What can we say about changes in the hydrologic cycle on 50-year timescales when we cannot predict rainfall next week? Eventually, perhaps, a great deal: the overall climate response to increasing atmospheric concentrations of greenhouse gases may prove much simpler and more predictable than the chaos of short-term weather. Quantifying the diversity of possible responses is essential for any objective, probability-based climate forecast, and this task will require a new generation of climate modelling experiments, systematically exploring the range of model behaviour that is consistent with observations. It will be substantially harder to quantify the range of possible changes in the hydrologic cycle than in global-mean temperature, both because the observations are less complete and because the physical constraints are weaker.

[1] We describe a new method for evaluating the radiative forcing, the climate feedback parameter (W m À2 K À1) and hence the effective climate sensitivity from any GCM experiment in which the climate is responding to a constant forcing. The method is simply to regress the top of atmosphere radiative flux against the global average surface air temperature change. This method does not require special integrations or off-line estimates, such as for stratospheric adjustment, to obtain the forcing, and eliminates the need for double radiation calculations and tropopause radiative fluxes. We show that for CO 2 and solar forcing in a slab model and an AOGCM the method gives results consistent with those obtained by conventional methods. For a single integration it is less precise but since it does not require a steady state to be reached its precision could be improved by running an ensemble of short integrations.

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.

Abstract. The scientific understanding of the Earth's climate system, including the central question of how the climate system is likely to respond to human-induced perturbations, is comprehensively captured in GCMs and Earth System Models (ESM). Diagnosing the simulated climate response, and comparing responses across different models, is crucially dependent on transparent assumptions of how the GCM/ESM has been driven -especially because the implementation can involve subjective decisions and may differ between modelling groups performing the same experiment. This paper outlines the climate forcings and setup ofCorrespondence to: C. D. Jones (chris.d.jones@metoffice.gov.uk) the Met Office Hadley Centre ESM, HadGEM2-ES for the CMIP5 set of centennial experiments. We document the prescribed greenhouse gas concentrations, aerosol precursors, stratospheric and tropospheric ozone assumptions, as well as implementation of land-use change and natural forcings for the HadGEM2-ES historical and future experiments following the Representative Concentration Pathways. In addition, we provide details of how HadGEM2-ES ensemble members were initialised from the control run and how the palaeoclimate and AMIP experiments, as well as the "emissiondriven" RCP experiments were performed.

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[1] The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO 2 , CH 4 , N 2 O, CFC-11, CFC-12, and the increased H 2 O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH 4 and N 2 O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.Citation: Collins, W. D., et al., (2006), Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the

letters to nature NATURE | VOL 399 | 10 JUNE 1999 | www.nature.com 569As depicted in Fig. 4, minimization of the total free energy per surfactant predicts equilibrium ribbons if the chirality or enantiomeric excess is large enough (specifically, if lS у g=2). Thus, despite the inevitable excess free energy associated with the edges, these ribbons may not grow beyond a certain preferred width W. In this case, the twist period T and ribbon width W are expected to vary inversely with the enantiomeric excess, to be well-defined (for example, their distributions are expected to be monodisperse), and the ratio T/W is expected to be of the order of 1, and nearly independent of the enantiomeric excess. Before these equilibrium structures are attained, however, it is expected that T varies inversely with the enantiomeric excess and is uncorrelated with W. Thus, the aspect ratio and monodispersity that we observe for ribbons formed by the pure 16-2-16 L-tartrate suggest that these are close to equilibrium.The non-equilibrium structures predicted for weakly chiral systems (specifically, if lS у g=2) are expected to exhibit correlated values of T and W: T/W varies inversely with enantiomeric excess, although T and W are expected to have polydisperse distributions. This corresponds well to the twisted ribbons formed by mixtures of D and L enantiomers, which exhibit such correlated, though polydisperse, geometrical parameters, and large T/W ratios. The observed stability for these helices probably results from kinetic effects which limit their growth.Thus, on the basis of molecular parameters such as chirality and anisotropy of the surfactant polar heads, this simple yet general model predicts that the observed twisted-ribbon shape is optimal, and that both the pitch and the width are expected to follow the trends that we observe on varying the chirality. Also important are the apparently qualitatively different behaviours for strongly and weakly chiral systems. The experimental observations and the theoretical model reported here may open the prospect of creating stable structures of variable pitch in amphiphilic bilayer systems.Ⅺ