syntheses of paleoclimaticdata from different sources and suites of climate-model simulations performed with different models are combined inan attempt to replicate a past “natural experiment” with the real climate system, such as those provided by the regularchanges in incoming solar radiation related to Earth’s orbital variations. Previous generations of data/model comparisonstudies have focused on key times in the paleoclimatic record, such as the Last Glacial Maximum (21,000 years ago) ormid-Holocene (6,000 years ago), but attention is now turning to the study of paleoclimatic variability as recorded in highresolutiontime series of paleoclimatic data and generated by long “transient” simulations with models.Paleoclimatic data/model comparisons contribute to our overall perspective on climate change, and can provide criticallyneeded information on how realistically climate models can simulate climate variability and change, what the role offeedbacks in the climate system are in amplifying or damping changes in the external controls of climate, and the generalcauses and mechanisms involved in climate change.the time history of the monsoon intensificationor deintensification, including the regionalscaleresponses of surface climate and vegetation(Claussen et al., 1999; Hales et al., 2006;Renssen et al., 2006). These simulations typicallyshow abrupt decreases in vegetation cover,and usually also in precipitation, around thetime of the observed vegetation change (5 ka),when insolation was changing only gradually.The initial success of EMICs in simulatingan abrupt climate and land-cover change inresponse to a gradual change in forcing influencedthe development of a conceptual modelthat proposed that strong nonlinear feedbacksbetween the land surface and atmosphere wereresponsible for the abruptness of the climatechange and, moreover, suggested the existenceof multiple stable states of the coupled climatevegetation-soilsystem that are maintained bypositive vegetation feedback (Claussen et al,1999; Foley et al., 2003). In such a system,abrupt transitions from one state to another (e.g.,from a green Sahara to a brown one), could occurunder relatively modest changes in externalforcing, with a green vegetation state and wetconditions reinforcing one another, and likewisea brown state reinforcing dry conditions andvice versa. The positive feedback involved inmaintaining the green or brown states wouldalso promote the conversion of large areas fromone state to the other at the same time.A different perspective on the way in whichabrupt changes in the land-surface cover of westAfrica may occur in response to gradual insolationchanges is provided by the simulations byLiu et al. (2006, 2007). They used a coupledAOVGCM (FOAM-LPJ) run in transientmode to produce a continuous simulation from6.5 ka to present. They combined a statisticalanalysis of vegetation-climate feedback inthe AOVGCM, and an analysis of a simpleconceptual model that relates a simple two-statedepiction of vegetation to annual precipitation(Liu et al., 2006), and argue that the short-term(i.e. year-to-year) feedback between vegetation97