Climate Change and Mid-Latitude Weather Extremes

Planetary Wave Resonance and Midlatitude Weather Extremes

Climate change is increasing the frequency and severity of summer extreme weather events in the densely populated mid-latitudes of the Northern Hemisphere. Many of these events are inextricably linked to anomalous behavior of the jet stream and meanders therein tied to Rossby or "planetary" waves. Yet, a comprehensive understanding of the impact climate change is having on planetary wave amplification associated with these warm-season extremes has remained elusive. Key challenges persist, including the rarity of real-world realizations of the phenomenon and its inadequate representation in current-generation climate models, hampering the ability to capture nonlinear interactions among physical drivers across scales.

Modified from Li et al. (2025)

Drawing on extended climate observations, reanalysis products, historical simulations, and model projections, our work has consistently indicated an increase in the frequency of planetary wave resonance—or its favorable conditions—strongly suggesting that anthropogenic warming is altering the background climate in ways that favor more resonant amplification events. Potential pathways include amplified Arctic warming, the El Niño–Southern Oscillation (ENSO), and land–atmosphere interactions. However, the precise role of each of the forcing in initiating, sustaining, or disrupting the conditions for resonance occurrence remains uncertain, meriting further investigation.

Relevant publications:

Li, X., M. E. Mann, M. F. Wehner, and S. Christiansen (2025), Increased frequency of planetary wave resonance events over the past half-century, Proceedings of the National Academy of Sciences, 122(25), e2504482122. [Article Link] [Code] [Associated Press] [The Guardian] [The Hill] [Penn Today]

Li, X., M. E. Mann, M. F. Wehner, S. Rahmstorf, S. Petri, S. Christiansen, and J. Carrillo (2024), Role of atmospheric resonance and land-atmosphere feedbacks as a precursor to the June 2021 Pacific Northwest Heat Dome event, Proceedings of the National Academy of Sciences, 121(4), e2315330121. [Article Link] [Penn Today] [Los Angeles Times] [Presentation at AGU23]

Guimarães, S. O., M. E. Mann, S. Rahmstorf, S. Petri, B. A. Steinman, D. J. Brouillette, S. Christiansen, and X. Li (2024), Increased projected changes in quasi-resonant amplification and persistent summer weather extremes in the latest multimodel climate projections, Scientific Reports, 14(1), 21991. [Article Link] [Code] [The Guardian]

Li, X., A. H. Lynch, D. A. Bailey, S. R. Stephenson, and S. Veland (2021), The impact of black carbon emissions from projected Arctic shipping on regional ice transport, Climate Dynamics, 57(9), 2453-2466. [Article Link]

From Wave Resonance to Urban Resilience

The large-scale nature of planetary wave resonance enables the study of a network of cities, providing new insights for developing resource mobilization strategies and emergency management plans in response to simultaneous, compound extreme weather events. A central policy question is: "How can we use climate science-based warning signals, such as planetary wave resonance, on a large scale, while also accounting for local adaptive capacities—such as mitigating the urban heat island effect—to ensure that scarce emergency resources are allocated equitably and efficiently across cities?"

Relevant publications or research report:

Li, X., M. E. Mann, S. Christiansen, and H. L. Kostick (2025), Rethinking extreme heat and urban resilience through scales connectivity. Policy Digest, Penn Kleinman Center for Energy Policy. (In review)

Liu, K., X. Li, S. Wang, and X. Gao (2022), Assessing the effects of urban green landscape on urban thermal environment dynamic in a semiarid city by integrated use of airborne data, satellite imagery and land surface model, International Journal of Applied Earth Observation and Geoinformation, 107, 102674. [Article Link]

Liu, K., X. Li, S. Wang, and Y. Li (2020), Investigating the impacts of driving factors on urban heat islands in southern China from 2003 to 2015, Journal of Cleaner Production, 254, 120141. [Article Link]

Liu, K., H. Su, X. Li, W. Wang, L. Yang, and H. Liang (2016), Quantifying spatial–temporal pattern of urban heat island in Beijing: an improved assessment using land surface temperature (LST) time series observations from LANDSAT, MODIS, and Chinese new satellite GaoFen-1, IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 9(5), 2028-2042. [Article Link]

Chen et al. (2025)

Coastal Hazards and Urbanization

Nor’easters, an important subclass of extratropical cyclones, pose significant threats to cities along the U.S. East Coast. These storms draw energy from both thermal contrasts (meridional and land-ocean) and oceanic latent heat release. Yet these contrasts are weakening under Arctic amplification, complicating how nor’easters respond to climate change.

