How Antarctic ice melt can be a tipping point for the whole planet’s climate

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Melting Antarctic ice can trigger effects on the other side of the globe.
NASA/Jane Peterson

Chris Turney, UNSW; Jonathan Palmer, UNSW; Peter Kershaw, Monash University; Steven Phipps, University of Tasmania, and Zoë Thomas, UNSW

Melting of Antarctica’s ice can trigger rapid warming on the other side of the planet, according to our new research which details how just such an abrupt climate event happened 30,000 years ago, in which the North Atlantic region warmed dramatically.

This idea of “tipping points” in Earth’s system has had something of a bad rap ever since the 2004 blockbuster The Day After Tomorrow purportedly showed how melting polar ice can trigger all manner of global changes.

But while the movie certainly exaggerated the speed and severity of abrupt climate change, we do know that many natural systems are vulnerable to being pushed into different modes of operation. The melting of Greenland’s ice sheet, the retreat of Arctic summer sea ice, and the collapse of the global ocean circulation are all examples of potential vulnerability in a future, warmer world.

Read more: Chasing ice: how ice cores shape our understanding of ancient climate.

Of course it is notoriously hard to predict when and where elements of Earth’s system will abruptly tip into a different state. A key limitation is that historical climate records are often too short to test the skill of our computer models used to predict future environmental change, hampering our ability to plan for potential abrupt changes.

Fortunately, however, nature preserves a wealth of evidence in the landscape that allows us to understand how longer time-scale shifts can happen.

Core values

One of the most important sources of information on past climate tipping points are the kilometre-long cores of ice drilled from the Greenland and Antarctic ice sheets, which preserve exquisitely detailed information stretching back up to 800,000 years.

The Greenland ice cores record massive, millennial-scale swings in temperature that have occurred across the North Atlantic region over the past 90,000 years. The scale of these swings is staggering: in some cases temperatures rose by 16℃ in just a few decades or even years.

Twenty-five of these major so-called Dansgaard–Oeschger (D-O) warming events have been identified. These abrupt swings in temperature happened too quickly to have been caused by Earth’s slowly changing orbit around the Sun. Fascinatingly, when ice cores from Antarctica are compared with those from Greenland, we see a “seesaw” relationship: when it warms in the north, the south cools, and vice versa.

Attempts to explain the cause of this bipolar seesaw have traditionally focused on the North Atlantic region, and include melting ice sheets, changes in ocean circulation or wind patterns.

But as our new research shows, these might not be the only cause of D-O events.

Our new paper, published today in Nature Communications, suggests that another mechanism, with its origins in Antarctica, has also contributed to these rapid seesaws in global temperature.

Tree of knowledge

The 30,000-year-old key to climate secrets.
Chris Turney, Author provided

We know that there have been major collapses of the Antarctic ice sheet in the past, raising the possibility that these may have tipped one or more parts of the Earth system into a different state. To investigate this idea, we analysed an ancient New Zealand kauri tree that was extracted from a peat swamp near Dargaville, Northland, and which lived between 29,000 and 31,000 years ago.

Through accurate dating, we know that this tree lived through a short D-O event, during which (as explained above) temperatures in the Northern Hemisphere would have risen. Importantly, the unique pattern of atmospheric radioactive carbon (or carbon-14) found in the tree rings allowed us to identify similar changes preserved in climate records from ocean and ice cores (the latter using beryllium-10, an isotope formed by similar processes to carbon-14). This tree thus allows us to compare directly what the climate was doing during a D-O event beyond the polar regions, providing a global picture.

The extraordinary thing we discovered is that the warm D-O event coincided with a 400-year period of surface cooling in the south and a major retreat of Antarctic ice.

When we searched through other climate records for more information about what was happening at the time, we found no evidence of a change in ocean circulation. Instead we found a collapse in the rain-bearing Pacific trade winds over tropical northeast Australia that was coincident with the 400-year southern cooling.

