Biogeochemical and Biophysical Responses of the Land Surface to a Sustained Thermohaline Circulation Weakening
Total Page：16
File Type：pdf, Size：1020Kb
1NOVEMBER 2004 HIGGINS 4135 Biogeochemical and Biophysical Responses of the Land Surface to a Sustained Thermohaline Circulation Weakening PAUL A. T. HIGGINS University of California, Berkeley, Berkeley, California (Manuscript received 20 November 2003, in ®nal form 14 May 2004) ABSTRACT Biotic responses to climate change may constitute signi®cant feedbacks to the climate system by altering biogeochemistry (e.g., carbon storage) or biophysics (i.e., albedo, evapotranspiration, and roughness length) at the land surface. Accurate projection of future climate change depends on proper accounting of these biological feedbacks. Similarly, projections of future climate change must include the potential for nonlinear responses such as thermohaline circulation (THC) weakening, which is increasingly evident in paleoclimate reconstructions and model experiments. This article uses of¯ine simulations with the Integrated Biosphere Simulator (IBIS) to determine long-term biophysical and biogeochemical responses to climate patterns generated by the third Hadley Centre Coupled Ocean±Atmosphere General Circulation Model (HadCM3) under forced THC weakening. Total land surface carbon storage decreases by 0.5% in response to THC weakening, suggesting that the biogeochemical response would not constitute a signi®cant climate feedback under this climate change scenario. In contrast, large regional and local changes in leaf area index (LAI) suggest that biophysical responses may constitute signi®cant feedbacks to at least local and regional climate. Indeed, the LAI responses do lead to changes in midday direct and diffuse beam albedo over large regions of the land surface. As a result, there are large local and regional changes in the land surface's capacity to absorb solar radiation. Changes in energy partitioning between sensible and latent heat ¯uxes also occur. However, the change in latent heat ¯ux from the land surface is primarily attributable to changes in precipitation that occur under forced THC weakening and not a result of the subsequent changes in vegetation. 1. Introduction system (Betts et al. 1997; Cox et al. 2000; Levis et al. 2000). In some region-speci®c cases these biotic respons- Structural and functional ecosystem responses to cli- es magnify climate changes by as much as a factor of 4 mate change may constitute signi®cant feedbacks to the (Ganopolski et al. 1998) and can greatly amplify even climate system through changes in biophysical and bio- geochemical characteristics of the land surface (Field small changes in external forcing (Claussen et al. 1999). and Avissar 1998; Pielke et al. 1998; Sellers et al. 1997). These atmosphere±biosphere interactions may be partic- Important biophysical characteristics of the land surface ularly important in arid regions (Foley et al. 2003; Hig- include 1) albedo, or re¯ectivity of incoming solar ra- gins et al. 2002; Nicholson 2000) as a consequence of diation, which depends on vegetation type and density; greater climate sensitivity to changes in transpiration, 2) evapotranspiration, which in¯uences atmospheric roughness length, and albedo (Nicholson 2000; Wang and moisture and partitioning of surface energy between Eltahir 2000a,c). As a result, projection of future climate sensible and latent heat (leaf area, rooting depth, and change depends on proper accounting of biophysical and photosynthetic rates all in¯uence rates of transpiration); biogeochemical responses at the land surface. and 3) surface roughness, which in¯uences friction be- Paleoclimate and model studies also increasingly sug- tween the atmosphere and the land surface and atmo- gest that changes in ocean circulation can trigger non- spheric mixing in the boundary layer. Biogeochemical linear climate responses to external forcing (Broecker changes (i.e., the carbon and nutrient cycles) can alter 2003; Clark et al. 2002; Hall and Stouffer 2001; Knutti carbon storage at the land surface and therefore in¯u- and Stocker 2002; Rahmstorf and Ganopolski 1999). ence atmospheric concentrations of greenhouse gases. For example, the paleoclimate record suggests that at Therefore, biophysical and biogeochemical responses least three modes of the ocean's thermohaline circulation can amplify or dampen external changes to the climate (THC) are stable: 1) the modern mode, 2) a weaker mode that was common during the previous glacial pe- riod, and 3) an ``off mode'' (Alley and Clark 1999). In Corresponding author address: Dr. Paul Higgins, University of California, Berkeley, 151 Hilgard Hall, Berkeley, CA 94720-3110. some models, multiple stable THC equilibria are also E-mail: [email protected] possible with the equilibria attained depending on tem- q 