New research shows that Antarctica’s largest floating ice shelf is highly sensitive to warming of the ocean



Since the last ice age, the ice sheet retreated over a thousand kilometres in the Ross Sea region, more than any other region on the continent.
Rich Jones, CC BY-ND

Dan Lowry, Victoria University of Wellington

Scientists have long been concerned about the potential collapse of the West Antarctic Ice Sheet and its contribution to global sea level rise. Much of West Antarctica’s ice lies below sea level, and warming ocean temperatures may lead to runaway ice sheet retreat.

This process, called marine ice sheet instability, has already been observed along parts of the Amundsen Sea region, where warming of the ocean has led to melting underneath the floating ice shelves that fringe the continent. As these ice shelves thin, the ice grounded on land flows more rapidly into the ocean and raises the sea level.

Although the Amundsen Sea region has shown the most rapid changes to date, more ice actually drains from West Antarctica via the Ross Ice Shelf than any other area. How this ice sheet responds to climate change in the Ross Sea region is therefore a key factor in Antarctica’s contribution to global sea level rise in the future.

Periods of past ice sheet retreat can give us insights into how sensitive the Ross Sea region is to changes in ocean and air temperatures. Our research, published today, argues that ocean warming was a key driver of glacial retreat since the last ice age in the Ross Sea. This suggests that the Ross Ice Shelf is highly sensitive to changes in the ocean.




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History of the Ross Sea

Since the last ice age, the ice sheet retreated more than 1,000km in the Ross Sea region – more than any other region on the continent. But there is little consensus among the scientific community about how much climate and the ocean have contributed to this retreat.

Much of what we know about the past ice sheet retreat in the Ross Sea comes from rock samples found in the Transantarctic Mountains. Dating techniques allow scientists to determine when these rocks were exposed to the surface as the ice around them retreated. These rock samples, which were collected far from where the initial ice retreat took place, have generally led to interpretations in which the ice sheet retreat happened much later than, and independently of, the rise in air and ocean temperatures following the last ice age.

But radiocarbon ages from sediments in the Ross Sea suggest an earlier retreat, more in line with when climate began to warm from the last ice age.

An iceberg floating in the Ross Sea – an area that is sensitive to warming in the ocean.
Rich Jones, CC BY-ND

Using models to understand the past

To investigate how sensitive this region was to past changes, we developed a regional model of the Antarctic ice sheet. The model works by simulating the physics of the ice sheet and its response to changes in ocean and air temperatures. The simulations are then compared to geological records to check accuracy.

Our main findings are that warming of the ocean and atmosphere were the main causes of the major glacial retreat that took place in the Ross Sea region since the last ice age. But the dominance of these two controls in influencing the ice sheet evolved through time. Although air temperatures influenced the timing of the initial ice sheet retreat, ocean warming became the main driver due to melting of the Ross Ice Shelf from below, similar to what is currently observed in the Amundsen Sea.

The model also identifies key areas of uncertainty of past ice sheet behaviour. Obtaining sediment and rock samples and oceanographic data would help to improve modelling capabilities. The Siple Coast region of the Ross Ice Shelf is especially sensitive to changes in melt rates at the base of the ice shelf, and is therefore a critical region to sample.




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Climate scientists explore hidden ocean beneath Antarctica’s largest ice shelf


Implications for the future

Understanding processes that were important in the past allows us to improve and validate our model, which in turn gives us confidence in our future projections. Through its history, the ice sheet in the Ross Sea has been sensitive to changes in ocean and air temperatures. Currently, ocean warming underneath the Ross Ice Shelf is the main concern, given its potential to cause melting from below.

Challenges remain in determining exactly how ocean temperatures will change underneath the Ross Ice Shelf in the coming decades. This will depend on changes to patterns of ocean circulation, with complex interactions and feedback between sea ice, surface winds and melt water from the ice sheet.

Given the sensitivity of ice shelves to ocean warming, we need an integrated modelling approach that can accurately reproduce both the ocean circulation and dynamics of the ice sheet. But the computational cost is high.

