Thursday, June 19, 2025

Warning signs on climate flashing bright red: top scientists

Human-induced warming increased over the last decade at a rate “unprecedented in the instrumental record”

By AFP
June 18, 2025


Dire future climate impacts worse than what the world has already experienced are already baked in over the next decade or two. - Copyright AFP Brendan SMIALOWSKI
Marlowe HOOD

From carbon pollution to sea-level rise to global heating, the pace and level of key climate change indicators are all in uncharted territory, more than 60 top scientists warned Thursday.

Greenhouse gas emissions from burning fossil fuels and deforestation hit a new high in 2024 and averaged, over the last decade, a record 53.6 billion tonnes per year — that is 100,000 tonnes per minute — of CO2 or its equivalent in other gases, they reported in a peer-reviewed update.

Earth’s surface temperature last year breached 1.5 degrees Celsius for the first time, and the additional CO2 humanity can emit with a two-thirds chance of staying under that threshold long-term — our 1.5C “carbon budget” — will be exhausted in a couple of years, they calculated.

Investment in clean energy outpaced investment in oil, gas and coal last year two-to-one, but fossil fuels account for more than 80 percent of global energy consumption, and growth in renewables still lags behind new demand.

Included in the 2015 Paris climate treaty as an aspirational goal, the 1.5C limit has since been validated by science as necessary for avoiding a catastrophically climate-addled world.

The hard cap on warming to which nearly 200 nations agreed was “well below” two degrees, commonly interpreted to mean 1.7C to 1.8C.

“We are already in crunch time for these higher levels of warming,” co-author Joeri Rogelj, a professor of climate science and policy at Imperial College London, told journalists in a briefing.

“The next three or four decades is pretty much the timeline over which we expect a peak in warming to happen.”

– ‘The wrong direction’ –

No less alarming than record heat and carbon emissions is the gathering pace at which these and other climate indicators are shifting, according to the study, published in Earth System Science Data.

Human-induced warming increased over the last decade at a rate “unprecedented in the instrumental record”, and well above the 2010-2019 average registered in the UN’s most recent Intergovernmental Panel on Climate Change (IPCC) report, in 2021.

The new findings — led by the same scientists using essentially the same methods — are intended as an authoritative albeit unofficial update of the benchmark IPCC reports underpinning global climate diplomacy.

They should be taken as a reality check by policymakers, the authors suggested.

“I tend to be an optimistic person,” said lead author Piers Forster, head of the University of Leed’s Priestley Centre for Climate Futures.

“But if you look at this year’s update, things are all moving in the wrong direction.”

The rate at which sea levels have shot up in recent years is also alarming, the scientists said.

After creeping up, on average, well under two millimetres per year from 1901 to 2018, global oceans have risen 4.3 mm annually since 2019.

– What happens next? –

An increase in the ocean watermark of 23 centimetres — the width of a letter-sized sheet of paper — over the last 125 years has been enough to imperil many small island states and hugely amplify the destructive power of storm surges worldwide.

An additional 20 centimetres of sea level rise by 2050 would cause one trillion dollars in flood damage annually in the world’s 136 largest coastal cities, earlier research has shown.

Another indicator underlying all the changes in the climate system is Earth’s so-called energy imbalance, the difference between the amount of solar energy entering the atmosphere and the smaller amount leaving it.

So far, 91 percent of human-caused warming has been absorbed by oceans, sparing life on land.

But the planet’s energy imbalance has nearly doubled in the last 20 years, and scientists do not know how long oceans will continue to massively soak up this excess heat.

Dire future climate impacts worse than what the world has already experienced are already baked in over the next decade or two.

But beyond that, the future is in our hands, the scientists made clear.

“We will rapidly reach a level of global warming of 1.5C, but what happens next depends on the choices which will be made,” said co-author and former IPCC co-chair Valerie Masson-Delmotte.

The Paris Agreement’s 1.5C target allows for the possibility of ratcheting down global temperatures below that threshold before century’s end.

Ahead of a critical year-end climate summit in Brazil, international cooperation has been weakened by the US withdrawal from the Paris Agreement.

