Shutdown Of Thermohaline Circulation Research Articles

Dynamic millennial‐scale climate changes in the northwestern Pacific over the past 40,000 years

Ice core records of polar temperatures and greenhouse gases document abrupt millennial‐scale oscillations that suggest the reduction or shutdown of thermohaline Circulation (THC) in the North Atlantic Ocean may induce the abrupt cooling in the northern hemisphere. It remains unknown, however, whether the sea surface temperature (SST) is cooling or warming in the Kuroshio of the Northwestern Pacific during the cooling event. Here we present an AMS 14C‐dated foraminiferal Mg/Ca SST record from the central Okinawa Trough and document that the SST variations exhibit two steps of warming since 21 ka — at 14.7 ka and 12.8 ka, and a cooling (∼1.5°C) during the interval of the Younger Dryas. By contrast, we observed no SST change or oceanic warming (∼1.5–2°C) during the episodes of Northern Hemisphere cooling between ∼21–40 ka. We therefore suggest that the “Antarctic‐like” timing and amplitude of millennial‐scale SST variations in the subtropical Northwestern Pacific between 20–40 ka may have been determined by rapid ocean adjustment processes in response to abrupt wind stress and meridional temperature gradient changes in the North Pacific.

Estimation of the economic impact of temperature changes induced by a shutdown of the thermohaline circulation: an application of FUND

The integrated assessment model FUND 2.8n is applied in an assessment to estimate the magnitude of the general market and non-market impacts of temperature changes caused by a possible shutdown of the thermohaline circulation (THC). The monetized impacts of this change in environmental conditions are determined for 207 individual countries for two scenarios: one warming scenario in which the THC weakens but remains intact, and another in which the THC breaks down. Eight different response patterns are identified. The dominant pattern is that a THC shutdown has an offsetting effect on the underlying warming trend. Depending on whether the impacts of warming are initially beneficial or detrimental, the economic effects of a THC shutdown show distinct regional variability. Key economic sectors affected are water resources and energy consumption, as well as cardiovascular and respiratory diseases among health impacts. The maximum national impact of a shutdown of the THC turns out to be of the magnitude of a few per cent of GDP, but the average global impact is much smaller. The results indicate that the temperature effect of a THC shutdown does not create an insurmountable economic threat on a global scale, but may cause severe damages to individual countries. However, a consideration of other climatic impacts such as precipitation and sea level changes is likely to alter the identified trends in economic development.

An iconic approach for representing climate change

International and national greenhouse gas emissions reduction goals implicitly rely in part on individuals undertaking voluntary emissions reductions through lifestyle decisions. Whilst there is widespread public recognition of climate change as an issue, there are many barriers – cognitive, psychological and social – preventing individuals from enacting lifestyle decarbonisation. More effective climate change communication approaches are needed which allow individuals to engage meaningfully with climate change, thus opening new prospects for lifestyle decarbonisation. This study presents an iconic approach to engagement, tested in the UK context, which allows individuals to approach climate change through their own personal values and experiences. The iconic approach harnesses the emotive and visual power of climate icons with a rigorous scientific analysis of climate impacts under a different climate future. Although some climate icons already exist – for example the Thermohaline Circulation shutdown – these ‘expert-led’ icons fail to effectively engage ‘non-experts’. We demonstrate that the non-expert-led iconic approach helps overcome some of the cognitive and affective barriers that impede action towards lifestyle decarbonisation.

