scholarly journals A Comparison of Climate Feedback Strength between CO2 Doubling and LGM Experiments

2009 ◽  
Vol 22 (12) ◽  
pp. 3374-3395 ◽  
Author(s):  
Masakazu Yoshimori ◽  
Tokuta Yokohata ◽  
Ayako Abe-Ouchi
Keyword(s):  
Water Vapor ◽  
Climate Sensitivity ◽  
Radiative Forcing ◽  
General Circulation ◽  
Lapse Rate ◽  
Climate Feedback ◽  
Water Vapor Feedback ◽  
Feedback Strength ◽  
Significant Difference ◽  
The Difference

Abstract Studies of the climate in the past potentially provide a constraint on the uncertainty of climate sensitivity, but previous studies warn against a simple scaling to the future. Climate sensitivity is determined by a number of feedback processes, and they may vary according to climate states and forcings. In this study, the similarities and differences in feedbacks for CO2 doubling, a Last Glacial Maximum (LGM), and LGM greenhouse gas (GHG) forcing experiments are investigated using an atmospheric general circulation model coupled to a slab ocean model. After computing the radiative forcing, the individual feedback strengths of water vapor, lapse-rate, albedo, and cloud feedbacks are evaluated explicitly. For this particular model, the difference in the climate sensitivity between the experiments is attributed to the shortwave cloud feedback, in which there is a tendency for it to become weaker or even negative in cooling experiments. No significant difference is found in the water vapor feedback between warming and cooling experiments by GHGs. The weaker positive water vapor feedback in the LGM experiment resulting from a relatively weaker tropical forcing is compensated for by the stronger positive lapse-rate feedback resulting from a relatively stronger extratropical forcing. A hypothesis is proposed that explains the asymmetric cloud response between the warming and cooling experiments associated with a displacement of the region of mixed-phase clouds. The difference in the total feedback strength between the experiments is, however, relatively small compared to the current intermodel spread, and does not necessarily preclude the use of LGM climate as a future constraint.

scholarly journals Separation of Contributions from Radiative Feedbacks to Polar Amplification on an Aquaplanet

2012 ◽  
Vol 25 (8) ◽  
pp. 3010-3024 ◽  
Author(s):  
Peter L. Langen ◽  
Rune Grand Graversen ◽  
Thorsten Mauritsen
Keyword(s):  
Water Vapor ◽  
Climate Sensitivity ◽  
General Circulation ◽  
Surface Albedo ◽  
Circulation Model ◽  
Lapse Rate ◽  
Low Latitude ◽  
Polar Amplification ◽  
Water Vapor Feedback ◽  
The Difference

Abstract When climate is forced by a doubling of CO2, a number of feedback processes are induced, such as changes of water vapor, clouds, and surface albedo. Here the CO2 forcing and concomitant feedbacks are studied individually using a general circulation model coupled to an aquaplanet mixed layer ocean. A technique for fixing the radiative effects of moisture and clouds by reusing these variables from 1 × CO2 and 2 × CO2 equilibrium climates in the model’s radiation code allows for a detailed decomposition of forcings, feedbacks, and responses. The cloud feedback in this model is found to have a weak global average effect and surface albedo feedbacks have been eliminated. As in previous studies, the water vapor feedback is found to approximately double climate sensitivity, but while its radiative effect is strongly amplified at low latitudes, the resulting response displays about the same degree of polar amplification as the full all-feedbacks experiment. In fact, atmospheric energy transports are found to change in a way that yields the same meridional pattern of response as when the water vapor feedback is turned off. The authors conclude that while the water vapor feedback does not in itself lead to polar amplification by increasing the ratio of high- to low-latitude warming, it does double climate sensitivity both at low and high latitudes. A polar amplification induced by other feedbacks in the system, such as the Planck and lapse rate feedbacks here, is thus strengthened in the sense of increasing the difference in high- and low-latitude warming.

scholarly journals Compensation between Model Feedbacks and Curtailment of Climate Sensitivity

