If warming exceeds 2°C, Antarctica’s melting ice sheets could raise seas 20 metres in coming centuries

During the Pliocene, up to one third of Antarctica’s ice sheet melted, causing sea-level rise of 20 metres.
from http://www.shutterstock.com, CC BY-ND

Georgia Rose Grant, GNS Science and Timothy Naish, Victoria University of Wellington

We know that our planet has experienced warmer periods in the past, during the Pliocene geological epoch around three million years ago.

Our research, published today, shows that up to one third of Antarctica’s ice sheet melted during this period, causing sea levels to rise by as much as 20 metres above present levels in coming centuries.

We were able to measure past changes in sea level by drilling cores at a site in New Zealand, known as the Whanganui Basin, which contains shallow marine sediments of arguably the highest resolution in the world.

Using a new method we developed to predict the water level from the size of sand particle moved by waves, we constructed a record of global sea-level change with significantly more precision than previously possible.

The Pliocene was the last time atmospheric carbon dioxide concentrations were above 400 parts per million and Earth’s temperature was 2°C warmer than pre-industrial times. We show that warming of more than 2°C could set off widespread melting in Antarctica once again and our planet could be hurtling back to the future, towards a climate that existed three million years ago.

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Overshooting the Paris climate target

Last week we saw unprecedented global protests under the banner of Greta Thunberg’s #FridaysForFuture climate strikes, as the urgency of keeping global warming below the Paris Agreement target of 2°C hit home. Thunberg captured collective frustration when she chastised the United Nations for not acting earlier on the scientific evidence. Her plea resonated as she reminded us that:

With today’s emissions levels, that remaining CO₂ budget [1.5°C] will be entirely gone in less than eight and a half years.

At the current rate of global emissions we may be back in the Pliocene by 2030 and we will have exceeded the 2°C Paris target. One of the most critical questions facing humanity is how much and how fast global sea levels will rise.

According to the recent special report on the world’s oceans and cryosphere by the Intergovernmental Panel on Climate Change (IPCC), glaciers and polar ice sheets continue to lose mass at an accelerating rate, but the contribution of polar ice sheets, in particular the Antarctic ice sheet, to future sea level rise remains difficult to constrain.

If we continue to follow our current emissions trajectory, the median (66% probability) global sea level reached by the end of the century will be 1.2 metres higher than now, with two metres a plausible upper limit (5% probability). But of course climate change doesn’t magically stop after the year 2100.

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Drilling back to the future

To better predict what we are committing the world’s future coastlines to we need to understand polar ice sheet sensitivity. If we want to know how much the oceans will rise at 400ppm CO₂, the Pliocene epoch is a good comparison.

Back in 2015, we drilled cores of sediment deposited during the Pliocene, preserved beneath the rugged hill country at the Whanganui Basin. One of us (Timothy Naish) has worked in this area for almost 30 years and identified more than 50 fluctuations in global sea level during the last 3.5 million years of Earth’s history. Global sea levels had gone up and down in response to natural climate cycles, known as Milankovitch cycles, which are caused by long-term changes in Earths solar orbit every 20,000, 40,000 and 100,000 years. These changes in turn cause polar ice sheets to grow or melt.

While sea levels were thought to have fluctuated by several tens of metres, up until now efforts to reconstruct the precise amplitude had been thwarted by difficulties due to Earth deformation processes and the incomplete nature of many of the cycles.

Our research used a well-established theoretical relationship between the size of the particles transported by waves on the continental shelf and the depth to the seabed. We then applied this method to 800 metres of drill core and outcrop, representing continuous sediment sequences that span a time period from 2.5 to 3.3 million years ago.

We show that during the Pliocene, global sea levels regularly fluctuated between five to 25 metres. We accounted for local tectonic land movements and regional sea-level changes caused by gravitational and crustal changes to determine the sea-level estimates, known as the PlioSeaNZ sea-level record. This provides an approximation of changes in global mean sea level.

Antarctica’s contribution to sea-level rise

Our study also shows that most of the sea-level rise during the Pliocene came from Antarctica’s ice sheets. During the warm Pliocene, the geography of Earth’s continents and oceans and the size of polar ice sheets were similar to today, with only a small ice sheet on Greenland during the warmest period. The melting of the Greenland ice sheet would have contributed at most five metres to the maximum 25 metres of global sea-level rise recorded at Whanganui Basin.

Of critical concern is that over 90% of the heat from global warming to date has gone into the ocean. Much of it has gone into the Southern Ocean, which bathes the margins of Antarctica’s ice sheet.

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Already, we are observing warm circumpolar deep water upwelling and entering ice shelf cavities in several sites around Antarctica today. Along the Amundsen Sea coast of West Antarctica, where the ocean has been heating the most, the ice sheet is thinning and retreating the fastest. One third of Antarctica’s ice sheet — the equivalent to up to 20 metres of sea-level rise — is grounded below sea level and vulnerable to widespread collapse from ocean heating.

Our study has important implications for the stability and sensitivity of the Antarctic ice sheet and its potential to contribute to future sea levels. It supports the concept that a tipping point in the Antarctic ice sheet may be crossed if global temperatures are allowed to rise by more than 2℃. This could result in large parts of the ice sheet being committed to melt-down over the coming centuries, reshaping shorelines around the world.

