U43A-0848
Expert Judgments About Transient Climate Response and Climate Sensitivity
We present results from detailed interviews with 14 leading climate scientists about transient climate response and "classic" climate sensitivity (the equilibrium global mean temperature response to a doubling of pre- industrial CO2). The interviews sought to explore the range of opinions within the climate research community about the physical processes that most contribute to the uncertainty in transient and equilibrium global mean surface air temperature response. We also elicited quantitative, probabilistic judgments about the temperature response to specified radiative forcing trajectories and about equilibrium climate sensitivity. A variety of questions were posed about the potential for uncertainty reduction given different levels of achievements in the climate sciences. The results reveal a rich diversity of expert opinion and a greater degree of disagreement than is often conveyed in scientific consensus documents.
U43A-0849
Why is climate sensitivity so unpredictable?
Uncertainties in projections of future climate change have not lessened substantially in the last decades. Both models and observations yield broad probability distributions for long-term increases in global mean temperature expected for doubling of CO2, with small but finite probabilities of very large increases. We show here that the shape of these probability distributions is an inevitable and general consequence of the nature of the climate system, and we derive a simple analytic form for the shape that fits recent published distributions very well. We show that the breadth of the distribution, and in particular, in the probability of large temperature increases, is relatively insensitive to decreases in uncertainties associated with the underlying climate processes.
U43A-0850
Climate Sensitivity and the Generation of Available Potential Energy
The current climate is characterized by an energy cycle which is forced by the generation of available potential energy G (Lorenz, 1955). G may be divided into its zonal and eddy components (Gz and Ge), which depend on the correlation between the latitudinal temperature and diabatic heating gradients in the case of Gz, and the meridional temperature and diabatic heating gradients for Ge. We expect that the temperature and heating fields will change as the climate changes, resulting in different correlations and thus different characteristic Gz and Ge. This study examines the generation of available potential energy for the output of two models with significantly different climate sensitivities - the NCAR CCSM3 model on the low end of climate sensitivity at 2.7K for doubled CO2, and the UKMO HadGEM1 on the high end at 4.4K for doubled CO2. Using the output submitted to PCMDI for the IPCC AR4, Gz and Ge for each model are computed for the twentieth century simulation, and compared to Gz and Ge computed for the 2xCO2 runs to explore how each model's energy generation relates to its climate sensitivity. Special attention is given to the partitioning between the various diabatic heating components.
U43A-0851
Maximum Likelihood Approach to the Estimate of Climate Sensitivity From the Climate Records Over the Past Century and Over the Past Millennium
To refine the estimate of the sensitivity of the Earth's climate system, a large ensemble of simple energy balance models are driven with estimated changes in radiative forcing over the past century and over the past millennium. The model outputs are evaluated against instrumental and proxy-based temperature data using a maximum likelihood profiling method. We demonstrate that, unlike similar analyses in the past, our method is not sensitive to the sampling strategy of the modeling parameters as long as the size of the ensemble is sufficiently large.
U43A-0852
Using the Radiative Kernel Technique to Calculate Climate Feedbacks in NCAR's Community Atmospheric Model
Climate models differ in their responses to imposed forcings, such as increased greenhouse gas concentrations, due to different climate feedback strengths. Climate feedbacks in NCAR's Community Atmospheric Model (CAM) are separated into two components: the change in climate components in response to an imposed forcing and the radiative kernel, the effect that climate changes have on the radiative budget at the top-of-the-atmosphere (TOA). This technique's usefulness depends on the linearity of the feedback processes. For the case of CO2 doubling, the sum of the effects of individual clear-sky components (water vapor, temperature, and surface albedo) on the TOA clear-sky flux is similar to the clear-sky flux changes directly calculated by CAM. When monthly averages are used rather than values from every time step, the global average TOA shortwave change is underestimated by a quarter as a result of intra-month correlations of surface albedo with the radiative kernel. The TOA longwave flux changes do not depend on the averaging period. The zonal averages are within 10% of the model-calculated values, while the global average differs by only 2%. Cloud radiative forcing (ΔCRF) is often used as a measure of cloud feedback strength. The net effect of the clear- sky feedbacks on ΔCRF is -1.6 W m-2, based on the kernel technique, while the total ΔCRF from CAM is -1.3 W m-2, indicating that clear-sky feedbacks contribute significantly to ΔCRF and make it more negative. Assuming linearity of the ΔCRF contributions, these results indicate that the net cloud feedback in CAM is positive.