Using a cyclone tracking approach combined with long-term reanalysis data, we found statistically significant trends in the maximum wind speeds of the most intense nor’easters, along with an increasing trend in area-averaged precipitation intensity. This intensification is expected due to increased storm moisture, fueled by warmer ocean temperatures, leading both to increased latent heating and greater coastal baroclinic instability.

Such changes carry serious implications for coastal urban centers. In highly developed areas with extensive impervious surfaces, accelerated runoff reduces drainage efficiency, amplifying the risk of flash flooding. As the most powerful nor’easters continue to intensify, these vulnerabilities further increase the exposure of densely populated coastal regions to extreme coastal hazards.

Relevant publications:

Chen, K., X. Li, M. M. Weaver, S. A. Christiansen, A. L. Horton, and M. E. Mann (2025), The intensification of the strongest nor’easters, Proceedings of the National Academy of Sciences, 122(29), e2510029122. [Article Link] [Code] [Inside Climate News]

Liu, K., X. Li, and S. Wang (2021), Characterizing the spatiotemporal response of runoff to impervious surface dynamics across three highly urbanized cities in southern China from 2000 to 2017, International Journal of Applied Earth Observation and Geoinformation, 100, 102331. [Article Link]

Climate Change Impacts at the Ice Edge

Modified from Li et al. (2024)

Daily Arctic Marine Accessibility Projections for Risk Assessment

Few transformations in Earth systems are as dramatic as those currently occurring in the Arctic. The rapid retreat of sea ice has spurred increased human activities in the region, raising expectations for both new opportunities and new challenges. Among the most critical issues is Arctic Ocean navigability, which underpins all Arctic operations. A central uncertainty lies in how evolving climate system characteristics interact with marine accessibility to influence projected probabilities of Arctic navigation, an uncertainty with cascading implications for operations, finance, and geopolitics.

To address this, I developed an ensemble of multi-model, multi-scenario daily projections of Arctic marine accessibility, which translates projected future sea ice concentration and thickness from Earth system models into a measure of navigational risk, providing a robust tool of broad utility as sea ice retreat redefines relationships across the Arctic. With it, I revealed the emergence of a new Transpolar Sea Route, which diverts traffic away from the Russian-administered Northern Sea Route, reducing navigational and financial risks as well as regulatory friction. However, this shift also introduces new risks for fragile marine ecosystems and vulnerable coastal communities.

Relevant publications:

Li, X., and A. H. Lynch (2024), Projections for Arctic marine accessibility: risk under climate change. Ocean and Coastal Law Journal, 29(2), 353. [Article Link] [University of Maine School of Law]

Li, X., and A. H. Lynch (2023), New insights into projected Arctic sea road: operational risks, economic values, and policy implications, Climatic Change, 176(4), 30. [Article Link] [Dialogue Earth]

Lynch, A. H., C. H. Norchi, and X. Li (2022), The interaction of ice and law in Arctic marine accessibility, Proceedings of the National Academy of Sciences, 119(26), e2202720119. [Article Link] [Dataset] [The Independent] [Daily Mail] [The Sunday Times] [New Scientist] [ScienceDaily] [The Hill] [EOS] [IMPACT Research at Brown 2023]

Goldstein, M. A., A. H. Lynch, X. Li, and C. H. Norchi (2022), Sanctions or sea ice: Costs of closing the Northern Sea Route, Finance Research Letters, 50, 103257. [Article Link] [Nature Correspondence] [Relevant Presentation at AGU21]