President Donald Trump’s dismantling of domestic climate policies means the United States is likely to fall short on its emissions reduction targets, and could sap the resolve of other countries to deepen their own pledges, experts say.


Climate change could cut crop yields up to a quarter


By AFP
June 18, 2025


Farmers in many regions are already experiencing longer dry spells, unseasonable heatwaves and erratic weather due to climate change - Copyright AFP/File STR


Marlowe HOOD

Climate change is on track to reduce by 11 percent in 2100 the yields that today provide two-thirds of humanity’s calories from crops, even taking into account adaptation to a warming world, scientists said Wednesday.

As soon as 2050, this “moderate” scenario in which greenhouse gas emissions peak around 2040 and slowly taper off — a trajectory aligned with current trends — would see global losses of nearly eight percent.

And if carbon pollution worsens, the loss of calories across the same six staples — corn, wheat, rice, soybeans, sorghum and cassava — rises to nearly a quarter by century’s end, the researchers reported in Nature.

More generally, every additional degree Celsius of warming reduces the world’s ability to produce food from these crops by 120 calories per person per day, or nearly five percent of current daily consumption, they calculated.

“If the climate warms by three degrees, that’s basically like everyone on the planet giving up breakfast,” said co-author Solomon Hsiang, a professor at the Stanford Doerr School of Sustainability in California.

The steepest losses will occur at the extremes of the agricultural economy: in modern, Big Ag breadbaskets that currently enjoy some of the world’s best growing conditions, and in subsistence farming communities that typically rely of small cassava harvests.

North America would be hit hardest, losing a fifth of yields by 2100 in the moderate carbon pollution scenario, and two-fifths if emissions from burning fossil fuels continue apace.

Working with more than a dozen scientists, Hsiang and co-leader Andrew Hultgren, an assistant professor at the University of Urbana-Champaign, sifted through data from more than 12,000 regions in 55 countries.



– Erratic weather –



Previous calculations of how a warming world will impact crop yields generally failed to consider the ways in which farmers would adapt, such as switching crop varieties, shifting planting and harvesting dates, and altering fertiliser use.

The scientists estimated such adjustments would offset about a third of climate related losses over the next 75 years in the scenario of rising emissions, but that residual impacts would still be devastating.

“Any level of warming, even when accounting for adaptation, results in global output losses for agriculture,” said Hultgren.

With the planet about 1.5C hotter than preindustrial levels in the late 1900s, farmers in many regions are already experiencing longer dry spells, unseasonable heatwaves and erratic weather that undermines yields.

The nutritional value of most crops also declines with hotter temperatures, earlier research has shown.

The study revealed sharp variations in the impact of global warming on different crops and regions.

In the “worst-case” scenario of rising carbon emissions, corn yields would plummet 40 percent by 2100 across the grain belt of the United States, eastern China, central Asia, and the Middle East.

For soybeans, yields in the US would decline by half, and increase by a fifth in Brazil.

Wheat losses would drop by a fifth in eastern and western Europe, and by 30 to 40 percent in other wheat-growing regions: China, Russia and North America.

Cassava would be hit hard everywhere it’s grown.

“Although cassava does not make up a large portion of global agricultural revenues, it is an important subsistence crop in low- and middle-income countries,” the researchers pointed out.

Among the six crops examined, rice is the only one that stands to benefit in a warmer climate, mainly due to warmer nights.

 

Ancient groundwater records reveal regional vulnerabilities to climate change



New study led by Woods Hole Oceanographic Institution builds on earlier work and shows the Southwest may be more sensitive to drying than the Pacific Northwest



Woods Hole Oceanographic Institution

Extracting dissolved gases 

image: 

Extracting dissolved gases from 20,000 year-old groundwater flowing from a supply well on a farm in Eastern Washington.

 

view more 

Credit: Rebecca Tyne/Woods Hole Oceanographic Institution and the University of Manchester




Woods Hole, Mass. (June 17, 2025) — During the last ice age, storms soaked the now-arid Southwestern U.S., while today’s rainy Pacific Northwest remained relatively dry. As global temperatures rose and ice sheets retreated, those storms shifted north—reshaping the climate patterns that define both regions today.

