iLEAPS Top10 research priorities

The iLEAPS SSC reviewed the iLEAPS Science Plan in 2010 and came up with ten most urgent research topics for the land-atmosphere community:

1. Ground-based ES observation system, observational networks

ES models are advancing to the point where they can begin to fully simulate the coupling between the physical, chemical, and biological processes in the climate system. Still, the limitations in the lack of suitable observations for developing and evaluating the quantitative relationships are hindering the progress for realistic simulations. The recent progress in understanding terrestrial biogeochemical feedbacks and their linkages has led to initial estimates of the potential magnitude of biogeochemical feedbacks associated with human-mediated changes in the biosphere. The ES components, interactions and feedbacks that are the focus of iLEAPS efforts include investigations of atmospheric, ecological and hydrological processes; surface fluxes of energy, aerosols, CO2, water, and organic and nitrogen compounds; ecohydrological disturbances and other factors that control the system, followed by efforts to improve their representation in ES models. Conceptually, the ES connections and interactions are clear. However, the processes controlling this coupled system are highly uncertain and not well quantified, precluding the full incorporation of these processes into ES models. One of the major challenges is linking the small-scale observations, used to improve our fundamental understanding of land ecosystem-atmosphere processes, to the regional-scale interactions that must be represented in ES models. Approaching this requires biochemical cellular studies, plant physiological enclosure studies, above-canopy micrometeorological towers, and airborne and satellite sensors. The integration of Earth observations from ground and from satellites, boundary layer measurements and modeling from the planning of measurement station location to the ES level is of importance and also requires collaboration among a variety of agencies and research communities. Further analysis is needed in the future in order to be able to analyze the complex interactions and feedbacks of the photosynthesis-forests-aerosol-clouds-climate system. In order to understand water and biogeochemical cycles, observing fluxes between different ES compartments, like atmosphere and ecosystems, is crucial. We also need to observe the stocks and processes in the atmosphere, biosphere and soils; concentrations of greenhouse gases, reactive trace gases, aerosols.


2. Boundary layer dynamics (heat, mass, biosphere-atmosphere interactions, atmospheric chemistry and aerosols)

Atmospheric boundary layer (ABL) fluxes describe the quantitatively measurable transport of radiation, momentum, thermal and moist energy, gases, and aerosol particles in and through the atmospheric boundary layer. These fluxes control the dynamical state and evolution of the ABL because they determine its stability and, consequently, how closely the surface layer is connected both to the overlying atmosphere and to the underlying land. That is why characterizing fluxes of mass, momentum, heat, moisture, and gases across the land-atmosphere interface is critical to understanding the vertical mixing of mass and material components, momentum, and heat. The wide variety of physical and chemical processes mediating the sources, sinks and transport of energy, momentum and material in the land-surface atmosphere system, either completely precludes direct measurement with currently available techniques, or requires careful adjustments and corrections to measured data. Often the combination of various measurement techniques with modelling will be the only realistic approach. Concerning boundary-layer processes the important question is to understand what are the relative contributions of biological activity and the evolution of boundary-layer state (e.g. height, stability and turbulent structure) in characterising the diurnal patterns of surface-atmosphere fluxes of various trace gases. The wide variety of physical and chemical processes mediating the sources, sinks and transport of energy, momentum and material in the land-surface atmosphere system, either completely precludes direct measurement with currently available techniques, or requires careful adjustments and corrections to measured data. iLEAPS will therefore promote further development of theories, models and measurement techniques to improve the quantification of land-atmosphere exchanges of various compounds for a wide range of land-surface structures.
Often the combination of various measurement techniques with modelling will be the only realistic approach, and hence the following questions need to be addressed:

1. Boundary-layer processes: what are the relative contributions of biological activity and the evolution of boundary-layer state (e.g. height, stability and turbulent structure) in characterising the diurnal patterns of surface-atmosphere fluxes of various trace gases?

2. Surface processes: landscapes are typically complex mosaics of varying vegetation types, snow and ice or bare soil. How well can the exchange characteristics of each of these surfaces be characterised, and how can they be combined to accurately represent their average exchange in larger-scale models?

