Can feedback analysis be used to uncover the physical origin of climate sensitivity and efficacy differences?

Different strengths and types of radiative forcings cause variations in the climate sensitivities and efficacies. To relate these changes to their physical origin, this study tests whether a feedback analysis is a suitable approach. For this end, we apply the partial radiative perturbation method. Combining the forward and backward calculation turns out to be indispensable to ensure the additivity of feedbacks and to yield a closed forcing-feedbackbalance at top of the atmosphere

Contrail cirrus supporting areas in model and observations

Contrails form and persist dependent on the surrounding moisture, temperature and pressure fields and on fuel and aircraft specific variables. After formation, contrail persistence requires only supersaturation relative to ice. The fractional area in which contrails can form is called potential contrail coverage. We introduce a potential contrail cirrus coverage equivalent to the cloud free supersaturated area. This field, simulated by the ECHAM4 climate model, agrees fairly well with estimates of supersaturation frequency as inferred from aircraft and satellite measurements. In areas where the two potential coverages are different, especially at lower flight levels, potential contrail coverage is not a valid estimate of maximum attainable contrail cirrus coverage. We parameterize both potential coverages consistently with the ECHAM4 cloud cover parameterization. A comparison of the potential contrail coverage with an earlier estimate reveals substantial differences especially at upper height levels in the tropics

Estimating the Effective Radiative Forcing of Contrail Cirrus

Evidence from previous climate model simulations has suggested a potentially low efficacy of contrails to force global mean surface temperature changes. In this paper, a climate model with a state-of-the-art contrail cirrus representation is used for fixed sea surface temperature simulations in order to determine the effective radiative forcing (ERF) from contrail cirrus. ERF is expected to be a good metric for intercomparing the quantitative importance of different contributions to surface temperature and climate impact. Substantial upscaling of aviation density is necessary to ensure statistically significant results from our simulations. The contrail cirrus ERF is found to be less than 50% of the respective instantaneous or stratosphere adjusted radiative forcings, with a best estimate of roughly 35%. The reduction of ERF is much more substantial for contrail cirrus than it is for a CO2 increase when both stratosphere adjusted forcings are of similar magnitude. Analysis of all rapid radiative adjustments contributing to the ERF indicates that the reduction is mainly induced by a compensating effect of natural clouds that provide a negative feedback. Compared to the CO2 reference case, a less positive combined water vapor and lapse rate adjustment also contributes to a more distinct reduction of contrail cirrus ERF, but not as much as the natural cloud adjustment. Based on the experience gained in this paper, respective contrail cirrus simulations with interactive ocean will be performed as the next step toward establishing contrail cirrus efficacy. ERF results of contrail cirrus from other climate models equipped with suitable parameterizations are regarded as highly desirable

Slow feedbacks resulting from strongly enhanced atmospheric methane mixing ratios in a chemistry-climate model with mixed-layer ocean

In a previous study the quasi-instantaneous chemical impacts (rapid adjustments) of strongly enhanced methane (CH4) mixing ratios have been analysed. However, to quantify the influence of the respective slow climate feedbacks on the chemical composition it is necessary to include the radiation-driven temperature feedback. Therefore, we perform sensitivity simulations with doubled and quintupled present-day (year 2010) CH4 mixing ratios with the chemistry-climate model EMAC (European Centre for Medium-Range Weather Forecasts, Hamburg version - Modular Earth Submodel System (ECHAM/MESSy) Atmospheric Chemistry) and include in a novel set-up a mixedlayer ocean model to account for tropospheric warming. Strong increases in CH4 lead to a reduction in the hydroxyl radical in the troposphere, thereby extending the CH4 lifetime. Slow climate feedbacks counteract this reduction in the hydroxyl radical through increases in tropospheric water vapour and ozone, thereby dampening the extension of CH4 lifetime in comparison with the quasi-instantaneous response. Changes in the stratospheric circulation evolve clearly with the warming of the troposphere. The Brewer-Dobson circulation strengthens, affecting the response of trace gases, such as ozone, water vapour and CH4 in the stratosphere, and also causing stratospheric temperature changes. In the middle and upper stratosphere, the increase in stratospheric water vapour is reduced with respect to the quasi-instantaneous response. We find that this difference cannot be explained by the response of the cold point and the associated water vapour entry values but by a weaker strengthening of the in situ source of water vapour through CH4 oxidation. However, in the lower stratosphere water vapour increases more strongly when tropospheric warming is accounted for, enlarging its overall radiative impact. The response of the stratosphere adjusted temperatures driven by slow climate feedbacks is dominated by these increases in stratospheric water vapour as well as strongly decreased ozone mixing ratios above the tropical tropopause, which result from enhanced tropical upwelling. While rapid radiative adjustments from ozone and stratospheric water vapour make an essential contribution to the effective CH4 radiative forcing, the radiative impact of the respective slow feedbacks is rather moderate. In line with this, the climate sensitivity from CH4 changes in this chemistry-climate model set-up is not significantly different from the climate sensitivity in carbon-dioxide-driven simulations, provided that the CH4 effective radiative forcing includes the rapid adjustments from ozone and stratospheric water vapour changes

