Our Common Future Under Climate Change

International Scientific Conference 7-10 JULY 2015 Paris, France

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Tuesday 7 July - 17:00-18:30 UPMC Jussieu - Amphi 15

1122 - Global warming hiatus

Parallel Session

Convener(s): C. Jeandron (Sauvons le Climat, TAVERNY, France), A. Berger (Louvain University, Louvain, Belgium)

17:00

Global warming and sea level rise

A. Cazenave (CNRS-CNES, Toulouse, France)

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Global warming and sea level rise

A. Cazenave (1)
(1) CNES, LEGOS, Toulouse, France

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It is now well established that the Earth‘s climate is warming because accumulation inside theatmosphere of green house gases produced by anthropogenic fossil fuel combustion and landuse change (mostly deforestation). Global warming has already several visible consequences,in particular increase of the ocean heat content, melting of glaciers, and ice mass loss from theGreenland and Antarctica ice sheets. Ocean warming and land ice melt in turn cause sea levelto rise. Sea level rise induced by global warming and its impacts in coastal zones has becomea question of growing interest for in the scientific community, and the media and public. Inthis presentation, we summarize the most up-to-date knowledge about climate change andassociated impacts on ocean warming, land ice melt and sea level rise. We also present sealevel projections for the 21st century under different warming scenarios, highlighting theregional variability that superimposes the global mean rise. Finally, we briefly discuss thecoastal zones case where climate-related sea level rise generally amplifies the vulnerability ofthese regions already impacted by many other factors due to natural phenomena and directanthropogenic forcing.

17:10

Greenland ice cores tell tales on the extent of the Greenland Ice Sheet during past warm climate periodes

D. Dahl-Jensen (Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark)

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Greenland ice cores tell tales on the extent of the Greenland Ice Sheet during past warm climate periodes

D. Dahl-Jensen (1)
(1) Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

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Knowledge on the long-term response of the Greenland ice sheet to climate warming duringpast interglacials is essential for estimating the potential of future rise in sea level. Duringthe last million years, the Greenland Ice Sheet (GRIS) has waxed and waned in response toglacial and interglacial periods. The deep ice cores through the Greenland ice sheet containice from the time ice covered the site. Ice from the last interglacial period (the Eemian, LIG)130 to 115 kyears before present is present in most of the deep ice cores and can be used todetermine both temperature and extent of the ice sheet during this warm interglacialperiod.Going to the bed, basal material enclosed in the ice cores contain DNA remnants that can beused to determine the ecosystems present before ice covered Greenland.The reaction of the Greenland ice sheet to climate changes and the sea level change frommass loss from the Greenland ice sheet is discussed based on the ice core findings.

17:20

Uncertainties in Mass Balance Studies of the Arctic Sea Ice Cover

J. Comiso (NASA Goddard Space Flight Centeer, Greenbelt, MD, United States of America)

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Uncertainties in Mass Balance Studies of the Arctic Sea Ice Cover

J. Comiso (1)
(1) NASA Goddard Space Flight Centeer, Cryospheric Sciences Laboratory, Code 615, Greenbelt, MD, United States of America

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The longest continuous observations of the global sea ice cover are those derived from satellite passive microwave sea ice data starting with the launch of the Nimbus-7 SMMR in October 1978 and followed by a series of DMSP/SSM/I sensors launched from July 1987 to the present.  These capabilities were further improved with the launch of Aqua/AMSR-E which provided higher resolution data from June 2002 to October 2011and followed by the launched of  GCOM-W/AMSR2 which has been in operation from 2012 up to the present.  Putting together an accurate time series data set has been a challenge because of differences in fields-of-view, antenna side-lobes, calibration and other physical attributes of the different sensors.  Through comparative analysis of overlapping data, however, some of these problems have been minimized and the accuracy in the assessments of the variability and trends of ice extent and ice area has been optimized.  The ice extent provides the sum of areas in the ocean that has ice cover of at least 15% ice concentration while ice area provides the actual area covered by ice and used to estimate the ice volume assuming that the average thickness of the ice cover is known.  The current record from 1978 to 2014 shows a trend in ice area of about 4.3 ± 0.16% per decade for the yearly average but a more drastic change is observed for the thick multiyear ice the trend of which is about 14.9±1.6% per decade suggesting large declines in the average ice thickness of this ice type. Measurements of ice thickness have been the source of greatest uncertainties since accurate, continuous and consistent time series measurements of global ice thickness are not available.  Historically, ice thickness measurements have been sparse and were done primarily using upward looking submarine sonar data which have been used to show a significant change in average winter ice thickness from about 3.6 m in 1980 to about 2.9m in 2000.  Ice freeboard measurements from ICESat/GLAS (January 2003-February 2010) provide more extensive coverage and indicate a change in winter thickness from about 2.4 m in 2004 to 1.9m in 2008.  More recent freeboard measurements from CryosSat2 (April 2010 to the present) provided a continuation of the satellite measurements and show a more stable ice cover.  Although retrieve thicknesses from freeboard measurements have indicated relatively low biases (less than 0.1m) compared to in situ and other measurements, there are large standard deviations (0.7m) in the comparative analysis suggesting that more accurate snow thickness and ice density data are needed.  The use of the improved measurements of average sea ice thickness separately for seasonal and multiyear ice in conjunction with passive microwave data would provide the desired estimate for average volume of the two major ice types.  Time series of such data are needed in mass balance studies of the Arctic sea ice cover and uncertainties associated with the use of current data sets will be provided. 

