Transient simulation of the last glacial inception. Part II: sensitivity and feedback analysis

The sensitivity of the last glacial-inception (around 115 kyr BP, 115,000 years before present) to different feedback mechanisms has been analysed by using the Earth system model of intermediate complexity CLIMBER-2. CLIMBER-2 includes dynamic modules of the atmosphere, ocean, terrestrial biosphere and inland ice, the last of which was added recently by utilising the three-dimensonal polythermal ice-sheet model SICOPOLIS. We performed a set of transient experiments starting at the middle of the Eemiam interglacial and ran the model for 26,000 years with time-dependent orbital forcing and observed changes in atmospheric CO₂ concentration (CO₂ forcing). The role of vegetation and ocean feedback, CO₂ forcing, mineral dust, thermohaline circulation and orbital insolation were closely investigated. In our model, glacial inception, as a bifurcation in the climate system, appears in nearly all sensitivity runs including a run with constant atmospheric CO₂ concentration of 280 ppmv, a typical interglacial value, and simulations with prescribed present-day sea-surface temperatures or vegetation cover—although the rate of the growth of ice-sheets growth is smaller than in the case of the fully interactive model. Only if we run the fully interactive model with constant present-day insolation and apply present-day CO₂ forcing does no glacial inception appear at all. This implies that, within our model, the orbital forcing alone is sufficient to trigger the interglacial–glacial transition, while vegetation, ocean and atmospheric CO₂ concentration only provide additional, although important, positive feedbacks. In addition, we found that possible reorganisations of the thermohaline circulation influence the distribution of inland ice.     

Ice-sheet growth and high-latitudes sea-surface temperature

The response of the LLN 2-D climate model to the insolation and CO₂ forcings during the Eemian interglacial is compared to reconstructions obtained from deep-sea cores drilled in the Norwegian Sea and in the North Atlantic. Both reconstructions and modeling results show a decrease of sea-surface temperature (SST) in the higher latitudes (70–75 °N zonal belt for the model and the Norwegian Sea for the proxy records), associated with a more moderate cooling at lower latitudes (50–55 °N and North Atlantic), at the middle of isotopic substage 5e, several millenia before the beginning of continental ice-sheet growth. Such a comparison between the simulated SST and ice volume of the Northern Hemisphere has been extended to the whole last glacial-interglacial cycle. The influence of the insolation forcing on SST and the shortcomings of the model due to its zonal character are discussed. Received: 6 July 1995/Accepted: 19 December 1995     

Simulated changes in vegetation distribution, land carbon storage, and atmospheric CO2 in response to a collapse of the North Atlantic thermohaline circulation

It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon cycle. The Lund–Potsdam–Jena Dynamic Global Vegetation Model is forced with climate perturbations from glacial freshwater experiments with the ECBILT-CLIO ocean–atmosphere–sea ice model. A reorganisation of the marine carbon cycle is not addressed. Modelled NA THC collapses and recovers after about a millennium in response to prescribed freshwater forcing. The initial cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the Hadley Centre model HadSM3 for different periods over the past 21 kyr. Changes in terrestrial storage vary between −67 and +50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric CO₂ are 6 and 13 ppmv for glacial and late Holocene conditions. Simulated changes in δ¹³C are between 0.15 and 0.25‰. These simulated carbon storage anomalies during a NA THC collapse depend on their magnitude on the CO₂ fertilisation feedback mechanism. The CO₂ changes simulated for glacial conditions are compatible with available evidence from marine studies and the ice core CO₂ record. The latter shows multi-millennial CO₂ variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4 in the South and cooling in the North.     

Long-term climate changes due to increased CO2 concentration in the coupled atmosphere-ocean general circulation model ECHAM3/LSG

 The long-term adjustment processes of atmosphere and ocean in response to gradually increased atmospheric CO₂ concentration have been analysed in two 850-year integrations with a coupled atmosphere-ocean general circulation model (AOGCM). In these experiments the CO₂ concentration has been increased to double and four times the initial concentration, respectively, and is kept fixed thereafter. Three characteristic time scales have been identified: a very fast response associated with processes dominated by the atmospheric adjustment, an intermediate time scale of a few decades connected with processes in the upper ocean, and adjustment processes with time scales of centuries and longer due to the inertia of the deep ocean. The latter in particular is responsible for a still ongoing adjustment of the atmosphere-ocean system at the end of the integrations after 850 years. After 60 years, at the time of CO₂ doubling, the global mean near-surface air temperature rises by 1.4 K. In spite of the constant CO₂ concentration during the following centuries the warming continues to 2.6 K after 850 years. The behaviour of the quadrupling run is similar: global mean near-surface air temperature increases by 3.8 K at the time of CO₂ quadrupling and by 4.8 K at the end of the simulation. The thermohaline circulation undergoes remarkable changes. Temporarily, the North Atlantic overturning circulation weakens by up to 30% in the CO₂ doubling experiment and up to 50% in the CO₂ quadrupling experiment. After reaching the minimum the North Atlantic overturning slowly recovers in both experiments. Received: 23 August 1999 / Accepted: 27 April 2000     

