Seasonal Dependence of the Effect of Arctic Greening on Tropical Precipitation

This paper examines the seasonal dependence of the effect of Arctic greening on tropical precipitation. In CAM3/CLM3 coupled to a mixed layer ocean, shrub and grasslands poleward of 60 degrees N are replaced with boreal forests. With darker Arctic vegetation, the absorption of solar energy increases, but primarily in boreal spring and summer since little insolation reaches the Arctic in boreal winter. The net energy input into the northern extratropics is partly balanced by southward atmospheric energy transport across the equator by an anomalous Hadley circulation, resulting in a northward shift of the tropical precipitation. In contrast, in boreal fall, the slight increase in insolation over the Arctic is more than offset by increased outgoing longwave radiation and reduced surface turbulent fluxes in midlatitudes, from the warmer atmosphere. As a result, the Northern Hemisphere atmosphere loses energy, which is compensated by a northward cross-equatorial atmospheric energy transport, leading to a southward shift of the tropical precipitation in boreal fall. Thus, although Arctic vegetation is changed throughout the year, its effect on tropical precipitation exhibits substantial seasonal variations.close00

Impact of Arctic greening on the seasonality of local and remote climate

Department of Urban and Environmental Engineering (Environmental Science And Engineering)With global warming, it is expected that vegetation type may also change, particularly in the northern high latitudes over the tundra region. Under a global warming scenario, grass and shrub type of vegetation are expected to shift to boreal forests over the Arctic. This study examines the impact of potential Arctic vegetation change with global warming on the seasonality of local Arctic and remote tropical climates, and compares its impact with the response to CO2 doubling. Three experiments are performed with the NCAR Community Atmosphere Model 3 (CAM3) coupled to a slab ocean, which are perturbed by 1) Arctic vegetation change from grass and shrub to boreal forest with present day CO2 concentrations (335ppmv), 2) present day vegetation type with doubling of CO2, and 3) Arctic vegetation change to darker species with doubling of CO2. With darker vegetation over the Arctic, the Arctic region becomes warmer throughout the year due to the surface albedo reduction. However, because little insolation reaches the Arctic in boreal winter, the Arctic warming in boreal summer is four times greater than in boreal winter. Evapotranspiration changes also act to maximize the warming in boreal summer. In contrast, the Arctic warming peaks in November in response to a doubling of CO2. As a result, as Arctic vegetation change is taken into consideration in addition to the doubling of CO2, the peak of Arctic warming shifts to August, resulting in a weaker seasonality in the Arctic amplification. Arctic warming driven by vegetation change results in imbalance of atmospheric energy budget between the hemispheres. Larger incoming insolation in the northern extra-tropics is balanced by increased outgoing longwave radiation (OLR) via warming the atmosphere, and some fraction reaches the subtropics by quasi-diffusive atmospheric eddy energy fluxes. As the northern subtropics become warmer relative to the southern subtropics, the Hadley circulation responds to transport energy southward across the equator because large horizontal temperature gradients cannot be sustained within the tropics due to the smallness of Coriolis parameter. Since the Hadley circulation transports energy in the direction of its upper branch, and moisture is transported northward across the equator, the time-mean inter-tropical convergence zone (ITCZ) is shifted northward. However, atmosphere cooling from June to September in local area can shift ITCZ to the south when Arctic incoming solar radiation is low.ope

