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

File 20170625 13461 lezup7
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.

Cabinet papers 1992-93: Australia reluctant while world moves towards first climate treaty

Marc Hudson, University of Manchester

Cabinet papers from 1992 and 1993 released today by the National Archives of Australia confirm that Australia was a reluctant player in international discussions about climate change and environmental issues under Prime Minister Paul Keating.

Internationally, it was an exciting time for the environment. In June 1992, the UN Earth Summit was held in Rio de Janeiro. Here the world negotiated the UN Framework Convention on Climate Change (which last year gave us the Paris Agreement) and opened the Convention on Biological Diversity for signing.

So what was Australia doing?

Australia stumbles towards climate policy

Domestically, the focus was on Ecologically Sustainable Development (ESD), a policy process begun by Prime Minister Bob Hawke. Working groups made up of corporate representatives, environmentalists and bureaucrats had beavered away and produced hundreds of recommendations.

By the final report in December 1991, the most radical recommendations (gasp – a price on carbon!) had been weeded out. Democrats Senator John Coulter warned of bureaucratic hostility to the final recommendations. Keating replaced Hawke in the same month.

The August 1992 meeting, where the ESD policies were meant to be agreed upon, was so disastrous that the environmentalists walked out and even the corporates felt aggrieved.

Two interim reports on the ESD process from the cabinet papers fill in some of the detail.

The first interim report, in March 1992, said that government departments had not been able to identify which recommendations to take on board. Cabinet moved the process on, but the only policies on the table were those that involved:

…little or no additional cost, cause minimal disruption to industry or the community, and which also offer benefits other than greenhouse related.

By May, federal ministers were told that the states and territories weren’t committed to either ESD or greenhouse gas policies.

The policy process rumbled on after the walkout, finally producing a National Strategy for Ecologically Sustainable Development and a National Greenhouse Response Strategy. The greenhouse strategy contained only – surprise! – toothless voluntary measures, which proved ineffective in keeping emissions down to 1990 levels.

The November 1992 minutes mildly note that:

Most major interest groups have voiced concerns about their lack of involvement in the drafting of the NGRS [greenhouse strategy] document. Officials made provision for community input through the public comment process and a public consultative forum held in August [the one the environmentalists walked out of]. Reaction from conservation groups is likely to be negative, given the limited changes made to many of the responses in the revised strategy. They are likely to want to see more concerted efforts in areas such as fuel efficiency and renewable energy sources.


With equal prescience, the document warns:

Coal producers and resource-intensive industries (eg. aluminium) may express concern about their prospects in the medium to long term.

There are not many surprises here. The dithering over climate and environmental policies has been well covered by Clive Hamilton, David Cox, Joan Staples and numerous academic papers (see here, here, here, and here).

And while we won’t know officially who said what for another 30 years, there are tantalising hints in Neal Blewett’s A Cabinet Diary. Published in 1999, it reveals the antagonism between the environment minister and others in the Keating cabinet.

The international stage

International climate policy was dominated by the US threat, under President George Bush senior, not to attend the Earth Summit if the UN Framework Convention on Climate Change (UNFCCC) included specific emission-reduction targets. The US attended, and the UNFCCC didn’t include targets.

In Australia, the cabinet papers point out, not for the first or last time, that:

Australia is the only developed megadiverse country; it is a major user and exporter of greenhouse gas producing fossil fuels and energy intensive products; it could be significantly affected by global environmental change.

In May 1992 cabinet endorsed in principle support for the UNFCCC. There are three ironies here.

First, it was a major concern that the media statement to accompany Environment Minister Ros Kelly’s signing should be amended to include the fact that:

The Convention does not bind any signatory to meet any greenhouse gas target by a specified date.

Second, the minutes note that:

A decision by Australia not to sign the Convention would be criticised by domestic environment interests and could also attract international criticism, particularly in the Pacific region.

In later years, Prime Minister John Howard would not worry about this when repeatedly nixing ratification of the Kyoto Protocol.

Third, an emphasis on assisting developing countries in the Asia-Pacific region with climate adaptation looks odd given there had been zero mention of greenhouse gases in a March 1992 discussion document of aid to Cambodia (that country is feeling the effects already).

Keating’s willingness to let Kelly sign the convention may have been related to the following:

The Convention contains several safeguards which protect Australia’s interests … [A]llowance is made for “the differences in Parties’ starting points and approaches, economic structures and resource bases, and the need to maintain strong and sustainable economic growth, available technologies and other individual circumstances”. Additionally, Parties are obliged to take into consideration the situation of Parties with economies that are highly dependent on the production, processing, export and use of fossil fuels. These two provisions will give relevant countries, including Australia, flexibility in fulfilling their obligations under the Convention.

And they probably thought they had more time than they actually did. The May 1992 note argues:

[The UNFCCC] is likely to take some years to obtain the necessary ratifications to bring it into force.

It took two. Australia ratified the treaty in December 1992, but not before noting that the UNFCCC would worry industry for being too strong, and environmental groups for being too weak. So no changes there.

What happened next

At least when it comes to climate policy, there are no real secrets worthy of the name. We have always known that the Australian state quickly retreated from its already hedged promise to take action, and told us all along that this was because we had a lot of coal.

While Australia’s international credibility has flatlined (with a brief bump from 2007 to 2009), two other things have soared over the last 25 years: Australia’s coal exports, and atmospheric concentrations of carbon dioxide. Both look set to continue their upward trend.

Reading the documents, it is striking how concerned the cabinet was to minimise its financial commitments (unsurprising, perhaps, given the overall state of the economy at the time), and just how unimportant the climate issue was to leaders who ask us to trust them on the long-term future of the country. It seems it was a distant abstraction that many didn’t really think was real. How times have changed.

The Conversation

Marc Hudson, PhD Candidate, Sustainable Consumption Institute, University of Manchester

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