Gauging water-vapour feedback

Nature ◽  
1989 ◽  
Vol 342 (6251) ◽  
pp. 736-737 ◽  
Author(s):  
Robert D. Cess
Keyword(s):  
Water Vapour ◽  
Water Vapour Feedback

Water vapour, CO 2 and insolation over the last glacial-interglacial cycles

1993 ◽  
Vol 341 (1297) ◽  
pp. 253-261 ◽  
Keyword(s):  
Water Vapour ◽  
Northern Hemisphere ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Last Glacial ◽  
Ice Volume ◽  
Water Vapour Feedback ◽  
Astronomical Forcing ◽  
The Last Glacial Maximum ◽  
The Last Glacial

A two-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea ice, the continents, the ice sheets and their underlying bedrock has been used to test the Milankovitch theory over the last two glacial-interglacial cycles. A series of sensitivity analyses have allowed us to understand better the internal mechanisms which drive the simulated climate system and in particular the feedbacks related to surface albedo and water vapour. It was found that orbital variations alone can induce, in such a system, feedbacks sufficient to generate the low frequency p art of the climatic variations over the last 122 ka. These simulated variations at the astronomical timescale are broadly in agreement with reconstructions of ice-sheet volume and of sea level independently obtained from geological data. Imperfections in the stimulated climate were the insufficient southward extent of the ice sheets and the too small hemispheric cooling at the last glacial maximum . These deficiencies were partly remedied in a further experiment by using the time-dependent atmospheric CO2 concentration given by the Vostok ice core in addition to the astronomical forcing. In this transient simulation, 70% of the Northern Hemisphere ice volume is related to the astronomical forcing and the related changes in the albedo, the rem aining 30% being due to the CO 2 changes. Analysis of the processes involved shows that variations of ablation are more important for the ice-sheet response than are variations of snow precipitation. A key mechanism in the deglaciation after the last glacial maxim um appears to be the ‘ageing’ of snow which significantly decreases its albedo. The other factors which play an important role are ice-sheet altitude, insolation, taiga cover, ice-albedo feedback, ice-sheet configuration (‘continentality’ and ‘desert’ effect), isostatic rebound, CO 2 changes and tem perature-water vapour feedback. Numerical experiments have also been carried out with a one-dimensional radiative-convective model in order to quantify the influence of the CO 2 changes and of the water vapour feedback on the climate evolution of the Northern Hemisphere over the last 122 ka. Results of these experiments indicate that 67% of the simulated cooling at the last glacial maximum can be attributed to the astronomical forcing and the subsequent surface albedo increase, the remaining 33% being associated with the reduced CO 2 concentration. Moreover, the water vapour feedback explains 40% of the simulated cooling in all the experiments done. The transient response of the clim ate system to both the astronomical and CO 2 forcing was also simulated by the LLN (Louvain-la-Neuve) 2.5-dimensional model over the two last glacial-interglacial cycles. It is particularly significant that spectral analysis of the simulated Northern Hemisphere global ice volume variations reproduces correctly the relative intensity of the peaks at the orbital frequencies. Except for variations with timescales shorter than 5 ka, the simulated long-term variations of total ice volume are comparable to that reconstructed from deep sea cores. For example, the model simulates glacial maxima of similar amplitudes at 134 ka BP and 15 ka BP, followed by abrupt deglaciations. The complete deglaciation of the three main Northern Hemisphere ice sheets, which is simulated around 122 ka BP, is in partial disagreement with reconstructions indicating that the Greenland ice sheet survived during the Eemian interglacial. The continental ice volume variations during the last 122 ka of the 200 ka simulation are, however, not significantly affected by this shortcoming.

scholarly journals Radiative forcing and feedback by forests in warm climates – a sensitivity study

2015 ◽  
Vol 6 (2) ◽  
pp. 2577-2615
Author(s):  
U. Port ◽  
M. Claussen ◽  
V. Brovkin
Keyword(s):  
Water Vapour ◽  
High Latitude ◽  
Radiative Forcing ◽  
Sensitivity Study ◽  
Lapse Rate ◽  
Order Effects ◽  
Early Eocene ◽  
Max Planck ◽  
Water Vapour Feedback ◽  
Dark Soil