The response of global terrestrial photosynthesis to rising CO2

In this dissertation I examine how photosynthesis on land has re- sponded to rising CO2 concentration in recent decades, and how we can use this knowledge to better predict the evolution of the climate- carbon system throughout the 21st century. More than three decades of satellite data reveal widespread and per- sistent changes in Earth’s ecosystems. The drivers underlying these changes and the implications for the terrestrial sink of anthropogenic carbon emissions are controversial. In the first part of this thesis, I examine a long-term satellite record of global leaf area observations (1981–2017) and identify clusters of significant change on the biome level. Using process-based models and a framework relying on causal theory, I disentangle and attribute vegetation changes to CO2-induced climatic changes and the CO2 fer- tilization effect. I show that 40% of Earth’s naturally vegetated surface is greening, predominately in the extratropics, and 14% is browning, mostly in the tropics. Although previous studies attributed the green- ing to CO2 fertilization, I show that only some biomes show a marked response to this effect, whereas many biomes bear the signature of climatic changes, i.e. warming and rainfall anomalies. The leaf area loss in the tropical forests due to increased droughts and long-term drying could be an early indicator of a slow-down in the terrestrial carbon sink. In the second part, I examine if the observed vegetation response to rising CO2 can be used to reduce uncertainty in the evolution of the carbon cycle. Using an approach called Emergent Constraint (EC), I combine satellite observations and multi-model simulations to de- rive an estimate for the increase in photosynthetic carbon fixation of northern ecosystems for 2×CO2 (3.4 ± 0.2 Pg C yr−1). Three compara- ble independent estimates from CO2 measurements and atmospheric inversions corroborate this result. The EC estimate is considerably larger than most model projections which suggests that the effect of rising CO2 concentration on photosynthesis in northern terrestrial ecosystems is underestimated. In the third part, I investigate the applicability of the EC method in a broader context of Earth system sciences. More and more EC estimates are being reported, however their robustness is controversial. By means of a thought experiment and analyses of a multi-model ensemble, I address the main caveats and highlight limitations as well as potential sources of uncertainty in the application of the EC method. All parts in this thesis highlight how the variety of observational data and the strength of process-based models in conjunction with new methods advance the understanding of the terrestrial biosphere.In dieser Dissertation untersuche ich, wie die terrestrische Photosyn- these auf die steigende CO2-Konzentration der letzten Jahrzehnte reagiert hat und wie wir dieses Wissen nutzen können, um die Ent- wicklung des Klima-Kohlenstoff-Systems im 21. Jahrhundert besser vorherzusagen. Satellitendaten aus mehr als drei Jahrzehnten zeigen, dass sich die Ökosysteme der Erde in großem Maßstab verändert haben. Die Treiber, die diesen Veränderungen zugrunde liegen und die damit verbunde- nen Auswirkungen auf die Landsenke von anthropogenen Kohlenstof- femissionen, sind umstritten. Im ersten Teil dieser Arbeit untersuche ich Langzeit-Satellitenbeo- bachtungen der globalen Vegetation (Blattfläche, 1981–2017). Unter Berücksichtigung verschiedener Biome identifiziere ich Regionen, die sich signifikant verändert haben. Mit Hilfe von prozessbasierten Mo- dellen und Kausaltheorie trenne ich die Auswirkungen des CO2- verursachten Düngungs- und Treibhauseffekts auf die Vegetation auf. Ich zeige, dass 40% der natürlich bewachsenen Erdoberfläche ergrünt, vornehmlich in den Extratropen, und 14% erbraunen, hauptsächlich in den Tropen. Frühere Studien haben den CO2-Düngungseffekt als Haupttreiber der Ergrünung identifiziert. Meine Analyse zeigt, dass nur einige Biome eine deutliche Reaktion auf diesen Effekt zeigen und dass Klimaveränderungen (Erwärmung und Niederschlagsanomalien) in vielen Ökosystemen eine stärkere Auswirkung haben. Der Verlust von Blattfläche in den Tropenwäldern durch häufiger auftretender Dürren und/oder stetigen Niederschlagsrückgang könnte ein Vorbote einer Abschwächung der terrestrischen Kohlenstoffsenke sein. Im zweiten Teil untersuche ich, ob die beobachteten Veränderungen in der Vegetation als Folge der steigenden CO2 Konzentration herange- zogen werden können, um die Entwicklung des globalen Kohlenstoff- kreislaufs besser abschätzen zu können. Unter der Zuhilfenahme von Emergent Constraints (EC) kombiniere ich Satellitenbeobachtungen und Multi-Modell-Simulationen, um die Zunahme der photosynthe- tischen Kohlenstoffaufname der Ökosysteme in den hohen Breiten für 2×CO2 vorherzusagen (3.4 ± 0.2 Pg C yr−1). Drei unabhängi- ge, vergleichbare Schätzungen, die auf CO2-Messungen und atmo- sphärischen Inversionen basieren, untermauern dieses Ergebnis. Die EC-basierte Schätzung ist wesentlich höher als die meisten Modell- vorhersagen, was darauf hindeutet, dass der Einfluss der steigenden CO2-Konzentration auf die Photosynthese der nördlichen terrestri- schen Ökosysteme unterschätzt werden könnte. Im dritten Teil untersuche ich die Anwendbarkeit der EC-Methode im allgemeinen Kontext der Erdsystemwissenschaften. Es werden mehr und mehr EC-Studien durchgeführt, die Ergebnisse sind jedoch kon- trovers. Anhand eines Gedankenexperiments und der Analyse eines Multi-Modell-Ensembles untersuche ich die wichtigsten Kritikpunkte und zeige mögliche Unsicherheitsquellen in der Anwendung der EC- Methode auf. Die vorliegende Dissertation zeigt, wie die Vielzahl an Beobachtungs- datensätzen und prozessbasierten Modellen mit neuen Methoden zum besseren Verständnis der terrestrischen Biosphäre verknüpft werden kann