17:30

Global Warming Hiatus 1998-2012 and the ice melting

A. Berger (Université Catholique de Louvain, Louvain la Neuve, Belgium), Q. Z. Yin (Université catholique de Louvain, , Belgium), H. Nifenecker (Sauvons le Climat, Vizille, France), J. Poitou (Université catholique de Louvain, Louvain-la-Neuve, Belgium)

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Global Warming Hiatus 1998-2012 and the ice melting
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17:40

What can we learn from the recent global warming hiatus about climate variability and climate models?

H. Douville (Météo-France, Toulouse, France), A. Voldoire (Météo-France, Toulouse, France)

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What can we learn from the recent global warming hiatus about climate variability and climate models?

H. Douville (1) ; A. Voldoire (1)
(1) Météo-France, CNRM-GAME, Toulouse, France

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The observed rise in global mean surface air temperature (GMST) has slowed down over the last 15 years, spurring outbreaks of skepticism regarding the nature of global warming and challenging the upper range transient response of the current-generation global climate models. Recent numerical studies have, however, tempered the relevance of the observed pause in global warming by highlighting the key role of tropical Pacific internal variability. Here we first show that many climate models overestimate the influence of the El Niño–Southern Oscillation on GMST, thereby shedding doubt on their ability to capture the tropical Pacific contribution to the hiatus. Moreover, model results can be quite sensitive to the experimental design. We argue that overriding the surface wind stress is more suitable than nudging the sea surface temperature for controlling the tropical Pacific ocean heat uptake and, thereby, the multidecadal variability of GMST. Using the former technique, our model captures several aspects of the recent climate evolution, including the weaker slowdown of global warming over land and the transition toward a negative phase of the Pacific Decadal Oscillation. Yet the observed global warming is still overestimated not only over the recent 1998–2012 hiatus period but also over former decades, thereby suggesting that the model might be too sensitive to the prescribed radiative forcings. Besides fully coupled ocean-atmosphere simulations, we therefore advocate the use of suitable partial-coupling techniques to control a fraction of the internal climate variability. This strategy could enable a more insightful comparison with the observed climate variability and, thereby, lead to stronger observational constraints on model sensitivity to the prescribed radiative forcings.

17:45

Eurasia winter cooling in a recent warming hiatus period of 1998-2012

C. Li, (Max Planck Institute for Meteorology, Hamburg, Germany), J. Marotzke, (Max Planck Institute for Meteorology, Hamburg, Germany)

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Eurasia winter cooling in a recent warming hiatus period of 1998-2012

C. Li, (1) ; J. Marotzke, (1)
(1) Max Planck Institute for Meteorology, The Ocean in the Earth System, Hamburg, Germany

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The global-mean surface temperature (GMST) has shown a comparatively small warming trend over 1998-2012, termed a hiatus [Meehl et al., 2011; Flato et al., 2013; Kosaka and Xie, 2013]. In addition to the warming hiatus of GMST in the recent decade, Cohen et al. [2012] found a significant winter (seasons refer to those for the Northern Hemisphere hereafter) cooling trend on the Northern-Hemisphere (NH) extropical land surface temperature. However, the relative magnitudes of the contribution from temperature trend at different latitude bands and also the mechanism of the NH mid-latitude winter cooling are poorly understood. Here we investigate the spatial pattern of surface temperature trends and the contribution of surface temperature trend reduction at different latitude bands with observational data, and the causation of NH mid-latitude winter cooling with large ensembles of AMIP-type sensitivity simulations.

