The 'End of the high seas,' or we watch the seas die

Even optimistic estimates for what might be achieved at December's Climate Change Conference (COP21) in Paris will not be enough to save the world's coral reefs, according to a Plenary session analysis presented at the Goldschmidt conference in Prague.

The left side of this photo shows a healthy reef at Heron Island. The right side shows
 an example of a degraded reef off Townsville after attack from Crown of Thorns
 and bleaching [Credit: Ove Hoegh-Guldberg/Global Change Institute
 at the University of Queensland]

Speaking to the world's major gathering of geochemists, Professor Peter F Sale (University of Windsor, Canada) spelled out the stark choice facing climate scientists in the run-up to the Paris conference. The stated aim for the COP21 climate conference is to limit a temperature increase to less than +2C by the end of the century.

According to Professor Sale: "Even if Paris is wildly successful, and a treaty is struck, ocean warming and ocean acidification are going to continue beyond the end of this century."

He continued: "This is now serious; I find it very unlikely that coral reefs as I knew them in the mid-1960s will still be found anywhere on this planet by mid-century. Instead, we will have algal-dominated, rubble-strewn, slowly eroding limestone benches. I see little hope for reefs unless we embark on a more aggressive emissions reduction plan. Aiming for CO2 at 350ppm, or a total warming of around 1C is scientifically defendable, and would give reefs a good chance; a number of coral reef scientists have called for this.

A goal of stabilising CO2 at less than 350ppm is also environmentally cautious. Getting to +2 degrees Celsius or so, overshooting along the way, is unwise, self-defeating, and may have far more serious consequences than are dreamed of by politicians happily negotiating minimalist responses to climate change

"As well as CO2 emissions, we must also deal with our other insults to the oceans. We have lost 90% of our commercial fish biomass since the 1940's, we are polluting coastal waters, and the great majority of marine protected areas are not being protected. Either we agree limits, which means the end of the' high seas', or we let large parts of the seas die."

Professor Sale summarised: "Knowing what we are doing, do we have the ethical right to eliminate an entire ecosystem from this planet? It's never been done before. But watching as our actions lead to the loss of all coral reefs on the planet is like removing all rainforests. I don't believe we have that right."

Professor Ove Hoegh-Guldberg, Coordinating Lead Author of section on 'The Ocean' within the latest IPCC (Intergovernmental Panel on Climate Change) report, said: "We need to wake up to the idea that business as usual, even clever taxation schemes, will not act fast enough to reduce global emissions. This is a global emergency, which requires us to decarbonise within the next 20 years, or face temperatures that will eliminate ecosystems like coral reefs, and indeed many systems that humans depend on."

"At the same time, dealing with non-climate stresses will be vitally important -- we must buy time by building resilience in Earth's biological systems, given that even more stringent activities will still result in much warmer and more acidic oceans, than today"

Coral expert, Professor John (Charlie) Veron, former chief scientist of the Australian Institute of Marine Science commented: "The extreme gravity of the current predicament is now widely acknowledged by reef and climate scientists. They also accept that only drastic action starting now will prevent wholesale destruction of reefs and other similarly affected ecosystems. Should humanity not be successful in preventing these threats from becoming reality no amount of management or expenditure will save future generations from the consequences of our failed guardianship."

Source: European Association of Geochemistry [August 17, 2015]

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1,800 years of global ocean cooling halted by global warming

Prior to the advent of human-caused global warming in the 19th century, the surface layer of Earth's oceans had undergone 1,800 years of a steady cooling trend, according to a new study. During the latter half of this cooling period, the trend was most likely driven by large and frequent volcanic eruptions.

Compared to the atmosphere, the oceans can absorb much more heat and trap it for
 longer periods of time. Thus the ocean can buffer short-term changes in global
 temperature. But when events such as volcanic eruptions cluster together
 in a relatively short period of time, the temperature changes can 
become prolonged [Credit: Kathryn Hansen/NASA]

An international team of researchers reported these findings in the August 17, 2015 issue of the journal Nature Geoscience. The study also indicates that the coolest temperatures occurred during the Little Ice Age--a period that spanned the 16th through 18th centuries and was known for cooler average temperatures over land.

The concurrence of cooling events on both land and sea suggests that a global cooling phenomenon was erased by subsequent human-caused global warming.

"Today, the Earth is warming about 20 times faster than it cooled during the past 1,800 years," said Michael Evans, second author of the study and an associate professor in the University of Maryland's Department of Geology and Earth System Science Interdisciplinary Center (ESSIC). "This study truly highlights the profound effects we are having on our climate today."

Compared to the atmosphere, the oceans can absorb much more heat and trap it for longer periods of time. Thus the ocean can buffer short-term changes in global temperature. But when events such as volcanic eruptions cluster together in a relatively short period of time, the temperature changes can become prolonged.

"Volcanic eruptions have a short-term cooling effect on the atmosphere, but our results showed that when volcanic eruptions occurred more frequently, there was long-term ocean cooling," said lead author Helen McGregor, an Australian Research Council (ARC) Future Fellow at the University of Wollongong in Australia. "With this research, we now have new insight into the century-scale global sea-surface temperature variations that came before human-made greenhouse gas forcing."