Variability of terrestrial carbon cycle and its interaction with climate under global warming

Land-atmosphere carbon exchange makes a significant contribution to the variability of atmospheric CO2 concentration on time scales of seasons to centuries. In this thesis, a terrestrial vegetation and carbon model, VEgetation-Global-Atmosphere-Soil (VEGAS), is used to study the interactions between the terrestrial carbon cycle and climate over a wide-range of temporal and spatial scales. The VEGAS model was first evaluated by comparison with FLUXNET observations. One primary focus of the thesis was to investigate the interannual variability of terrestrial carbon cycle related to climate variations, in particular to El Niño-Southern Oscillation (ENSO). Our analysis indicates that VEGAS can properly capture the response of terrestrial carbon cycle to ENSO: suppression of vegetative activity coupled with enhancement of soil decomposition, due to predominant warmer and drier climate patterns over tropical land associated with El Niño. The combined affect of these forcings causes substantial carbon flux into the atmosphere. A unique aspect of this work is to quantify the direct and indirect effects of soil wetness vegetation activities and consequently on land-atmosphere carbon fluxes. Besides this canonic dominance of the tropical response to ENSO, our modeling study simulated a large carbon flux from the northern mid-latitudes, triggered by the 1998-2002 drought and warming in the region. Our modeling indicates that this drought could be responsible for the abnormally high increase in atmospheric CO2 growth rate (2 ppm/yr) during 2002-2003. We then investigated the carbon cycle-climate feedback in the 21st century. A modest feedback was identified, and the result was incorporated into the Coupled Carbon Cycle Climate Model Inter-comparison Project (C4MIP). Using the fully coupled carbon cycle-climate simulations from C4MIP, we examined the carbon uptake in the Northern High Latitudes poleward of 60˚N (NHL) in the 21st century. C4MIP model results project that the NHL will be a carbon sink by 2100, as CO2 fertilization and warming stimulate vegetation growth, canceling the effect of enhancement of soil decomposition by warming. However, such competing mechanisms may lead to a switch of NHL from a net carbon sink to source after 2100. All these effects are enhanced as a result of positive carbon cycle-climate feedbacks

Extratropical forcing and tropical rainfall distribution: energetics framework and ocean Ekman advection