Modelling the concentration of atmospheric CO2 during the Younger Dryas climate event

 The Younger Dryas (YD, dated between 12.7–11.6 ky BP in the GRIP ice core, Central Greenland) is a distinct cold period in the North Atlantic region during the last deglaciation. A popular, but controversial hypothesis to explain the cooling is a reduction of the Atlantic thermohaline circulation (THC) and associated northward heat flux as triggered by glacial meltwater. Recently, a CH₄-based synchronization of GRIP δ¹⁸O and Byrd CO₂ records (West Antarctica) indicated that the concentration of atmospheric CO₂ (CO^atm ₂) rose steadily during the YD, suggesting a minor influence of the THC on CO^atm ₂ at that time. Here we show that the CO^atm ₂ change in a zonally averaged, circulation-biogeochemistry ocean model when THC is collapsed by freshwater flux anomaly is consistent with the Byrd record. Cooling in the North Atlantic has a small effect on CO^atm ₂ in this model, because it is spatially limited and compensated by far-field changes such as a warming in the Southern Ocean. The modelled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP (Northern Hemisphere) and from Byrd and Vostok (East Antarctica) during the YD. δ¹³C depletion and PO₄ enrichment are predicted at depth in the North Atlantic, but not in the Southern Ocean. This could explain a part of the controversy about the intensity of the THC during the YD. Potential weaknesses in our interpretation of the Byrd CO₂ record in terms of THC changes are discussed. Received: 27 May 1998 / Accepted: 5 November 1998     

Internal Variability as a Cause of Qualitative Intermodel Disagreement on Anthropogenic Climate Changes

Summary The qualitative agreement of two climate models, HADCM2 and ECHAM3, on the response of surface climate to anthropogenic climate forcing in the period 2020 – 2049 is studied. Special attention is paid to the role of internal climate variability as a source of intermodel disagreement. After illustrating the methods in an intermodel comparison of simulated changes in June–August mean precipitation, some global statistics are presented. Excluding surface air temperature, the four-season mean proportion of areas in which the two models agree on the sign of the climatic response is only 53 – 60% both for increases in CO₂ alone and for increases in CO₂ together with direct radiative forcing by sulphate aerosols, but somewhat larger, 59 – 70% for the separate aerosol effect. In areas where the response is strong (at least twice the standard error associated with internal variability) in both models, the agreement is better and the contrast between the different forcings becomes more marked. The proportion of agreement in such areas is 57 – 75% for the response to increases in CO₂ alone, 64 – 84% for the response to combined CO₂ and aerosol forcing, and as high as 88 – 94% for the separate aerosol effect. The relatively good intermodel agreement for aerosol-induced climate changes is suggested to be associated with the uneven horizontal distribution of aerosol forcing. Received December 2, 1998 Revised May 5, 1999     

Dangerous anthropogenic interference, dangerous climatic change, and harmful climatic change: non-trivial distinctions with significant policy implications

Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC) calls for stabilization of greenhouse gas (GHG) concentrations at levels that prevent dangerous anthropogenic interference (DAI) in the climate system. However, some of the recent policy literature has focused on dangerous climatic change (DCC) rather than on DAI. DAI is a set of increases in GHGs concentrations that has a non-negligible possibility of provoking changes in climate that in turn have a non-negligible possibility of causing unacceptable harm, including harm to one or more of ecosystems, food production systems, and sustainable socio-economic systems, whereas DCC is a change of climate that has actually occurred or is assumed to occur and that has a non-negligible possibility of causing unacceptable harm. If the goal of climate policy is to prevent DAI, then the determination of allowable GHG concentrations requires three inputs: the probability distribution function (pdf) for climate sensitivity, the pdf for the temperature change at which significant harm occurs, and the allowed probability (“risk”) of incurring harm previously deemed to be unacceptable. If the goal of climate policy is to prevent DCC, then one must know what the correct climate sensitivity is (along with the harm pdf and risk tolerance) in order to determine allowable GHG concentrations. DAI from elevated atmospheric CO₂ also arises through its impact on ocean chemistry as the ocean absorbs CO₂. The primary chemical impact is a reduction in the degree of supersaturation of ocean water with respect to calcium carbonate, the structural building material for coral and for calcareous phytoplankton at the base of the marine food chain. Here, the probability of significant harm (in particular, impacts violating the subsidiary conditions in Article 2 of the UNFCCC) is computed as a function of the ratio of total GHG radiative forcing to the radiative forcing for a CO₂ doubling, using two alternative pdfs for climate sensitivity and three alternative pdfs for the harm temperature threshold. The allowable radiative forcing ratio depends on the probability of significant harm that is tolerated, and can be translated into allowable CO₂ concentrations given some assumption concerning the future change in total non-CO₂ GHG radiative forcing. If future non-CO₂ GHG forcing is reduced to half of the present non-CO₂ GHG forcing, then the allowable CO₂ concentration is 290–430 ppmv for a 10% risk tolerance (depending on the chosen pdfs) and 300–500 ppmv for a 25% risk tolerance (assuming a pre-industrial CO₂ concentration of 280 ppmv). For future non-CO₂ GHG forcing frozen at the present value, and for a 10% risk threshold, the allowable CO₂ concentration is 257–384 ppmv. The implications of these results are that (1) emissions of GHGs need to be reduced as quickly as possible, not in order to comply with the UNFCCC, but in order to minimize the extent and duration of non-compliance; (2) we do not have the luxury of trading off reductions in emissions of non-CO₂ GHGs against smaller reductions in CO₂ emissions, and (3) preparations should begin soon for the creation of negative CO₂ emissions through the sequestration of biomass carbon.     

Late Pliocene to Pleistocene sensitivity of the Greenland Ice Sheet in response to external forcing and internal feedbacks

The timing and nature of ice sheet variations on Greenland over the last ～5 million years remain largely uncertain. Here, we use a coupled climate-vegetation-ice sheet model to determine the climatic sensitivity of Greenland to combined sets of external forcings and internal feedbacks operating on glacial-interglacial timescales. In particular, we assess the role of atmospheric pCO₂, orbital forcing, and vegetation dynamics in modifying thresholds for the onset of glaciation in late Pliocene and Pleistocene. The response of circum-Arctic vegetation to declining levels of pCO₂ (from 400 to 200 ppmv) and decreasing summer insolation includes a shift from boreal forest to tundra biomes, with implications for the surface energy balance. The expansion of tundra amplifies summer surface cooling and heat loss from the ground, leading to an expanded summer snow cover over Greenland. Atmospheric and land surface fields respond to forcing most prominently in late spring-summer and are more sensitive at lower Pleistocene-like levels of pCO₂. We find cold boreal summer orbits produce favorable conditions for ice sheet growth, however simulated ice sheet extents are highly dependent on both background pCO₂ levels and land-surface characteristics. As a result, late Pliocene ice sheet configurations on Greenland differ considerably from late Pleistocene, with smaller ice caps on high elevations of southern and eastern Greenland, even when orbital forcing is favorable for ice sheet growth.     

Temperature and snow-melt controls on interannual variability in carbon exchange in the high Arctic

Summary Net Ecosystem CO₂ Exchange (NEE) was studied during the summer season (June–August) at a high Arctic heath ecosystem for 5 years in Zackenberg, NE Greenland. Integrated over the 80 day summer season, the heath is presently a sink ranging from −1.4 g C m⁻² in 1997 to −23.3 g C m⁻² in 2003. The results indicate that photosynthesis might be more variable than ecosystem respiration on the seasonal timescale. The years focused on in this paper differ climatically, which is reflected in the measured fluxes. The environmental conditions during the five years strongly indicated that time of snow-melt and air temperature during the growing season are closely related to the interannual variation in the measured fluxes of CO₂ at the heath. Our estimates suggest that net ecosystem CO₂ uptake is enhanced by 0.16 g C m⁻² per increase in growing degree-days during the period of growth. This study emphasises that increased summer time air temperatures are favourable for this particular ecosystem in terms of carbon accumulation.     