Ultimately, these integrated projections of the Southern Ocean and Antarctic ice sheet will help policymakers and communities to develop meaningful adaptation strategies for cities and coastal infrastructure exposed to the risk of rising seas.The Conversation

Dan Lowry, PhD candidate, Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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How solar heat drives rapid melting of parts of Antarctica’s largest ice shelf



Scientists measured the thickness and basal melt of the Ross Ice Shelf.
Supplied, CC BY-ND

Craig Stewart, National Institute of Water and Atmospheric Research

The ocean that surrounds Antarctica plays a crucial role in regulating the mass balance of the continent’s ice cover. We now know that the thinning of ice that affects nearly a quarter of the West Antarctic Ice Sheet is clearly linked to the ocean.

The connection between the Southern Ocean and Antarctica’s ice sheet lies in ice shelves – massive slabs of glacial ice, many hundreds of metres thick, that float on the ocean. Ice shelves grind against coastlines and islands and buttress the outflow of grounded ice. When the ocean erodes ice shelves from below, this buttressing action is reduced.

While some ice shelves are thinning rapidly, others remain stable, and the key to understanding these differences lies within the hidden oceans beneath ice shelves. Our recently published research explores the ocean processes that drive melting of the world’s largest ice shelf. It shows that a frequently overlooked process is driving rapid melting of a key part of the shelf.




Read more:
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Ocean fingerprints on ice sheet melt

Rapid ice loss from Antarctica is frequently linked to Circumpolar Deep Water (CDW). This relatively warm (+1C) and salty water mass, which is found at depths below 300 metres around Antarctica, can drive rapid melting. For example, in the south-east Pacific, along West Antarctica’s Amundsen Sea coast, CDW crosses the continental shelf in deep channels and enters ice shelf cavities, driving rapid melting and thinning.

Interestingly, not all ice shelves are melting quickly. The largest ice shelves, including the vast Ross and Filchner-Ronne ice shelves, appear close to equilibrium. They are largely isolated from CDW by the cold waters that surround them.

The satellite image shows that strong offshore winds drive sea ice away from the north-western Ross Ice Shelf, exposing the dark ocean surface. Solar heating warms the water enough to drive melting. Figure modified from https://www.nature.com/articles/s41561-019-0356-0.
Supplied, CC BY-ND

The contrasting effects of CDW and cold shelf waters, combined with their distribution, explain much of the variability in the melting we observe around Antarctica today. But despite ongoing efforts to probe the ice shelf cavities, these hidden seas remain among the least explored parts of Earth’s oceans.




Read more:
Climate scientists explore hidden ocean beneath Antarctica’s largest ice shelf


It is within this context that our research explores a new and hard-won dataset of oceanographic observations and melt rates from the world’s largest ice shelf.

Beneath the Ross Ice Shelf

In 2011, we used a 260 metre deep borehole that had been melted through the north-western corner of the Ross Ice Shelf, seven kilometres from the open ocean, to deploy instruments that monitor ocean conditions and melt rates beneath the ice. The instruments remained in place for four years.

The observations showed that far from being a quiet back water, conditions beneath the ice shelf are constantly changing. Water temperature, salinity and currents follow a strong seasonal cycle, which suggests that warm surface water from north of the ice front is drawn southward into the cavity during summer.

Melt rates at the mooring site average 1.8 metres per year. While this rate is much lower than ice shelves impacted by warm CDW, it is ten times higher than the average rate for the Ross Ice Shelf. Strong seasonal variability in the melt rate suggests that this melting hotspot is linked to the summer inflow.

Summer sea surface temperature surrounding Antarctica (a) and in the Ross Sea (b) showing the strong seasonal warming within the Ross Sea polynya. Figure modified from https://www.nature.com/articles/s41561-019-0356-0.
Supplied, CC BY-ND

To assess the scale of this effect, we used a high-precision radar to map basal melt rates across a region of about 8,000 square kilometres around the mooring site. Careful observations at around 80 sites allowed us to measure the vertical movement of the ice base and internal layers within the ice shelf over a one-year interval. We could then determine how much of the thinning was caused by basal melting.

Melting was fastest near the ice front where we observed short-term melt rates of up to 15 centimetres per day – several orders of magnitude higher than the ice shelf average rate. Melt rates reduced with distance from the ice front, but rapid melting extended far beyond the mooring site. Melting from the survey region accounted for some 20% of the total from the entire ice shelf.

