Hot Science: Ocean Acidification, Rhinos at Risk, & Declining Snowpack

Welcome to Hot Science, a joint effort of World Resources Institute and Bezos Earth Fund. This edition features studies published in January 2024 and is not meant to be comprehensive. Rather, this newsletter aims to highlight ten of the most significant climate science papers published in leading peer-reviewed journals each month, with a focus on global temperature rise and surface albedo; GHG emissions and carbon removals; climatic changes across ice, permafrost, and the ocean; extreme weather events; and climate risks and impacts. If you find this email useful, please encourage your colleagues and friends to subscribe to Hot Science.

Severe acidification of the Southern Ocean may upend food chains across marine protected areas by 2100

The world’s ocean currently absorbs roughly a quarter of all anthropogenic CO2 emissions, but as this GHG dissolves, it increases the acidity of seawater. Declining pH levels, in turn, will likely harm the Southern Ocean’s tiniest organisms — plankton may struggle to grow, krill hatch rates could fall steeply, and the shells of clams, mollusks, and scallops may suffer significant damage. Even relatively small changes at the bottom of the food chain can cascade upwards, threatening Antarctic whales, penguins and other marine predators. 

Relying on a high-resolution ocean-sea ice-biogeochemistry model, a recent study published in Nature Communications finds that, under a high emissions scenario (SSP5-RCP8.5), acidification across the Southern Ocean’s existing and proposed marine protected areas could more than double by the end of the century, with acidification occurring throughout the coastal water column. The authors note that only in the lowest emissions scenario (SSP1-RCP2.6) can the world avoid severe acidification in Antarctica’s waters and its resulting impacts on marine biodiversity.

Ocean warming reaches an all-time high (again)

Analyzing data collected primarily from a global network of buoys across the ocean, the authors of a new study estimate that ocean heat content, which describes the amount of heat stored in the upper 2,000 meters of the water column, surpassed 2022’s record-breaking value by upwards of 15 zettajoules (a measure of energy). 

For comparison, about half a zettajoule of energy powers the entire global economy each year. And while the ocean heat content has risen steadily in recent decades, the upper 100 meters of seawater saw substantial warming this past year, with the onset of El Niño. The annual average sea surface temperature in 2023, for example, was 0.23°C higher than in 2022 and 0.54°C higher than the annual average between 1981 and 2020. 

The paper also finds extreme anomalies in salinity, and that stratification of ocean waters reached a record high in 2023. Such trends not only fuel extreme weather events and sea level rise, but also reduce the mixing of surface waters with deeper waters, which impacts carbon cycling, heat uptake and marine life.

Climate change is causing drought in the Amazon, grasslands and shrublands

Research led by scientists from the Imperial College of London has concluded that climate change, not El Niño, was the cause of last year’s extreme drought in the Amazon River Basin. Their study looks at drought conditions from June to November 2023, finding that although El Niño reduced precipitation, increased global temperatures drove the drought. 

Additional research on drought published in Proceedings of the National Academy of Sciences found a decrease in aboveground plant growth for grasslands and shrublands that were experimentally exposed to intensified drought conditions. Both studies provide examples of how the impacts from climate change, especially drought, are often initially underestimated, and frequently much more severe than expected.

Western forests bearing the brunt of climate stress in the western US

A recent study untangles the interplay of positive and negative drivers shaping the global forest carbon sink. Analyzing 20 years of forest productivity trends in the United States, the research reveals a stark contrast: as the eastern region experiences a boost in forest productivity due to CO2 fertilization effects, the western United States faces a decline in productivity amidst severe warming and decreased precipitation, which negates the impact of fertilization. 

Employing non-linear models on extensive forest inventory data, the study finds that adverse climate change impacts on tree growth are likely responsible for the productivity decline in the water-limited forests of the western U.S. These findings underscore the vulnerability of the global forest carbon sink to climate change, emphasizing the need for policies addressing geographical variations in climate stress to sustain these vital ecosystems and their crucial role in carbon sequestration.

Rhinos pushed beyond their limits with future warming

Rhinoceros populations are already dwindling due to poaching and land use change. A new study explores the future risk that climate change poses to black and white rhino populations in five large national parks in southern Africa, a region which harbors 80% of the world’s remaining black rhinos and 92% of white rhinos. The study assessed the impacts of two warming scenarios (IPCC’s mid- and high-level warming scenarios, RCP4.5 and RCP8.5) and found significant negative impacts on both species, with temperature proving to be a greater driver of loss than precipitation. The authors found that under a high-emissions scenario the two species will have “zero probability of occurrence” in the national parks assessed. They conclude by calling upon the conservation field to consider such impacts when managing rhino populations.