New research published in Science Advances reveals that groundwater levels responded differently in the two regions during this dramatic shift. While the Pacific Northwest saw little change in water table depth despite increased rainfall, the Southwest experienced significant groundwater loss. The findings suggest that Southwestern aquifers—critical to millions of people—may be more vulnerable to future climate shifts.

“On average, climate models suggest the Southwestern U.S. may get drier while the Pacific Northwest may get wetter by the end of the century,” said Alan Seltzer, associate scientist at Woods Hole Oceanographic Institution (WHOI) and lead author of the study.

Seltzer and his co-authors, including seven WHOI-affiliated scientists, built new records of groundwater levels from the Last Glacial Termination, a period of warming, ice sheet loss, and major environmental change that occurred between 20,000 and 11,000 years ago.

“The last ice age gives us a window to explore groundwater dynamics that might be quite relevant to future change,” said Seltzer.

Groundwater is Earth’s largest source of usable freshwater, supplying up to half of the water people use for drinking, agriculture, and industry. But with millions of wells at risk of drying up due to our changing climate, understanding how groundwater responds to long-term climate shifts is critical for future planning.

Modern groundwater records are limited to just the last few centuries and are complicated by human activity. To examine longer-term trends, the research team analyzed fossil groundwater from 17 wells across Washington and Idaho, dating back as far as 20,000 years. Using a novel method developed by Seltzer, they measured isotopes of xenon and krypton—noble gases sensitive to gravitational separation—to calculate past water table depths.

The team’s analysis showed that the Pacific Northwest’s groundwater levels remained remarkably stable from the Last Ice Age through the early Holocene, despite increased precipitation. They combined the results with previous work led by Seltzer, which found water table levels in Southern California dropped sharply in response to a loss of precipitation during the deglaciation.

“Going back in time to large amplitude changes helps us understand the behavior of a system, like groundwater, which we may struggle to capture with short modern records,” Seltzer said.

To validate these findings, the researchers compared the ancient groundwater data to simulations from an Earth system model that includes large-scale groundwater processes. “The model gave almost exactly the same answer as the isotope measurements,” said Seltzer. “This was an exciting result that suggests even relatively simple groundwater models can capture key dynamics.”

The study not only underscores the vulnerability of Southwestern aquifers but also demonstrates how combining paleoclimate data with modern models can improve future water resource planning worldwide.

While this study focused on western North America, using these model simulations combined with the new insights from the ancient water table depth records, we were able to map out areas of concern globally,” said co-author Kris Karnauskas, who is an associate professor of Atmospheric and Oceanic Sciences at CU Boulder. “By going beyond just precipitation, these results should help direct research and adaptation efforts to regions with heightened water insecurity in the future.”

An associated study on fossil groundwater, led by Seltzer’s lab in collaboration with the University of Manchester, was published just a week prior and focused on geological insights from ancient groundwater in the Pacific Northwest.  Published in the journal Nature Geoscience, the study analyzes groundwater from 17 wells in the Palouse Basin Aquifer that spans the Washington-Idaho border, and makes use of a new analytical technique pioneered at WHOI to identify volcanic gas input to the aquifer despite a lack of modern volcanic or tectonic activity in the region. These findings could give scientists a better idea of the geological and chemical processes that take place deep inside the Earth, indicating that diffuse gas fluxes from the shallow mantle may occur broadly throughout volcanically inactive regions.

###

About Woods Hole Oceanographic Institution

The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Massachusetts, dedicated to marine research, engineering, and higher education. Established in 1930, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate an understanding of the ocean’s role in the changing global environment. Top scientists, engineers, and students collaborate on more than 800 concurrent projects worldwide—both above and below the waves—pushing the boundaries of knowledge and possibility. 

 

 

Arctic peatlands expanding as climate warms



University of Exeter
Drone image of the research team in an Arctic peatland 

image: 

Drone image of the research team in an Arctic peatland

view more 

Credit: Dan Charman / ICAAP




Peatlands across the Arctic are expanding as the climate warms, new research shows.

