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Research

Modeling of the Earth system

• Climate-vegetation interactions; Carbon cycle responses to the climate evolution of the geological past

• Biosphere-atmosphere feedbacks mediated by vegetation; Climate impacts of vegetation cover

The Sahara Desert experienced wet and vegetated conditions in the past. The vegetation-atmosphere feedbacks play an important role in sustaining vegetation cover in that region. We perform model simulations to reproduce herbaceous and woody vegetation types in North Africa 6,000 years ago. We further investigate separately the relative importance of various climate forcings (precipitation, temperature, radiation, and soil temperature) in inducing the “Green Sahara.”

The terrestrial biosphere plays a key role in mitigating climate change by sequestering the anthropogenic carbon emissions in the atmosphere. The projected change in biosphere-atmosphere carbon exchange remains highly uncertain due to competing effects of elevated CO2 concentration and the accompanying climate forcing. Studying how the carbon balance changed in the past cold and warm climate transitions can help to constrain these uncertainties. We perform model simulations to reproduce the vegetation patterns and terrestrial carbon variations in these climate states, which are consistent with paleo-vegetation and carbon cycle-related reconstructions. We further investigate separately the climate forcing effects (precipitation, temperature and CO2 concentration) and take into account the different land-sea distributions.

Refs:

Lu, Z., Miller, P., Zhang, Q., Wårlind, D., Nieradzik, L., Sjolte, J., Li, Q. and Smith, B. (2019). Vegetation pattern and terrestrial carbon variation in past warm and cold climates. Geophysical Research Letters.
Lu, Z., Miller, P., Zhang, Q., Zhang, Q., Wårlind, D., Nieradzik, L., Sjolte, J. and Smith, B. (2018). Dynamic Vegetation Simulations of the Mid-Holocene Green Sahara. Geophysical Research Letters.
Lu, Z., Chen, D., Wyser, K., Fuentes-Franco, R., Olin, S., Zhang, Q., Wu, M. and Ahlström, A. (2023). Natural decadal variability of global vegetation growth in relation to major decadal climate modes. Environmental Research Letters.

Climate and climate variability

• Paleoclimate climate sensitivity under variations of external forcings (Orbital parameters, Icesheet/Sea-ice, Greenhouse gases, Meltwater discharge, etc.)

• El Niño–Southern Oscillation dynamics and ocean-atmospheric interaction; Global monsoon change mechanisms; Tropical-extratropical/high latitude teleconnection

The El Niño Southern Oscillation (ENSO) is Earth’s dominant source of interannual climate variability. To improve our understanding of its response to global warming, it is paramount to determine its past behavior. We analyze a series of climate model simulations for the past 21,000 years. Consistent with most reconstructions, our results suggest an orbitally induced strengthening of ENSO during the Holocene epoch. During the early deglaciation, ENSO characteristics change drastically in response to meltwater discharges and the resulting changes in the Atlantic Meridional Overturning Circulation. Increasing deglacial atmospheric CO2 concentrations tend to weaken ENSO, whereas retreating glacial ice sheets intensify ENSO.

Modern ENSO events tend to peak at the end of calendar year (phase-locking).We show that in an idealized NCAR-CCSM3 simulation of the climate of the last 300,000 years, ENSO seasonal phase locking is shifted periodically following the precessional forcing: ENSO tends to peak in boreal winter when perihelion is near vernal equinox, but to peak in boreal summer when perihelion lies in between autumnal equinox and winter solstice. The mechanism for the change of ENSO’s phase locking is found to be caused by the change of seasonality of the growth rate, or the intensity of ocean-atmosphere feedbacks, of ENSO.

We combine central Pacific fossil coral oxygen isotope reconstructions with a multimodel ensemble of transient Holocene global climate simulations to investigate the multi-year ENSO evolution during the Holocene (beginning ~11,700 years ago). We find that, over the past ~7,000 years, in proxies the ratio of multi-year to single-year ENSO events increased by a factor of 5, associated with a longer ENSO period (from 3.5 to 4.1 years). More frequent multi-year ENSO events and prolonged ENSO periods are being caused by a shallower thermocline and stronger upper-ocean stratification in the Tropical Eastern Pacific in the present day.

Refs:

Liu, Z., Lu, Z., Wen, X., Otto-Bliesner, B. L., Timmermann, A., and Cobb, K. M. (2014). Evolution and forcing mechanisms of El Niño over the past 21,000 years. Nature.
Lu, Z., Liu, Z., and Zhu, J. (2016). Abrupt intensification of ENSO forced by deglacial ice-sheet retreat in CCSM3. Climate Dynamics.
Lu, Z. and Liu, Z. (2018). Orbital modulation of ENSO seasonal phase locking. Climate Dynamics.
Lu, Z., Liu, Z., and Zhu, J. and Cobb K. M. (2018). A Review of Paleo El Niño Southern Oscillation. Atmosphere.
Lu, Z. and Liu, Z. (2018). Examining El Niño in the Holocene: Implications and Challenges. National Science Review
Lu, Z., Liu, Z., Chen, G. and Guan J. (2019). Prominent precession-band variance in ENSO intensity over the last 300,000 years. Geophysical Research Letters.
Lu, Z., Schultze, A., Carré, M., Brierley, C., Hopcroft, P. O., Zhao, D., ... & Zhang, Q. (2025). Increased frequency of multi-year El Niño-Southern Oscillation events across the Holocene. Nature Geoscience.

Interactions between climate, ecosystem and renewable energy

• To quantify how massive solar panels and wind turbines can exert the impacts on the atmospheric and oceanic circulations, extreme weather, vegetation distribution and associated terrestrial carbon cycling by performing ESM simulations.

• To identify optimized spatial planning for future large-scale solar/wind farm installment that reduces harmful climate and ecological consequences, and assess its mitigation effects.

I use Earth System models (ESM, coupled ocean-atmosphere-terrestrial ecosystem) simulations to analyze the coupled ocean-atmosphere-terrestrial ecosystem reorganize in response to the large-scale solar and wind farms, as they would change land surface properties such as albedo and friction. These results will greatly advance our understanding of the mechanisms for the interaction processes in the Earth system regarding similar land-cover land-use changes. They also help to find potential suitable locations for large-scale solar farm installment that optimize the energy production while minimizing harmful climate, ecological consequences.

Refs:

Lu, Z., Zhang, Q., Miller, P. A., Zhang, Q., Berntell, E., and Smith, B. (2021). Impacts of large‐scale Sahara solar farms on global climate and vegetation cover. Geophysical Research Letters.
Power, K.*, Lu, Z.*, and Zhang, Q. (2023). Impacts of large-scale Saharan solar farms on the global terrestrial carbon cycle. Environmental Research Letters.
Long, J.*, Lu, Z.*, Miller, P. A., Pongratz, J., Guan, D., Smith, B., Zhu, Z., Xu, J., and Zhang, Q. (2024). Large-scale photovoltaic solar farms in the Sahara affect solar power generation potential globally. Communications Earth & Environment.

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