H. Renssen

The role of solar forcing upon climate change.

Evidence for millennial-scale climate changes during the last 60,000 years has been found in Greenland ice cores and North Atlantic ocean cores. Until now, the cause of these climate changes remained a matter of debate. We argue that variations in solar activity may have played a significant role in forcing these climate changes. We review the coincidence of variations in cosmogenic isotopes (14C and 10Be) with climate changes during the Holocene and the upper part of the last Glacial, and present two possible mechanisms (involving the role of solar UV variations and solar wind/cosmic rays) that may explain how small variations in solar activity are amplified to cause significant climate changes. Accepting the idea of solar forcing of Holocene and Glacial climatic shifts has major implications for our view of present and future climate. It implies that the climate system is far more sensitive to small variations in solar activity than generally believed.

The sharp rise of 14C around 800 cal BC: possible causes, related climatic teleconnections and the impact on human environments.

In this study we report on accelerator mass spectrometry (AMS) wiggle-match dating of selected macrofossils from organic deposits ca. 800 cal BC (ca. 2650 BP). Based on paleological, archaeological and geological evidence, we found that the sharp rise of atmospheric (super 14) C between 850 and 760 cal BC corresponds to the following related phenomena: 1. In European raised bog deposits, the changing spectrum of peat forming mosses and a sharp decline in decomposition of the peat indicate a sudden change from relatively dry and warm to cool, moist climatic conditions. 2. As a consequence of climate change, there was a fast and considerable rise of the groundwater table so that peat growth started in areas that were already marginal from a hydrological point of view. 3. The rise of the groundwater table in low-lying areas of the Netherlands resulted in the abandonment of settlement sites. 4. The contemporaneous earliest human colonization of newly emerged salt marshes in the northern Netherlands (after loss of cultivated land) may have been related to thermal contraction of ocean water, causing a temporary stagnation in the relative sea-level rise. Furthermore, there is evidence for synchronous climatic change in Europe and on other continents (climatic teleconnections on both hemispheres) ca. 2650 BP. We discuss reduced solar activity and the related increase of cosmic rays as a cause for the observed climatological phenomena and the contemporaneous rise in the (super 14) C-content of the atmosphere. Cosmic rays may have been a factor in the formation of clouds and precipitation, and in that way changes in solar wind were amplified and the effects induced abrupt climate change.

The 8.2 kyr BP event simulated by a Global Atmosphere—Sea-Ice—Ocean Model

Seven freshwater perturbation experiments were performed with a global atmosphere-sea-ice-ocean model to study the mechanism behind the 8.2 kyr BP Holocene cooling event. These experiments differed in initial state and duration of the applied freshwater pulse, while the amount of freshwater was kept constant (4.67x10(14) m(3)). One of the scenarios, with freshwater added to the Labrador Sea at a rate of 0.75 Sv during 20 years, resulted in weakening of the North Atlantic thermohaline circulation during 320 years and surface cooling varying from 1 to 5 degreesC over adjacent continents. This result is consistent with proxy data, suggesting that a meltwater-induced weakening of the thermohaline circulation caused the event. Moreover, our results indicate that the time-scale of the meltwater release and the initial state are important, as both have a strong effect on the magnitude and duration of the produced model response.

Making sense of palaeoclimate sensitivity

Many palaeoclimate studies have quantified pre-anthropogenic climate change to calculate climate sensitivity (equilibrium temperature change in response to radiative forcing change), but a lack of consistent methodologies produces a wide range of estimates and hinders comparability of results. Here we present a stricter approach, to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change. Over the past 65 million years, this reveals a climate sensitivity (in K W−1 m2) of 0.3–1.9 or 0.6–1.3 at 95% or 68% probability, respectively. The latter implies a warming of 2.2–4.8 K per doubling of atmospheric CO2, which agrees with IPCC estimates.

Climatic change in Chile at around 2700 BP and global evidence for solar forcing: a hypothesis

Palaeoecological and geological evidence for changing atmospheric circulation patterns in Chile indicates that equatorward and poleward shifts of the Southern Westerlies (Pacific precipitation source) were an important factor during Weichselian and Holocene climate change. We focus on the evidence and possible causes of considerable climate change in the Holocene at around 2700 BP, which was associated with a steep rise in atmospheric radiocarbon content, indicating an abrupt decrease in solar activity. Climate change may have been caused by the lowering of solar irradiation through two amplifying factors, namely (1) increased cosmic ray intensity, stimulating cloud formation and precipitation, and (2) reduced solar UV intensity, causing a decline of stratospheric ozone production and cooling as a result of less absorption of sunlight. A decrease in the latitudinal extent of Hadley Cell circulation may have occurred with concomitant equatorward relocation of mid-latitude storm tracks, which brought about northward movement of vegetation belts and advance of glaciers.

The two major warming phases of the last deglaciation at ; 14.7 and ; 11.5 ka cal BP in Europe: climate reconstructions and AGCM experiments