2010 ◽  
Vol 23 (11) ◽  
pp. 3009-3018 ◽  
Author(s):  
Peter Huybers
Keyword(s):  
Confidence Interval ◽  
Climate Sensitivity ◽  
Radiative Forcing ◽  
General Circulation ◽  
Circulation Model ◽  
Lapse Rate ◽  
Statistical Interpretation ◽  
Intergovernmental Panel ◽  
Ocean Heat Uptake ◽  
Feedback Strength

Abstract The spread in climate sensitivity obtained from 12 general circulation model runs used in the Fourth Assessment of the Intergovernmental Panel on Climate Change indicates a 95% confidence interval of 2.1°–5.5°C, but this reflects compensation between model feedbacks. In particular, cloud feedback strength negatively covaries with the albedo feedback as well as with the combined water vapor plus lapse rate feedback. If the compensation between feedbacks is removed, the 95% confidence interval for climate sensitivity expands to 1.9°–8.0°C. Neither of the quoted 95% intervals adequately reflects the understanding of climate sensitivity, but their differences illustrate that model interdependencies must be understood before model spread can be correctly interpreted. The degree of negative covariance between feedbacks is unlikely to result from chance alone. It may, however, result from the method by which the feedbacks were estimated, physical relationships represented in the models, or from conditioning the models upon some combination of observations and expectations. This compensation between model feedbacks—when taken together with indications that variations in radiative forcing and the rate of ocean heat uptake play a similar compensatory role in models—suggests that conditioning of the models acts to curtail the intermodel spread in climate sensitivity. Observations used to condition the models ought to be explicitly stated, or there is the risk of doubly calling on data for purposes of both calibration and evaluation. Conditioning the models upon individual expectation (e.g., anchoring to the Charney range of 3° ± 1.5°C), to the extent that it exists, greatly complicates statistical interpretation of the intermodel spread.

Robustness of Dynamical Feedbacks from Radiative Forcing: 2% Solar versus 2 × CO2 Experiments in an Idealized GCM

2012 ◽  
Vol 69 (7) ◽  
pp. 2256-2271 ◽  
Author(s):  
Ming Cai ◽  
Ka-Kit Tung
Keyword(s):  
Water Vapor ◽  
Radiative Forcing ◽  
Lower Troposphere ◽  
Radiative Heating ◽  
Climate Feedback ◽  
Response Analysis ◽  
High Latitudes ◽  
Water Vapor Feedback ◽  
The Tropics ◽  
Heating Profile

Abstract Despite the differences in the spatial patterns of the external forcing associated with a doubling CO2 and with a 2% solar variability, the final responses in the troposphere and at the surface in a three-dimensional general circulation model appear remarkably similar. Various feedback processes are diagnosed and compared using the climate feedback–response analysis method (CFRAM) to understand the mechanisms responsible. At the surface, solar radiative forcing is stronger in the tropics than at the high latitudes, whereas greenhouse radiative forcing is stronger at high latitudes compared with the tropics. Also solar forcing is positive everywhere in the troposphere and greenhouse radiative forcing is positive mainly in the lower troposphere. The water vapor feedback strengthens the upward-decreasing radiative heating profile in the tropics and the poleward-decreasing radiative heating profile in the lower troposphere. The “evaporative” and convective feedbacks play an important role only in the tropics where they act to reduce the warming at the surface and lower troposphere in favor of upper-troposphere warming. Both water vapor feedback and enhancement of convection in the tropics further strengthen the initial poleward-decreasing profile of energy flux convergence perturbations throughout the troposphere. As a result, the large-scale dynamical poleward energy transport, which acts on the negative temperature gradient, is enhanced in both cases, contributing to a polar amplification of warming aloft and a warming reduction in the tropics. The dynamical amplification of polar atmospheric warming also contributes additional warming to the surface below via downward thermal radiation.

scholarly journals How Well Do We Understand and Evaluate Climate Change Feedback Processes?