3. Problems of measurement: aerodynamic methods of surface exchange measurements at scales from soil chambers to boundary layer budgets typically assume simple flow conditions (e.g. horizontal homogeneity and/or stationarity) and conservative species. How can these techniques be extended to remove these restrictions?

4. Data assimilation, model-data fusion and multiple constraint approaches: can process models be developed for non-conservative species, terrain forced heterogeneity and non-stationarity, that will serve as a basis for data assimilation approaches so as to extend measurement methods?


3. The role of land-use induced land-cover changes (LULCC) in modulating climate and climate changes

Land use/land cover change (LULCC) effects on climate include direct alterations in surface solar and longwave radiation and in atmospheric turbulence which result in changes in the fluxes of momentum, heat, water vapor, and carbon dioxide as well as other trace gases and both inorganic and biogenic aerosols including dust between vegetation, soils, and the atmosphere. This direct biogeophysical radiative impact of LULCC since preindustrial times is small relative to other global climate forcings. Reasoning of this kind has led to the role of LULCC being mostly omitted from the climate models used in previous IPCC assessments of climate projections and historical reconstructions. However, there are a number of observational studies that document a major role of LULCC in altering surface fluxes, surface, and near-surface variables, and boundary layer dynamics. The evidence for a significant effect of LULCC on climate at local scales is therefore convincing. Where LULCC has been intensive, the regional impact is likely to be at least as important as greenhouse gas and aerosol forcings. Human vulnerability to forcings such as climate change is realized locally and regionally and the conclusion that LULCC is a significant regional scale driver of climate is sufficient to require its incorporation into past, present, and future climate model simulations. Still, the modeling project LUCID show that agreement between the models' responses to similar LULCC is hard to obtain. Part of this dispersion can be attributed to the way land-surface models represent the anthropogenic land. At a broad level, it is unclear what the relative importance of biophysical vs biogeochemical effects are and how this relationship changes by region and by particular type of land use change. Further work is necessary to determine whether climate is sensitive to the spatial pattern of land use, and if so at what scale, and in which regions, this sensitivity exists. This type of knowledge could usefully inform integrated modelling studies by establishing the detail with which land use change patterns need to be modelled in order to capture potential influences on climate. Earth system models and integrated assessment models differ among themselves and from each other in their present day and historical land use and land cover data. The issue of land-use as a climate driver has largely failed to gain traction because a) the intrinsic difference in the scales at which land-use decisions are made and those usually implied when discussing climate and b) the evidence to suggest that more detailed representations of land use would lead to increases in predictability, and/or large magnitude impacts, at climate relevant scales is scarce.  Many of these impacts of land use occur on medium-long time scales against a background of high variability, implying they are intrinsically difficult to quantify.


4. Regional emphasis (e.g. high latitudes)

iLEAPS has drawn the modelling community’s attention to regional differences in climate change effects. Pitman et al. (2011) state that although global climate models simulate the Earth's climate impressively and reliably at scales of continents and greater, they exclude a suite of important processes that are locally and/or regionally important. In areas where some of these regional drivers act strongly, existing regional projections may be wrong, and significantly wrong, because they do not include regionally significant processes. Such local and regional processes include fire, irrigation, land cover change (including crops and urban landscapes), and the emissions of biogenic volatile organic compounds by vegetation (Fig. 5). Many of these interact within the atmosphere via dynamical, physical, and chemical mechanisms that lead to boundary-layer feedbacks. Eventually, improving climate models for policy-relevant spatial scales will require adding regionally important processes into the climate models. Some processes will be more important than others and the case is already clear for land cover change. For other processes, a framework is required to develop modules to represent these processes and to examine and test them in well designed international programs. This will develop an understanding of which are important, where they are important, how important they are, and how they interact with the increasing greenhouse gases. Pitman et al. (2011) recommend establishing a framework to identify key regional climate drivers, and then building, testing, evaluating, and choosing modules to represent key regional climate drivers well before the 6th assessment report of the IPCC. Beer et al. (2010) also emphasise the importance of regional processes: they show that water availability has a significant effect on terrestrial gross primary production (GPP) over 70% of savannahs, shrublands, grasslands, and agricultural areas and that this feedback effect implies a high susceptibility of these ecosystems’ productivity to projected changes of precipitation over the 21st century; on the other hand, tropical and boreal forests will be robust against precipitation changes. Missing feedbacks such as this could help explain the large between-model variation in GPP simulated by state-of-the-art process-oriented biosphere models used for climate predictions. Most likely, the association of GPP and climate in process-oriented models can be improved by including negative feedback mechanisms (e.g., adaptation) that might stabilize the systems.