Li, X., S. R. Stephenson, A. H. Lynch, M. A. Goldstein, D. A. Bailey, and S. Veland (2021), Arctic shipping guidance from the CMIP6 ensemble on operational and infrastructural timescales, Climatic Change, 167(1), 23. [Article Link] [Presentation at AGU21]

Vilkitsky Strait Animation Credit: Arctic Sea Ice Blog

Remote Sensing of Sea Ice Hazards

The Arctic Ocean is transitioning towards an ice pack that is younger, thinner, and more mobile. Driven by ocean waves and winds, ice floes present significant navigational hazards in ice-covered waters—particularly during the melt season, when floes fragment and drift, and within shallow, narrow straits where conditions become especially treacherous.

At the opposite pole, our high-resolution satellite observations revealed a positive feedback in Antarctic sea ice melt, operating along the edges of individual floes. These fine-scale processes highlight the importance of floe-scale dynamics for assessing polar navigability and the associated risks for ship operations, resupply missions, and tourism in these extreme environments.

Relevant publications:

Doddridge, E. W., W. R. Hobbs, M. Auger, P. W. Boyd, S. M. T. Chua, S. Cook, S. Corney, L. Emmerson, A. D. Fraser, P. Heil, N. Kelly, D. Lannuzel, X. Li, G. Liniger, R. A. Massom, A. Meyer, P. Reid, C. Southwell, P. Spence, A. Steketee, K. M. Swadling, N. Teder, B. Wienecke, P. Wongpan, and K. Yamazaki (2025), Impacts of Antarctic summer sea-ice extremes, PNAS Nexus, 4(7). [Article Link] [The Guardian] [The Conversation]

Gupta, M., H. Reagan, Y. Koo, S. M. T. Chua, X. Li, and P. Heil (2025), Inferring the seasonality of sea ice floes in the Weddell Sea using ICESat-2. The Cryosphere, 19(3), 1241-1257. [Article Link]

Climate Change and the Terrestrial Water Cycle

Modified from Liu et al. (2023)

Remote Sensing and Machine Learning of Terrestrial Water Storage

Climate change affects not only the exchange of energy and water between the land surface and the atmosphere—a process fundamental to Earth’s climate and water cycle—but also the storage of water at and below the surface (terrestrial water storage, TWS), thereby reshaping where, when, and how much water is available for human use.

By integrating satellite observations, reanalysis datasets, and Earth system model outputs with machine learning approaches, we show that TWS responds adversely to vegetation greening in drylands, a signal projected to intensify by the end of the century. Cropland-dominated regions with intensive irrigation are at the frontline of this change. Irrigation, designed to supplement rainfall to meet crop water demand, increasingly relies on groundwater as droughts become more frequent and intense and surface water availability declines under climate change. This reliance accelerates groundwater depletion, threatening to erode its societal benefits. Achieving accurate and reliable estimation of irrigation water use to guide more efficient management remains an urgent challenge—one we address through approaches such as data-driven ensemble learning and theory-guided strategies.

Relevant publications:

Bo, Y., X. Li, K. Liu, S. Wang, D. Li, Y. Xu, and M. Wang (2024), Hybrid theory-guided data driven framework for calculating irrigation water use of three staple cereal crops in China, Water Resources Research, 60(3), e2023WR035234. [Article Link]

Liu, K., Y. Bo, X. Li, S. Wang, and G. Zhou (2024), Uncovering current and future variations of irrigation water use across China using machine learning, Earth’s Future, 12(3), e2023EF003562. [Article Link] [Code]

Liu, K., X. Li, S. Wang, and G. Zhou (2023), Past and future adverse response of terrestrial water storages to increased vegetation growth in drylands. npj Climate and Atmospheric Science, 6(1), 113. [Article Link]

Liu, K., X. Li, and X. Long (2021), Trends in groundwater changes driven by precipitation and anthropogenic activities on the southeast side of the Hu Line, Environmental Research Letters, 16(9), 094032. [Article Link]

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