Browsing by Author "Banerjee, Asmita"

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    Clumped-Isotope Constraint on Upper-Tropospheric Cooling During the Last Glacial Maximum
    (Wiley, 2022) Banerjee, Asmita; Yeung, Laurence Y.; Murray, Lee T.; Tie, Xin; Tierney, Jessica E.; Legrande, Allegra N.
    Ice cores and other paleotemperature proxies, together with general circulation models, have provided information on past surface temperatures and the atmosphere's composition in different climates. Little is known, however, about past temperatures at high altitudes, which play a crucial role in Earth's radiative energy budget. Paleoclimate records at high-altitude sites are sparse, and the few that are available show poor agreement with climate model predictions. These disagreements could be due to insufficient spatial coverage, spatiotemporal biases, or model physics; new records that can mitigate or avoid these uncertainties are needed. Here, we constrain the change in upper-tropospheric temperature at the global scale during the Last Glacial Maximum (LGM) using the clumped-isotope composition of molecular oxygen trapped in polar ice cores. Aided by global three-dimensional chemical transport modeling, we exploit the intrinsic temperature sensitivity of the clumped-isotope composition of atmospheric oxygen to infer that the upper troposphere (effective mean altitude 10–11 km) was 6–9°C cooler during the LGM than during the late preindustrial Holocene. A complementary energy balance approach supports a minor or negligible steepening of atmospheric lapse rates during the LGM, which is consistent with a range of climate model simulations. Proxy-model disagreements with other high-altitude records may stem from inaccuracies in regional hydroclimate simulation, possibly related to land-atmosphere feedbacks.
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    Evaluating changes in high altitude temperature and atmospheric circulation during the last deglaciation using clumped isotopic composition of oxygen in polar ice cores
    (2023-04-21) Banerjee, Asmita; Yeung, Laurence
    The last deglacial period, spanning 21,000 to 10,000 years before present, has been studied extensively to quantify Earth system responses to changes in climate forcings like greenhouse gas concentrations. During this time, the Earth system underwent near-synchronous changes: atmospheric greenhouse gas concentrations and surface temperatures increased, ice volume and sea ice extent decreased resulting in sea level rise while atmospheric and ocean circulation patterns underwent drastic changes. Paleoclimate archives are used extensively to understand the causes and quantify the magnitude of these past changes. However, most of these studies are focused on the surface. Little information exists about the vertical profile of the atmosphere, namely how high-altitude temperatures and stratosphere-to-troposphere transport fluxes evolve with a rapidly evolving climate. Understanding the evolution of the vertical thermal structure of the atmosphere is necessary for quantifying how temperature lapse rates change with changing climate. Furthermore, air mass exchange between the stratosphere and the troposphere governs the chemistry of both regions and is expected to accelerate in a warming world. Thus, evaluating how high-altitude temperatures and atmosphere circulation evolved in the past is crucial for predicting their changes in the future. This dissertation evaluates the potential for a novel ice core proxy record, clumped isotopic composition of molecular oxygen measured in occluded air in polar ice cores, to provide constraints on how high-altitude temperatures and stratosphere-to-troposphere transport evolved during the last deglacial period. Clumped isotopic composition of oxygen, denoted by Δ36, is the proportional abundance of two heavy isotopes of oxygen, i.e., 18O18O in O2 and its formation is sensitive to the thermal and photochemical properties of the atmosphere. Isotope exchange reactions in the stratosphere and troposphere and mass exchange between the two governs the net surface Δ36 value. Evaluation of changing clumped isotopic composition of O2 during the deglacial period can provide insights on how upper-tropospheric temperatures and/or atmospheric circulation evolved through this time. The Last Glacial Maximum (LGM) spanning 21,000 to 18,000 years before present is first studied in Chapter 2. Polar ice core clumped isotopic compositions are measured and factors affecting the measured values are evaluated. Measurements are complemented with results from a global three-dimensional chemical transport model to infer changes in upper-tropospheric temperatures during this time. Finally, computed upper-tropospheric temperatures are compared with existing records of global surface temperature change during the LGM to infer changes in the temperature lapse rate during this time. The evolution of clumped isotopic composition of O2 during the Bølling Allerød warm period and Younger Dryas cold stadial spanning 15,000-11,000 years before present is investigated in Chapter 3. Both these periods represent abrupt centennial scale changes in the Earth’s climate, notably in the Northern Hemisphere. Ice core measurements indicate that Δ36 values reach pre-Industrial levels during this time, much before global surface temperatures. Combination of the measured values with sensitivity experiments indicate the non-linear relationship between Earth’s surface and high-altitude temperatures during periods of abrupt climate change, particularly ones that alter the cryospheric extent in the Northern Hemisphere. The results presented here indicates the role of Northern Hemisphere ice cover in governing the thermal structure of the atmosphere and lapse rate feedback. Finally, in Chapter 4, changes in atmospheric circulation, specifically, air mass exchange between the stratosphere and the troposphere during Heinrich Stadial 1 (HS1) is evaluated. HS1 (18000-14700 years before present) is thought to be a consequence of slowdown of the Atlantic Meridional Overturning Circulation (AMOC), a thermohaline circulation responsible for meridional heat transport. Slowdown of the AMOC affects the meridional temperature gradient and in turn, affects atmospheric circulation. Measurements of clumped isotopic composition of O2 in the ice core record indicates an abrupt increase during Heinrich Stadial 1, that is attributed to enhanced stratosphere-to-troposphere transport fluxes of high Δ36 bearing O2. The results presented here observationally constrain the relationship between increased meridional temperature gradient and air mass exchange between the stratosphere and the troposphere. Subsequent investigations using three-dimensional chemical transport models and imposed meridional temperature gradients may provide a more cohesive understanding of the mechanisms relating the two.
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    Tropospheric Ozone During the Last Interglacial
    (Wiley, 2022) Yan, Yuzhen; Banerjee, Asmita; Murray, Lee T.; Tie, Xin; Yeung, Laurence Y.
    The history of tropospheric O3, an important atmospheric oxidant, is poorly constrained because of uncertainties in its historical budget and a dearth of independent records. Here, we estimate the mean tropospheric O3 burden during the Last Interglacial period (LIG; 115 to 130 thousand years ago) using a record of the clumped isotopic composition of O2 (i.e., Δ36 values) preserved in Antarctic ice. The measured LIG Δ36 value is 0.03 ± 0.02‰ (95% CI) higher than the late pre-industrial Holocene (PI; 1,590–1,850 CE) value and corresponds to a modeled 9% reduction in LIG tropospheric O3 burden (95% CI: 3%–15%), caused in part by a substantial reduction in biomass burning emissions during the LIG relative to the PI. These results are consistent with the hypothesis that late-Pleistocene megafaunal extinctions caused woody and grassy fuels to accumulate on land, leading to enhanced biomass burning in the preindustrial Holocene.