Pitman AJ, Arneth A, and Ganzeveld L 2011. Regionalizing global climate models. International Journal of Climatology. doi: 10.1002/joc.2279

Beer C, Reichstein M, Tomelleri E, Ciais P, Jung M, Carvalhais N, Rödenbeck N, Arain MA, Baldocchi D, Bonan GB, Bondeau A, Cescatti A, Lasslop G, Lindroth A, Lomas M, Luyssaert S, Margolis H, Oleson KW,  Roupsard O, Veenendaal E, Viovy N, Williams C, Woodward FI, and Papale D 2010. Terrestrial gross carbon dioxide uptake: global distribution and covariation with climate. Science 329, 834, doi:10.1126/science.1184984


5. Integrative model evaluation (including climate-carbon models)

Process-oriented models are a primary tool being used to project future states of climate and ecosystems in the Earth system in response to anthropogenic and other forcing. Earth system models receive tremendous attention, especially in the context of the 5th assessment report of the IPCC (Intergovernmental Panel on Climate Change). However, intercomparison of model scenarios indicate large uncertainties regarding predictions of global interactions between atmosphere and biosphere. Rigorous scientific testing of these models is essential but very challenging, largely because it is neither technically nor ethically possible to perform global earth-scale experiments – i.e. we do not have replicate Earths for hypothesis testing. Hence, model evaluations have to rely on monitoring data such as ecological observation networks on concentrations, fluxes and isotopes, global remote sensing on land and ocean surface states, paleo proxy data, or small-scale manipulative experiments and process studies. Hence, one iLEAPS Top10 research priority is to develop strong and efficient model evaluation strategies or “stress tests”, which take advantage of all the relevant information in the observed data. In this context is crucial to avoid apparent falsifications, i.e. “false alarms” which likely to occur when individual system processes (e.g. in the model) are compared to the overall emergent system behaviour (e.g. of the observed world). This iLEAPS research prirority is pursued by a strong integration of recent advances in pattern-oriented and system-oriented approaches which impose multidimensional and multi-scale constraints on Earth system models. Such novel generation of model-evaluation tools will give rise to assessments of future IPCC projections in order to distinguish plausible simulation trajectories from less plausible projections.


6. Extreme events vs. gradual change and adaptation (e.g. dry spells, heatwaves); impacts on biosphere, role of biosphere, regional feedbacks, non-linearity

Climate is associated with a certain probability distribution of weather events. The least likely weather events are called “extreme events”. Extreme weather in one region (e.g. a heat wave, drought) may be normal in another. In both regions nature and society are adapted to the regional weather averaged over longer periods, but much less to extremes. For example, tropical African temperatures could severely damage vegetation or human health if they occurred in Northern Europe. Impacts of extreme events are felt strongly by ecosystems and society and may be destructive. Small changes in climate will not necessarily have a large impact as extremes. Nature and society are often particularly ill prepared for extremes, but both have an often un-quantified ability to adapt to moderate shifts to new mean weather and climate conditions (IPCC).