Georgia Rose Grant, Postdoctoral Research Assistant, Paleontology Team, GNS Science and Timothy Naish, Professor, Victoria University of Wellington

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

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Melting Antarctic ice sheets and sea level rise: a warning from the future

Andrew Glikson, Australian National University

The remote location of the Antarctic and Greenland polar ice sheets may leave us with the impression that developments in these regions have little effect on the climate and life in the temperate zones of the Earth, where most of us live. We may therefore be forgiven for asking why should we care when these changes are projected to unfold over tens to hundreds of years.

However, the stability of the polar regions is critical for maintaining a planet with the conditions that allowed the emergence of humans, agriculture and civilisation, as well as many other species. The polar ice sheets serve as “thermostats” of global temperatures from which cold air and cold ocean currents emanate, moderating the effects of solar radiation. The ice sheets regulate sea levels, store volumes of ice whose melting would raise sea level by up to 61 metres.

Unfortunately, what’s happening with the polar ice sheets now ought to warn humanity of what is to come.

For example, a recent paper suggested that melting Antarctic ice sheets could lead to 0.6-3.0 m of sea level rise by the year 2300. This is based on modelling of greenhouse gas emissions out to 2300.

If greenhouse gas emissions continue unchecked, the world may warm by 8–10℃ by 2300. Such a temperature rise could raise sea levels by tens of meters over hundreds of years.

The recent paper only looked at sea level rise from melting Antarctic ice sheets and does not take into account sea level rise contributions from the Greenland ice sheet (currently about 280 billion tonnes per year), which would more than double the Antarctic contribution.

Antarctic warming: Red represents areas where temperatures have increased the most during the last 50 years, particularly in West Antarctica.
NASA

Peering into the past to see the future

Much of the discussion in the paper and related papers appears to assume linear global warming – that is, little change to the rate of warming over time.

Little mention is made of feedbacks which could increase the rate of warming. Such feedbacks could arise from reducing albedo, where solar radiation usually strongly reflected by ice is replaced by strong absorption by water.

Other feedback processes associated with warming include methane release from permafrost and bogs; loss of vegetation; and fires.

In a recent article, former NASA climate scientist James Hansen and a large group of climate scientists point to observations arising from detailed studies of the recent history of the atmosphere-ocean-ice sheet system.

The climate records of the past — specifically, the Holocene (from about 10,000 years ago) and the Eemian interglacial period (about 115,000 to 130,000 years ago) — are closely relevant to future climate projections. These records include evidence for rapid disintegration of ice sheets in contact with the oceans as a result of feedback processes resulting in sea level rise to 5-9 m above current levels. All this during a period when mean global temperatures were near to only 1℃ above pre-industrial temperatures.

Sea levels reflect the overall global temperature and thus of global climate conditions. As shown by the position of the circles in the chart below, the ratio of sea level rise (SL) to temperature rise (TR) during the glacial-interglacial cycles was approximately between 10-15 metres per 1℃.

Plots of Temperature rise (relative to the pre-industrial age) vs relative sea level rise in (meters).

By contrast from around 1800 to the present sea level rose by an approximate ratio of 0.2-0.3 m per 1℃. This suggests significant further rise towards an equilibrium state between sea level and temperature. Thus, the points in the right-hand circle represent long-term temprature-sea level equilibria in the past while points in the left-hand circle represent where we’re at now, namely at an incipient stage moving toward future temprature-sea level equilibrium.

Why should long term climate change matter?

Due to the extreme rate of CO₂ and temperature rise during the 20th century relative to earlier events and the non-linearity of climate change trends the timing of sea level rise may be difficult to estimate.

Even on conservative estimates, current global warming is bound to have major consequences for human civilisation and for nature, as follows:

• Further melting of the ice sheets will destroy the climate conditions which allowed agriculture and the rise of civilisation in the first place.

• The lower parts of the world’s great rivers (Po, Rhine, Nile, Ganges, Indus, Mekong, Yellow, Mississippi, Amazon), where more than 3 billion people live and the bulk of agriculture and industry are located, sit no more than a few metres above sea level.

• Further melting of the Antarctic and Greenland ice sheets can only result in sea level rises on the scale of tens of metres, changing the continent-ocean map of Earth.

Global temperatures have already risen 0.9℃ and continental temperatures 1.5℃ degrees above pre-industrial levels. If we account for the cooling effect of sulphur aerosols from industrial pollution, greenhouse gases have already contributed 2℃ of global warming. The current rate of global warming, faster than any observed in the geological record, is already having a major effect in many parts of the world in terms of droughts, fires, and storms.

According to James Hansen burning all the fossil fuels on Earth would result in warming of 20℃ over land areas and a staggering 30℃ at the poles, making “most of the planet uninhabitable by humans”.

In 2009 Joachim Hans Schellnhuber, Director of the Potsdam Climate Impacts Institute and Climate Advisor to the German Government, stated: “We’re simply talking about the very life support system of this planet”, constituting one of the most critical warnings science has ever issued to our species.

Mitigation plans proposed by governments would slow down the rate of carbon emissions but continuing emissions as well as feedbacks from ice melt, warming oceans, methane release and fires would continue to push temperatures upwards.

An effective technology required for global cooling efforts, if technically possible, would require investment on a scale not less than the trillions of dollars currently poured into armaments and war in the name of defence (more than $1.6 trillion in 2014).

Which planet do current decision makers think we are living on?

Andrew Glikson, Earth and paleo-climate scientist, Australian National University

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