Intense tropical rainfall occurs in a narrow belt near the equator, called the inter-tropical convergence zone (ITCZ). In the past decade, the atmospheric energy budget has been used to explain changes in the zonal-mean ITCZ position. The energetics framework provides a mechanism for extratropics-to-tropics teleconnections, which have been postulated from paleoclimate records. In atmosphere models coupled with a motionless slab ocean, the ITCZ shifts toward the warmed hemisphere in order for the Hadley circulation to transport energy toward the colder hemisphere. However, recent studies using fully coupled models show that tropical rainfall can be rather insensitive to extratropical forcing when ocean dynamics is included. Here, we explore the effect of meridional Ekman heat advection while neglecting the upwelling effect on the ITCZ response to prescribed extratropical thermal forcing. The tropical component of Ekman advection is a negative feedback that partially compensates the prescribed forcing, whereas the extratropical component is a positive feedback that amplifies the prescribed forcing. Overall, the tropical negative feedback dominates over the extratropical positive feedback. Thus, including Ekman advection reduces the need for atmospheric energy transport, dampening the ITCZ response. We propose to build a hierarchy of ocean models to systematically explore the full dynamical response of the coupled climate system

Spatiotemporal analysis of vegetation variability and its relationship with climate change in China

This paper investigated spatiotemporal dynamic pattern of vegetation, climate factor, and their complex relationships from seasonal to inter-annual scale in China during the period 1982–1998 through wavelet transform method based on GIMMS data-sets. First, most vegetation canopies demonstrated obvious seasonality, increasing with latitudinal gradient. Second, obvious dynamic trends were observed in both vegetation and climate change, especially the positive trends. Over 70% areas were observed with obvious vegetation greening up, with vegetation degradation principally in the Pearl River Delta, Yangtze River Delta, and desert. Overall warming trend was observed across the whole country (\u3e98% area), stronger in Northern China. Although over half of area (58.2%) obtained increasing rainfall trend, around a quarter of area (24.5%), especially the Central China and most northern portion of China, exhibited significantly negative rainfall trend. Third, significantly positive normalized difference vegetation index (NDVI)–climate relationship was generally observed on the de-noised time series in most vegetated regions, corresponding to their synchronous stronger seasonal pattern. Finally, at inter-annual level, the NDVI–climate relationship differed with climatic regions and their long-term trends: in humid regions, positive coefficients were observed except in regions with vegetation degradation; in arid, semiarid, and semihumid regions, positive relationships would be examined on the condition that increasing rainfall could compensate the increasing water requirement along with increasing temperature. This study provided valuable insights into the long-term vegetation–climate relationship in China with consideration of their spatiotemporal variability and overall trend in the global change process

Study on Regional Responses of Pan-Arctic Terrestrial Ecosystems to Recent Climate Variability Using Satellite Remote Sensing

I applied a satellite remote sensing based production efficiency model (PEM) using an integrated AVHRR and MODIS FPAR/LAI time series with a regionally corrected NCEP/NCAR reanalysis surface meteorology and NASA/GEWEX shortwave solar radiation inputs to assess annual terrestrial net primary productivity (NPP) for the pan-Arctic basin and Alaska from 1983 to 2005. I developed a satellite remote sensing based evapotranspiration (ET) algorithm using GIMMS NDVI with the above meteorology inputs to assess spatial patterns and temporal trends in ET over the pan-Arctic region. I then analyzed associated changes in the regional water balance defined as the difference between precipitation (P) and ET. I finally analyzed the effects of regional climate oscillations on vegetation productivity and the regional water balance. The results show that low temperature constraints on Boreal-Arctic NPP are decreasing by 0.43% per year ( P \u3c 0.001), whereas a positive trend in vegetation moisture constraints of 0.49% per year ( P = 0.04) are offsetting the potential benefits of longer growing seasons and contributing to recent drought related disturbances in NPP. The PEM simulations of NPP seasonality, annual anomalies and trends are similar to stand inventory network measurements of boreal aspen stem growth ( r = 0.56; P = 0.007) and atmospheric CO2 measurement based estimates of the timing of growing season onset (r = 0.78; P \u3c 0.001). The simulated monthly ET results agree well (RMSE = 8.3 mm month-1; R2 = 0.89) with tower measurements for regionally dominant land cover types. Generally positive trends in ET, precipitation and available river discharge measurements imply that the pan-Arctic terrestrial water cycle is intensifying. Increasing water deficits occurred in some boreal and temperate grassland regions, which agree with regional drought records and recent satellite observations of vegetation browning and productivity decreases. Climate oscillations including Arctic Oscillation and Pacific Decadal Oscillation influence NPP by regulating seasonal patterns of low temperature and moisture constraints to photosynthesis. The pan-Arctic water balance is changing in complex ways in response to climate change and variability, with direct linkages to terrestrial carbon and energy cycles. Consequently, drought induced NPP decreases may become more frequent and widespread, though the occurrence and severity of drought events will depend on future water cycle patterns