2006 ◽  
Vol 19 (15) ◽  
pp. 3445-3482 ◽  
Author(s):  
Sandrine Bony ◽  
Robert Colman ◽  
Vladimir M. Kattsov ◽  
Richard P. Allan ◽  
Christopher S. Bretherton ◽  
...  
Keyword(s):  
Climate Change ◽  
Climate Sensitivity ◽  
General Circulation ◽  
External Perturbation ◽  
General Circulation Models ◽  
Lapse Rate ◽  
Climate Feedback ◽  
Climate Feedbacks ◽  
Intergovernmental Panel ◽  
Global Estimates

Abstract Processes in the climate system that can either amplify or dampen the climate response to an external perturbation are referred to as climate feedbacks. Climate sensitivity estimates depend critically on radiative feedbacks associated with water vapor, lapse rate, clouds, snow, and sea ice, and global estimates of these feedbacks differ among general circulation models. By reviewing recent observational, numerical, and theoretical studies, this paper shows that there has been progress since the Third Assessment Report of the Intergovernmental Panel on Climate Change in (i) the understanding of the physical mechanisms involved in these feedbacks, (ii) the interpretation of intermodel differences in global estimates of these feedbacks, and (iii) the development of methodologies of evaluation of these feedbacks (or of some components) using observations. This suggests that continuing developments in climate feedback research will progressively help make it possible to constrain the GCMs’ range of climate feedbacks and climate sensitivity through an ensemble of diagnostics based on physical understanding and observations.

scholarly journals Climate Feedbacks in CCSM3 under Changing CO2 Forcing. Part II: Variation of Climate Feedbacks and Sensitivity with Forcing

2013 ◽  
Vol 26 (9) ◽  
pp. 2784-2795 ◽  
Author(s):  
Alexandra K. Jonko ◽  
Karen M. Shell ◽  
Benjamin M. Sanderson ◽  
Gokhan Danabasoglu
Keyword(s):  
Water Vapor ◽  
Temperature Change ◽  
Climate Sensitivity ◽  
General Circulation ◽  
Coupled Model ◽  
Lapse Rate ◽  
Climate Feedbacks ◽  
Cloud Feedbacks ◽  
Response To Temperature ◽  
Fully Coupled

Abstract Are equilibrium climate sensitivity and the associated radiative feedbacks a constant property of the climate system, or do they change with forcing magnitude and base climate? Using the radiative kernel technique, feedbacks and climate sensitivity are evaluated in a fully coupled general circulation model (GCM) for three successive doublings of carbon dioxide starting from present-day concentrations. Climate sensitivity increases by 23% between the first and third CO2 doublings. Increases in the positive water vapor and cloud feedbacks are partially balanced by a decrease in the positive surface albedo feedback and an increase in the negative lapse rate feedback. Feedbacks can be decomposed into a radiative flux change and a climate variable response to temperature change. The changes in water vapor and Planck feedbacks are due largely to changes in the radiative response with climate state. Higher concentrations of greenhouse gases and higher temperatures lead to more absorption and emission of longwave radiation. Changes in cloud feedbacks are dominated by the climate response to temperature change, while the lapse rate and albedo feedbacks combine elements of both. Simulations with a slab ocean model (SOM) version of the GCM are used to verify whether an SOM-GCM accurately reproduces the behavior of the fully coupled model. Although feedbacks differ in magnitude between model configurations (with differences as large as those between CO2 doublings for some feedbacks), changes in feedbacks between CO2 doublings are consistent in sign and magnitude in the SOM-GCM and the fully coupled model.

scholarly journals Evolution and Trend of the Outgoing Longwave Radiation Spectrum

2009 ◽  
Vol 22 (17) ◽  
pp. 4637-4651 ◽  
Author(s):  
Yi Huang ◽  
V. Ramaswamy
Keyword(s):  
Water Vapor ◽  
Outgoing Longwave Radiation ◽  
General Circulation ◽  
Circulation Model ◽  
Longwave Radiation ◽  
Natural Variability ◽  
Lapse Rate ◽  
Vapor Concentration ◽  
Geophysical Fluid Dynamics Laboratory ◽  
The Difference