Read more: Two centuries of continuous volcanic eruption may have triggered the end of the ice age.

To explore how melting Antarctic ice might cause such dramatic change in the global climate, we used a climate model to simulate the release of large volumes of freshwater into the Southern Ocean. The model simulations all showed the same response, in agreement with our climate reconstructions: regardless of the amount of freshwater released into the Southern Ocean, the surface waters of the tropical Pacific nevertheless warmed, causing changes to wind patterns that in turn triggered the North Atlantic to warm too.

The ConversationFuture work is now focusing on what caused the Antarctic ice sheets to retreat so dramatically. Regardless of how it happened, it looks like melting ice in the south can drive abrupt global change, something of which we should be aware in a future warmer world.

Chris Turney, Professor of Earth Sciences and Climate Change, UNSW; Jonathan Palmer, Research Fellow, School of Biological, Earth and Environmental Sciences., UNSW; Peter Kershaw, Emeritus Professor, Earth, Atmosphere and Environment, Monash University; Steven Phipps, Palaeo Ice Sheet Modeller, University of Tasmania, and Zoë Thomas, Research Associate, UNSW

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


Contributions to sea-level rise have increased by half since 1993, largely because of Greenland’s ice

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Water mass enters the ocean from glaciers such as this along the Greenland coast.

John Church, UNSW; Christopher Watson, University of Tasmania; Matt King, University of Tasmania; Xianyao Chen, and Xuebin Zhang, CSIRO

Contributions to the rate of global sea-level rise increased by about half between 1993 and 2014, with much of the increase due to an increased contribution from Greenland’s ice, according to our new research.

Our study, published in Nature Climate Change, shows that the sum of contributions increased from 2.2mm per year to 3.3mm per year. This is consistent with, although a little larger than, the observed increase in the rate of rise estimated from satellite observations.

Globally, the rate of sea-level rise has been increasing since the 19th century. As a result, the rate during the 20th century was significantly greater than during previous millennia. The rate of rise over the past two decades has been larger still.

The rate is projected to increase still further during the 21st century unless human greenhouse emissions can be significantly curbed.

However, since 1993, when high-quality satellite data collection started, most previous studies have not reported an increase in the rate of rise, despite many results pointing towards growing contributions to sea level from the ice sheets of Greenland and Antarctica. Our research was partly aimed at explaining how these apparently contradictory results fit together.

Changes in the rate of rise

In 2015, we completed a careful comparison of satellite and coastal measurements of sea level. This revealed a small but significant bias in the first decade of the satellite record which, after its removal, resulted in a slightly lower estimate of sea-level rise at the start of the satellite record. Correcting for this bias partially resolved the apparent contradiction.

In our new research, we compared the satellite data from 1993 to 2014 with what we know has been contributing to sea level over the same period. These contributions come from ocean expansion due to ocean warming, the net loss of land-based ice from glaciers and ice sheets, and changes in the amount of water stored on land.

Previously, after around 2003, the agreement between the sum of the observed contributions and measured sea level was very good. Before that, however, the budget didn’t quite balance.

Using the satellite data corrected for the small biases identified in our earlier study, we found agreement with the sum of contributions over the entire time from 1993 to 2014. Both show an increase in the rate of sea-level rise over this period.

The total observed sea-level rise is the sum of contributions from thermal expansion of the oceans, fresh water input from glaciers and ice sheets, and changes in water storage on land.

After accounting for year-to-year fluctuations caused by phenomena such as El Niño, our corrected satellite record indicates an increase in the rate of rise, from 2.4mm per year in 1993 to 2.9mm per year in 2014. If we used different estimates for vertical land motion to estimate the biases in the satellite record, the rates were about 0.4mm per year larger, changing from 2.8mm per year to 3.2mm per year over the same period.

Is the whole the same as the sum of the parts?