2004 American Meteorological Society Unauthenticated | Downloaded 09/27/21 01:26 PM UTC 4136 JOURNAL OF CLIMATE VOLUME 17 perature and salt conditions, details of surface forcing, sition, land-use patterns, and species invasions further initial THC strength, and additional model parameters complicate projections of ecosystem responses to global (Dijkstra and Neelin 2000; Rahmstorf 1996; Schneider change disturbance. This article presents simulated bio- and Thompson 2000; Stocker and Schmittner 1997). geochemical and biophysical responses of the land sur- Projection of future climate change requires assessment face to a sustained THC weakening scenario under con- of and accounting for the biophysical and biogeochem- stant GHG concentrations. This is a necessary ®rst step ical responses of the land surface that could result from for projecting biological responses and feedbacks to fu- such nonlinear climate change. ture climate changes. Climate simulations performed with the third Hadley Centre Coupled Ocean±Atmosphere General Circula- tion Model (HadCM3) under preindustrial greenhouse 2. Methods gas (GHG) forcing suggest that THC weakening could a. Climate scenarios alter temperature and precipitation throughout the world (Vellinga and Wood 2002; Vellinga et al. 2002). Vellinga I use four previously described climate scenarios un- and Wood (2002) imposed a large salinity disruption in der constant CO2 (280 ppm) that vary in monthly tem- the North Atlantic, which caused an initial decrease in perature and precipitation (Higgins and Vellinga 2004). HadCM3's THC from 20 Sverdrups (1 Sv [ 106 m3 s21) Temperature and precipitation scenarios are based on to 0 Sv in the North Atlantic at 488N and 666-m depth. monthly means from the third decade of two simulations Over the next 120 yr of the simulation, THC recovers from HadCM3 (described above), a control simulation to its full strength as the off mode is not stable in in which THC remains stable and active, and a THC HadCM3 under preindustrial climate conditions and weakening simulation in which a prescribed salinity de- model parameters. During the ®rst decade annually av- cline greatly reduces THC (Vellinga and Wood 2002; eraged temperature decreases by 68±88C in the atmo- Vellinga et al. 2002). The four climate scenarios are 1) sphere above the northwest Atlantic while THC strength a control scenario in which temperature and precipita- remains below roughly 6 Sv. Over subsequent decades, tion are taken from HadCM3's control simulation, 2) a climate system teleconnections slowly spread through- weakened THC scenario in which both temperature and out the world while the strength of THC continues to precipitation are taken from HadCM3's weakened THC recover. During the third decade, portions of western simulation, 3) weakened THC temperature combined Europe cool by over 38 with 18±28C cooling occurring with HadCM3's control precipitation, and 4) weakened throughout much of Europe and in parts of Asia, north- THC precipitation combined with HadCM3's control ern Africa, and North America. Large shifts in precip- temperature. Since temperature and precipitation in¯u- itation in the Tropics also occur during the third decade ence one another (temperature affects precipitation by as a consequence of a shift in the intertropical conver- altering atmospheric circulation, relative humidity, and gence zone (ITCZ; Vellinga and Wood 2002; Vellinga evapotranspiration, while precipitation affects temper- et al. 2002). ature by altering the net energy available to the surface Attribution of these climate responses to THC weak- and the partitioning of that energy between sensible and ening requires statistical distinction of the forced re- latent heat ¯uxes), these last two scenarios are not phys- sponse (signal) from the internal variability of the cli- ically consistent. By isolating temperature and precip- mate system (noise) (Chervin and Schneider 1976). The itation changes, however, this approach provides a mea- HadCM3 climate experiments identify statistically sig- sure of ecosystem sensitivity to each. ni®cant climate responses to THC weakening as anom- Climate in HadCM3 does not reach equilibrium for alies that exceed 2 times the decadal mean standard THC weakening as recovery occurs over subsequent deviation from the control, sampled over 300 yr (Vel- decades in this model implementation. As previously linga and Wood 2002; Vellinga et al. 2002). By this test, described (Higgins and Vellinga 2004), this study uses the Northern Hemisphere temperature changes and the the third decade of the transient HadCM3 scenarios in ITCZ shift are both signi®cant. order to balance the ongoing THC recovery with the Previous examination of ecosystem responses to this slow spread of climate teleconnections throughout the scenario demonstrates that large structural