Influences of the NAO on the North Atlantic CO_2 Fluxes in Winter and Summer on the Interannual Scale

The differences in the influences of the North Atlantic Oscillation (NAO) on the air–sea CO_2 fluxes (f CO_2) in the North Atlantic (NA) between different seasons and between different regions are rarely fully investigated. We used observation-based data of f CO_2, surface-ocean CO_2partial pressure (p CO_(2sea)), wind speed and sea surface temperature(SST) to analyze the relationship between the NAO and f CO_2 of the subtropical and subpolar NA in winter and summer on the interannual time scale. Based on power spectrum estimation, there are significant interannual signs with a 2–6 year cycle in the NAO indexes and area-averaged f CO_2 anomalies in winter and summer from 1980 to 2015. Regression analysis with the 2–6 year filtered data shows that on the interannual scale the response of the f CO_2 anomalies to the NAO has an obvious meridional wave-train-like pattern in winter, but a zonal distribution in summer. This seasonal difference is because in winter the f CO_2anomalies are mainly controlled by the NAO-driven wind speed anomalies, which have a meridional distribution pattern, while in summer they are dominated by the NAO-driven SST anomalies, which show distinct zonal difference in the subtropical NA. In addition, in the same season, there are different factors controlling the variation of p CO_(2sea)in different regions. In summer, SST is important to the interannual variation of p CO_(2sea)in the subtropical NA, while some biogeochemical variables probably control the p CO_(2sea) variation in the subpolar NA.     

A transient climate change simulation with greenhouse gas and aerosol forcing: projected climate to the twenty-first century

 The potential climatic consequences of increasing atmospheric greenhouse gas (GHG) concentration and sulfate aerosol loading are investigated for the years 1900 to 2100 based on five simulations with the CCCma coupled climate model. The five simulations comprise a control experiment without change in GHG or aerosol amount, three independent simulations with increasing GHG and aerosol forcing, and a simulation with increasing GHG forcing only. Climate warming accelerates from the present with global mean temperatures simulated to increase by 1.7 °C to the year 2050 and by a further 2.7 °C by the year 2100. The warming is non-uniform as to hemisphere, season, and underlying surface. Changes in interannual variability of temperature show considerable structure and seasonal dependence. The effect of the comparatively localized negative radiative forcing associated with the aerosol is to retard and reduce the warming by about 0.9 °C at 2050 and 1.2 °C at 2100. Its primary effect on temperature is to counteract the global pattern of GHG-induced warming and only secondarily to affect local temperatures suggesting that the first order transient climate response of the system is determined by feedback processes and only secondarily by the local pattern of radiative forcing. The warming is accompanied by a more active hydrological cycle with increases in precipitation and evaporation rates that are delayed by comparison with temperature increases. There is an “El Nino-like” shift in precipitation and an overall increase in the interannual variability of precipitation. The effect of the aerosol forcing is again primarily to delay and counteract the GHG-induced increase. Decreases in soil moisture are common but regionally dependent and interannual variability changes show considerable structure. Snow cover and sea-ice retreat. A PNA-like anomaly in mean sea-level pressure with an enhanced Aleutian low in northern winter is associated with the tropical shift in precipitation regime. The interannual variability of mean sea-level pressure generally decreases with largest decreases in the tropical Indian ocean region. Changes to the ocean thermal structure are associated with a spin-down of the Atlantic thermohaline circulation together with a decrease in its variability. The effect of aerosol forcing, although modest, differs from that for most other quantities in that it does not act primarily to counteract the GHG forcing effect. The barotropic stream function in the ocean exhibits modest change in the north Pacific but accelerating changes in much of the Southern Ocean and particularly in the north Atlantic where the gyre spins down in conjunction with the decrease in the thermohaline circulation. The results differ in non-trivial ways from earlier equilibrium 2 × CO₂ results with the CCCma model as a consequence of the coupling to a fully three-dimensional ocean model and the evolving nature of the forcing. Received: 24 September 1998 / Accepted: 8 October 1999     

Evaluation of ocean and climate models using present‐day observations and forcing

