Climate feedbacks - setting the research agenda

Constraints on aerosol forcing

Professor Bjorn Stevens, Max Planck Institute for Meteorology, Germany

Abstract

Uncertainty in aerosol forcing, and the spectre of strong radiative forcing by anthropogenic aerosols have raised fears that dangerously large climate sensitivities are being masked by short-lived aerosol forcing. I argue that present understanding of the atmospheric aerosol and the historical temperature record is inconsistent with a large-aerosol forcing, and that forcing magnitudes more negative than -1 W/m^2 are unlikely. If correct, these arguments would reduce current estimates of uncertainty in radiative forcing by anthropogenic aerosols by a factor of two.

• Audio recording (mp3)

An assessment of radiative forcing, radiative adjustments and radiative feedbacks in climate models

Professor Brian Soden, University of Miami, USA

Abstract

Radiative kernels are used to separate direct radiative forcing from radiative adjustments to that forcing in order to quantify the magnitude and intermodel spread of tropospheric and stratospheric adjustments. The direct radiative forcing due to quadrupling CO₂ is found to have an intermodel spread of ~3 W m^-2, which is comparable to spread in the radiative adjustments to that forcing. However, intermodel differences in these quantities are negatively correlated, resulting in an intermodel spread in total forcing of only ~4 W m^-2. In contrast to previous studies, relatively small estimates of cloud adjustments are obtained which are both positive and negative. It is shown that the regional patterns in all tropospheric adjustments tend to oppose the radiative feedback. This compensation between feedback and adjustment is closely tied to spatial inhomogeneities in the initial rate of surface warming and can be explained assuming a linear feedback with no adjustment, suggesting that much of the spatial variation in the adjustment may be an artifact of assuming the methodology. Even when assuming that the global-mean estimates of the tropospheric adjustments are valid, neglecting them introduces little uncertainty in estimates of the total forcing, feedback or effective climate sensitivity relative to the intermodel spread in these values.

• Audio recording (mp3)

Modelling ice-sheets as climate forcing and feedback

Professor Jonathan Gregory, University of Reading and Met Office Hadley Centre, UK

Abstract

Ice-sheets are very sensitive to climate change through its effect on their surface mass balance i.e. mostly snowfall minus surface melting, and through changes in basal melting at marine margins having an effect of the dynamics of grounded ice. Ice-sheet mass changes are a matter of practical concern because of their influence on sea level; the effect of anthropogenic climate change on the ice-sheets could produce changes in sea-level of many metres over future centuries, modulated regionally by the consequent changes in the geoid. Ice-sheets influence climate through other effects than sea-level: absorbed solar radiation is reduced by high albedo, surface temperature is cooled by raised elevation, precipitation is enhanced upwind and suppressed downwind of an ice-sheet, tropospheric circulation is altered by ice-sheet topography in ways which may affect climate remote from the ice-sheet, and freshwater input may influence ocean circulation. If ice-sheets are treated as a boundary condition for climate, changes in ice-sheets are a forcing of climate change, but if ice-sheets and climate are regarded as a coupled system, ice-sheet changes give feedbacks on climate change. In either case, it is valuable to quantify the effects. The coupled approach is essential to achieve a full understanding of the enormous and complex changes which take place during glacial cycles, and is needed for accurate projections of some aspects of anthropogenic climate change. Coupled AOGCM--ice-sheet models offer the most physically comprehensive treatment. This is a relatively new endeavour. There are many technical challenges and no doubt much to be learned scientifically.

• Audio recording (mp3)

Towards improving uncertainty in future evolution of the Arctic sea ice and associated impacts

Dr Julienne Stroeve, University of Colorado, USA

Abstract

The modern satellite passive microwave record provides consistent estimates of Arctic sea ice extent since late 1970s. These data document downward linear trends in Arctic sea ice extent for all months, but with the largest trend for September at -86,000 km2 yr-1 between 1979 and 2014. Less clear is how the thickness has changed over time as long-term consistent data sets are not available. However, based on the data that is available, it is clear that the Arctic sea ice has thinned over the last few decades, making the Arctic more vulnerable to melting out each summer.

All climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) show that Arctic sea ice extent will continue to decline, the eventual outcome being an essentially seasonally-ice Arctic Ocean. While the CMIP5 models better capture the observed mean state of the Arctic sea ice extent than the CMIP3 models did, the models remain poor at representing the spatial pattern of the ice thickness and general atmospheric circulation pattern, which bears on the timing of when seasonally ice-free conditions may happen. Given the influence the ice cover plays on the overlying atmosphere, with feedbacks on lower latitudes, improving model performance is essential to making good decisions within an adaptive framework.