Climate sensitivity of radiative impacts from transport systems

Comparing individual components of a total climate impact is traditionally done in terms of radiative forcing. However, the climate impact of transport systems includes contributions that are likely to imply climate sensitivity parameters distinctly different from the “reference value” for a homogeneous CO2 perturbation. We propose to introduce efficacy factors for each component into the assessment. The way of proceeding is illustrated using aviation as an example, and prospects for evaluating the other transport system in the EU project QUANTIFY are given

Towards Determining the Contrail Cirrus Efficacy

Contrail cirrus has been emphasized as the largest individual component of aircraft climate impact, yet respective assessments have been based mainly on conventional radiative forcing calculations. As demonstrated in previous research work, individual impact components can have different efficacies, i.e., their effectiveness to induce surface temperature changes may vary. Effective radiative forcing (ERF) has been proposed as a superior metric to compare individual impact contributions, as it may, to a considerable extent, include the effect of efficacy differences. Recent climate model simulations have provided a first estimate of contrail cirrus ERF, which turns out to be much smaller, by about 65%, than the conventional radiative forcing of contrail cirrus. The main reason for the reduction is that natural clouds exhibit a substantially lower radiative impact in the presence of contrail cirrus. Hence, the new result suggests a smaller role of contrail cirrus in the context of aviation climate impact (including proposed mitigation measures) than assumed so far. However, any conclusion in this respect should be drawn carefully as long as no direct simulations of the surface temperature response to contrail cirrus are available. Such simulations are needed in order to confirm the power of ERF for assessing contrail cirrus efficacy

Feedback Analyses of Equilibrium Climate Change Simulations

Performing equilibrium climate change simulations is a standard method to study the forcing-response relationship within the climate system. An important finding is th different climate models simulate a considerably different global mean response to the same kind of forcing, and previous studies have focussed to explore the respective reason. Radiative feedbacks are essential in controlling the global response and a model's distinctive climate sensitivity. The cloud feedback has been identified to be of particular importance. Respective evidence in equilibrium simulations is valid for transient model simulations as well. Another key issue is to explain the distinctive global mean response of one and the same model to the same amount of forcing induced by different forcings mechanisms. This so-called efficacy characterising a certain forcing can also be related to the specific acting of feedbacks, and not necessarily to the cloud feedback only. Several ways to explore different efficacy of different forcings have been developed, and it is not clear which method is best suited for the purpose. In this talk we will point out some merits and shortcomings of the "partial radiative perturbation method". It has been applied to equilibrium climate change simulations with a coupled chemistry-climate model. The particular focus will be on CO2 increase simulations

Radiative forcing and rapid atmospheric adjustments induced by contrail cirrus

The sustainability of worldwide air traffic forms an important issue due to its expected large growth rates in the coming decades. Contrail cirrus is regarded to be the largest contributor to aviation climate impact and thus plays an important role in considerations towards limiting aviation induced climate change. Here, we present results from global climate model simulations, designed to determine the adjusted radiative forcing (RFadj) and the effective radiative forcing (ERF) of contrail cirrus. For a 2050 air traffic scenario a RFadj of 160 mWm-2 was determined, which corresponds to an increase by a factor of more than 3 compared to 2006 values (49 mWm-2) and thus highlights the largely growing impact of air traffic in a future climate. However, as has been indicated by earlier studies, the efficacy of RFadj of linear contrails in forcing surface temperature is significantly reduced and it stands to reason that this might hold for contrail cirrus as well. For this reason we also performed ERF simulations which account for further rapid radiative adjustments in the atmosphere, not included in RFadj, and thus may form a better metric for estimating surface temperature changes. ERF of contrail cirrus is found to be severely reduced by between 50 and 75% (best estimate about 65%), compared to RFadj. In a subsequent feedback analysis the rapid adjustments, which are physically responsible for the reduced ERF, have been determined. A large negative cloud adjustment, due to a decline of natural cirrus cover, is found to be the main driver of the substantial reduction. For a CO2 doubling simulation, the reduction of ERF in comparison to the RFadj is found to be much smaller