Scientists used satellite data, drones and on-the-ground observations to assess the edges of existing peatlands (waterlogged ecosystems that store vast amounts of carbon).

The study – led by the University of Exeter – found peatlands in the European and Canadian Arctic have expanded outwards in the last 40 years.

While this could slow climate change by storing carbon, the researchers warn that extreme future warming could cause widespread loss of peatlands – releasing that carbon and further accelerating the climate crisis.

“The Arctic has warmed faster than the rest of the planet, with average temperatures increasing by about 4°C in the last four decades,” said Dr Katherine Crichton.

“This has improved growing conditions for plants, causing ‘greening’ of the Arctic. We wanted to identify if this greening could be from peatland plant communities.

“We know from paleo records that warmer periods in Earth’s history led to more carbon being stored in peatlands.

“Our new study puts these pieces together to examine whether our warming climate is causing peatland expansion – and we find strong evidence that it is.”

Peatlands cover just 3% of Earth’s surface but they store about 600 billion tons of carbon – more than all the world’s forest biomass combined.

The Arctic has large peatland areas but these peter out in the far north, where harsh conditions limit plant growth.

In the new study, researchers examined 16 sites – a range of peatlands in both the low and high Arctic – and compared data from 1985-95 with the last 15-20 years.

They found strong evidence of expansion at more than two thirds of sites (measured by “peak-summer greening” – increased growth of peatland-forming plants at the edges of existing peatlands).

The largest changes were found in places with the highest increases in summer temperature, such as the Norwegian islands of Svalbard.

“Our findings suggest Arctic peatlands are an increasingly important natural carbon sink, at least in the near term,” said Professor Karen Anderson, from the Environment and Sustainability Institute on Exeter’s Penryn Campus in Cornwall.

“But if temperatures continue to rise, we are likely to see changes in rainfall, and we are not sure how sustainable new or existing peatlands will be. Plus we could see increases of methane emissions at the same time.

“So – while our study gives us some positive news – it does not detract from the urgent need to reduce greenhouse gas emissions and stabilise our climate.”

 

The story behind the study

This study took researchers on an unexpected journey that included COVID lockdowns, polar bear safety training and dragging a canoe overland.

Like many research projects, it started with pilot studies – one extracting and analysing peatland samples in Canada and Finland, the other testing “remote sensing” with drones and satellites.

The team wanted to combine these to find out how climate change is affecting Arctic peatlands. They started applying for funding in 2013, and got their first rejection in 2015. Two more rejections came the following year. In 2018, they finally got a grant – and the project started in summer 2019.

Dr Crichton used Google Earth Engine to identify possible study sites, and Professor Angela Gallego-Sala went on the first fieldwork expedition – to Svalbard, where she received training on avoiding encounters with hungry polar bears.

With the research finally making progress, COVID lockdowns halted fieldwork and lab work. While this hampered the project, Dr Crichton’s computer-based work could continue. “I was still at my desk using Google Earth Engine,” she said. “Lockdown didn’t make any difference to the work I was doing.”

So Dr Crichton continued identifying fieldwork sites, analysing data and applying for permits – paving the way for fieldwork in Canada in 2021-22. On one of those expeditions, Professor Gallego-Sala stayed at a basic research station on Bylot Island where the washing facility was a “half-frozen lake”. She said: “It was light all the time. You could do fieldwork all day long and all night if you wanted to.”

From that research station, the team visited remote sites via helicopter. Many sites had no name, and the pilot wanted names in order to arrange pick-ups – so sites got informal names including “Glacial Nirvana” and “Angela’s Paradise”. At each site, the team extracted peatland cores to learn about the history of the peatland and how it might be changing.

At Salluit in northern Canada, the team had an Inuit guide for expeditions out into the peatland – during which they saw wildlife including black bears and reindeer, and caught fish and mussels for dinner each evening. When the team laid out their plan one day, the guide shrugged and said: “You can go wherever you want.” He did not mention that their plan would leave their canoe stuck on a large area of sand at low tide.