Our results show that the largest contribution to sea-level rise – about 1mm per year – comes from the ocean expanding as it warms. This rate of increase stayed fairly constant over the time period.

The second-largest contribution was from mountain glaciers, and increased slightly from 0.6mm per year to 0.9mm per year from 1993 to 2014. Similarly, the contribution from the Antarctic ice sheet increased slightly, from 0.2mm per year to 0.3mm per year.

Strikingly, the largest increase came from the Greenland ice sheet, as a result of both increased surface melting and increased flow of ice into the ocean. Greenland’s contribution increased from about 0.1mm per year (about 5% of the total rise in 1993) to 0.85mm per year (about 25% in 2014).

Greenland’s contribution to sea-level rise is increasing due to both increased surface melting and flow of ice into the ocean.
NASA/John Sonntag, CC BY

The contribution from land water also increased, from 0.1mm per year to 0.25mm per year. The amount of water stored on land varies a lot from year to year, because of changes in rainfall and drought patterns, for instance. Despite this, rates of groundwater depletion grew whereas storage of water in reservoirs was relatively steady, with the net effect being an increase between 1993 and 2014.

So in terms of the overall picture, while the rate of ocean thermal expansion has remained steady since 1993, the contributions from glaciers and ice sheets have increased markedly, from about half of the total rise in 1993 to about 70% of the rise in 2014. This is primarily due to Greenland’s increasing contribution.

What is the future of sea level?

The satellite record of sea level still spans only a few decades, and ongoing observations will be needed to understand the longer-term significance of our results. Our results also highlight the importance of the continued international effort to better understand and correct for the small biases we identified in the satellite data in our earlier study.

Nevertheless, the satellite data are now consistent with the historical observations and also with projected increases in the rate of sea-level rise.

Ocean heat content fell following the 1991 volcanic eruption of Mount Pinatubo. The subsequent recovery (over about two decades) probably resulted in a rate of ocean thermal expansion larger than from greenhouse gases alone. Thus the underlying acceleration of thermal expansion from human-induced warming may emerge over the next decade or so. And there are potentially even larger future contributions from the ice sheets of Greenland and Antarctica.

The ConversationThe acceleration of sea level, now measured with greater accuracy, highlights the importance and urgency of cutting greenhouse gas emissions and formulating coastal adaptation plans. Given the increased contributions from ice sheets, and the implications for future sea-level rise, our coastal cities need to prepare for rising sea levels.

Sea-level rise will have significant impacts on coastal communities and environments.
Bruce Miller/CSIRO, CC BY

John Church, Chair professor, UNSW; Christopher Watson, Senior Lecturer, Surveying and Spatial Sciences, School of Land and Food, University of Tasmania; Matt King, Professor, Surveying & Spatial Sciences, School of Land and Food, University of Tasmania; Xianyao Chen, Professor, and Xuebin Zhang, Senior research scientist, CSIRO

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

The Himalaya shows off how fast it can melt, too


Last week, we got the news that the West Antarctic ice sheet is ditching us. Then, on Sunday, another fresh study told us that Greenland is also melting away rather fast. And now glaciology brings us a new report, on what’s going on at the so-called “third pole” (so called because it has more snow and ice than anywhere outside of the polar regions): the Himalaya mountain range. Seemingly unwilling to get left behind, it’s been shedding its icy stocks, too.

The report, from the International Centre for Integrated Mountain Development (ICMOD), found that Nepal’s glaciers lost 24 percent of their volume between 1977 and 2010. It did also find that the number of glaciers increased by 11 percent over that period, but it turns out even that’s not good news! It attributes the increase to the fact that the big glaciers tend to break into smaller ones once they become…

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Antarctic Ice Melt Increasing Rising Sea Levels

The link below is to an article that looks at the rising threat of the Antarctic ice melt to sea levels around the world.

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Article & Videos: Arctic Sea Ice Melt

The link below is to a very informative article (with videos embedded) reporting on the record sea ice melt in the Arctic.

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