Paul R. Holland’s research while affiliated with British Antarctic Survey and other places

Anomalous freshwater fluxes from the Greenland and Antarctic ice sheets and ice shelves are impacting the surrounding oceans and we need to be able to account for these effects in climate model simulations over the historical period and beyond. In previous phases of the Coupled Model Intercomparison Project (CMIP), models mostly either assumed that the ice sheets were in mass balance, or that discharge from the ice sheets was constant, but in neither case was the observed increasing discharge properly accounted for. In this paper, we present an updateable dataset of absolute and anomalous freshwater mass fluxes from both ice sheets. These fluxes can be implemented in historical climate simulations as a forcing for models that do not (yet) include interactive ice sheets, or used to evaluate models that do. We also make recommendations for how climatological and anomalous fluxes can be implemented in climate models that may have different approaches to interactions with the ice sheets. These forcings are available for CMIP7 simulations and should lead to more robust and coherent simulation of sea surface temperature, sea ice and regional sea level trends in the recent historical period, and improve the credibility of projections.

The retreat of the Antarctic Ice Sheet is conventionally attributed to increased ocean melting of ice shelves, potentially enhanced by internal instability from grounding lines near retrograde bed slopes. Ocean melting is enhanced by increased intrusion of modified Circumpolar Deep Water (mCDW) into ice shelf cavities. Upwelling from the release of subglacial meltwater can enhance mCDW’s melting ability, though its efficacy is not well understood and is not represented in current ice sheet loss projections. Here we quantify this process during an exceptional subglacial lake drainage event under Thwaites Glacier. We found that the buoyant plume from the subglacial discharge temporarily doubled the rate of ocean melting under Thwaites, thinning the ice shelf. These events likely contributed to Thwaites’ rapid thinning and grounding line retreat during that period. However, simulations and observations indicate that a steady subglacial water release would more efficiently enhance basal melt rates at Thwaites, with melt rate increasing like the square root of the subglacial discharge. Thus, it remains unclear whether increased subglacial flooding events provide a stabilizing influence on West Antarctic ice loss by reducing the impact of subglacial water on ocean melting, or a destabilizing influence by triggering rapid changes at the grounding zone.

Sea-ice floating in the Arctic ocean is a constantly moving, growing and melting layer. The seasonal cycle of sea-ice volume has an average amplitude of $10\,000\,\mathrm{km}^3$ or 9 trillion tonnes of sea ice. The role of dynamic redistribution of sea ice is observable during winter growth by the incorporation of satellite remote sensing of ice thickness, concentration and drift. Recent advances in the processing of CryoSat-2 radar altimetry data have allowed for the retrieval of summer sea-ice thickness. This allows for a full year of a purely remote sensing-derived ice volume budget analysis. Here, we present the closed volume budget of Arctic sea ice over the period October 2010–May 2022 revealing the key contributions to summer melt and minimum sea-ice volume and extent. We show the importance of ice drift to the inter-annual variability in Arctic sea-ice volume and the regional distribution of sea ice. The estimates of specific areas of sea-ice growth and melt provide key information on sea-ice over-production, the excess volume of ice growth compared to melt. The statistical accuracy of each key region of the Arctic is presented, revealing the current accuracy of knowledge of Arctic sea-ice volume from observational sources.

Ice shelves buttress the grounded ice sheet, restraining its flow into the ocean. Mass loss from these ice shelves occurs primarily through ocean-induced basal melting, with the highest melt rates occurring in regions that host basal channels – elongated, kilometre-wide zones of relatively thin ice. While some models suggest that basal channels could mitigate overall ice shelf melt rates, channels have also been linked to basal and surface crevassing, leaving their cumulative impact on ice-shelf stability uncertain. Due to their relatively small spatial scale and the limitations of previous satellite datasets, our understanding of how channelised melting evolves over time remains limited. In this study, we present a novel approach that uses CryoSat-2 radar altimetry data to calculate ice shelf basal melt rates, demonstrated here as a case study over Pine Island Glacier (PIG) ice shelf. Our method generates monthly Digital Elevation Models (DEMs) and melt maps with a 250 m spatial resolution. The data show that near the grounding line, basal melting preferentially melts a channel's western flank 50 % more than its eastern flank. Additionally, we find that the main channelised geometries on PIG are inherited upstream of the grounding line and play a role in forming ice shelf pinning points. These observations highlight the importance of channels under ice shelves, emphasising the need to investigate them further and consider their impacts on observations and models that do not resolve them.