An important and obvious “point of reference” for examining the tradeoffs and balance between the vulnerability to extreme events and the potential adaptation to moderate changes are the global forests.  Plants are sessile and hence cannot migrate during their ontogeny when local conditions become unfavorable for their growth or survival. This selected for a remarkable ability of plants to acclimate to a changing environment through physiological, molecular, and genetic responses. Trees in particular are sensitive to climate change due to their longevity, extending over time-spans from decades to centuries. For example, extreme, rare climate events (e.g. a drought year once a decade) inevitably influence tree communities more than annuals and geophytes that can, for example, avoid germination under such events. Consistent, directional climate change taking place today on a global scale highlights the importance of tree acclimation (Allen et al. 2009). The relatively high rate of changes in air temperatures and precipitation also increases the importance of the ability of a genotype to express different phenotypes in different environments (Bradshaw 1965, Schlichting 1986, Richards et al. 2006, Nicotra et al. 2010).

Climate predictions indicate drying trends associated with reduced precipitation in many regions (Christensen et al. 2007).  Observed increases in global drought severity evaporative demand in Israel are expected to intensify during the 21st century. Evidence for impact of warming and drying on forests around the world is already accumulating at an alarming pace. For example in 2008 alone, and just in the Mediterranean regions, at least five examples of distinct drought-induced forest stand dieback and decline were observed, including Abies cephalonica forests in Greece (Raftoyannis et al. 2008); Cedrus atlantica in Algeria and Morroco; Quercus, Pinus, and Juniper spp. in Turkey; and Quercus suber in France (Allen et al. 2009 and references therein). Impacts on Pinus sect. halepensis (including P. brutia) following the drought years of 1999-2000 are evident in Israel, Greece (Korner et al. 2005; Sarris et al. 2007) and Spain (Penuelas et al. 2001; Martinez-Vilalta and Pinol 2002). Additional implications of climate change on tree growth include reduced ability of forests to sequester carbon (Ciais et al. 2005) as well as shifting the timing of the growth season (Chmielewski and Rotzer 2001). But while the effect of rapid climate change is evident, it can also be significantly ameliorated by existing phenotypic plasticity in a range of ecophysiological traits, both within and across provenances. This could provide sufficient basis to promote growth and confer resistance to long-term potential warming and drying.

Allen CD, Macalady AK, Chenchouni H, Bachelet D, McDowell N, Vennetier M, Kitzberger T, Rigling A, Breshears DD, Hogg EH et al (2009). A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecol Management 259:660-684.

Bradshaw AD. 1965. Evolutionary significance of phenotypic plasticity in plants. Advances in Genetics 13:115-155.

Chmielewski FM, Rotzer T (2001) Response of tree phenology to climate change across Europe. Agri Forest Meteorology 108:101-112.

Christensen JH, Hewitson B, Busuic A, Chen A, Gao X, Held I, Jones R, Kolli RK, Kwon WT, Laprise R et al (2007) Regional climate projections. In: Climate Change 2007: The Physical Science Basis. Contributions of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, eds Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tingor M, Miller HL (Cambridge University Press, Cambridge, United Kingdom/New York, NY).

Ciais P, Reichstein M, Vivoy N, Granier A, Ogee J, Allard V, Aubinet M, Buchmann N, Bernhofer C, Carrara A et al. (2005) Europe-wide reduction in primary productivity caused by the heat and drought in 2003. Nature 437:529–533.

Korner C, Sarris D, Christodoulakis D (2005) Long-term increase in climatic dryness in the East-Mediterranean as evidenced for the island of Samos. Regional Environ Change J 5:27–36.

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Martinez-Vilalta J, Pinol J (2002) Drought-induced mortality and hydraulic architecture in pine populations of the NE Iberian Peninsula. Forest Ecol and Manag 161:247–256.

Nicotra AB, Atkin OK, Bonser SP, Davidson AM, Finnegan EJ, Mathesius U, Poot P, Purugganan MD, Richards CL, Valladares F, van Kleunen M (2010) Plant phenotypic plasticity in a changing climate. Trends in Plant Sci 15:684-92.

Palutikof JP, Goodes CM, Guo X (1994) Climate change, potential evapotranspiration and moisture availability in the Mediterranean basin. International j climatology 14:853-869.

