Are glaciers behind perplexing shift in paleoclimate Ice Age patterns?

In early August, at the Goldschmidt Conference on geochemistry, a team of scientists from Columbia University presented evidence from seafloor cores that suggest that a million years ago ice sheets in the Northern Hemisphere began sticking to their bedrock. The team proposes that as the glaciers grew thicker, it led to a global cooling that disrupted both the Atlantic Meridional Overturning Circulation (AMOC) and the ice age cycle. But how exactly might glaciers have been involved in this perplexing shift in paleoclimate ice age patterns?

As skeptics of anthropogenic climate change often note, Earth’s climate changes and has changed before. Aside from humans’ unabashed consumption of greenhouse gases, a wide variety of natural factors cause shifts in this complex system. For instance, scientists have long acknowledged how tiny changes in the Earth’s orbit around the sun, collectively known as the Milankovitch Cycles, drive the coming and going of ice ages. As the Milankovitch Cycles interact, the planet’s movements displace the incoming solar radiation across the globe, dramatically affecting the Earth’s climate system and the advancement and retreat of glaciers.

Glaciers in the North Atlantic, such as this one in the Johan Petersen Fjord of eastern Greenland, may have driven a global cooling a million years ago (Source: Ray Swi-hymn/Flickr).

For a while, ice ages were known to occur steadily every 40,000 years. However, a million years ago, that metronome inexplicably got off course. Instead of periods of intense glaciation occurring every 40,000 years, it shifted to every 100,000 years. But the likely culprit, the Milankovitch Cycles, hadn’t changed a million years ago. It didn’t add up.

And that’s not all. Around the same time, the massive AMOC— the conveyor belt that brings warm, shallow water to the North Atlantic, where it cools and sinks to the sea floor before returning south— nearly collapsed. Were these events related? If so, how and what was behind them?

These questions have perplexed scientists for years, as was apparent even at last month’s conference. But through an analysis of the chemical composition of basin-wide ocean sediment cores over several years, geochemist Steve Goldstein from Columbia University, who led the study presented at Goldschmidt, found unique shifts in isotopic signals that reflect a slower turn of the AMOC 950,000 years ago. 

For the present study, the team examined five more ocean cores, in addition to two analyzed earlier in the decade, that also demonstrated signs of a weak AMOC. The group believes two of the cores from the North Atlantic indicate possible triggers for the AMOC crisis. They suggest that such a slowdown could have rapidly cooled the North Atlantic region, in turn lengthening the ice age rhythm.

Peter Clark, a glaciologist at Oregon State University in Corvallis, has advanced this hypothesis as the only plausible explanation for many years, wrote Paul Voosen in Science last month. Three million years ago, a sustained warming period allowed for the build-up of thick soil in the Northern Hemisphere. Ice sheets would often collapse as the soil acted as an oiled buffer. But repeated glaciations wore down the warm protective layer and enabled glaciers to dig deeper into older rock that stabilized them and helped them thicken and advance.

Aerial shot of a large glacier in Greenland (Source: Leon Weber/Flickr).

But as exciting as the findings may be, not everyone is sold on the hypothesis. Climate scientist Amy Clement from the University of Miami told GlacierHub it sounded like an interesting concept, but she has problems with how the AMOC idea is applied in the modern climate. Clement explains how some argue that variations in the AMOC strength control the North Atlantic surface temperature on these multi-decadal timescales.

“The problems are (1) timescale and (2) magnitude. On these short timescales, the AMOC doesn’t seem to be the driver,” she noted to GlacierHub. “Instead we think the North Atlantic surface temperatures are controlled by external forcing (some natural, such as the sun and volcanoes) and some anthropogenic (such as greenhouse gases and aerosols).”

Others including Henrieka Detlef, a paleoclimatologist at Cardiff University in the U.K., told Science that while she accepts something important happened in the North Atlantic to lead to AMOC crisis, she has yet to see conclusive evidence that northern ice sheets were increasing in thickness prior to the AMOC slowdown.

Still, most agree that ice age rhythm shifts were likely caused by more than one trigger. The Columbia team is confident that thickening ice sheets in addition to other factors played a role in the perplexing transition. “The interactions between the different components of the Earth’s climate are elusive, but understanding them is crucial for reconstructing past changes,” Maayan Yehudai, part of the research group and a graduate student at Columbia, told GlacierHub. “We still have a long way to go as scientists before we can characterize them perfectly, but I think this is another important step forward on this account.”

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Roundup: Shifting Ice Age, Plane Wrecks and Receding Streamflows

Mysterious Shifts in Ice Age Rhythms

From Science: “About 1 million years ago, one of Earth’s most important metronomes mysteriously shifted: Ice ages went from occurring every 40,000 years to every 100,000 years. At the same time, the ‘conveyor belt’ of warming currents in the North Atlantic Ocean slowed sharply. Last week, scientists at the Goldschmidt Conference presented a clue to these twin mysteries: evidence that glaciers in the Northern Hemisphere suddenly began to stick to their beds. Growing thicker, they might have triggered a cooling that disrupted the conveyor belt and allowed the 100,000-year cycle that we see today to take root.”

Read more here.

The Russell Glacier in Greenland. After ancient glaciers scoured away soil and reached bedrock, they may have grown thicker, triggering a shift in the ice age cycles (Source: Jason Edwards/Getty Images).


