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Environment

Aug 20, 2019

Antarctica’s largest floating ice shelf is highly sensitive to warming of the ocean

Much of West Antarctica’s ice lies below sea level, and warming ocean temperatures may lead to runaway ice sheet retreat.

WRITTEN BY

Dan Lowry

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 with 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.

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.

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.

Cover photo: 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

 

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Environment

Sep 04, 2019

The Great Barrier Reef outlook is ‘very poor’. We have one last chance to save it.

It’s official. The outlook for the Great Barrier Reef has been downgraded from “poor” to “very poor” by the Australian government’s own experts.

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WRITTEN BY

Terry Hughes

That’s the conclusion of the latest five-yearly report from the Great Barrier Reef Marine Park Authority, released on Friday. The report assessed literally hundreds of scientific studies published on the reef’s declining condition since the last report was published in 2014.

The past five years were a game-changer. Unprecedented back-to-back coral bleaching episodes in 2016 and 2017, triggered by record-breaking warm sea temperatures, severely damaged two-thirds of the reef. Recovery since then has been slow and patchy.

Fish swimming among coral on the Great Barrier Reef.
AAP

Looking to the future, the report said “the current rate of global warming will not allow the maintenance of a healthy reef for future generations […] the window of opportunity to improve the reef’s long-term future is now”.

But that window of opportunity is being squandered so long as Australia’s and the world’s greenhouse gas emissions continue to rise.

The evidence on the reef’s condition is unequivocal

A logical national response to the outlook report would be a pledge to curb activity that contributes to global warming and damages the reef. Such action would include a ban on the new extraction of fossil fuels, phasing out coal-fired electricity generation, transitioning to electrified transport, controlling land clearing and reducing local stressors on the reef such as land-based runoff from agriculture.

But federal Environment Minister Sussan Ley’s response to the outlook report suggested she saw no need to take dramatic action on emissions, when she declared: “it’s the best managed reef in the world”.

Major coral bleaching events in 2016 and 2017 have devastated the reef.

The federal government’s lack of climate action was underscored by another dire report card on Friday. Official quarterly greenhouse gas figures showed Australia’s greenhouse gas emissions have risen to the highest annual levels since the 2012-13 financial year.

But rather than meaningfully tackle Australia’s contribution to climate change, the federal government has focused its efforts on fixing the damage wrought on the reef. For example as part of a A$444 million grant to the Great Barrier Reef Foundation, the government has allocated $100 million for reef restoration and adaptation projects over the next five years or so.

Solutions being supported by the foundation include a sunscreen-like film to float on the water to prevent light penetration, and gathering and reseeding coral spawn Separately, Commonwealth funds are also being spent on projects such as giant underwater fans to bring cooler water to the surface.

But the scale of the problem is much, much larger than these tiny interventions.


Climate change is not the only threat to the reef

The second biggest impact on the Great Barrier Reef’s health is poor water quality, due to nutrient and sediment runoff into coastal habitats. Efforts to address that problem are also going badly.

This was confirmed in a confronting annual report card on the reef’s water quality, also released by the Commonwealth and Queensland governments on Friday.

The Great Barrier Reef attained world heritage status in the 1980s.
AAP

It showed authorities have failed to reach water quality targets set under the Reef 2050 Plan – Australia’s long-term plan for improving the condition of the reef.

For example the plan sets a target that by 2025, 90% of sugarcane land in reef catchments should have adopted improved farming practices. However the report showed the adoption had occurred on just 9.8% of land, earning the sugarcane sector a grade of “E”.

So yes, the reef is definitely in danger

The 2019 outlook report and other submissions from Australia will be assessed next year when the UNESCO World Heritage Committee meets to determine if the Great Barrier Reef should be listed as “in danger” – an outcome the federal government will fight hard to avoid.

An in-danger listing would signal to the world that the reef was in peril, and put the federal government under greater pressure to urgently prevent further damage. Such a listing would be embarrassing for Australia, which presents itself as a world’s-best manager of its natural assets.

Environment activists engaged in a protest action to bring attention to the dangers facing the Great Barrier Reef.
AAP

The outlook report maintains that the attributes of the Great Barrier Reef
that led to its inscription as a world heritage area in 1981 are still intact, despite the loss of close to half of the corals in 2016 and 2017.

But by any rational assessment, the Great Barrier Reef is in danger. Most of the pressures on the reef are ongoing, and some are escalating – notably anthropogenic heating, also known as human-induced climate change.

Read more:
Great Barrier Reef Foundation chief scientist: science will lie at the heart of our decisions

And current efforts to protect the reef are demonstrably failing. For example despite an ongoing “control” program, outbreaks of the damaging crown-of-thorns starfish – triggered by poor water quality – have spread throughout the reef.

The federal government has recently argued that climate change should not form the basis for an in-danger listing, because rising emissions are not the responsibility of individual countries. The argument comes despite Australia having one of the highest per capita emissions rates in the world.

But as Australia’s greenhouse gas emissions continue to rise – an outcome supported by government policy – the continued downward trajectory of the Great Barrier Reef is inevitable.The Conversation

Terry Hughes, Distinguished Professor, James Cook University

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

Cover photo: A supplied image obtained Thursday, June 6, 2013 of holiday makers in the Great Barrier Reef, Tropical North Queensland, October 2008. ReefLive, a live 12-hour interactive online show about the reef, will be broadcast on YouTube from 10am (AEST) on Friday to coincide with World Ocean Day on Saturday. (AAP Image/Supplied by Tourism and Events Queensland, Richard Fitzpatrick) NO ARCHIVING, EDITORIAL USE ONLY

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