Vegetation Dynamics Revealed by Remote Sensing and Its Feedback to Regional and Global Climate

This book focuses on some significant progress in vegetation dynamics and their response to climate change revealed by remote sensing data. The development of satellite remote sensing and its derived products offer fantastic opportunities to investigate vegetation changes and their feedback to regional and global climate systems. Special attention is given in the book to vegetation changes and their drivers, the effects of extreme climate events on vegetation, land surface albedo associated with vegetation changes, plant fingerprints, and vegetation dynamics in climate modeling

Observational Constraints on the Response of High‐Latitude Northern Forests to Warming

Since the 1960s, carbon cycling in the high‐latitude northern forest (HLNF) has experienced dramatic changes: Most of the forest is greening and net carbon uptake from the atmosphere has increased. During the same time period, the CO₂ seasonal cycle amplitude (SCA) has increased by ~50% or more. Disentangling complex processes that drive these changes has been challenging. In this study, we substitute spatial sensitivity to temperature for time to quantify the impact of temperature increase on gross primary production (GPP), total ecosystem respiration (TER), the fraction of Photosynthetic Active Radiation (fPAR), and the resulted contribution of these changes in amplifying the CO₂ SCA over the HLNF since 1960s. We use the spatial heterogeneity of GPP inferred from solar‐induced chlorophyll Fluorescence in combination with net ecosystem exchange (NEE) inferred from column CO₂ observations made between 2015 and 2017 from NASA's Orbiting Carbon Observatory‐2. We find that three quarters of the spatial variations in GPP can be explained by the spatial variation in the growing season mean temperature (GSMT). The long term hindcast captures both the magnitude and spatial variability of the trends in observed fPAR. We estimate that between 1960 and 2010, the increase in GSMT enhanced both GPP and the SCA of NEE by ~20%. The calculated enhancement of NEE due to increase in GSMT contributes 56–72% of the trend in the CO₂ SCA at high latitudes, much larger than simulations by most biogeochemical models

The remote response of the South Asian Monsoon to reduced dust emissions and Sahara greening during the middle Holocene

Previous studies based on multiple paleoclimate archives suggested a prominent intensification of the South Asian Monsoon (SAM) during the mid-Holocene (MH, similar to 6000 years before present). The main forcing that contributed to this intensification is related to changes in the Earth's orbital parameters. Nonetheless, other key factors likely played important roles, including remote changes in vegetation cover and airborne dust emission. In particular, northern Africa also experienced much wetter conditions and a more mesic landscape than today during the MH (the so-called African Humid Period), leading to a large decrease in airborne dust globally. However, most modeling studies investigating the SAM changes during the Holocene overlooked the potential impacts of the vegetation and dust emission changes that took place over northern Africa. Here, we use a set of simulations for the MH climate, in which vegetation over the Sahara and reduced dust concentrations are considered. Our results show that SAM rainfall is strongly affected by Saharan vegetation and dust concentrations, with a large increase in particular over northwestern India and a lengthening of the monsoon season. We propose that this re- mote influence is mediated by anomalies in Indian Ocean sea surface temperatures and may have shaped the evolution of the SAM during the termination of the African Humid Period