Academic literature on the topic 'Anthropogenic forcing'

Doutoramento em Engenharia Florestal - Instituto Superior de Agronomia - UL<br>Vegetation fires are an important component of the earth’s system land processes and have a significant impact on the vegetation and CO2 dynamics. The global fire patterns are not thoroughly explored and the drivers of fire regimes in global scale are interconnected. However, several modelling assumptions are contradicted by exploring those relationships partially. At global scale, fire extent is fuel limited, with climatic variables showing both positive and negative influence on fuel moisture conditions, and humans showing a negative net effect. When isolating the influence of population density and assuming spatial nonstationarity, the human impact is very detailed and reflects the main land use activities with emphasis on cropland and rangeland management at continental scale. The footprint of fire into the Earth system can be measured in terms of radiative forcing from pre and post-fire albedo changes, with the forest biomes driving the extremes on annual basis. Additionally this thesis explores the patterns and the trends of contemporary fire activity. Contrary to previous studies, the results show non-monotonic patterns at grid cell level. The findings of this thesis give a better insight into the spatial variability and the controls of fire at global scale using satellite derived datasets with a focus to the anthropogenic land use activities

We present revised probability density functions (PDF) for climate system properties (climate sensitivity, rate of deep-ocean heat uptake, and the net aerosol forcing strength) that include the effect on 20th century temperature changes of natural as well as anthropogenic forcings. The additional natural forcings, primarily the cooling by volcanic eruptions, affect the PDF by requiring a higher climate sensitivity and a lower rate of deep-ocean heat uptake to reproduce the observed temperature changes. The estimated 90% range of climate sensitivity is 2.4 to 9.2 K. The net aerosol forcing strength for the 1980s decade shifted towards positive values to compensate for the now included volcanic forcing with 90% bounds of -0.7 to -0.16 W/m2. The rate of deep-ocean heat uptake is also reduced with the effective diffusivity, Kv, ranging from 0.25 to 7.3 cm2/s. This upper bound implies that many coupled atmosphere-ocean GCMs mix heat into the deep ocean (below the mixed layer) too efficiently.<br>Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/).<br>This work was supported in part by the NOAA Climate Change Data and Detection Program with support from DOE, the Joint Program on the Science and Policy of Global Change at MIT, and the Office of Science (BER) DOE Grant No.DE-FG02-93ER61677.