U43A-0853
Climatic Feedbacks Between Tropical Cyclones, Temperature and Vertical Ocean Mixing
Vertical ocean mixing in the tropics is important to the climate system because it contributes to redistributing momentum, heat, and nutrients. Tropical cyclones are efficient vertical ocean mixers. Furthermore, recent studies suggest the frequency and intensity of tropical cyclones are largely controlled by temperature, thus these events may provide a causal link for the observed relationship between temperature and vertical mixing. Strong feedbacks between climate and tropical cyclones have been proposed by Emanuel, and some support is found in our work. Currently tropical cyclone-induced ocean mixing is not included in existing conceptual or numerical models of the climate system; therefore, accurate representation of any feedbacks is not possible. Here we provide evidence for the existence of climatic feedbacks between tropical cyclone activity, vertical ocean mixing and temperature, and we discuss implications for past and future climate scenarios. Potential differences between glacial and greenhouse climates will be discussed, and the potential importance of this feedback through Earth's history will be emphasized.
U43A-0854
Quantification of subsurface heat storage in the GCM ECHO-g: Effects of shallow bottom boundary placement
Recent studies indicate that shallow bottom boundary conditions (BBCs) used in state-of-the-art GCMs impose an artificial limit to the amount of heat that can be absorbed by the subsurface. Since this is an important issue for determining the energy partitioning among climate model subsystems. To better quantify this effect, the energy accumulation from the ECHO-g soil model is compared to the energy accumulation in a finite difference land- surface model (FDLSM) driven by the ECHO-g based IPCC A2 and B2 future climate simulation. The FDLSM is run with a BBC at the same depth as the ECHO-g soil model (10m) to verify that the soil models are thermodynamically equivalent. A run with a deep, causally detached BBC is also carried out. Results show that the deep FDLSM run captures several times more energy than the ECHO-g soil model for the time period 1991- 2100 CE. The spatial distribution of the FDLSM enhanced heat storage is described. These results suggest that shallow BBCs in GCMs prevent large amounts of heat from being stored in the subsurface and that this effect could be relevant in simulations of future climate change.
U43A-0855
Spatial sensitivity of heat storage to bottom boundary placement in climate models: Global variability
Proper execution of n-dimensional climate models requires an appropriate bottom boundary condition placement (BBCP). In order for the subsurface to store a realistic quantity of heat, models must impose BBCPs that are sufficiently deep so as not to significantly perturb the quasi-steady state thermal profile. Previous work has shown that a BBCP of 10m can deprive the subsurface of 4.5 to 5.5 times the heat otherwise stored during a 110-year future climate scenario with a causally detached BBCP. In an effort to ascertain the spatial dependence of this phenomenon, temperatures from all continental grids (1552) of the state-of-the-art General Circulation Model (GCM) ECHO-g are each used to force an independent Land-Surface Model (LSM). Two distinct future ECHO-g climate scenarios are used as skin boundary conditions (A2 and B2) for the LSM, as well as two bottom boundary conditions: a zero-flux condition, and a globally interpolated geothermal flux data set. These 4 permutations of skin/bottom boundaries are used to examine subsurface heat storage discrepancies in climate models under an array of realistic conditions that includes ECHO-g soil moisture data, and freeze/thaw events at high latitudes. Results indicate the presence of a spatial dependence of heat storage on BBCP, and that this dependence may be used to quantify regional climatic effects. It is expected that these results will lead to further understanding of the role BBCP plays in producing plausible estimations of past or future climate at the global scale.