• Audio recording (mp3)

Emergent constraints on climate-carbon cycle feedbacks

Professor Peter Cox, University of Exeter, UK

Abstract

Earth System Models (ESMs) are designed to project changes in the climate-carbon cycle system over the coming centuries. However, ESMs still suffer from a significant timescale problem – we need constraints on the huge range of projected changes in the climate-carbon system over the next century, but the contemporary observational data we have relates to much shorter timescales. One way around this problem is to look for relationships between the more extensive observations of short-term variability and the longer-term sensitivity of the ES to anthropogenic forcing. According to the Fluctuation-Dissipation Theorem (FDT), such relationships should be common in a large-class of systems including the ES (Leith, 1975). In principle it should even be possible to get good estimates of ES sensitivities to external forcing purely by analysing the temporal correlations evident in climate observations – unfortunately this typically requires a prohibitively long time-series of observations.

An alternative approach to utilising the constraints embodied in short-term variability relies on “Emergent Constraints”. An Emergent Constraint is a relationship between some ES sensitivity to anthropogenic forcing and an observable feature of the ES. We call it emergent because it emerges from the ensemble of ESMs, and it is described as a constraint because it enables an observation to constrain the estimate of the sensitivity in the real world. As an example, we will describe an emergent constraint on the sensitivity of tropical land-carbon storage to warming, for both the current (CMIP5, Wenzel et al., 2014) and previous generation ESMs (C⁴MIP, Cox et al., 2013).

• Audio recording (mp3)

Attributing climate feedbacks between short lived forcers and well mixed greenhouse gases

Professor Philippe Ciais, Laboratoire des Sciences du Climat et de l'Environnement, France

Abstract

Using the OSCAR model designed as a substitute model of complex 3D Earth System Models, we will look at the attribution of carbon climate feedbacks to emissions of well mixed greenhouse gases vs. those of short lived climate forcers such as aerosols and tropospheric ozone precursors. The use of a full attribution framework of climate change causality and feedbacks down to emissions reveals a significant contribution of short lived forcers to long term climate change that reaches way beyond the lifetime in the atmosphere of aerosols and tropospheric ozone. This contribution is explained through interactions between the sensitivity of the carbon cycle to climate change, and the forcing caused by emissions of short lived forcers.

• Audio recording (mp3)

What can and can't we learn from observed climate change about sensitivity?

Professor Gabriele Hegerl FRS, University of Edinburgh, UK

Abstract

The observed and palaeoclimatic record provides powerful evidence that climate responds to changes in the earth's energy balance. This talk discusses evidence for the equilibrium and the transient climate response from the instrumental period and briefly summarises evidence from the palaeoclimatic record. The limitations and needs and opportunities for advance of each line of evidence are discussed. The evidence supports that climate significantly responds to forcing, and that feedbacks enhance the climate response to forcing. Some recent analyses suggest the most likely value of climate sensitivity to be lower than the 'best estimate' from full climate models. However, natural climate variability in the record, and uncertainty in the climate response to aerosol forcing may play a role in these lower estimates. The transient climate response, which accounts for ocean heat uptake during evolving warming, is less affected by these uncertainties and more informative about near-term future warming.

• Audio recording (mp3)

Extreme warm conditions and climate feedbacks

Dr Valérie Masson-Delmotte, Laboratoire des Sciences du Climat et de l'Environnement, France

Abstract

In summer 2012, widespread melt was observed above the Greenland ice sheet. It was caused by a rare atmospheric river event, transporting heat and moisture from the subtropics towards the ice sheet; local snow-albedo and cloud feedbacks have been suggested to act as key amplifiers, enhancing surface melt. Such extreme warm conditions are projected to be recurrent within this century, if greenhouse gas emissions continue to grow, calling for an improved understanding and simulation of polar climate feedbacks under these warm conditions. In a different context, the last interglacial period was also marked by very warm summer conditions at mid to high latitudes. The initial driver of last interglacial northern hemisphere warming is the well-known orbital forcing, providing an extraordinary case study to investigate the climate and Earth system feedbacks at play. In the IPSL climate model, climate feedbacks operating in summer and in the high latitudes of the northern hemisphere have similar magnitudes during the last interglacial period and for projected future climate change in response to increased atmospheric greenhouse gas concentrations. I will argue that this should be a key target period for systematic model-data intercomparisons. Moreover, I will argue that reconstructions, observations and simulations of water stable isotopes provide an integrated framework in which fingerprints of climate feedbacks associated with changes in the atmospheric water cycle could be better constrained. Finally, I will argue that several key aspects of the last interglacial period remain poorly understood, such as the mechanisms leading to Antarctic warming, and its contribution to sea level rise. This illustrates the critical role of Earth system feedbacks which are not embedded in state-of-the-art climate models.

• Audio recording (mp3)