The bigger picture

Why is this region of the shelf melting so much more quickly than elsewhere? As is so often the case in the ocean, it appears that winds play a key role.

During winter and spring, strong katabatic winds sweep across the western Ross Ice Shelf and drive sea ice from the coast. This leads to the formation of an area that is free of sea ice, a polynya, where the ocean is exposed to the atmosphere. During winter, this area of open ocean cools rapidly and sea ice grows. But during spring and summer, the dark ocean surface absorbs heat from the sun and warms, forming a warm surface pool with enough heat to drive the observed melting.

Although the melt rates we observe are far lower than those seen on ice shelves influenced by CDW, the observations suggest that for the Ross Ice Shelf, surface heat is important.

Given this heat is closely linked to surface climate, it is likely that the predicted reductions in sea ice within the coming century will increase basal melt rates. While the rapid melting we observed is currently balanced by ice inflow, glacier models show that this is a structurally critical region where the ice shelf is pinned against Ross Island. Any increase in melt rates could reduce buttressing from Ross Island, increasing the discharge of land-based ice, and ultimately add to sea levels.

While there is still much to learn about these processes, and further surprises are certain, one thing is clear. The ocean plays a key role in the dynamics of Antarctica’s ice sheet and to understand the stability of the ice sheet we must look to the ocean.The Conversation

Craig Stewart, Marine Physicist, National Institute of Water and Atmospheric Research

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Melting Himalayan glaciers: a big drop in a bucket that’s already full


Anthony Dosseto, University of Wollongong

A new report has warned that even if global warming is held at 1.5℃, we will still lose a third of the glaciers in the Hindu Kush-Himalaya (HKH) region. What does that mean for rivers that flow down these mountains, and the people who depend on them?

The HKH region is home to the tallest mountains on Earth, and also to the source of rivers that sustain close to 2 billion people. These rivers supply agriculture with water and with sediments that fertilise soils in valleys and the floodplain.

Some of these rivers are hugely culturally significant. The Ganges (or Ganga), for instance, which flows for more than 2,525km from the western Himalayas into the Bay of Bengal, is personified in Hinduism as the goddess Gaṅgā.

The Ganga River at Rishikesh, as it exits the Himalayas.
Anthony Dosseto



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When it rains, it pours… literally

Before we get to the effect of melting glaciers on Himalayan rivers, we need to understand where they get their water.

For much of Himalayas, rain falls mostly during the monsoon active between June and September. The monsoon brings heavy rain and often causes devastating floods, such as in northern India in 2013, which forced the evacuation of more than 110,000 people.

2013 floods in Uttarakhand, India.

But the summer monsoon is not the only culprit for devastating floods. Landslides can dam the river, and when this dam bursts it can cause dramatic, unpredictable flooding. Some of those events have been linked to folk stories of floods in many cultures around the world. In the Himalayas, a study tracking the 1,000-year history of large floods showed that heavy rainfall and landslide-dam burst are the main causes.

When they melt, glaciers can also create natural dams, which can then burst and send floods down the valley. In this way, the newly forecast melting poses an acute threat.

The potential problem is worsened still further by the Intergovernmental Panel on Climate Change’s prediction that the frequency of extreme rainfall events will also increase.

Come hell or high water

What will happen to Himalayan rivers when the taps are turned to high in this way? To answer this, we need to look into the past.

For tens of thousands of years, rivers have polished rocks and laid down sediments in the lower valleys of the mountain range. These sediments and rocks tell us the story of how the river behaves when the tap opens or closes.

Rock surfaces tell us where the river was carving into its bed.
Anthony Dosseto

Some experts propose that intense rain tends to trigger landslides, choking the river with sediments which are then dumped in the valleys. Others suggest that the supply of sediments to the river generally doesn’t change much even in extreme rainfall events, and that the main effect of the extra flow is that the river erodes further into its bed.

The most recent work supports the latter theory. It found that 25,000-35,000 years ago, when the monsoon was much weaker than today, sediments were filling up Himalayan valleys. But more recently (3,000-6,000 years ago), rock surfaces were exposed during a period of strong monsoon, illustrating how the river carved into its bed in response to higher rainfall.