Most climate models do not reproduce the 1979–2014 increase in Antarctic sea ice cover. This was a contributing factor in successive Intergovernmental Panel on Climate Change reports allocating low confidence to model projections of sea ice over the 21st century. We show that recent rapid declines bring observed sea ice area trends back into line with the models and confirm that discrepancies exist for earlier periods. This demonstrates that models exhibit different skill for different timescales and periods. We discuss possible interpretations of this linear trend assessment given the abrupt nature of recent changes and discuss the implications for future research.

The reality of human-induced climate change is unequivocal and exerts an ever-increasing global impact. Access to the latest scientific information on current climate change and projection of future trends is important for planning adaptation measures and for informing international efforts to reduce emissions of greenhouse gases (GHGs). Identification of hazards and risks may be used to assess vulnerability, determine limits to adaptation, and enhance resilience to climate change. This article highlights how recent research programs are continuing to elucidate current processes and advance projections across major climate systems and identifies remaining knowledge gaps. Key findings include projected future increases in monsoon rainfall, resulting from a changing balance between the rainfall-reducing effect of aerosols and rainfall-increasing GHGs; a strengthening of the storm track in the North Atlantic; an increase in the fraction of precipitation that falls as rain at both poles; an increase in the frequency and severity of El Niño Southern Oscillation (ENSO) events, along with changes in ENSO teleconnections to North America and Europe; and an increase in the frequency of hazardous hot-humid extremes. These changes have the potential to increase risks to both human and natural systems. Nevertheless, these risks may be reduced via urgent, science-led adaptation and resilience measures and by reductions in GHGs.

The Amundsen Sea in West Antarctica features rapidly thinning ice shelves, large polynyas, and sizable spring phytoplankton blooms. Although considerable effort has gone into characterizing heat fluxes between the Amundsen Sea, its associated ice shelves, and the overlying atmosphere, the effect of the phytoplankton blooms on the distribution of heat remains poorly understood. In this modeling study, we implement a feedback from biogeochemistry onto physics into MITgcm‐BLING and use it to show that high levels of chlorophyll—concentrated in the Amundsen Sea Polynya and the Pine Island Polynya—have the potential to increase springtime surface warming in polynyas by steepening the attenuation profile of solar radiation with depth. The chlorophyll‐associated warm anomaly (on average between +0.2° ${}^{\circ}$C and +0.3° ${}^{\circ}$C) at the surface is quickly dissipated to the atmosphere, by increases in longwave, latent and sensible heat loss from open water areas. Outside of the coastal polynyas, the summertime warm anomaly leads to an average sea ice thinning of 1.7 cm across the region, and stimulates up to 20% additional seasonal melting near the fronts of ice shelves. The accompanying cold anomaly, caused by shading of deeper waters, persists year‐round and affects a decrease in the volume of Circumpolar Deep Water on the continental shelf. This cooling ultimately leads to an average sea ice thickening of 3.5 cm and, together with associated changes to circulation, reduces basal melting of Amundsen Sea ice shelves by approximately 7% relative to the model scenario with no phytoplankton bloom.

Plain Language Summary Changes in the West Antarctic Ice Sheet mass balance (i.e., the difference between the gain and loss of ice mass) are partly influenced by large‐scale winds, and in particular, a climatological low‐pressure feature located off the West Antarctic coast called the Amundsen Sea Low (ASL). Yet, although the long‐term strengthening of the ASL since the mid‐20th century has been demonstrated to be related to anthropogenic forcing, our understanding of the variability of the ASL on time‐scales of decades is poorly known. In this paper, we therefore investigate the origins of this variability since 1870, and quantify the relative contributions of human‐caused climate changes and natural variability of the climate system. For this purpose, we use several ensembles of model simulations as well as new climate reconstructions that combine paleoclimate records with model simulations using a statistical method. Our results indicate that the multi‐decadal variability of the ASL is strongly driven by tropical variability in the Indo‐Pacific through atmospheric connections between this region and the Amundsen Sea. Our reconstruction, when compared with a large ensemble of model simulations, indicates that since 1950, human‐induced climate forcing has become a dominant driver of long‐term ASL variability, contributing equally to tropical variability.