Abstract

The most common method used to evaluate climate models involves spinning them up under perpetual present‐day forcing and comparing the model results with present‐day observations. This approach clearly ignores any potential long‐term memory of the model ocean to past climatic conditions. Here we examine the validity of this approach through the 6000‐year integration of a coupled atmosphere–ocean–sea‐ice model. The coupled model is initially spun‐up with atmospheric CO₂ concentrations and orbital parameters applicable for 6KBP. The model is then integrated forward in time to 2100. Results from this transient coupled model simulation are compared with the results from two additional simulations, in which the model is spun up with perpetual 1850 (preindustrial) and 1998 (present‐day) atmospheric CO₂ concentrations and orbital parameters. This comparison leads to substantial differences between the equilibrium climatologies and the transient simulation, even at 1850 (in weakly ventilated regions), prior to any significant changes in atmospheric CO₂. When compared to the present‐day equilibrium climatology, differences are very large: the global mean surface air and sea surface temperatures are ,0.5°C and ,0.4°C colder, respectively, deep ocean temperatures are substantially cooler, Southern Hemisphere sea‐ice cover is 38% larger, and the North Atlantic conveyor 16% weaker in the transient case. These differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene, as well as to its large thermal inertia. It is also demonstrated that a ‘cold start’ global warming simulation (one that starts from a 1998 equilibrium climatology) underestimates the global temperature increase at 2100 by ,10%. Our results question the accuracy of current techniques for climate model evaluation and underline the importance of using paleoclimatic simulations in parallel with present‐day simulations in this evaluation process.     

Closure of the Panama Seaway during the Pliocene: implications for climate and Northern Hemisphere glaciation

The “Panama Hypothesis” states that the gradual closure of the Panama Seaway, between 13 million years ago (13 Ma) and 2.6 Ma, led to decreased mixing of Atlantic and Pacific water Masses, the formation of North Atlantic Deep water and strengthening of the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere (NH) high latitudes, culminating in the intensification of Northern Hemisphere Glaciation (NHG) during the Pliocene, 3.2–2.7 Ma. Here we test this hypothesis using a fully coupled, fully dynamic ocean-atmosphere general circulation model (GCM) with boundary conditions specific to the Pliocene, and a high resolution dynamic ice sheet model. We carry out two GCM simulations with “closed” and “open” Panama Seaways, and use the simulated climatologies to force the ice sheet model. We find that the models support the “Panama Hypothesis” in as much as the closure of the seaway results in a more intense Atlantic thermohaline circulation, enhanced precipitation over Greenland and North America, and ultimately larger ice sheets. However, the volume difference between the ice sheets in the “closed” and “open” configurations is small, equivalent to about 5 cm of sea level. We conclude that although the closure of the Panama Seaway may have slightly enhanced or advanced the onset of NHG, it was not a major forcing mechanism. Future work must fully couple the ice sheet model and GCM, and investigate the role of orbital and CO₂ effects in controlling NHG.     

Factors influencing anthropogenic carbon dioxide uptake in the North Atlantic in models of the ocean carbon cycle

The uptake and storage of anthropogenic carbon in the North Atlantic is investigated using different configurations of ocean general circulation/carbon cycle models. We investigate how different representations of the ocean physics in the models, which represent the range of models currently in use, affect the evolution of CO₂ uptake in the North Atlantic. The buffer effect of the ocean carbon system would be expected to reduce ocean CO₂ uptake as the ocean absorbs increasing amounts of CO₂. We find that the strength of the buffer effect is very dependent on the model ocean state, as it affects both the magnitude and timing of the changes in uptake. The timescale over which uptake of CO₂ in the North Atlantic drops to below preindustrial levels is particularly sensitive to the ocean state which sets the degree of buffering; it is less sensitive to the choice of atmospheric CO₂ forcing scenario. Neglecting physical climate change effects, North Atlantic CO₂ uptake drops below preindustrial levels between 50 and 300 years after stabilisation of atmospheric CO₂ in different model configurations. Storage of anthropogenic carbon in the North Atlantic varies much less among the different model configurations, as differences in ocean transport of dissolved inorganic carbon and uptake of CO₂ compensate each other. This supports the idea that measured inventories of anthropogenic carbon in the real ocean cannot be used to constrain the surface uptake. Including physical climate change effects reduces anthropogenic CO₂ uptake and storage in the North Atlantic further, due to the combined effects of surface warming, increased freshwater input, and a slowdown of the meridional overturning circulation. The timescale over which North Atlantic CO₂ uptake drops to below preindustrial levels is reduced by about one-third, leading to an estimate of this timescale for the real world of about 50 years after the stabilisation of atmospheric CO₂. In the climate change experiment, a shallowing of the mixed layer depths in the North Atlantic results in a significant reduction in primary production, reducing the potential role for biology in drawing down anthropogenic CO₂.     