Sediments laid down in Himalayan valleys support agriculture, but also tell us the ancient story of rivers that carried them.
Anthony Dosseto

So what does the past tell us about the future of Himalayan rivers? More frequent extreme rainfall events mean more floods, of course. But a stronger monsoon also means rivers will cut deeper into their beds, instead of fertilising Himalayan valleys and the Indo-Gangetic plain with sediments.




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What about glaciers melting? For as long as there are glaciers, this will increase the amount of meltwater in the rivers each spring (until 2060, according the report, after which there won’t be any meltwater to talk about). So this too will contribute to rivers carving into their beds instead of distributing sediments. It will also increase the risk of flooding from outburst of glacial lake dams.

So what is at stake? The melting glaciers? No. Given thousands or millions of years, it seems likely that they will one day return. But on a more meaningful human timescale, what is really at stake is us – our own survival. Global warming is reducing our resources, and making life more perilous along the way. The rivers of the Himalayas are just one more example.The Conversation

Anthony Dosseto, Associate Professor, University of Wollongong

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Satellites reveal melting of rocks under volcanic zone, deep in Earth’s mantle



File 20170705 9733 ityqvm
Mount Ngauruhoe, in the foreground, and Mount Ruapehu are two of the active volcanoes in the Taupo volcanic zone.
Guillaume Piolle/Wikimedia Commons, CC BY-ND

Simon Lamb, Victoria University of Wellington and Timothy Stern, Victoria University of Wellington

Volcanoes erupt when magma rises through cracks in the Earth’s crust, but the exact processes that lead to the melting of rocks in the Earth’s mantle below are difficult to study.

In our paper, published today in the journal Nature, we show how it is possible to use satellite measurements of movements of the Earth’s surface to observe the melting process deep below New Zealand’s central North Island, one of the world’s most active volcanic regions.

Rifting in the Taupo volcanic zone

The solid outer layer of the Earth is known as the crust, and this overlies the Earth’s mantle. But these layers are not fixed. They are broken up into tectonic plates that slowly move relative to each other.

It is along the boundaries of the tectonic plates that most of the geological action at the Earth’s surface occurs, such as earthquakes, volcanic activity and mountain building. This makes New Zealand a particularly dynamic place, geologically speaking, because it straddles the boundary between the Australian and Pacific plates.

The central region of the North Island is known as the Taupo volcanic zone, or TVZ. It is named after Lake Taupo, the flooded crater of the region’s largest volcano, and it has been active for two million years. Several volcanoes continue to erupt regularly.

The TVZ is the southern tip of a zone of expansion, or rifting, in the Earth’s crust that extends offshore for thousands of kilometres, all the way north in the Pacific Ocean to Tonga. Offshore, this takes place through sea floor spreading in the Havre Trough, creating both new oceanic crust and a narrow sliver of a plate right along the edge of the Australian tectonic plate. Surprisingly, this spreading is going on at the same time as the adjacent Pacific tectonic plate is sliding beneath the Australian plate in a subduction zone, triggering some of the major earthquakes in the region.

Sea floor spreading results in melting of the Earth’s mantle, but it is very difficult to observe this process directly in the deep ocean. However, sea floor spreading in the Havre Trough transitions abruptly onshore into the volcanic activity in the TVZ. This provides an opportunity to observe the melting in the Earth’s mantle on land.

Lake Taupo is the caldera of the region’s largest volcano.
NASA/Wikimedia Commons, CC BY-ND

In general, volcanic activity happens whenever there is molten rock at depth, and therefore the volcanism in the North Island indicates vast volumes of molten rock beneath the surface. However, it has been a tricky problem to understand exactly what is causing the melting in the first place, because the underlying rocks are buried by thick layers of volcanic material.

We have tackled this problem using data from Global Positioning System (GPS) sensors, some of which form part of New Zealand’s GeoNet network and some that have been used in measurement campaigns since 1995. The sensors measure horizontal and vertical shifts in the Earth’s surface to millimetre precision, and our research is based on data collected over the past two decades.