In addition to the tropical Pacific cooling over 1998-2012, we find the GMST warming hiatus is strongly influenced by a pronounced DJF cooling trend over 1998-2011 in NH mid-latitude,  especially in Eurasia. However, an absent of the strong mid-latitude winter cooling trend in the previous simulations with restoring the observed SST over tropical Pacific in a coupled climate model Kosaka and Xie [2013] underlines that mechanisms other than cooling in the tropical Pacific must contribute to the warming hiatus in recent decade. In the present study, we explore the impact of the dramatic loss of Arctic sea ice on the NH large-scale circulation changes and the NH mid-latitude winter cooling trend. We found that the Arctic sea ice does not drive systemic changes on the NH large-scale circulation as Arctic Oscillation (AO), Pacific/North American Pattern (PNA) and the Eurasia winter blocking frequency in the past decades. And the observed DJF cooling trend over 1998-2012 is a random internal variability and belongs to a extreme climate events. Although the dramatic loss of Arctic sea ice does not drive systematic cooling trend of Eurasia winter climate, but it can enhance the variability of Eurasia winter climate, and thus increasing the possibility of the Eurasia winter reaching an extreme cold trend over 1998-2012.

17:50

Impacts of External Forcing on the Decadal Climate Variability in CMIP5 Simulations

Y. Yu, (Institute of atmospheric physics,Chinese Academy of Sciences, Beijing, China), Y. Song, (Institute of atmospheric physics,Chinese Academy of Sciences, Beijing, China)

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Impacts of External Forcing on the Decadal Climate Variability in CMIP5 Simulations

Y. Yu, (1) ; Y. Song, (1)
(1) Institute of atmospheric physics,Chinese Academy of Sciences, Beijing, China

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Decadal climate variability is usually regarded as an internal variability in the climate system. However, using the coupled simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), we have demonstrated that the external radiative forcing plays an important role in modulating decadal variability of the global mean surface air temperature (SAT). In historical runs, the standard deviations of the global mean SAT exhibit robust increases relative to pre-industrial runs, indicating that external forcing acts on decadal variability of the global mean SAT through enhancing amplitude and modulating phase. By comparing model results using different external forcing agents, we find the natural-forcing agent has the strongest impact on the decadal timescale. Every type of simulation (e.g., the pre-industrial, historical, natural forcing and anthropogenic forcing runs) from almost all the CMIP5 models exhibits a high correlation between the net shortwave (SW) radiative flux at the top-of-atmosphere (TOA) and the global mean SAT with a 13 month lag. However, after taking the multi-model ensemble mean for the TOA SW and the SAT, respectively, the correlations from external-forcing run are much higher than those from pre-industrial runs. This is because that the decadal SAT anomalies from multiple models cancel each other out in the pre-industrial runs without external forcing, but generally follow decadal evolution of the external forcing with a 13 month lag. The most significant responses to external forcing are found in the tropical Indian and Pacific oceans, through with different physical mechanisms for the natural and greenhouse gas forcing agents.

17:55

Change Points of Global Temperature

N. Cahill (University College Dublin, Dublin, Ireland), S. Rahmstorf (Potsdam Institute for Climate Impact Research, Potsdam, Germany)

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Change Points of Global Temperature

N. Cahill (1) ; S. Rahmstorf (2)
(1) University College Dublin, Mathematical Science, Dublin, Ireland; (2) Potsdam Institute for Climate Impact Research, Potsdam, Germany

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We aim to address the question of whether or not there is a recent “hiatus”, “pause” or “slowdown” of global temperature rise. Using a statistical technique known as change point analysis we identify the statistically significant changes in four global temperature records and estimate the rates of temperature rise before and after these changes occur. In each case the results indicate that three change points are enough to accurately capture the variability in the data with no evidence of any significant change in the global warming trend since ~1970. We conclude that a hiatus or pause cannot be statistically justified.

18:00

Impact of initial conditions and atmospheric model resolution in predicting “Climate Hiatuses”

S. Corti (National Research Council (CNR), Bologna, France)

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Impact of initial conditions and atmospheric model resolution in predicting “Climate Hiatuses”

S. Corti (1)
(1) National Research Council (CNR), Institute of Atmospheric Science and Climate (ISAC), Bologna, France

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The results from a set of multi-year hindcasts carried out with the ECMWF coupled system at two different resolutions of the atmospheric component will be presented. The first experiment consists of a control series of ensemble hindcasts with the atmospheric model integrated at T255 with 91 levels in vertical (this is the current resolution of the ECMWF System4). The ocean resolution is the standard NEMO-ORCA1. In the sensitivity experiment the atmospheric resolution has increased to T511 (the ocean resolution is the same). By comparing the control and the sensitivity experiment we estimate the impact of the increased atmospheric resolution on forecast quality.