An Integrated Assessment of changes in the thermohaline circulation

This paper discusses the risks of a shutdown of the thermohaline circulation (THC) for the climate system, for ecosystems in and around the North Atlantic as well as for fisheries and agriculture by way of an Integrated Assessment. The climate model simulations are based on greenhouse gas scenarios for the 21st century and beyond. A shutdown of the THC, complete by 2150, is triggered if increased freshwater input from inland ice melt or enhanced runoff is assumed. The shutdown retards the greenhouse gas-induced atmospheric warming trend in the Northern Hemisphere, but does not lead to a persistent net cooling. Due to the simulated THC shutdown the sea level at the North Atlantic shores rises by up to 80 cm by 2150, in addition to the global sea level rise. This could potentially be a serious impact that requires expensive coastal protection measures. A reduction of marine net primary productivity is associated with the impacts of warming rather than a THC shutdown. Regional shifts in the currents in the Nordic Seas could strongly deteriorate survival chances for cod larvae and juveniles. This could lead to cod fisheries becoming unprofitable by the end of the 21st century. While regional socioeconomic impacts might be large, damages would be probably small in relation to the respective gross national products. Terrestrial ecosystem productivity is affected much more by the fertilization from the increasing CO2 concentration than by a THC shutdown. In addition, the level of warming in the 22nd to 24th century favours crop production in northern Europe a lot, no matter whether the THC shuts down or not. CO2 emissions corridors aimed at limiting the risk of a THC breakdown to 10% or less are narrow, requiring departure from business-as-usual in the next few decades. The uncertainty about THC risks is still high. This is seen in model analyses as well as in the experts’ views that were elicited. The overview of results presented here is the outcome of the Integrated Assessment project INTEGRATION.

ENSO Suppression due to Weakening of the North Atlantic Thermohaline Circulation*

Abstract Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Niño–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections.

Ocean Gyres and Abrupt Change in the Thermohaline Circulation: A Conceptual Analysis

Abstract The implications are investigated of representing ocean gyre circulations by a diffusion term in the Stommel and Rooth box models of the thermohaline circulation (THC) in one and two hemispheres, respectively. The approach includes mostly analytical solution and study of the bifurcation structure, but also numerical integration and feedback analysis. Sufficient diffusion (gyre strength) eliminates multiple equilibria from either model, highlighting the need for accurate gyre circulation strength in general circulation models (GCMs) when considering the potential for abrupt climate change associated with THC shutdown. With diffusion, steady-state flow strength in the Rooth model depends on freshwater forcing (i.e., implied atmospheric water vapor transport) in both hemispheres, not only on that in the upwelling hemisphere, as in the nondiffusive case. With asymmetric freshwater forcing, two solutions (strong stable and weak unstable) are found with sinking in the hemisphere with stronger forcing and one solution with sinking in the other hemisphere. Under increased freshwater forcing the two solutions in the hemisphere with stronger forcing meet in a saddle-node bifurcation (if diffusion is sufficiently strong to prevent a subcritical Hopf bifurcation first), followed by flow reversal. Thus, the bifurcation structure with respect to freshwater forcing of the diffusive Rooth model of two-hemisphere THC is similar to that of the Stommel model of single-hemisphere THC, albeit with a very different dynamical interpretation. Gyre circulations stabilize high-latitude sinking in the Stommel model. In the Rooth model, gyre circulations only stabilize high-latitude sinking if the freshwater forcing is weaker in the sinking hemisphere than in the upwelling hemisphere, by an amount that increases with diffusion. The values of diffusion and freshwater forcing at which qualitative change in behavior occurs correspond to the range of the values used in and obtained with GCMs, suggesting that this analysis can provide a conceptual foundation for analyzing the stability of the interhemispheric THC, and also for the potential of the Atlantic THC to undergo abrupt change.

Global warming and thermohaline circulation stability.

The Atlantic thermohaline circulation (THC) plays an important role in global climate. Theoretical and palaeoclimatic evidence points to the possibility of rapid changes in the strength of the THC, including a possible quasi-permanent shutdown. The climatic impacts of such a shutdown would be severe, including a cooling throughout the Northern Hemisphere, which in some regions is greater in magnitude than the changes expected from global warming in the next 50 years. Other climatic impacts would likely include a severe alteration of rainfall patterns in the tropics, the Indian subcontinent and Europe. Modelling the future behaviour of the THC focuses on two key questions. (i) Is a gradual weakening of the THC likely in response to global warming, and if so by how much? (ii) Are there thresholds beyond which rapid or irreversible changes in the THC are likely? Most projections of the response of the THC to increasing concentrations of greenhouse gases suggest a gradual weakening over the twenty-first century. However, there is a wide variation between different models over the size of the weakening. Rapid or irreversible THC shutdown is considered a low-probability (but high-impact) outcome; however, some climate models of intermediate complexity do show the possibility of such events. The question of the future of the THC is beset with conceptual, modelling and observational uncertainties, but some current and planned projects show promise to make substantial progress in tackling these uncertainties in future.