An Observationally-Based Quantification of Climate Feedbacks

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment

Abstract

The Earth's climate sensitivity / how much it warms as greenhouses gases increase, is arguably the most important 'unknown' in predictions of climate change. Models give a range of approximately 1.5 - 4.5 K for the increase in equilibrium global mean temperature expected when carbon dioxide is doubled. Recently scientists have attempted to use combinations of observations and models to constrain this range / but if anything the range has increased. Uncertainties, mainly in the cloud feedback but also in other feedbacks such as water vapour and ice, account for these large differences between the climate models. These climate feedbacks act to either amplify or reduce the initial effects of the climate change mechanism. Water vapour is the largest positive feedback and acting alone is believed to increase by an amount which roughly doubles the effectiveness of the initial greenhouse gas perturbation. Prime objective: - To evaluate the four main feedback terms in the climate system using observed varaibles. The feedbacks evaluated will be 1) water vapour, 2) clouds (specifically cloud amount, cloud height and cloud optical depth), 3) lapse-rate and 4) surface albedo. A variety of global-scale observations will be combined from many sources and these will be incorporated into offline radiative transfer calculations to gauge the role of these feedbacks in modifying the global energy balance. Uncertainty assessment: - Both the proposed methodology and other more conventional methodologies of calculating climate feedbacks will be assessed in climate model simulations from project partners at the Hadley Centre. These feedback calculations with their model output will be of direct benefit to the Centre who to date have not calculated these feedback terms within their model. These model and data comparisons will be used to: test and assess assumptions used in the proposed methodology, and to quantify realistic uncertainties for each of the feedback terms. - A parallel energy budget calculation by project partners at the NASA Goddard Institute for Space Studies (GISS) will also be used to gauge uncertainty estimates from our analyses. Secondary objectives: - The second aim of the project employs similar methodologies to those of the prime aim to analyse feedbacks on both shorter timescales and on regional scales, and will also analyse feedbacks for different regimes. This work will be used to design diagnostic tests of feedback mechanisms in climate models. Here we will make use of the regime analysis of feedbacks already undertaken by the Hadley Centre. - The third aim of the study is to test the linear model of climate feedbacks: here we will use two different methodologies to evaluate the linear and non linear components of these feeback terms, testing assumptions of non-linearity. Additional output: - We will produce a synthetic dataset of the top-of-atmosphere fluxes, which we will make available to the wider community for their own model evaluation exercises. In summary the project will attempt to quantify some of the largest 'unknowns' in our predictions of global climate change. It will also develop diagnostic tests for feedback analysis in climate models. Overall it will lead to better and more trustworthy climate model predictions, which would not only be of great benefit to the climate modelling community, it would also benefit policy makers who need to rely on the accuracy of such climate model predictions.

Publications

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Rap A (2015) Satellite constraint on the tropospheric ozone radiative effect in Geophysical Research Letters

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Sherwood S (2015) Adjustments in the Forcing-Feedback Framework for Understanding Climate Change in Bulletin of the American Meteorological Society

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Zelinka M (2014) Quantifying components of aerosol-cloud-radiation interactions in climate models in Journal of Geophysical Research: Atmospheres

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Crook J (2014) Comparison of surface albedo feedback in climate models and observations in Geophysical Research Letters

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Gordon N (2013) An observationally based constraint on the water-vapor feedback in Journal of Geophysical Research: Atmospheres

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Otto A (2013) Energy budget constraints on climate response in Nature Geoscience

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Andrews T (2010) Precipitation, radiative forcing and global temperature change in Geophysical Research Letters

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Murphy D (2009) An observationally based energy balance for the Earth since 1950 in Journal of Geophysical Research: Atmospheres

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Murphy D. M. (2009) The Earth's energy budget and aerosol radiative forcing in GEOCHIMICA ET COSMOCHIMICA ACTA

 
Description Recent observations help constrain the water vapour feedback and the surface reflectance feedback on climate. However, they are not good enough to constrain the cloud feedback
Exploitation Route Climate policy around how much the world will warm. Gives confidence in climate model projections
Sectors Environment

 
Description Used in IPCC reports as citations in several chapters of the working group 1 report
First Year Of Impact 2013
Sector Environment
Impact Types Policy & public services

 
Description IPCC report authorship and citations
Geographic Reach Multiple continents/international 
Policy Influence Type Citation in systematic reviews
Impact IPCC reports influence the UNFCCC climate negotiations and government policy on climate change worldwide
URL http://unfccc.int/meetings/lima_dec_2014/meeting/8141.php
 
Description Public debate with Piers Corbyn London 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Supporters
Results and Impact Debate with Piers Corbyn organised by debating society - Conway Hall
Year(s) Of Engagement Activity 2016
URL https://conwayhall.org.uk/issue/vol-122-no-1/