Climate processes

Abstract:

<p>A wide range of aerosol effects on precipitation have been proposed, from the scale of individual clouds to that of the globe.</p><p>This presentation, based on the findings of an expert workshop under the umbrella of the GEWEX Aerosol Precipitation initiative, reviews the evidence and scientific consensus behind these effects and the underlying set of physical mechanisms, categorised into i) radiative effects via modification of radiative fluxes and the energy balance and ii) microphysical effects via modification of cloud droplets and ice crystals.</p><p>There exists broad consensus and strong theoretical evidence that, because global mean precipitation is constrained by energetics and surface evaporation, aerosol radiative effects (aerosol-radiation interactions and aerosol-cloud interactions) act as drivers of precipitation changes. Likewise, aerosol radiative effects cause well-documented shifts of large-scale precipitation patterns, such as the Inter-Tropical Convergence Zone (ITCZ). The extent to which aerosol effects on precipitation are applicable at smaller scales and driven or buffered by compensating microphysical and dynamical mechanisms and budgetary constraints is less clear. Although there exists broad consensus and strong evidence that suitable aerosol perturbations increase cloud droplet numbers, reducing the efficiency of warm rain formation across cloud regimes, the overall aerosol effect on cloud microphysics and dynamics as well as the subsequent impact on local, regional and global precipitation is less constrained.</p><p>This presentation provides a review of the physical mechanisms of aerosol effects on precipitation backed up by evidence from recent cloud-resolving and global modelling simulations as well as from satellite observations.</p>

Abstract:

<p>The spatial pattern of anthropogenic aerosol has changed markedly over the historical period and is expected to continue evolving in the coming decades. Additionally, the global composition of anthropogenic aerosol is expected to become relatively more absorbing because policy measures often target sources of scattering and absorbing aerosols differently. However, despite these historical and future changes,  relatively little attention has been given to the potential climatic impacts of the evolving spatial pattern of absorbing aerosol.</p><p>In this talk, we will present results from a large ensemble of idealised aerosol absorption experiments with a state-of-the-art climate model to show that the global-mean effective radiative forcing (ERF) from absorbing aerosol strongly depends on their location, driven by rapid adjustments of clouds and circulation. Furthermore, by viewing absorbing aerosol as a localised diabatic heating source we will provide an explanation for this location-dependence of ERF in terms of simple atmospheric dynamics. We will also demonstrate how this approach can be used to understand the sensitivity of local and global precipitation to realistic and idealised changes in the spatial pattern of absorbing aerosol. </p><p>Our results have implications for understanding the climatic impacts of regional aerosol absorption and demonstrate the utility of an ensemble approach to understanding the impacts of variations in the spatial pattern of aerosol.</p>

Abstract:

<p>Successive IPCC reports have assessed the level of human-induced warming above preindustrial, but much less emphasis has been placed on quantifying the rate of anthropogenic warming, despite the rate of warming being a key variable for ambitious policymaking. The decadal global temperature anomaly trend can be considered a combination of the forced responses from the full range of radiatively-active pollutants, plus the additional trend introduced by natural variability over the previous decade.</p><p>The global temperature anomaly trend likely increased in the 2010s, following a temporary pause through the 2000s. Estimates of the globally averaged radiative forcing (RF) timeseries, which are used to attribute the anthropogenic contribution to this recent behaviour, suggest a 50% increase in the anthropogenic RF trend, which largely arises from aerosol RF trend changes since 2000. When these RF timeseries are used to complete a global temperature anomaly attribution (following the technique outlined in the IPCC’s Special Report on the Global Warming of 1.5°C), they suggest that the attributed anthropogenic warming rate has increased by between 50 and 100% since 2000, pushing the estimated rate of net anthropogenic warming up to around 0.3°C/decade since 2010.</p><p>We study the global observational evidence supporting the aerosol trends presented in these RF datasets, and thus aim to determine the likely anthropogenic contribution to the perceived warming acceleration behaviour since 2000. We argue that while observations do support the claim that RF trends are partly responsible for the warming trend (and importantly do support the best-estimate RF trend estimates in this ensemble), observational evidence is circumstantial, with a counterhypothesis that aerosol RFs make only a small contribution to the warming trend since 2000 consistently failing to be disproven across the full ensemble of RFs.</p><p>This occurs because observed trends in radiative fluxes and global temperatures are significantly influenced by internal variability, principally ENSO and PDO, precluding a clearer assessment of the externally forced behaviour over the short global observational records we have. In light of this uncertainty, considerable caution is required in predictions or policy judgments that depend on the precise current anthropogenic warming trend, such as the time remaining to, or the outstanding carbon budget consistent with, a warming of 1.5°C, since these may be influenced considerably by recent changes in aerosol forcing behaviour.</p>

Abstract:

<p>The self-aggregation of convection in idealised models has been widely studied. Work has been done to identify key physical mechanisms responsible for both driving and maintaining aggregation in a range of idealised radiative-convective equilibrium (RCE) models. These idealised models are typically run without any land, rotation, variation in sea-surface temperatures (SSTs), or a diurnal cycle. Due to these idealisations, a key question in the study of convective aggregation is how these convective processes and mechanisms manifest in the real-world. Several studies have tried to tackle this question by increasing the complexity of processes in the idealised models, such as SST gradients, adding a slab ocean, adding a diurnal cycle, or adding an aerosol diabatic heating perturbation. Particularly, the inclusion of interactive ocean surfaces has been shown to strongly impact the formation of aggregated clusters.</p><p>The interactions between land surfaces and aggregation are currently less well understood. Early studies have found that convective aggregation may favour land areas over oceans, and that soil moisture feedbacks can act to oppose the aggregation altogether. Thus, in this study we investigate the relationship between land, oceans, and aggregation, addressing the following questions:</p><ul><li>How does the inclusion of an idealised island into a global RCE model impact the aggregation of convection?</li> <li>Are the physical mechanisms responsible for the aggregation similar to those seen in land-free simulations?</li> <li>How sensitive are these results to our choice of land parameters, such as initial soil moisture, surface temperature, soil type, and land topography?</li> </ul>

Abstract:

This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiäla in Finland. The review is complemented by short-term observations from networks and large experiments. The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction. Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink. It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.