The three female researchers had to push the stranded boat overland, while the guide sat in it. “We pushed it a long way through the sand,” Professor Gallego-Sala said. “It was pretty tough – but it was also hilarious, and we managed to get it out.”

“Meanwhile, I’m still sat at my computer by the way,” said Dr Crichton, laughing. But this work provided a crucial component – allowing comparison between peatland cores and long-term satellite data that shows peatland edges getting greener as vegetation spreads.

Professor Gallego-Sala added: “Going out for fieldwork is a short time in comparison to the rest of the work. There is lots of lab work to analyse the samples, then extensive data analysis before the findings can be written into a published paper.”

 

The study is part of a project called Increased Accumulation in Arctic Peatlands (ICAAP), funded by the Natural Environment Research Council.

The paper, published in the journal Communications Earth and Environment, is entitled: “Satellite data indicates recent Arctic peatland expansion with warming.”

 

Sea ice plays important role in variability of carbon uptake by Southern Ocean




University of East Anglia
Sea ice in the Southern Ocean. 

image: 

Sea ice in the Southern Ocean.

view more 

Credit: Elise Droste (UEA)





New research reveals the importance of winter sea ice in the year-to-year variability of the amount of atmospheric CO2 absorbed by a region of the Southern Ocean.

In years when sea ice lasts longer in winter, the ocean will overall absorb 20% more CO2 from the atmosphere than in years when sea ice forms late or disappears early. This is because sea ice protects the ocean from strong winter winds that drive mixing between the surface of the ocean and its deeper, carbon-rich layers.

The findings, based on data collected in a coastal system along the west Antarctic Peninsula, show that what happens in winter is crucial in explaining this variability in CO2 uptake.

The study was led by scientists at the University of East Anglia (UEA), in collaboration with colleagues from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI, Germany), British Antarctic Survey (BAS, UK) and Institute of Marine Research (IMR, Norway). It is published today in the journal Communications Earth & Environment.

The global ocean takes up about a quarter of all CO2 that humans emit into the atmosphere. The Southern Ocean is responsible for about 40% of this and the researchers wanted to know why it varies so much from year to year.

Lead author Dr Elise Droste, of UEA’s School of Environmental Sciences, said: “Our picture of the Southern Ocean’s carbon cycle is incomplete, and so we cannot predict whether its atmospheric CO2 uptake – and therefore its contribution to climate change mitigation – will increase, decrease, or remain the same in the future.

“Whatever it does, it will affect what our climate will look like and how fast it will change. To improve predictions, our work suggests that we need to look at how sea ice affects the exchange of carbon between the deep and shallow parts of the ocean. To do this, we need more wintertime observations in the Southern Ocean.”

Much of the Southern Ocean surrounding the west Antarctic Peninsula is covered by sea ice in winter, which disappears in spring and summer. In spring and summer, phytoplankton growth and melt water lead to low CO2 concentrations at the ocean surface. This allows the Southern Ocean to absorb large amounts of atmospheric CO2, significantly reducing the global impact of anthropogenic emissions.

In winter, as sea ice forms, the ocean underneath mixes with deeper waters that contain lots of ‘natural’ carbon that has been in the ocean for centuries. This can cause CO2 at the ocean surface to increase to the point where it can be released into the atmosphere.

Sea ice blocks a large amount of this CO2 ‘outgassing’. However, it is part of the natural seasonal cycle that some CO2 does escape the ocean. This seasonal balance means that the total amount of CO2 absorbed by the Southern Ocean within one year often depends on how much CO2 is absorbed in summer and how much is released in winter.

“We don’t have a good grasp on what is driving this year-to-year variability, which is making it difficult to fully understand the system and to improve the predictability of how the ocean’s COuptake will change in the future,” said Dr Droste. “One major reason is because we have relatively little data on the Southern Ocean, particularly in the wintertime.

“It is extremely challenging to collect observations in the harsh weather and sea conditions of the Southern Ocean, not to mention sea ice cover making much of it inaccessible, even for the strongest icebreaker. However, this study takes us a step in the right direction.”