Seasonal variations of net CO2 exchange in European Arctic ecosystems

Summary  The carbon dioxide exchange in arctic and subarctic terrestrial ecosystems has been measured using the eddy-covariance method at sites representing the latitudinal and longitudinal extremes of the European Arctic sea areas as part of the Land Arctic Physical Processes (LAPP) project. The sites include two fen (Kaamanen and Kevo) and one mountain birch ecosystems in subarctic northern Finland (69° N); fen, heathland, and snowbed willow ecosystems in northeastern Greenland (74° N); and a polar semidesert site in Svalbard (79° N). The measurement results, which are given as weekly average diurnal cycles, show the striking seasonal development of the net CO₂ fluxes. The seasonal periods important for the net CO₂ fluxes, i.e. winter, thaw, pre-leaf, summer, and autumn can be identified from measurements of the physical environment, such as temperature, albedo, and greenness. During the late winter period continuous efflux is observed at the permafrost-free Kaamanen site. At the permafrost sites, efflux begins during the thaw period, which lasts about 3–5 weeks, in contrast to the Kaamanen site where efflux continues at the same rate as during the winter. Seasonal efflux maximum is during the pre-leaf period, which lasts about 2–5 weeks. The summer period lasts 6 weeks in NE Greenland but 10–14 weeks in northern Finland. During a high summer week, the mountain birch ecosystem had the highest gross photosynthetic capacity, GP _max, followed by the fen ecosystems. The polar semidesert ecosystem had the lowest GP _max. By the middle of August, noon uptake fluxes start to decrease as the solar elevation angle decreases and senescence begins within the vascular plants. At the end of the autumn period, which lasts 2–5 weeks, topsoil begins to freeze at the end of August in Svalbard; at the end of September at sites in eastern Greenland; and one month later at sites in northern Finland. Received March 1, 2000 Revised October 2, 2000     

A European pattern climatology 1766–2000

Using monthly independently reconstructed gridded European fields for the 500 hPa geopotential height, temperature, and precipitation covering the last 235 years we investigate the temporal and spatial evolution of these key climate variables and assess the leading combined patterns of climate variability. Seasonal European temperatures show a positive trend mainly over the last 40 years with absolute highest values since 1766. Precipitation indicates no clear trend. Spatial correlation technique reveals that winter, spring, and autumn covariability between European temperature and precipitation is mainly influenced by advective processes, whereas during summer convection plays the dominant role. Empirical Orthogonal Function analysis is applied to the combined fields of pressure, temperature, and precipitation. The dominant patterns of climate variability for winter, spring, and autumn resemble the North Atlantic Oscillation and show a distinct positive trend during the past 40 years for winter and spring. A positive trend is also detected for summer pattern 2, which reflects an increased influence of the Azores High towards central Europe and the Mediterranean coinciding with warm and dry conditions. The question to which extent these recent trends in European climate patterns can be explained by internal variability or are a result of radiative forcing is answered using cross wavelets on an annual basis. Natural radiative forcing (solar and volcanic) has no imprint on annual European climate patterns. Connections to CO₂ forcing are only detected at the margins of the wavelets where edge effects are apparent and hence one has to be cautious in a further interpretation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The cold event 8200 years ago documented in oxygen isotope records of precipitation in Europe and Greenland

 Stable oxygen isotope ratios of ostracod valves in Late Glacial and Holocene sediments of core AS 92-5 from deep lake Ammersee (southern Germany) reflect variations of mean oxygen isotope ratios in past atmospheric precipitation. The record reconfirms the strong similarity of climate evolution in Europe and Greenland during the last deglaciation. For the first time in Europe, we find a 200-year-long negative δ¹⁸O-excursion, which is contemporaneous with the strongest negative δ¹⁸O-excursion in the Greenland ice around 8.2 ky before present. The 8.2 ky isotopic event on both sides of the North Atlantic ocean is interpreted as a cold period, most probably induced by a perturbation of the North Atlantic thermohaline circulation. We discuss two possible triggering mechanisms: (1) weak forcing (as proposed by Alley et al.), and (2) forcing by a strong and sudden freshwater pulse from the collapse of the Hudson Ice Dome. Received: 27 May 1997 / Accepted: 21 July 1997     