Bien qu’historiquement inattendu, la calotte Antarctique contribue à l’élévation du niveau des mers depuis plusieurs décennies. Les satellites révèlent que la calotte perd de la masse via la fonte massive induite par l’océan au droit des terminaisons flottantes de ses glaciers, notamment dans les secteurs d’Amundsen et d’Aurora. Le rôle du forçage anthropique dans cette fonte reste néanmoins largement méconnu en raison de la variabilité naturelle, très présente dans la région, et de la durée d’observations relativement restreinte.L’intrusion des eaux profondes circumpolaires relativement chaudes aux abords de la calotte en mer d’Amundsen est avérée depuis le début des relevés en 1994 et l’étude des sédiments suggère que plusieurs glaciers ont subi un retrait de leur ligne d’échouage depuis les années 40. Néanmoins, ces données ne permettent pas d’arbitrer si les conditions pré-industrielles sont compatibles avec des conditions océaniques froides ou chaudes en mer d’Amundsen ou si cette dernière a basculé dans un état chaud tardivement en raison des changements induits par le forçage anthropique. A l’aide d’un ensemble de simulations océaniques du secteur d’Amundsen avec différentes perturbations idéalisées des flux atmosphériques, nous montrons que les conditions pré-industrielles peuvent être associées, au moins de manière intermittente, avec des conditions froides en mer d’Amundsen et de faibles taux de fonte. A l’échelle multi-décennale, les transitions entre deux états résultent principalement des changements des flux de flottabilité en surface qui affectent la stratification thermohaline en modulant la production nette de glace de mer.Pour autant, si ces premiers résultats sont compatibles avec la thèse d’une influence anthropique, ils ne permettent pas d’écarter la thèse de l’influence de la variabilité interne basse fréquence, très marquée dans la région. Un second volet s’attache donc à caractériser l’effet de cette variabilité sur la perte de masse future de l’Antarctique. Cette étude est fondée sur un ensemble de simulations de contribution de la calotte au niveau des mers, d’ici 2100 sous le scénario médian SSP2-4.5, issu d’un modèle de calotte forcé par plusieurs membres de trois modèles climatiques. Nos expériences montrent que la variabilité interne affecte la contribution de l’Antarctique d’ici la fin du siècle de plus de 45% à 93% selon le modèle climatique, alors que leur représentation de la variabilité interne, à la fois océanique et atmosphérique, est certainement sous-estimée. L’influence de la variabilité atmosphérique prédomine et les secteurs d’Amundsen et d’Aurora, siège des principales pertes actuelles, sont les secteurs les plus affectés. Nos résultats suggèrent que l’impact de la variabilité interne sur l’évolution de la calotte doit être systématiquement exploré via l’utilisation de plusieurs membres climatiques.La tentative d’identification des rôles respectifs des influences anthropique et naturelle dans les pertes de masses observées dans les régions d’Amundsen et d'Aurora parachève ces travaux. A cette fin, nous reconstruisons, via une méthode originale d’initialisation, des ensembles d'évolutions de la calotte depuis 1850 avec et sans forçage anthropique pour sept membres d’un même modèle climatique. Comme escompté lors des deux premières phases d’étude, les trajectoires historiques sont impactées par la variabilité interne et certains membres climatiques permettent l’obtention d’états pré-industriels froids par intermittence en mer d'Amundsen. La détection du signal anthropique est ensuite estimée en comparant les trajectoires avec et sans forçage anthropique.Ces travaux ouvrent la voie (i) à la détection/attribution des changements de masse de la calotte et (ii) à l'intégration des modèles de calottes au sein des modèles de climat en proposant une méthodologie d'initialisation de la calotte dès l'ère pré-industrielle<br>The Antarctic Ice Sheet has contributed to global sea level rise over the last few decades, which was historically unexpected. Satellites indicate that the ice sheet is losing mass through massive ocean-induced melting beneath the floating ice shelves, particularly in the Amundsen and Aurora sectors. Nevertheless, the role of anthropogenic forcing remains largely unclear due to the natural climate variability, which is particularly strong in this region, as well as the relatively short duration of observations.The intrusion of relatively warm circumpolar deep waters beneath the ice shelves in the Amundsen Sea has been observed since the first records in 1994, and the analysis of sediments suggests that several glaciers have experienced a retreat of their grounding line since the 1940s. However, these data do not allow any assessment of whether pre-industrial conditions were compatible with cold or warm oceanic conditions in the Amundsen Sea, or whether the Amundsen Sea switched into a warm state later due to changes triggered by anthropogenic forcing. Using a set of ocean simulations of the Amundsen Sea with various idealised perturbations of atmospheric fluxes, we show that pre-industrial conditions may be associated, at least intermittently, with cold conditions and low melt rates. On multi-decadal timescales, transitions between warm and cold states are primarily driven by changes in surface buoyancy fluxes, which modify thermohaline stratification through the modulation of net sea-ice production.Although these first results suggest a possible anthropogenic influence, they do not rule out the influence of the strong low-frequency internal climate variability. Hence, the second part of the project focuses on the impact of this variability on future Antarctic mass loss. This work is based on a set of simulations of the ice-sheet contribution to sea level rise, by 2100 under the SSP2-4.5 mid-range scenario, generated from an ice-sheet model forced with several members of three climate models. Our experiments show that internal variability affects the Antarctic contribution to sea level rise by the end of the century by more than 45% to 93% depending on the climate model, even though both oceanic and atmospheric internal variability are probably underestimated. The influence of atmospheric variability prevails and the Amundsen and Aurora sectors, which are currently experiencing the largest losses, are the most affected. Our results suggest that the impact of internal variability on the ice-sheet changes should be systematically explored using several climate members.Finally, we attempt to identify the relative roles of anthropogenic and natural influences in the observed mass losses in the Amundsen and Aurora regions. For this purpose, we use an original initialisation method to reconstruct sets of ice-sheet changes since 1850, with and without anthropogenic forcing, for seven members of a single climate model. As expected from the first two study phases, the historical trajectories are sensitive to internal variability, and some climate model members enable intermittent cold pre-industrial states in the Amundsen Sea. The detection of the anthropogenic signal is then evaluated through the comparison of trajectories with and without anthropogenic forcing.This work paves the way for (i) the detection/attribution of ice-sheet mass changes and (ii) for integrating ice-sheet models into climate models, by providing an initialisation method to initialise the ice sheet from the pre-industrial era

L'érosion mécanique des surfaces continentales, ou "détritisme", résulte du forçage climatique mais peut être amplifiée par le forçage anthropique. Cette érosion des sols, et sa compréhension, représentent aujourd'hui un questionnement sociétal majeur. Le présent travail s'est donc intéresse aux relations étroites liant climat, Homme et détritisme, dans les environnements continentaux holocènes. Associée à une démarche analytique multiparamètres, couplant quantification et modélisation de l'érosion des sols, l'étude intégrée et la comparaison de différents systèmes lacustres d'Europe occidentale, d'altitude et de piedmont, alpins et pyrénéens, a permis d'obtenir les informations suivantes. A long terme, la bipartition climatique Holocène (Optimum Climatique/Néoglaciaire) s'illustre par une augmentation de l'humidité, généralisée en Europe occidentale, et de l'ordre de 800 mm/an dans les Alpes françaises. Cette transition résulterait d'un relais entre le forçage solaire et le couplage océan/atmosphère. A court terme, l'Holocène est ponctué de périodes plus humides ou plus sèches, synchrones a l'échelle de l'Europe occidentale, et culminant avec le Petit Age Glaciaire. La présence humaine est d'abord mise en évidence dans les systèmes de piedmont et est synchrone a l'échelle des Alpes (Néolithique). Elle parait plus tardive dans les sites de haute altitude (Age du Bronze). L'implantation humaine en altitude et en piedmont est régulée par l'accessibilité aux sites, mais également par des rétroactions climatiques négatives. En piedmont, ces rétroactions négatives ne sont effectives que jusqu'à l'Age du Fer. Si le détritisme est dans un premier temps principalement controlé par le climat, il subit les conséquences de l'anthropisation des le Néolithique dans les Préalpes. Cette anthropisation est limitée aux systèmes de piedmont, ou elle explique jusqu'à 50% de l'érosion des sols, notamment pendant l'Age du Bronze, l'Age du Fer et le dernier siècle.