Abstract The variability and change occurring in the outgoing longwave radiation (OLR) spectrum are investigated by using simulations performed with a Geophysical Fluid Dynamics Laboratory coupled atmosphere–ocean–land general circulation model. First, the variability in unforced climate (natural variability) is simulated. Then, the change of OLR spectrum due to forced changes in climate is analyzed for a continuous 25-yr time series and for the difference between two time periods (1860s and 2000s). Spectrally resolved radiances have more pronounced and complex changes than broadband fluxes. In some spectral regions, the radiance change is dominated by just one controlling factor (e.g., the window region and CO2 band center radiances are controlled by surface and stratospheric temperatures, respectively) and well exceeds the natural variability. In some other spectral bands, the radiance change is influenced by multiple and often competing factors (e.g., the water vapor band radiance is influenced by both water vapor concentration and temperature) and, although still detectable against natural variability at certain frequencies, demands stringent requirements (drift less than 0.1 K decade−1 at spectral resolution no less than 1 cm−1) of observational platforms. The difference between clear-sky and all-sky radiances in the forced climate problem offers a measure of the change in the cloud radiative effect, but with a substantive dependence on the temperature lapse rate change. These results demonstrate that accurate and continuous observations of the OLR spectrum provide an advantageous means for monitoring the changes in the climate system and a stringent means for validating climate models.

scholarly journals An Assessment of the Primary Sources of Spread of Global Warming Estimates from Coupled Atmosphere–Ocean Models

2008 ◽  
Vol 21 (19) ◽  
pp. 5135-5144 ◽  
Author(s):  
Jean-Louis Dufresne ◽  
Sandrine Bony
Keyword(s):  
Global Warming ◽  
Water Vapor ◽  
General Circulation ◽  
Lapse Rate ◽  
Climate Feedback ◽  
Ocean Heat Uptake ◽  
Feedback Analysis ◽  
Cloud Feedbacks ◽  
Rate Feedback ◽  
Heat Uptake

Abstract Climate feedback analysis constitutes a useful framework for comparing the global mean surface temperature responses to an external forcing predicted by general circulation models (GCMs). Nevertheless, the contributions of the different radiative feedbacks to global warming (in equilibrium or transient conditions) and their comparison with the contribution of other processes (e.g., the ocean heat uptake) have not been quantified explicitly. Here these contributions from the classical feedback analysis framework are defined and quantified for an ensemble of 12 third phase of the Coupled Model Intercomparison Project (CMIP3)/Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled atmosphere–ocean GCMs. In transient simulations, the multimodel mean contributions to global warming associated with the combined water vapor–lapse-rate feedback, cloud feedback, and ocean heat uptake are comparable. However, intermodel differences in cloud feedbacks constitute by far the most primary source of spread of both equilibrium and transient climate responses simulated by GCMs. The spread associated with intermodel differences in cloud feedbacks appears to be roughly 3 times larger than that associated either with the combined water vapor–lapse-rate feedback, the ocean heat uptake, or the radiative forcing.

The Evolution of Climate Sensitivity and Climate Feedbacks in the Community Atmosphere Model

2012 ◽  
Vol 25 (5) ◽  
pp. 1453-1469 ◽  
Author(s):  
A. Gettelman ◽  
J. E. Kay ◽  
K. M. Shell
Keyword(s):  
Climate Sensitivity ◽  
Radiative Forcing ◽  
Ocean Model ◽  
Lapse Rate ◽  
Shallow Convection ◽  
Climate Feedbacks ◽  
Cloud Feedbacks ◽  
Community Atmosphere Model ◽  
Atmosphere Model ◽  
The Difference

The major evolution of the National Center for Atmospheric Research Community Atmosphere Model (CAM) is used to diagnose climate feedbacks, understand how climate feedbacks change with different physical parameterizations, and identify the processes and regions that determine climate sensitivity. In the evolution of CAM from version 4 to version 5, the water vapor, temperature, surface albedo, and lapse rate feedbacks are remarkably stable across changes to the physical parameterization suite. However, the climate sensitivity increases from 3.2 K in CAM4 to 4.0 K in CAM5. The difference is mostly due to (i) more positive cloud feedbacks and (ii) higher CO2 radiative forcing in CAM5. The intermodel differences in cloud feedbacks are largest in the tropical trade cumulus regime and in the midlatitude storm tracks. The subtropical stratocumulus regions do not contribute strongly to climate feedbacks owing to their small area coverage. A “modified Cess” configuration for atmosphere-only model experiments is shown to reproduce slab ocean model results. Several parameterizations contribute to changes in tropical cloud feedbacks between CAM4 and CAM5, but the new shallow convection scheme causes the largest midlatitude feedback differences and the largest change in climate sensitivity. Simulations with greater cloud forcing in the mean state have lower climate sensitivity. This work provides a methodology for further analysis of climate sensitivity across models and a framework for targeted comparisons with observations that can help constrain climate sensitivity to radiative forcing.