U43A-0856
Sensitivity of Arctic Sea Ice to Global Warming Forcing in Observations and Climate Model Simulations
Response of Arctic sea ice coverage to global warming forcing exhibits a large diversity across climate models and across climate model ensemble members, as shown in our recent study for the IPCC AR4. This constitutes a major source of uncertainty in estimating climate sensitivity. Arctic sea ice, as an essential component of climate system, greatly impacts global energy balance due to its high radiative reflectance and associated albedo feedback. Extension and retreat of sea ice coverage accordingly contribute to variability of and change in global mean surface air temperature. In particular, Arctic sea ice coverage shrinks rapidly in recent decades and this shrinkage would be accelerated under global warming scenario. This would lead to an intensification of albedo feedback and, in turn, a further amplification of surface air temperature increase over the Arctic region. In order to improve understanding of the origin of the above-mentioned uncertainty, we analyzed sensitivity of Arctic sea ice cover to the change in surface air temperature in observations and in 20 IPCC AR4 climate models, including their multiple ensemble members for each model. The results show divergent sensitivity across models and even across ensemble members in the same model, with most of them differing from observations. Also, the sensitivity differs in climate change. Detailed and quantitative analysis from this study also helps better understanding and reducing uncertainty in assessment of climate change feedback processes and future climate change projections.
U43A-0857
Total Solar Irradiance Time Series since 1975
Total solar irradiance measurements from different satellites are available since late 1978 and may be extended by reliable models back to the minimum between cycles 19 and 20. This time series shows a slight downward trend of the values of the three minima and its significance will be discussed. This trend is confirmed by the analysis of the average over each cycle which is supposed to influence the long-term climate change. It is also manifested in all solar activity related indices in a very similar way and is found only after about 1980, before they all show an upward trend. The corresponding NH temperature record does not show such a change in trend. For the determination of the climate sensitivity this has to be taken into account. Thus, the use of the 11-year cycle modulation only, as performed by Camp and Tung (GRL, Doi: 10.1029/2007GL030207) may need some modification. Moreover, they used the result of a model and not the measured values. A possibility to overcome these drawbacks is proposed.
U43A-0858
Climate sensitivity to carbon emissions
Climate sensitivity defines the climate system response to an imposed radiative forcing, often characterized as the equilibrium temperature change in response to a doubling of atmospheric carbon dioxide. Climate sensitivity includes the direct effect of a change in forcing, in addition to the net effect of positive and negative climate feedbacks. However, the traditional concept of climate sensitivity is unable to accommodate carbon cycle feedbacks, which affect the level of carbon dioxide in the atmosphere directly, rather than the climate response to a given CO2 level. Earth system models which include interactive carbon cycle components have shown that carbon cycle feedbacks can vary substantially between models, and that this adds an additional level of uncertainty to that described by climate sensitivity. In this study, I propose a new measure of climate response, which is referenced to carbon emissions, rather than concentrations. This quantity (the Emissions Climate Response, or ECR) includes the direct carbon cycle response to CO2 emissions, the climate response to CO2 concentrations, as well as any feedbacks between climate change and carbon sinks. Using an intermediate complexity Earth system model, I illustrate the concept of the ECR with reference to the more traditional quantities of equilibrium climate sensitivity and transient climate response. I also present a framework within which the total climate response can be separated into its component carbon cycle and climate sensitivities.
U43A-0859
On the Sensitivity of the Oceanic pCO2 to Recent and Future Climate Induced Changes in Marine Carbon Tracers
A review is given of recent modeling studies with the ocean carbon cycle HAMOCC to evaluate the role of climate- induced changes of the carbon cycle for the oceanic pCO2. Single contributing terms for the total change in pCO2, including the temperature effect and non-temperature effects (DIC, ALK, S), are analyzed. For this purpose, an adjoint carbon cycle model has been developed and applied. Adjoint sensitivities indicate that in the North Atlantic changes in the seasonal pCO2 are predominantly controlled by DIC variations, whereas changes in the North Pacific are correlated about equally to changes in temperature and in DIC. Moreover, the adjoint simulation reveals that the temperature and non-temperature effects are mostly balanced in the equatorial regions, whereas in the Southern Ocean more non-linear processes control the pCO2. The implications of the results will be discussed in respect to past and future climate changes. Specifically, future long-term climate projections with a comprehensive earth system model indicate a significant sensitivity of the change in air-sea Δ pCO2 for the tropics and Southern Ocean to changes in the climate. In the Southern Ocean, these changes are partially linked to non-temperature effects, whereas in the tropics a temperature effect is predominant.
U43A-0860
What can be learnt from the comparison between ice core temperature records and climate model simulations?