The Mediterranean region has experienced persistent drying since the middle of the 20th Century and global climate models project further drying in the future as a consequence of increasing greenhouse gases. The Mediterranean region is also known to oscillate between decades of relatively wet and dry conditions due to the strong influence of multidecadal North Atlantic Oscillation (NAO). It is therefore of great importance to understand the relationship between forced long-term drying resulting from human influences and those due to natural variability. To this end, we used observations, reanalyses and comprehensive global climate models in this thesis research. The roles of anthropogenic climate change and internal climate variability in causing the Mediterranean region's late 20th Century extended winter drying trend were examined using 20th Century observations as well as 19 coupled climate models from the CMIP3. The drying was strongly influenced by the robust positive trend in the NAO from the 1960s to the 1990s. Model simulations and observations were used to assess the probable relative roles of radiative forcing and internal variability in explaining the circulation trend that drove much of the precipitation change. It was concluded that the radiatively forced trends were a small fraction of the total observed trends. Instead it was argued that the robust trends in the observed NAO and Mediterranean rainfall during this period were largely due to multidecadal internal variability with a small contribution from the external forcing. Differences between the observed and NAO associated precipitation trends are consistent with those expected as a response to radiative forcing. The radiatively forced trends in circulation and precipitation are expected to strengthen in the current century and these results highlight the importance of their contribution to future precipitation changes in the region. The Mediterranean precipitation climatology and trend were further examined by comparing the newest generation of global climate models (CMIP5) used in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, to the previous generation (CMIP3) and to observations over the latter half of the 20th Century for both the summer and winter half years. The observed drying trend since 1950 was predominantly due to winter drying, with very little contribution from the summer. However, in the CMIP5 multimodel mean, the precipitation trend since 1950 is evenly divided throughout the seasonal cycle. This may indicate that in observation, multidecadal internal variability, particularly that associated with the NAO, dominates the wintertime trend. An estimate of the observed externally forced trend showed that winter drying dominated in observations but the spatial patterns were grossly similar to the multimodel mean trend. The similarity was particularly robust in the eastern Mediterranean region, indicating a radiatively forced component being stronger there. These results also revealed modest improvement for the CMIP5 multimodel ensemble in representation of the observed six-month winter and summer climatology. We further explored the detailed mechanisms leading to the NAO-associated precipitation change, such as the role of the change in mean circulation versus that of the storm tracks in the regional moisture budget, which had not been investigated previously. We employed a moisture budget analysis using 15 CMIP5 models and the ERA-Interim Reanalysis to investigate the relationship between the NAO and the various moisture budget terms for the six-month winter and summer. Compared with the ERA-Interim, the models performed well in their simulation of the relationship between the naturally varying NAO and the large-scale moisture budget. Our results indicated that the shift in the midlatitude transient eddies induced modest moisture convergence, rather than divergence, over the Mediterranean under a positive NAO. The reduction in precipitation in this region during a positive NAO was dominated by the mean moisture divergence, which opposed the transient contribution. There were significant differences between the patterns of NAO-induced moisture budget anomaly and changes due to external radiative forcing. Under radiative forcing there was enhanced evaporation over the Mediterranean Sea, Italy and eastern Europe and drying by the shift in the wintertime storms over nearly all of Europe and the Mediterranean. Under a positive phase of the NAO, on the other hand, there was modest reduction in evaporation and wetting by the storms over the Mediterranean, and drying over northern Europe. The dependence of the Mediterranean moisture budget on the NAO was similarly explored in the summer half of the year and in this season the models exhibited more disagreement with observations, but otherwise showed the similar results as winter. The stronger anthropogenic induced drying signal over the eastern Mediterranean provided a basis to examine the possible cause and impact of the recent severe and persistent drought in Syria that occurred directly prior to the uprising of 2011. The drought devastated Syrian agriculture, resulting in food shortages, widespread unemployment, the collapse of rural social structure and a mass migration of agricultural refugees to Syria's urban areas. Anger at the government's failure to ameliorate conditions was one spark for the uprising that evolved into civil war. We found that though droughts occur periodically in Syria due to natural causes it is likely that the recent drought was more extreme due to the century long drying trend caused by increased radiative forcing. It was estimated that the anthropogenic trend made a drought of such severity several times more likely. Droughts as persistent as the recent one are projected to be commonplace in a future warmer world.