Net CO2 exchange of a subarctic mountain birch ecosystem

Summary  Net ecosystem CO₂ exchange was measured over a mountain birch forest in northern Finland throughout the growing season. The maximal net CO₂ uptake rate of about − 0.5 mg(CO₂) m⁻² s⁻¹ was observed at the end of July. The highest nocturnal respiration rates in early August were 0.2 mg(CO₂) m⁻² s⁻¹. The daily CO₂ balances during the time of maximal photosynthesis were about −15 g(CO₂) m⁻² d⁻¹. The mountain birch forest acted as a net sink of CO₂ from 30 June to 28 August. During that period the net CO₂ balance was −448 g(CO₂)m⁻². The interannual representativeness of the observed balances was studied using a simplified daily balance model, with daily mean global radiation and air temperature as the input parameters. The year-to-year variation in the phenological development was parameterised as a function of the cumulative effective temperature sum. The daily balance model was used for estimating the variability in the seasonal CO₂ balances due to the timing of spring and meteorological factors. The sink term of CO₂ in 1996 was lower than the 15-year mean, mainly due to the relatively late emergence of the leaves. Received October 11, 1999 Revised April 25, 2000     

Quantifying the role of biosphere-atmosphere feedbacks in climate change: coupled model simulations for 6000 years BP and comparison with palaeodata for northern Eurasia and northern Africa

 The LMD AGCM was iteratively coupled to the global BIOME1 model in order to explore the role of vegetation-climate interactions in response to mid-Holocene (6000 y BP) orbital forcing. The sea-surface temperature and sea-ice distribution used were present-day and CO₂ concentration was pre-industrial. The land surface was initially prescribed with present-day vegetation. Initial climate “anomalies” (differences between AGCM results for 6000 y BP and control) were used to drive BIOME1; the simulated vegetation was provided to a further AGCM run, and so on. Results after five iterations were compared to the initial results in order to identify vegetation feedbacks. These were centred on regions showing strong initial responses. The orbitally induced high-latitude summer warming, and the intensification and extension of Northern Hemisphere tropical monsoons, were both amplified by vegetation feedbacks. Vegetation feedbacks were smaller than the initial orbital effects for most regions and seasons, but in West Africa the summer precipitation increase more than doubled in response to changes in vegetation. In the last iteration, global tundra area was reduced by 25% and the southern limit of the Sahara desert was shifted 2.5 °N north (to 18 °N) relative to today. These results were compared with 6000 y BP observational data recording forest-tundra boundary changes in northern Eurasia and savana-desert boundary changes in northern Africa. Although the inclusion of vegetation feedbacks improved the qualitative agreement between the model results and the data, the simulated changes were still insufficient, perhaps due to the lack of ocean-surface feedbacks. Received: 5 December 1996 / Accepted: 16 June 1997     

Evidence bearing on the mechanism of rapid deglaciation

Six northeast Atlantic cores contain planktonic foraminiferal records implying a very abrupt glacial/interglacial surface-ocean warming roughly coincident with the last deglaciation (isotopic termination II) at 127 000 yr B.P. These faunal composition curves have, however, been substantially altered by sediment mixing processes on the sea floor; they are translated downward in the core record and made to look steeper than they actually were. The reason for this abnormally large mixing impact is an interval of sediment with very low to negligible concentrations of all microfossils (surface ocean and bottom living). These low concentrations reflect a several-thousand-year interval of low productivity and little or no life in the overlying surface waters. We interpret this thorough suppression of productivity as a consequence of meltwater and icebergs flooding into the subpolar Atlantic gyre from the surrounding Northern Hemisphere ice sheets during deglaciation. The meltwater influx inhibited warm-season productivity by maintaining a well-stratified low-salinity surface layer; in winter, the low salinity layer froze, stopping nutrientrich deep waters from surfacing in normal cold-season convection. The earth's orbital configuration during this deglaciation created an unusually strong summer insolation maximum and winter insolation minimum in the Northern Hemisphere. Rapid melting and disintegration of the Northern Hemisphere ice sheets induced by strong summer insolation apparently created the meltwater influx; combined with very low winter insolation, the presence of this low-salinity meltwater layer led to unusually extensive sea-ice formation. The existence of a large region of winter sea ice across the subpolar North Atlantic during deglaciation implies a reduced supply of moisture in winter to the wasting Northern Hemisphere ice sheets. This includes the loss of winter moisture both locally from ice-covered northern waters and regionally from low-latitude winter storms no longer penetrating northward. The winter sea-ice cover thus acts as an amplifier providing positive feedback to the insolation-driven deglaciation process.