Penuelas J, Lloret F, Montoya R (2001) Severe drought effects on Mediterranean woody flora in Spain. Forest Sci 47:214–218.

Raftoyannis Y, Spanos I, Radoglou K (2008) The decline of Greek fir (Abies cephalonica Loudon): Relationships with root condition. Plant Biosystems 142:386–90.

Richards CL, Bossdorf O, Muth NZ, Gurevitch J, Pigliucci M (2006) Jack of all trades, master of some? On the role of phenotypic plasticity in plant invasions. Ecology Lett 9: 981–993.

Sarris D, Christodoulakis D, Korner C (2007) Recent decline in precipitation and tree growth in the eastern Mediterranean. Global Change Biol 13:1187–2000.

Schiller G, Atzmon N (2009) Performance of Aleppo pine (Pinus halepensis) provenances grown at the edge of the Negev desert: A review. J Arid Environ 73:1051-1057.

Schlichting CD (1986) The evolution of phenotypic plasticity in plants. Ann Rev Ecol Syst 17:667-693.


7. Interactions among Managed Environments, Climate, and Societies

Human-dominated environments are becoming – or have become – equally important in the Earth System as natural ones. Studying the interactions and feedbacks among climate and natural ecosystems is no longer sufficient to adequately describe the functioning of the Earth System and its components. Human-dominated environments include various types of land use such as managed forests, agricultural fields, grasslands and pastures used to produce food for livestock but also oceans, seas, lakes, rivers, coastlines, and deltas used for transport, fishing, and irrigation as well as urban environments with varying amounts of vegetation ranging from sparsely inhabited villages to towns, cities, and megacities. All these environments have significant and in some cases crucial interactions with climate, air quality, ecosystem services and biodiversity of the region on a potentially very large spatial and temporal scale. Examples include pollution impacts from managed ecosystems and urban communities that in turn affect local or regional biogeochemical cycles and global circulation: for instance, poor air quality in cities may reduce photosynthesis and thereby plant transpiration, resulting in an increase in temperature and decrease in moisture and, potentially, in precipitation, a major cleansing mechanism; emissions of volatile organic and inorganic compounds from fertilisers, pollution, and vegetation that influence regional aerosol loading and, consequently, rain patterns downwind; and pollution deposition that can influence cloud processes and boundary-layer processes in the region. However, in order to provide solutions for sustainable management of these environments, there is urgent need to also look at the economic and social drivers behind land-use decisions, management and transport practices, and urban development. Therefore, it is imperative to integrate societies and economy into feedback loops such as forests-aerosols-climate-precipitation or soil-vegetation-hydrology-reactive compounds-radiative forcing. Economy drives management practices both in forestry, agriculture, fishing, and aquaculture and is involved in several indirect ways in the complex dynamics among societies and all these managed environments.


8. How are human drivers changing LEAP (e.g. land use effects), impact studies (liaise with social sciences, coupled ESMs with decent DGVMs, societal-relevance indicators (biomass, water stress indices, ...)

In a world that is facing continued population growth and climate change, human actions mediated through the global land system are critical for the supply and sustainable use of a number of ecosystem services. The land system plays a fundamental role in biogeochemical and biophysical climate regulation, food security, biodiversity and fresh water supply. It is important to understand that land-based ecosystem services are not supplied independently. A change in one service’s quantity or quality, for example through climate change, or human management, will affect other services. Land system change is therefore one of the key human dimension issues in understanding the functioning of the earth system and its response to global environmental change. Research has not yet established quantitatively whether land use change-climate dynamics are a significant component in the various feedback loops. If at all, human activities have been considered as an external driver to provide the emissions necessary for climate or atmospheric chemistry simulation experiments, ignoring the possibility that anthropogenic activities not only impact on the earth system, but also in turn respond to system changes. Traditionally, natural and socioeconomic sciences have operated relatively separately, dealing with specific questions in the context of human-land use-climate interactions on different space and time scales and using different methods. Some dynamic global vegetation models (DGVMs) have started to incorporate agricultural and pastoral ecosystems and their management into their process representations for applications at regional or global scales. This will eventually enable the quantitative representation of processes on managed land to be included in simulations of biophysical and biogeochemical ecosystem processes. But, these developments are hampered by uncertain parameterisation of human management activities and prescription of land use/cover change scenarios as external drivers that do no take into account the actual yield calculations in the DGVMs. Applying behavioural models of land use change at global scale levels should include the necessary realism to improve our capacity to understand the global coupled human–biophysical land system. Such an approach would allow pertinent questions to be addressed on the relative roles of socio-economic decision-making against climate change on land use/land management change in a globalised world.