Glacier Recession and Summer Streamflow in the Cascades

From Water Resources Research: “The Pacific Northwest (PNW) is the most highly glacierized region in the conterminous United States (858 glaciers; 466 km2)… We applied a high‐resolution glacio‐hydrological model to predict glacier mass balance, glacier area, and river discharge for the period 1960‐2099…Results show that the rate of regional glacier recession will increase, but the runoff from glacier melt and its relative contribution to streamflow display both positive and negative trends.”

Read more here.

North Cascades glaciers (Source: Sean Munson/Flickr).


Melting Ice Uncovers 1946 Wreckage of U.S. Plane

From New York Times: “After an emergency landing on a Swiss glacier, the group of 12 Americans drank melted snow and survived on rations of one chocolate bar a person until daring pilots shuttled them to safety after five days marooned on the ice. Relics of that harrowing adventure and the successful rescue of all those onboard…resurfaced after more than 70 years this month when scorching summer temperatures in Europe caused the glacial ice to recede.”

Read more here.

Some of the debris on Wednesday from a plane crash 70 years ago on the Gauli Glacier in Switzerland (Source: Anthony Anex/Shutterstock).
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New Study Warns: Rapid Sea Level Rise, Superstorms Likely

Existing climate change assessments could be underestimating the amount of future sea level rise, as well as the likelihood of other phenomenons like increased superstorms and glacier loss, warns a new high-profile paper in Atmospheric Chemistry and Physics. The study, by longtime climate scientist James Hansen and 18 co-authors, has gained attention recently for its radical projections of climate change impacts.

Photo: Dr. James Hansen

To conduct research for the paper, titled Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 ◦C global warming is highly dangerous, Hansen and the other authors combined ancient climate data with new satellite readings and an updated climate model to show that ice melt is occurring more quickly than previously thought. Instead of incrementally melting, the ice sheets around the Earth’s poles actually melt at a non-linear rate, losing mass rapidly, according to Hansen and his team.

“We have uncovered information and a partial understanding of feedbacks in the climate system, specifically interactions between the ocean and the ice sheets. These feedbacks raise questions about how soon we will pass points of no return, in which we lock in consequences that cannot be reversed on any time scale that people care about. Consequences include sea level rise of several meters, which we estimate would occur this century or at latest next century, if fossil fuel emissions continue at a high level,” Hansen says in a video released about the paper. “That would mean loss of all coastal cities, most of the world’s large cities and all their history.”

Hansen notes that a positive feedback loop is created as ice melt influences the structure of the ocean’s layers. As cold freshwater runoff from exit glaciers flows into the ocean, it lowers the density of the surface water. This change of density shuts down the normal circulation in which cold salty water sinks and brings warm water to the surface, releasing the heat it carries into the atmosphere. But when heat stays in the ocean at a depth where ice shelves contact the trapped warm water, a feedback loop occurs. The warm water next to the deep ice makes the ice melt even faster.

Stratification and precipitation amplifying feedbacks. Source: Hansen et al, 2015

Thus the ice melt in these regions causes further loss of ice sheets in direct contact with the ocean, which contributes to more rapid movement of exit glaciers and to faster sea level rise. In addition to quickening ice melt, the feedback loop also contributes to shutting down the ocean’s circulation, trapping warm water between layers of cold water in polar regions. The feedback loop creates a greater temperature gradient by increasing temperature differences between high and low latitudes, which increases the likelihood of superstorms.

Source: 5 Meters of sea level rise

Earth’s ice sheets are melting quickly, and the rate of melt is also expected to increase exponentially. As a result, we could see the sea level rise up to five meters, or about 16 feet, by the end of this century if no emissions reduction actions are taken. This puts many of the world’s coastal cities in danger of flooding, including cities like Miami, London, New York, Miami and Shanghai.

The paper forecasts a greater increase in sea level within a shorter period of time than other research has found. In its 2013 Fifth Assessment Report (AR5), the U.N’s Intergovernmental Panel on Climate Change (IPCC) predicts closer to three feet of sea level rise occurring at or after 2100.

“The models that were run for the IPCC report did not include ice melt,” Hansen said at a news conference.

But the paper has received criticism. Hansen and the other researchers first released their research as a discussion paper in the European Geosciences Union (EGU) Open Access journals. This made the paper visible to the public before the peer review process was finished, which is atypical of scientific research and generated some criticism.

There has been contention about Hansen findings within the scientific community, which can be seen not only in the papers reviews and comments but also playing out across Twitter and in the news. In an op-ed on the paper in the New York Times, the environmental journalist Andy Revkin quoted  the climate journalist Eric Holthaus, who succinctly sums up the negative responses in the tweet, appended below.

Gravity satellite ice sheet mass measurements for greenland and antarctic ice sheets. Photo: Hansen

Concern has been expressed that the predictions made in the paper are too extreme. For one, some critics found the assumptions, such as exponential rates of ice loss, to be improbable. Others raised objections to the particular way in which paleoclimate data was used to suggest future conditions. Kevin Trenberth of the National Center for Atmospheric Research strongly criticized the study, saying that “there are way too many assumptions and extrapolations for anything here to be taken seriously other than to promote further studies.” As the extensive comments on blog posts here and here show, the paper by Hansen and his team has attracted a great deal of attention, and sparked lengthy debates in the scientific community.

At a February 2012 TED talk titled Why I must speak out about climate change Hansen said: “Clearly I haven’t got this message across. The science is clear. I need your help to communicate the gravity and urgency of this situation and its solutions more effectively. We owe it to our children and grandchildren.”

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