Abstract. The biogeophysical effect of forests in a climate with permanent high-latitude ice cover has already been investigated. We extend this analysis to warm, basically ice-free climates, and we choose the early Eocene, some 54 to 52 million years ago, as paradigm for such type of climate. We use the Max Planck Institute for Meteorology Earth System Model to evaluate the radiative forcing of forests and the feedbacks triggered by forests in early Eocene and pre-industrial climate, respectively. To isolate first-order effects, we compare idealised simulations in which all continents are covered either by dense forests or by deserts with either bright or dark soil. In comparison with desert continents covered by bright soil, forested continents warm the planet in the early Eocene climate and in the pre-industrial climate. The warming can be attributed to different feedback processes, though. The lapse-rate – water-vapour feedback is stronger in early Eocene climate than in pre-industrial climate, but strong and negative cloud-related feedbacks nearly outweigh the positive lapse-rate – water-vapour feedback in the early Eocene climate. Subsequently, global mean warming by forests is weaker in the early Eocene climate than in the pre-industrial climate. Sea-ice related feedbacks are weak in the almost ice-free climate of the early Eocene, thereby leading to a weaker high-latitude warming by forests than in the pre-industrial climate. When the land is covered with dark soils, forests cool the early Eocene climate stronger than the pre-industrial climate because the lapse-rate and water-vapour feedbacks are stronger in the early Eocene climate. Cloud-related feedbacks are equally strong in both climates. We conclude that radiative forcing by forests varies little with the climate state, while most subsequent feedbacks depend on the climate state.

Positive water vapour feedback in climate models confirmed by satellite data

Nature ◽  
1991 ◽  
Vol 349 (6309) ◽  
pp. 500-503 ◽  
Author(s):  
D. Rind ◽  
E.-W. Chiou ◽  
W. Chu ◽  
J. Larsen ◽  
S. Oltmans ◽  
...  
Keyword(s):  
Water Vapour ◽  
Satellite Data ◽  
Climate Models ◽  
Water Vapour Feedback

scholarly journals Radiative forcing by forest and subsequent feedbacks in the early Eocene climate

2015 ◽  
Vol 11 (2) ◽  
pp. 997-1029 ◽  
Author(s):  
U. Port ◽  
M. Claussen ◽  
V. Brovkin
Keyword(s):  
Water Vapour ◽  
Radiative Forcing ◽  
Bare Soil ◽  
Lapse Rate ◽  
Early Eocene ◽  
Max Planck ◽  
State Dependency ◽  
Cloud Feedbacks ◽  
Current Climate ◽  
Water Vapour Feedback

Abstract. Using the Max Planck Institute for Meteorology Earth System Model, we investigate the forcing of forests and the feedback triggered by forests in the pre-industrial climate and in the early Eocene climate (about 54 to 52 million years ago). Other than the interglacial, pre-industrial climate, the early Eocene climate was characterised by high temperatures which led to almost ice-free poles. We compare simulations in which all continents are covered either by dense forest or by bare soil. To isolate the effect of soil albedo, we choose either bright soils or dark soils, respectively. Considering bright soil, forests warm in both, the early Eocene climate and the current climate, but the warming differs due to differences in climate feedbacks. The lapse-rate and water-vapour feedback is stronger in early Eocene climate than in current climate, but strong and negative cloud feedbacks and cloud masking in the early Eocene climate outweigh the stronger positive lapse-rate and water-vapour feedback. In the sum, global mean warming is weaker in the early Eocene climate. Sea-ice related feedbacks are weak in the almost ice-free climate of the early Eocene leading to a weak polar amplification. Considering dark soil, our results change. Forests cools stronger in the early Eocene climate than in the current climate because the lapse-rate and water-vapour feedback is stronger in the early Eocene climate while cloud feedbacks and cloud masking are equally strong in both climates. The different temperature change by forest in both climates highlights the state-dependency of vegetation's impact on climate.

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