scholarly journals Aerosol lifetime and climate change

2012 ◽  
Vol 12 (7) ◽  
pp. 16493-16514 ◽  
Author(s):  
G.-J. Roelofs
Keyword(s):  
Climate Change ◽  
Water Vapor ◽  
Climate Sensitivity ◽  
Radiative Forcing ◽  
Climate Models ◽  
Climate Model ◽  
Hydrological Cycle ◽  
Lapse Rate ◽  
Fundamental Relation ◽  
Wide Range

Abstract. The dominant removal mechanism for atmospheric aerosol is activation of particles to cloud droplets and subsequent wet deposition in precipitation. The atmospheric lifetime of aerosol is thus closely coupled to the atmospheric cycling time of water vapor. Changes of hydrological cycle characteristics resulting from climate change therefore directly affect aerosol lifetime, and thus the radiative forcing exerted by aerosol. This study expresses the coupling between water vapor and aerosol lifetimes and their temperature sensitivities in fundamental equations and in terms of the efficiency of processing of air by precipitating clouds. Based on climate model simulations these temperature sensitivities are estimated to be on the order of +5.3% K−1, but this may be an overestimation. Generally, shifting spatial and temporal patterns of aerosol (precursor) emissions and precipitation, and changes in aerosol activation efficiency probably influence aerosol lifetimes more than climate change itself, resulting in a wide range of simulated aerosol lifetime sensitivities between aerosol-climate models. It is possible that the climate sensitivity of models plays a role. It can be argued that climate sensitivity is intrinsically coupled with the simulated (temperature sensitivity of the) aerosol lifetime through the distribution of water vapor and aerosol between the lower and upper troposphere. This implies a fundamental relation between various feedback forcings (water vapor, lapse rate, cloud) and the aerosol forcing, illustrating the key role of the hydrological cycle in different aspects of the climate system.

scholarly journals Physical Mechanisms of Tropical Climate Feedbacks Investigated Using Temperature and Moisture Trends*

2015 ◽  
Vol 28 (22) ◽  
pp. 8968-8987 ◽  
Author(s):  
A. J. Ferraro ◽  
F. H. Lambert ◽  
M. Collins ◽  
G. M. Miles
Keyword(s):  
Water Vapor ◽  
Climate Models ◽  
Tropical Climate ◽  
Lapse Rate ◽  
Climate Feedback ◽  
Climate Projections ◽  
Tropospheric Temperature ◽  
Regional Pattern ◽  
Rate Feedback ◽  
Water Vapor Feedback

Abstract Tropical climate feedback mechanisms are assessed using satellite-observed and model-simulated trends in tropical tropospheric temperature from the MSU/AMSU instruments and upper-tropospheric humidity from the HIRS instruments. Despite discrepancies in the rates of tropospheric warming between observations and models, both are consistent with constant relative humidity over the period 1979–2008. Because uncertainties in satellite-observed tropical-mean trends preclude a constraint on tropical-mean trends in models regional features of the feedbacks are also explored. The regional pattern of the lapse rate feedback is primarily determined by the regional pattern of surface temperature changes, as tropical atmospheric warming is relatively horizontally uniform. The regional pattern of the water vapor feedback is influenced by the regional pattern of precipitation changes, with variations of 1–2 W m−2 K−1 across the tropics (compared to a tropical-mean feedback magnitude of 3.3–4 W m−2 K−1). Thus the geographical patterns of water vapor and lapse rate feedbacks are not correlated, but when the feedbacks are calculated in precipitation percentiles rather than in geographical space they are anticorrelated, with strong positive water vapor feedback associated with strong negative lapse rate feedback. The regional structure of the feedbacks is not related to the strength of the tropical-mean feedback in a subset of the climate models from the CMIP5 archive. Nevertheless the approach constitutes a useful process-based test of climate models and has the potential to be extended to constrain regional climate projections.

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