Bending of the earth’s surface

The GPS measurements in the Taupo volcanic zone reveal that it is widening east-west at a rate of 6-15 millimetres per year – in other words, the region, overall, is expanding, as we anticipated from our previous geological understanding. But it was surprising to discover that, at least for the past 15 years, a roughly 70-kilometre stretch is undergoing strong horizontal contraction and is also rapidly subsiding, quite the opposite of what one might anticipate.

Also unexpectedly, the contracting zone is surrounded by regions that are expanding, but also uplifting. Trying to make sense of these observations turned out to be the key to our new insight into the process of melting beneath the TVZ.

We found that the pattern of contraction and subsidence, together with expansion and uplift, in the context of the overall rifting of the TVZ, could be explained by a simple model that involves the bending and curving of an elastic upper crust, pulled downwards or pushed upwards by an underlying vertical driving force. The size of the region that is behaving like this, extending for about 100 kilometres in width and 200 kilometres in length, requires this force to originate nearly 20 kilometres underground, in the Earth’s mantle.

This diagram illustrates a patch of suction stress along the axis of the underlying upwelling mantle flow beneath the Taupo volcanic zone.
Simon Lamb, CC BY-ND

Melting the mantle

When tectonic plates drift apart on the sea floor, the underlying mantle rises up to fill the gap. This upwelling triggers melting, and the reason for this is that hot, but solid, mantle rocks undergo a reduction in pressure as they move upwards and closer to the Earth’s surface. This drop in pressure, rather than a change in temperature, begins the melting of the mantle.

But there is another property of this upwelling mantle flow, because it also creates a suction force that pulls down the overlying crust. This force comes about because as part of the flow, the rocks have to effectively “turn a corner” near the surface from a predominantly vertical flow to a predominantly horizontal one.

It turns out that the strength of this force depends on how stiff or sticky the mantle rocks are, measured in terms of viscosity (it is difficult to drive the flow of highly viscous or sticky fluids, but easy in runny ones).

Experimental studies have shown that the viscosity of rocks deep in the Earth is very sensitive to how much molten material they contain, and we propose that changes in the amount of melt provide a powerful mechanism to change the viscosity of the upwelling mantle. If mantle rocks don’t contain much melt, they will be much stickier, causing the overlying crust to be pulled down rapidly. If the rocks have just melted, then this makes the flow of the rocks runnier, allowing the overlying crust to spring back up again.

We also know that the movements that we observe at the surface with GPS must be relatively short lived, geologically speaking, lasting for no more than a few hundred or few thousand years. Otherwise they would result in profound changes to the landscape and we have no evidence for that.

Using GPS, we can not only measure the strength of the suction force, but we can “see” where, for how long, and by how much the underlying mantle is melting. This melt will eventually rise up through the crust to feed the overlying volcanoes.

This research helps us to understand how volcanic systems work on a variety of time scales, from human to geological. In fact, it may be that the GPS measurements made over just the last two decades have captured a change in the amount of mantle melt at depth, which could herald the onset of increased volcanic activity and associated risk in the future. But we don’t have measurements over a long enough time period yet to make any confident predictions.

The ConversationThe key point here is, nevertheless, that we have entered a new era whereby satellite measurements can be used to probe activity 20 kilometres beneath the Earth’s surface.

Simon Lamb, Associate Professor in Geophysics, Victoria University of Wellington and Timothy Stern, Professor of Geophysics, Victoria University of Wellington

This article was originally published on The Conversation. Read the original article.

Volcanoes under the ice: melting Antarctic ice could fight climate change



File 20170615 24988 wlh6r4
Furious winds keep the McMurdo Dry Valleys in Anarctica free of snow and ice. Calcites found in the valleys have revealed the secrets of ancient subglacial volcanoes.
Stuart Rankin/Flickr, CC BY-NC

Silvia Frisia, University of Newcastle

Iron is not commonly famous for its role as a micronutrient for tiny organisms dwelling in the cold waters of polar oceans. But iron feeds plankton, which in turn hold carbon dioxide in their bodies. When they die, the creatures sink to the bottom of the sea, safely storing that carbon.

How exactly the iron gets to the Southern Ocean is hotly debated, but we do know that during the last ice age huge amounts of carbon were stored at the bottom of the Southern Ocean. Understanding how carbon comes to be stored in the depth of the oceans could help abate CO2 in the atmosphere, and Antarctica has a powerful role.