Atmospheric water vapor from climate change has not increased as previously projected

A study published in Proceedings of the National Academy of Sciences investigated differences between observed and model-based historical hydroclimate trends. The observational data shows that, on average, previously projected increases in atmospheric moisture have not occurred, resulting in drier conditions. This trend was mostly present for arid and semi-arid regions. Large discrepancies between projections and actual observations could have huge implications for our ability to accurately project future hazards from climate change and may mean that some regions are more vulnerable to secondary effects than previously expected.

Steep declines in snowpack across the Northern Hemisphere may heighten water risks for millions of people

Climate change’s influence on snowpack (the total mass of snow on the ground) is complicated. Warming can increase precipitation — and, therefore, winter snowfall — while simultaneously accelerating snowmelt. Until recently, scientists lacked a clear understanding of where, when and by how much climate change impacts snowpack. But by programming a machine learning model to assess thousands of observations and climate model experiments focused on temperature, precipitation, snowpack and runoff data, authors of a new Nature article found that global warming has triggered sharp declines in March snowpack across roughly 20% of the Northern Hemisphere’s river basins between 1981 and 2020.

What’s more, they discovered that once average winter temperatures across these basins reach -8°C, they cross a tipping point, and snowpack losses accelerate nonlinearly. Already, some of these basins have warmed beyond this threshold and are home to major cities, some of which depend on snowpack melt for their water during the spring and summer. When these frozen reservoirs of water fail to accumulate during the winter, the risk of drought increases.

Previous studies underestimate Greenland Ice Sheet loss by as much as twenty percent

Scientists have recently pieced together a 120-m-resolution picture of the Greenland Ice Sheet’s extent across every month for the past four decades. They found that current estimates of mass balance of the ice sheet have underestimated loss by as much as 20%. The Ice Sheet has lost 1930 sq miles (5,091 sq km) between 1985-2022. While the ice loss the authors have identified is not significantly impacting sea level rise, it is large enough to impact ocean circulation and heat energy distribution.

Arctic albedo falling even faster than sea ice is receding

Recent summer measurements in the Arctic reveal a 20% to 35% decrease in reflected sunlight (albedo). This reduction coincides with a 7–9% decrease in Arctic sea ice coverage, indicating that not only is sea ice receding, but the albedo of the remaining sea ice is also falling, which can happen as the ice becomes weathered due to warming temperatures. The data, collected using previously unpublished pixelated radiometers on Global Positioning System satellites between 2014 and 2019, emphasizes the potential role of sea ice albedo in Arctic amplification, the phenomenon wherein the Arctic warms much more rapidly than the global average temperature.

Connection established between warming and diarrheal illness

It is well established that weather can affect pathogen survival and distribution. A new study assesses the impact of warming on Campylobacter infections, a type of gastrointestinal infection that is found worldwide and that is the most common bacterial food-borne disease in developed countries. These infections can be deadly among the elderly, the immunosuppressed and children. The authors use both England and Wales as a case study and find that there is a strong association between infection and maximum air temperature, among other factors.

Warming temperatures alter eyespots in the meadow brown butterfly

Climate change is impacting the world in unique and often unpredictable ways. Researchers from the United Kingdom investigated the effects of temperature on the number of eyespots present on the meadow brown butterfly at three different field sites. Their results, published in Ecology and Evolution, show that higher temperatures during larval/pupal development result in fewer eyespots. Females that developed at 11°C had a median of six spots, whereas those developing at 15°C had three spots. The authors suggest that fewer hindwing spots may be an advantage for butterflies camouflaging in dry, brown grass. They conclude that future warming will further drive the loss of butterfly spots each year, demonstrating the unforeseen ways in which climate change is altering our planet.

Biodiversity has limited impact on peatlands stability under drought

A recent study challenges prevailing assumptions about the relationship between biodiversity and ecosystem stability in peatlands. Based on an experiment that subjected different peat moss species mixtures to varying water table conditions, the study suggests that peat moss displays resilience in the face of mild drought conditions, and is somewhat more resilient given higher species diversity. Severe drought conditions, however, compromise ecosystem function regardless of biodiversity. The findings underscore the vulnerability of peatlands to severe droughts, emphasizing the critical role of water levels in maintaining their essential function as carbon sinks.