The study draws on data for 2010-2020, a time series led and maintained by BAS, which collects year-round measurements along the west Antarctic Peninsula. At Rothera, the UK’s Antarctic research station, ocean scientists measured physical aspects of the seawater in Ryder Bay and collected samples for nutrient and CO2 analysis, carried out at both Rothera and UEA.

Using other physical and chemical data collected at the same time, the team was able to study why years with long sea ice duration differed from those with short sea ice duration.

Dr Hugh Venables, from BAS, said: “A series of ocean scientists have wintered at Rothera on the Antarctic Peninsula to collect these and other samples, from either a small boat or a sea ice sledge, to build a unique time series of year-round oceanographic conditions for the last 25 years.

“This important result shows the importance of this winter sampling and will hopefully lead to more year-round sampling in the Southern Ocean, both by humans and autonomous technology.”

Prof Dorothee Bakker, Professor in Marine Biogeochemistry at UEA, added: “The fact that this data has been collected throughout the year at the same location allows us to investigate which mechanisms are important to explain the year-to-year variability of CO2 uptake by the ocean at this particular location, but we can also use these insights to better understand how the rest of the Southern Ocean works.”

The study also involved scientists from the National Institute of Oceanography and Applied Geophysics (Italy) and University of Gothenburg (Sweden). It was supported by funding from the UK’s Natural Environment Research Council and European Union’s Horizon 2020 research and innovation programme.

‘Sea ice controls net ocean uptake of carbon dioxide by regulating wintertime stratification’, Elise Droste et al, is published in Communications Earth & Environment on June 18.


 

When Earth iced over, early life may have sheltered in meltwater ponds



Modern-day analogs in Antarctica reveal ponds teeming with life similar to early multicellular organisms




Massachusetts Institute of Technology

Meltwater Shelter 

image: 

A cyanobacterial mat fragment sampled from the periphery of a meltwater pond. 

view more 

Credit: Roger Summons




When the Earth froze over, where did life shelter? MIT scientists say one refuge may have been pools of melted ice that dotted the planet’s icy surface. 

In a study appearing in Nature Communications, the researchers report that 635 million to 720 million years ago, during periods known as “Snowball Earth,” when much of the planet was covered in ice, some of our ancient cellular ancestors could have waited things out in meltwater ponds. 

The scientists found that eukaryotes — complex cellular lifeforms that eventually evolved into the diverse multicellular life we see today — could have survived the global freeze by living in shallow pools of water. These small, watery oases may have persisted atop relatively shallow ice sheets present in equatorial regions. There, the ice surface could accumulate dark-colored dust and debris from below, which enhanced its ability to melt into pools. At temperatures hovering around 0 degrees Celsius, the resulting meltwater ponds could have served as habitable environments for certain forms of early complex life. 

The team drew its conclusions based on an analysis of modern-day meltwater ponds. Today in Antarctica, small pools of melted ice can be found along the margins of ice sheets. The conditions along these polar ice sheets are similar to what likely existed along ice sheets near the equator during Snowball Earth. 

The researchers analyzed samples from a variety of meltwater ponds located on the McMurdo Ice Shelf in an area that was first described by members of Robert Falcon Scott's 1903 expedition as “dirty ice.” The MIT researchers discovered clear signatures of eukaryotic life in every pond. The communities of eukaryotes varied from pond to pond, revealing a surprising diversity of life across the setting. The team also found that salinity plays a key role in the kind of life a pond can host: Ponds that were more brackish or salty had more similar eukaryotic communities, which differed from those in ponds with fresher waters. 

“We’ve shown that meltwater ponds are valid candidates for where early eukaryotes could have sheltered during these planet-wide glaciation events,” says lead author Fatima Husain, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “This shows us that diversity is present and possible in these sorts of settings. It’s really a story of life’s resilience.”

The study’s MIT co-authors include Schlumberger Professor of Geobiology Roger Summons and former postdoct Thomas Evans, along with Jasmin Millar of Cardiff University, Anne Jungblut at the Natural History Museum in London, and Ian Hawes of the University of Waikato in New Zealand. 