Icebergs and atmospheric dust are believed to have been the major sources of this micronutrient in the past. However, in research published in Nature Communications, my colleagues and I examined calcite crusts from Antarctica, and found that volcanoes under its glaciers were vital in delivering iron to the ocean during the last ice age.

Today, glacial meltwaters from Greenland and the Antarctic peninsula supply iron both in solution and as tiny particles (less than 0.0001mm in diameter), which are readily consumed by plankton. Where glaciers meet bedrock, minute organisms can live in pockets of relatively warm water. They are able to extract “food” from the rock, and in doing so release iron, which then can be carried by underwater rivers to the sea.

Volcanic eruptions under the ice can create underwater subglacial lakes, which, at times, discharge downstream large masses of water that travel to the ice margin and beyond, carrying with them iron in particle and in solution.

The role of melting ice in climate change is as yet poorly understood. It’s particularly pertinent as scientists predict the imminent collapse of part of the Larsen C ice shelf.

Researchers are also investigating how to reproduce natural iron fertilisation in the Southern Ocean and induce algal blooms. By interrogating the volcanic archive, we learn more about the effect that iron fertilisation from meltwater has on global temperatures.

A polished wafer of the subglacial calcites. The translucent, crystalline layers formed while in pockets of water, providing nourishment to microbes. The opaque calcite with rock fragments documents a period when waters discharged from a subglacial lake formed by a volcanic eruption, carrying away both iron in solution and particles of iron.
Supplied

The Last Glacial Maximum

During the Last Glacial Maximum, a period 27,000 to 17,000 years ago when glaciers were at their greatest extent worldwide, the amount of CO2 in the atmosphere was lowered to 180 parts per million (ppm) relative to pre-industrial levels (280 ppm).

Today we are at 400 ppm and, if current warming trends continue, a point of no return will be reached. The global temperature system will return to the age of the dinosaurs, when there was little difference in temperature from the equator to the poles.

If we are interested in providing a habitable planet for our descendants, we need to mitigate the quantity of carbon in the atmosphere. Blooms of plankton in the Southern Ocean boosted by iron fertilisation were one important ingredient in lowering CO2 in the Last Glacial Maximum, and they could help us today.

The Last Glacial Maximum had winds that spread dust from deserts and icebergs carrying small particles into the Southern Ocean, providing the necessary iron for algal blooms. These extreme conditions don’t exist today.

Hidden volcanoes

Neither dust nor icebergs alone, however, explain bursts of productivity recorded in ocean sediments in the Last Glacial Maximum. There was another ingredient, only discovered in rare archives of subglacial processes that could be precisely dated to the Last Glacial Maximum.

Loss of ice in Antartica’s Dry Valleys uncovered rusty-red crusts of calcite plastered on glacially polished rocks. The calcites have tiny layers that can be precisely dated by radiometric techniques.

A piece of subglacial calcite coating pebbles. This suggests that the current transporting the pebbles was quite fast, like a mountain stream. The pebbles were deposited at the same time as the opaque layer in the calcite formed.
Supplied

Each layer preserves in its chemistry and DNA a record of processes that contributed to delivering iron to the Southern Ocean. For example, fluorine-rich spherules indicate that underwater vents created by volcanic activity injected a rich mixture of minerals into the subglacial environment. This was confirmed by DNA data, revealing a thriving community of thermophiles – microorganisms that live in very hot water only.

Then, it became plausible to hypothesise that volcanic eruptions occurred subglacially and formed a subglacial lake, whose waters ran into an interconnected system of channels, ultimately reaching the ice margin. Meltwater drained iron from pockets created where ice met bedrock, which then reached the ocean – thus inducing algal blooms.

We dated this drainage activity to a period when dust flux does not match ocean productivity. Thus, our study indicates that volcanoes in Antarctica had a role in delivering iron to the Southern Ocean, and potentially contributed to lowering CO2 levels in the atmosphere.

The ConversationOur research helps explain how volcanoes act on climate change. But it also uncovers more about iron fertilisation as a possible way to mitigate global warming.

Silvia Frisia, Associate Professor, School of Environmental and Life Sciences , University of Newcastle

This article was originally published on The Conversation. Read the original article.