During the last deglaciation two distinct warming phases occurred in the N Atlantic region at similar to 14.7 and similar to 11.5 ka cal BP. These two shifts are the transitions from (1) GS-2a (Greenland Stadial 2a) to GI-1e (Greenland Interstadial 1e) and (2) GS-1 to the Preboreal. In this study we characterise these two important climate transitions by comparing maps of January and July temperatures for Europe acquired with two independent methods: (1) simulations with the ECHAM4 atmospheric general circulation model in T42 resolution and (2) temperature reconstructions based on geological and palaeoecological data. We also compare estimated lake level changes with simulated P-E (effective precipitation) values. These comparisons enable quantification of the climate change during the two phases. January temperatures increased by as much as 20 degreesC in NW Europe from values between - 25 degreesC and - 15 degreesC in both GS-2a and GS-1 to temperatures between - 5 degreesC and 5 degreesC in both GI-le and the Preboreal. During July the changes were smaller, as the July temperatures increased in NW Europe by 3-5 degreesC from about 10 degreesC to 15 degreesC in both GS-2a and GS-1 to values of 13 degreesC to 17 degreesC in both GI-1e and the Preboreal. In S Europe the increase in July temperature was less intense. Our analysis suggests that the effective precipitation remained at the same level during the 14.7 ka cal BP transition, whereas a small increase is inferred for some regions for the 11.5 ka cal BP shift. This small effect in effective precipitation is explained by comparable increases in precipitation and evaporation during both transitions. We infer that the strong increase in January temperatures was forced by changes in the N Atlantic Ocean, as the variations in sea surface temperatures and the position of the sea ice margin determined the temperature chan-e over land. The increase in July temperatures was mainly driven by two factors: the increase in insolation and the deglaciation in Scotland and Scandinavia. The insolation changes were gradual (2 to 3 W/m(2)) compared to the changes in the N Atlantic Ocean, explaining the relatively small temperature increase during July compared to January, In regions that were deglaciated during the two climate transitions, July temperatures appeared to have increased by up to 10 degreesC. Our results suggest that the registration of the magnitude of the two climate shifts in terrestrial proxy records was geographically different due to the changing environmental conditions; variations in the N Atlantic sea ice limit appear to be the most important. This implies that reconstructed temperature curves from different places in Europe should show different magnitudes. Moreover, it is to be expected that the timing of the major warming phases is spatially different, as this timing is mainly determined by the position of the sea ice and land ice margins relative to the place of interest

Modeling the effect of freshwater pulses on the early Holocene climate: The influence of high‐frequency climate variability

freshwater (4.67 10 14 m 3 ) into the Labrador Sea at three different constant rates: 1.5 Sv (1 Sv = 1 10 6 m 3 s 1 )i n 10 years, 0.75 Sv in 20 years, and 0.3 Sv in 50 years. For each rate, five ensemble experiments have been performed, varying in initial conditions. The freshwater pulses produce a weakening of the thermohaline circulation. The perturbed state is in agreement with proxy evidence for the 8.2 ka event. Two types of recovery of the thermohaline circulation occurred, differing in time-scale: (1) 200 years and (2) >200 years. In the experiments with 10 year and 20 year pulses, both types of recovery were observed. This suggests that the model response is unpredictable in the range of parameters studied here. It is hypothesized that the unpredictability is associated with annual-to-decadal climate variability. Our results demonstrate that several types of recovery may exist with the same kind of perturbation. The interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere–sea ice–ocean system. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 4255 Oceanography: General: Numerical modeling; 4532 Oceanography: Physical: General circulation; 4215 Oceanography: General: Climate and interannual variability (3309); 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: Holocene, thermohaline circulation, predictability, meltwater, coupled climate model

The impact of the North Atlantic Ocean on the Younger Dryas climate in northwestern and central Europe

The main results of multiproxy climate reconstructions for the Younger Dryas and experiments carried out with an atmospheric general circulation model are discussed. Quantitative temperature inferences for northwestern and central Europe show that winter conditions were remarkably extreme, with values of 20–30°C below that of today. Annual temperature ranges with a continental signature (30–34°C) were reconstructed for the entire study area. Compared with today, the greatest cooling was experienced in the seaboard area. The comparison of the climate reconstructions and simulation experiments indicates that sea-ice in the North Atlantic Ocean played a decisive role in the climate of the study area by cooling the surface air temperatures and by controlling the position of the storm track. Surface air circulation was westerly even during Younger Dryas winters. A slight northward shift of the mean Atlantic sea-ice margin during winter may explain the proposed subdivision of the Younger Drays into a phase of maximum cold and humidity succeeded by a less cold and relatively dry phase.

Investigation of the relationship between permafrost distribution in NW Europe and extensive winter sea-ice cover in the North Atlantic Ocean during the cold phases of the Last Glaciation

Atmospheric model simulations with different extents of sea-ice are compared with reconstructed European mean annual temperatures derived from permafrost indicators. Analysis of the results suggest that during cold phases of the Last Glacial, the southern margin of permafrost in western Europe was controlled by the latitude of the winter sea-ice margin in the North Atlantic Ocean. In this case reconstructions of permafrost extent in Europe may be used to constrain past winter sea-ice conditions in the North Atlantic Ocean. Accordingly, extensive North Atlantic sea-ice cover southwards to at least 50degreesN is inferred during four phases of the Last Glaciation: (1) Early Pleniglacial (74-59 cal kyr BP), (2) the Hasselo Stadial (41.5-40 cal kyr BP), (3) the LGM (23-19 cal kyr BP) and (4) the Younger Dryas (12.7-11.5 cal kyr BP). The extensive sea-ice cover for the phase of maximum cold disagrees with recent studies suggesting a relatively warm North Atlantic during the LGM, while it agrees with the original CLIMAP reconstruction. Moreover, the estimate for the Younger Dryas cooling conflicts with reconstructions based on marine proxy data

Sensitivity of global river discharges under Holocene and future climate conditions

A comparative analysis of global river basins shows that some river discharges are more sensitive to future climate change for the coming century than to natural climate variability over the last 9000 years. In these basins (Ganges, Mekong, Volta, Congo, Amazon, Murray-Darling, Rhine, Oder, Yukon) future discharges increase by 6-61%. These changes are of similar magnitude to changes over the last 9000 years. Some rivers (Nile, Syr Darya) experienced strong reductions in discharge over the last 9000 years (17-56%), but show much smaller responses to future warming. The simulation results for the last 9000 years are validated with independent proxy data.