Results of the global sea surface temperature compilation from Ocean2k: A cooling over the past two millenium was reversed only in the most recent two centuries. Fifty-seven previously published and publicly available marine sea surface temperature reconstructions were combined and compiled into 200-year brackets, represented by the boxes. The thin horizontal lines dividing each box are the median of the values in that box. The thick blue line is the median of these values weighted for differences in the region of the global ocean in which they were found. (More in Figure 2a in the paper and Supplementary Table S13). [Credit: Modified version of Fig 2a in McGregor et al. 
Nature Geoscience 2015]

The scientists are the first to combine 57 previously published marine surface temperature reconstructions that cover all of the world's oceans, from near-polar to tropical regions. The team compiled the data within 200-year brackets to observe long-term trends, and then compared the findings to land-based reconstructions, which revealed similar cooling trends.

"No matter how we divided the data set, the cooling trend stands out as a robust signal," McGregor said.

To investigate the cause of the cooling trend, the researchers turned to climate models. They examined how sea-surface temperatures reacted to various "forcing" factors, such as changes in solar output, Earth's orbit, land use, volcanic activity and greenhouse gases. Only volcanic events resulted in a cooling trend that matched the team's real-world observations.

Understanding how forcing factors changed ocean temperatures in the past can open a window into future climate change.

"Model simulations by others have shown us that the oceans can impart a substantial delay in the warming of the surface climate," said Evans, who is also the lead of the Ocean2k working group of the Past Global Changes (PAGES) program. "With much of the heat from global warming entering our oceans, recent ocean surface warming may foreshadow additional future warming, in the same way ocean cooling appeared as a long-term response to large and frequent volcanic events in recent centuries."

"We are still learning how the oceans mediate climate variations," Evans added. "Further work combining both observations and simulations of ocean climate will refine our understanding of the ocean's role in climate change."

Source: University of Maryland [August 17, 2015]

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Heat release from stagnant deep sea helped end last Ice Age

The build-up and subsequent release of warm, stagnant water from the deep Arctic Ocean and Nordic Seas played a role in ending the last Ice Age within the Arctic region, according to new research led by a UCL scientist.

Calving ice sheet in Spitzbergen 
[Credit: David Thornalley]

The study, published today in Science, examined how the circulation of the ocean north of Iceland -- the combined Arctic Ocean and Nordic Seas, called the Arctic Mediterranean -- changed since the end of the last Ice Age (~20,000-30,000 years ago).

Today, the ocean is cooled by the atmosphere during winter, producing large volumes of dense water that sink and flush through the deep Arctic Mediterranean. However, in contrast to the vigorous circulation of today, the research found that during the last Ice Age, the deep Arctic Mediterranean became like a giant stagnant pond, with deep waters not being replenished for up to 10,000 years.

This is thought to have been caused by the thick and extensive layer of sea ice and fresh water that covered much of the Arctic Mediterranean during the Ice Age, preventing the atmosphere from cooling and densifying the underlying ocean.

Dr David Thornalley (UCL Geography) said: "As well as being stagnant, these deep waters were also warm. Sitting around at the bottom of the ocean, they slowly accumulated geothermal heat from the seafloor, until a critical point was reached when the ocean became unstable.

"Suddenly, the heat previously stored in the deep Arctic Mediterranean was released to the upper ocean. The timing of this event coincides with the occurrence of evidence for a massive release of meltwater into the Nordic Seas. We hypothesize that this input of melt water was caused by the release of deep ocean heat, which melted icebergs, sea-ice and surrounding marine-terminating ice sheets."

A schematic of the changes in the Arctic Mediterranean at the
 end of the last Ice Age [Credit: UCL Geography]

This study highlights the important impact that changes in ocean circulation can have on climate, due to the ocean's capacity to redistribute vast quantities of heat around the globe. For example, scientists are currently concerned that ongoing changes in ocean circulation may result in warmer subsurface water that will cause enhanced melting and retreat of certain ice sheets in Greenland and Antarctica.

Dr Thornalley added: "To help predict the role of the ocean in future climate change, it is useful to investigate how ocean circulation changed in the past and what the associated climate effects were."

In this study, researchers from UCL, Woods Hole Oceanographic Institute and other partner institutions analysed the composition of calcite shells of small single-celled organisms (called foraminifera) that are found in ocean floor sediment. The shells of these organisms record the chemistry of the deep ocean at the time they were living, enabling the researchers to reconstruct past changes in ocean circulation.

By measuring the radiocarbon content of these shells, the research team was able to determine how rapidly deep water was being formed in the Arctic Mediterranean. A number of different techniques were then used to constrain past temperature changes, including measuring the ratio of magnesium and calcium, and the arrangement of isotopes of carbon and oxygen within the calcite shells of the foraminifera, both of which vary according to the temperature of the water in which the foraminifera grew.

A warmer, deep Arctic Mediterranean during glacial times has been suggested in previous studies, too. As summarised by co-author Dr Henning Bauch (GEOMAR/Germany) "It is good to see that new, independent proxy data would give strong support now to these former hypotheses."

Source: University College London [August 13, 2015]

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