9. Aerosols and climate

Human actions, such as emission policy, forest management and land use changes, as well as various natural feedback mechanisms involving the biosphere and atmosphere, have substantial impacts on the complicated couplings between atmospheric aerosols and climate. Atmospheric aerosol particles influence the Earth's radiation balance directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei. The interaction between atmospheric aerosols and climate system is the dominant uncertainty in predicting the radiative forcing and future climate. The aerosol direct radiative effect has been shown to be associated with considerable model diversity and thus causing uncertainty in the AR4 total radiative forcing. Uncertainty in the computation of the direct effect is due to several factors involved, among which dominate the vertical distribution of aerosol, the relative position and interaction of clouds and aerosols, the amount of aerosol absorption, the anthropogenic aerosol load, humidity growth and possibly also details of the radiative transfer calculation itself. One of the major uncertainty factor for direct aerosol forcing is aerosol absorption due to black and brown carbon. The interplay of meteorological and dynamical parameters with microphysical and chemical parameters of aerosols and clouds lead to changes in cloud optical depth and thus to radiative climate forcing, i.e. the first indirect effect of aerosols on climate (Twomey effect). For instance, models in the iLEAPS project EUCAARI have estimated by scaling simulated clear and cloud-sky forcing that a global cloudy-sky (aerosol indirect effect) is -0.7 +- 0.5 W m-2, compared to -1.5 W m-2 of AR4. Better understanding and quantifying of the aerosol effects in the atmosphere requires detailed information on how different sources and atmospheric transformation processes modify the properties of atmospheric particles and the concentration of trace gases. It also requires the development of advanced instrumentation and methodologies for measuring and validating atmospheric composition changes and understanding key atmospheric processes.


10. Complex dynamics among human and natural systems

The Human-Earth system has now reached a state where socially-mediated stocks and flows of material, energy and information are as important to the planet’s functioning as natural stocks and flows.  In today’s world, combined socioeconomic and climate change impacts lead to important trade-offs in ecosystem functioning and ecosystem services that will significantly influence climate and human societies. We call this state the Anthropocene [1].  The fundamentally interacting nature of these social and natural forces defines the human-earth system as a ‘complex adaptive system’, a type of dynamical system which displays strongly non-linear behaviour such as thresholds or tipping points, their opposites such as lock-in and resistance to change, breakdowns in conventional notions of cause and effect and a range of other ‘pathological’ characteristics including learning, internal reorganization and resilience to external forcing. Two decades of research in Complex Systems Science has told us that the dynamics of complex adaptive systems are characterized by emergence (at a fundamental level, global system behaviour is more than the sum of the parts) and self-organization (the system behaviour is not imposed from outside but arises spontaneously from its internal interactions).  Such systems have their own inexorable trajectories and attempts to alter these trajectories by policy interventions based on incomplete or incorrect understanding of their dynamics can be futile or disastrous.

Modern challenges in global management and policy are often described as ‘wicked problems’ because solutions developed when the world was simpler are increasingly being seen to fail.  From a systems stance, we can often see such failures as the result of a naïve application of an inappropriate solution to a system whose dynamics are misunderstood either partially or completely. This theme aims to understand the human-earth system as a complex adaptive system and thereby to propose different approaches to some major environmental issues as well as to problems in global governance and management that are at the heart of sustainability science and the Future Earth Initiative.