Polar plunge

Snowball Earth is the colloquial term for periods of time in Earth history during which the planet iced over. It is often used as a reference to the two consecutive, multi-million-year glaciation events which took place during the Cryogenian Period, which geologists refer to as the time between 635 and 720 million years ago. Whether the Earth was more of a hardened snowball or a softer “slushball” is still up for debate. But scientists are certain of one thing: Most of the planet was plunged into a deep freeze, with average global temperatures of minus 50 degrees Celsius. The question has been: How and where did life survive?

“We’re interested in understanding the foundations of complex life on Earth. We see evidence for eukaryotes before and after the Cryogenian in the fossil record, but we largely lack direct evidence of where they may have lived during,” Husain says. “The great part of this mystery is, we know life survived. We’re just trying to understand how and where.”

There are a number of ideas for where organisms could have sheltered during Snowball Earth, including in certain patches of the open ocean (if such environments existed), in and around deep-sea hydrothermal vents, and under ice sheets. In considering meltwater ponds, Husain and her colleagues pursued the hypothesis that surface ice meltwaters may also have been capable of supporting early eukaryotic life at the time. 

“There are many hypotheses for where life could have survived and sheltered during the Cryogenian, but we don’t have excellent analogs for all of them,” Husain notes. “Above-ice meltwater ponds occur on Earth today and are accessible, giving us the opportunity to really focus in on the eukaryotes which live in these environments.”

Small pond, big life

For their new study, the researchers analyzed samples taken from meltwater ponds in Antarctica. In 2018, Summons and colleagues from New Zealand traveled to a region of the McMurdo Ice Shelf in East Antarctica, known to host small ponds of melted ice, each just a few feet deep and a few meters wide. There, water freezes all the way to the seafloor, in the process trapping dark-colored sediments and marine organisms. Wind-driven loss of ice from the surface creates a sort of conveyer belt that brings this trapped debris to the surface over time, where it absorbs the sun’s warmth, causing ice to melt, while surrounding debris-free ice reflects incoming sunlight, resulting in the formation of shallow meltwater ponds.

The bottom of each pond is lined with mats of microbes that have built up over years to form layers of sticky cellular communities. 

“These mats can be a few centimeters thick, colorful, and they can be very clearly layered,” Husain says. 

These microbial mats are made up of cyanobacteria, prokaryotic, single-celled photosynthetic organisms that lack a cell nucleus or other organelles. While these ancient microbes are known to survive within some of the the harshest environments on Earth including meltwater ponds, the researchers wanted to know whether eukaryotes — complex organisms that evolved a cell nucleus and other membrane bound organelles — could also weather similarly challenging circumstances. Answering this question would take more than a microscope, as the defining characteristics of the microscopic eukaryotes present among the microbial mats are too subtle to distinguish by eye. 

To characterize the eukaryotes, the team analyzed the mats for specific lipids they make called sterols, as well as genetic components called ribosomal ribonucleic acid (rRNA), both of which can be used to identify organisms with varying degrees of specificity. These two independent sets of analyses provided complementary fingerprints for certain eukaryotic groups. As part of the team’s lipid research, they found many sterols and rRNA genes closely associated with specific types of algae, protists, and microscopic animals among the microbial mats. The researchers were able to assess the types and relative abundance of lipids and rRNA genes from pond to pond, and found the ponds hosted a surprising diversity of eukaryotic life. 

“No two ponds were alike,” Husain says. “There are repeating casts of characters, but they’re present in different abundances. And we found diverse assemblages of eukaryotes from all the major groups in all the ponds studied. These eukaryotes are the descendants of the eukaryotes that survived the Snowball Earth. This really highlights that meltwater ponds during Snowball Earth could have served as above-ice oases that nurtured the eukaryotic life that enabled the diversification and proliferation of complex life — including us — later on.”

This research was supported in part by the NASA Exobiology Program, the Simons Collaboration on the Origins of Life, and a MISTI grant from MIT-New Zealand.

###

Written by Jennifer Chu, MIT News