The impact of initial conditions relative to external forcings in multi-year integrations is further assessed using specifically designed sensitivity experiments. They consist, for each atmospheric resolution, of three sets of ensemble hindcasts for three initial dates in 1988, 1994 and 2002 using either the external forcings from the ‘correct’ decade or swapping the forcings between the three decades. By comparing the three sets of integrations, the impact of external forcing versus initial conditions on the predictability over multi-annual time scales is estimated.  In particular we estimate the sensitivity of the model to initial conditions and horizontal resolution in predicting the multi-year climate oscillations that modulate the global warming trend, also known as “Climate Hiatuses”.

18:05

Multi-model simulations of radiative forcing of aerosols and ozone during the 1990-2015 period

G. Myhre (CICERO, Oslo, Norway), B. H. Samset (CICERO, Oslo, Norway), Ø. Hodnebrog, (CICERO, Oslo, Norway), R. B. Skeie (CICERO, Oslo, Norway), Z. Klimont, (IIASA, Laxenburg, Austria), G. Faluvegi (GISS, New York, United States of America), D. Shindell (Duke University, Durham, United States of America), M. Flanner (University of Michigan, Ann Arbor, United States of America), D. Olivie (Met.no, Oslo, Norway), S. Tsyro (Met.no, Oslo, Norway), M. Schulz (Met.no, Oslo, Norway), R. Cherian (University of Leipzig, Leipiz, Germany), J. Quaas (University of Leipzig, Leipiz, Germany), J. Mülmenstädt (University of Leipzig, Leipiz, Germany), T. Takemura (Kyushu University, Kyushu, Japan), J. Schnell (University of California, Irvine, Irvine, United States of America), M. Prather (University of California, Irvine, Irvine, United States of America)

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Multi-model simulations of radiative forcing of aerosols and ozone during the 1990-2015 period

G. Myhre (1) ; BH. Samset (2) ; Ø. Hodnebrog, (1) ; RB. Skeie (1) ; Z. Klimont, (3) ; G. Faluvegi (4) ; D. Shindell (5) ; M. Flanner (6) ; D. Olivie (7) ; S. Tsyro (7) ; M. Schulz (7) ; R. Cherian (8) ; J. Quaas (8) ; J. Mülmenstädt (8) ; T. Takemura (9) ; J. Schnell (10) ; M. Prather (10)
(1) CICERO, Oslo, Norway; (2) CICERO, Climate System, Oslo, Norway; (3) IIASA, Laxenburg, Austria; (4) GISS, New York, United States of America; (5) Duke University, Durham, United States of America; (6) University of Michigan, Ann Arbor, United States of America; (7) Met.no, Oslo, Norway; (8) University of Leipzig, Leipiz, Germany; (9) Kyushu University, Kyushu, Japan; (10) University of California, Irvine, Irvine, United States of America

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Seven global models have simulated the changes in aerosols and ozone over the 1990-2015 period, and the associated radiative forcing. The models have used updated emission data from the EU-project Eclipse. The modelled changes in aerosol abundance at the surface in terms of PM2.5 show broad agreement with observations, whereas they generally underestimate the overall increase in surface ozone over US and Europe. We focus on model-mean radiative forcing and the spread among the models, and compare this to estimated radiative forcing in IPCC AR5. The models show remarkably good agreement for the direct aerosol effect of sulphate, and for carbonaceous aerosols from fossil fuel and biofuel sources. However, for nitrate the differences are large.  Over the 1990-2015 period, the models simulate a positive direct aerosol effect radiative forcing of sulphate and black carbon, due to a reduction the sulphate abundance and increase in black carbon abundance. The model mean shows relatively small changes in the indirect aerosol effect over the investigated period. It is indicated that including semi-direct effect of black carbon is important, even though few models have been able to quantify this forcing. Overall the models have a more strongly positive aerosol radiative forcing over the 1990-2015 period than IPCC AR5. Similarly the ozone radiative forcing over the same period is simulated in this study to have a more positive radiative forcing than IPCC AR5. Based on these multi-model simulations using updated emission data of aerosols and ozone, we find no indications that these forcing agents are driving the global warming hiatus. 
 

18:10

Panel discussion

J. Comiso (NASA Goddard Space Flight Centeer, Greenbelt, MD, United States of America), N. Cahill (University College Dublin, Dublin, Ireland), . Y. Yu (Chinese Academy of Science, Beijing, China), H. Douville (Météo-France, Toulouse, France), C. Li, S. Corti (National Research Council (CNR), Bologna, France), A. Cazenave (Laboratoire d’Etudes en Géophysique et Océanographie Spatiales, Toulouse, France), A. Berger (Université Catholique de Louvain, Louvain la Neuve, Belgium), D. Dahl-Jensen (Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark)

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Panel discussion
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