Past temperature reconstructions are available from Greenland and Antarctica using different temperature reconstruction methods applied on ice cores and boreholes. Here we focus on two time periods : (i) the temperature increase from the Last Glacial Maximum to present-day, (ii) the temperature difference between the last interglacial period and present-day. For these two periods, ice-core data are compared to the magnitude of temperature change simulated for central Greenland and central eastern Antarctica by coupled ocean-atmosphere state-of-the-art climate models. Model-data comparisons suggest that glacial-interglacial temperature changes in Antarctica may have global relevance in terms of climate sensitivity. These comparisons further show that climate models may under- estimate the polar warming, possibly due to missing past forcings such as those associated with land surface and dust changes. For the last interglacial period, ice cores show that Greenland and Antarctica were significantly warmer than during the late Holocene -although with probably different optima periods. In response to the 130 ka or 125 ka BP orbital forcing, state-of-the-art climate models fail to capture the reconstructed Antarctic warming, which raises questions regarding the mechanisms linking past Antarctic and northern hemisphere climate variations.
U43A-0861
Climate sensitivity estimated from LGM ensemble simulations
Recent studies have shown that it is not possible to reduce the uncertainty range of climate sensitivity (CS) by focusing on last Century warming or on present day climatology. Alternative approaches of estimating the sensitivity of the Earth system comprise the use of paleo-data, ideally based on a period with a radiative forcing and global mean temperature pronouncedly different to modern day climate, such as the Last Glacial Maximum (LGM, 21kyrs B.P.). Yet the forcing of the glacial climate is of different nature compared to the forcing causing future climate changes. Thus it had been questioned to what extent the past is a good analogue for the future. Purely proxy-data based estimates of CS turn out to be biased in case the sensitivity of the climate system to past and future forcings is pronouncedly different. Model-based studies do account for differences in the feedback behaviour between simulated glacial cooling and 2xCO2 warming, but currently do not give a consistent picture of the asymmetry in the feedback strengths. In our study we combine the knowledge from LGM paleo-data with model simulations of present day, 2xCO2, and glacial climate. For this purpose we used a fully-coupled model of intermediate complexity (CLIMBER-2) that is fast enough to allow for running various ensemble experiments (order of 1000 model runs). By having simultaneously perturbed a set of 11 model parameters that strongly affect the model's fast feedbacks (water vapour, clouds, albedo and lapse rate) we could cover a broad range of climate sensitivities. We then have used LGM paleo-data from the tropics and from Antarctica to constrain the set of models being consistent with the glacial climate. Our results suggest that very high estimates of CS (larger than 5° C) are hard to reconcile with proxy-evidence from the LGM. We especially address the question of model dependence of our results by analyzing the simulated feedback strengths. We compare these key model characteristics with recent GCM results and discuss the issue of asymmetry in the feedback strengths between glacial and 2xCO2 conditions.
U43A-0862
Lessons from the Mid-Pleistocene on Orbital-Scale Tropics-Subtropics Phase Locking
Established meterological theory of the meridional extent of the tropics and their transition into the midlatitudes suggests that tropical and subtropical SSTs should go in and out of phase with the wax and wane of Northern Hemisphere ice sheets. The large range in the amplitude of ice volume changes during Mid-Pleistocene gives us an opportunity to test this theory with the paleorecord. In this talk, we present a mid-Pleistocene subtropical North Atlantic SST record (Ocean Drilling Program site 1058) derived from planktonic foraminiferal Mg/Ca. We examine quantitatively the phase relation between this new subtropical SST record and published records from the deep tropics. We conclude that throughout this interval of the mid-Pleistocene when there were large variations in the maximum extent of glaciation, the tropics and subtropics varied in concert but with the sub-tropics exhibiting substantially higher amplitude of variability. These results are of particular importance in light of current developments in the theory of Tropics-Midlatitude transition in the zonally symmetric atmosphere. Specifically, comparison of the paleorecords may ultimately shed light on the relative predictive power of traditional (angular-momentum conserving) theories versus more recent, "eddy-centric" ideas about the meridional extent of the tropics. The angular momentum conserving theory would have the tropics extend to a fixed relative for a fixed tropical heating rate. Conversely, eddy-based theories suggest a more plastic extent of the tropics, with greater sensitivity to high latitude boundary conditions. These results may in turn give insight to future tropical climates under putative global warming and shrinking polar ice caps.
U43A-0863 [WITHDRAWN]
The Paleocene-Eocene Thermal Maximum and Deep Sea Carbonate Compensation: Implications for Long-term Effects of Anthropogenic Carbon Emissions
The release of thousands of Pg C during the Paleocene-Eocene Thermal Maximum (55.5 Mya) represents a natural carbon cycle experiment that should provide insight into the potential carbon cycle response to current and future anthropogenic emissions. Unabated emission of fossil fuel CO2 over the next 3 centuries will release as much as 4500 Pg C to the atmosphere. Much of this carbon will eventually be absorbed by ocean, and buffered by massive dissolution of seafloor carbonates. Numerical simulations of this scenario often show similar patterns. Over the first several centuries, pCO2 rapidly rises peaking between 1800 and 2000 ppm. The total dissolved carbon in the ocean also rises, as pH and [CO32-] decline. Following peak emissions, pCO2 begins to decline and within several thousand years stabilizes, but at levels (350-500 ppm) significantly higher than the pre-anthropogenic steady state (Archer, 2005). This latter feature appears to be a consequence of the primary buffering process by which [CO32-] is initially restored, that is through the dissolution of seafloor carbonates, which leaves both ocean DIC and alkalinity at levels significantly higher than pre-anthropogenic (Tyrrell et al., 2007). Pelagic sediment records that span the PETM appear to exhibit several features that are consistent with the above pattern. The initial release of carbon, as represented by the negative carbon isotope excursion (CIE), triggered shoaling of the CCD and massive carbonate dissolution, followed by a gradual recovery of deep-sea carbonate chemistry. Though a detailed proxy record of pCO2 across the PETM does not yet exist, an indirect record, sea surface temperatures, suggests pCO2 remained high for tens of thousands of years after the peak input of isotopically depleted carbon as inferred from the CIE. This feature of the PETM is consistent with the simulated anthropogenic perturbation, and therefore lends some credibility to the prediction of high pCO2 for thousands of years after anthropogenic emissions have ceased. One feature that is not always reproduced in simulations, however, is a subsequent phase of carbonate oversaturation, a feature that has been attributed to a slow negative feedback, the weathering of silicate rocks (Dickens et al., 1997).
U43A-0864
On the Linearity of Climate Sensitivity on Geological Timescales: A Case Study for the Modern and the Eocene
Climate Sensitivity is a key parameter for quantifying the response of the entire Earth System to a perturbation in radiative forcing, via a doubling of atmospheric CO2 concentration. However, Climate Sensitivity is not a constant, in that it depends on the background state of the system. For example, due to the non-linear nature of climatic feedbacks, it is expected to have a different value if doubling CO2 from a preindustrial level (×1 to ×2), to doubling CO2 from an already elevated level (e.g. ×2 to ×4). Climate Sensitivity is also expected to have varied in Earth's history, where potential feedback mechanisms, in particular ice-albedo, may have been considerably different to modern (e.g. at the Last Glacial Maximum, 21,000 years ago, or in the icesheet-free world of the Eocene, 50 million years ago). On geological timescales, different configurations of ocean basins could also have led to different circulation states, with potential impacts on the linearity of the response of the system to increasing CO2 concentrations. In this paper, we investigate Climate Sensitivity on geological timescales. In particular, we investigate the non- linearity of Climate Sensitivity under modern-day and Eocene boundary conditions. Using the UK Met Office fully coupled GCM (HadCM3L), we carry out simulations at ×1, ×2, ×4, ×8, and ×16 CO2 concentrations, relative to pre-industrial. We then carry out an identical suite of CO2 simulations but under Eocene boundary conditions (i.e. solar constant, paleotopography and paleobathymetry appropriate for 50 million years ago). For the modern, we find that the climate sensitivity of the model is remarkably linear, with a value of 2.5°C over the range from ×1 to ×16 CO2 concentration. Perhaps counterintuitively, the Eocene Climate Sensitivity is greater than at modern - 3.5°C, despite there being less ice in the system. Additionally, the Eocene Climate Sensitivity is non-linear over the range considered. This is due to changes in the ocean circulation which take place at elevated CO2. At low CO2, the Eocene ocean circulation is in a state in which deep water formation occurs round Antarctica. However, at higher CO2, the deep water formation switches off, and the THC collapses, resulting in a state in which the deep ocean heats up only diffusively, the surface waters becoming relatively warm.