Mathematics predicts a sixth mass extinction

In the past 540 million years, the Earth has endured five mass extinction events, each involving processes that upended the normal cycling of carbon through the atmosphere and oceans. These globally fatal perturbations in carbon each unfolded over thousands to millions of years, and are coincident with the widespread extermination of marine species around the world.

Scientists have analyzed significant changes in the carbon cycle over the last 540 million years, including the five mass 
extinction events. They have identified 'thresholds of catastrophe' in the carbon cycle that, if exceeded, would lead
 to an unstable environment, and ultimately, mass extinction [Credit: MIT]

The question for many scientists is whether the carbon cycle is now experiencing a significant jolt that could tip the planet toward a sixth mass extinction. In the modern era, carbon dioxide emissions have risen steadily since the 19th century, but deciphering whether this recent spike in carbon could lead to mass extinction has been challenging. That's mainly because it's difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today's disruptions, which have taken place over just a little more than a century.

Now Daniel Rothman, professor of geophysics in the MIT Department of Earth, Atmospheric and Planetary Sciences and co-director of MIT's Lorenz Center, has analyzed significant changes in the carbon cycle over the last 540 million years, including the five mass extinction events. He has identified "thresholds of catastrophe" in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.

In a paper published in Science Advances, he proposes that mass extinction occurs if one of two thresholds are crossed: For changes in the carbon cycle that occur over long timescales, extinctions will follow if those changes occur at rates faster than global ecosystems can adapt. For carbon perturbations that take place over shorter timescales, the pace of carbon-cycle changes will not matter; instead, the size or magnitude of the change will determine the likelihood of an extinction event.

Taking this reasoning forward in time, Rothman predicts that, given the recent rise in carbon dioxide emissions over a relatively short timescale, a sixth extinction will depend on whether a critical amount of carbon is added to the oceans. That amount, he calculates, is about 310 gigatons, which he estimates to be roughly equivalent to the amount of carbon that human activities will have added to the world's oceans by the year 2100.

Does this mean that mass extinction will soon follow at the turn of the century? Rothman says it would take some time -- about 10,000 years -- for such ecological disasters to play out. However, he says that by 2100 the world may have tipped into "unknown territory."

"This is not saying that disaster occurs the next day," Rothman says. "It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction."

History follows theory

Rothman had previously done work on the end-Permian extinction, the most severe extinction in Earth's history, in which a massive pulse of carbon through the Earth's system was involved in wiping out more than 95 percent of marine species worldwide. Since then, conversations with colleagues spurred him to consider the likelihood of a sixth extinction, raising an essential question:

"How can you really compare these great events in the geologic past, which occur over such vast timescales, to what's going on today, which is centuries at the longest?" Rothman says. "So I sat down one summer day and tried to think about how one might go about this systematically."

He eventually derived a simple mathematical formula based on basic physical principles that relates the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change. He hypothesized that this formula should predict whether mass extinction, or some other sort of global catastrophe, should occur.

Rothman then asked whether history followed his hypothesis. By searching through hundreds of published geochemistry papers, he identified 31 events in the last 542 million years in which a significant change occurred in Earth's carbon cycle. For each event, including the five mass extinctions, Rothman noted the change in carbon, expressed in the geochemical record as a change in the relative abundance of two isotopes, carbon-12 and carbon-13. He also noted the duration of time over which the changes occurred.

He then devised a mathematical transformation to convert these quantities into the total mass of carbon that was added to the oceans during each event. Finally, he plotted both the mass and timescale of each event.

"It became evident that there was a characteristic rate of change that the system basically didn't like to go past," Rothman says.

In other words, he observed a common threshold that most of the 31 events appeared to stay under. While these events involved significant changes in carbon, they were relatively benign -- not enough to destabilize the system toward catastrophe. In contrast, four of the five mass extinction events lay over the threshold, with the most severe end-Permian extinction being the farthest over the line.

"Then it became a question of figuring out what it meant," Rothman says.

A hidden leak

Upon further analysis, Rothman found that the critical rate for catastrophe is related to a hidden process within the Earth's natural carbon cycle. The cycle is essentially a loop between photosynthesis and respiration. Normally, there is a "leak" in the cycle, in which a small amount of organic carbon sinks to the ocean bottom and, over time, is buried as sediment and sequestered from the rest of the carbon cycle.

Rothman found that the critical rate was equivalent to the rate of excess production of carbon dioxide that would result from plugging the leak. Any additional carbon dioxide injected into the cycle could not be described by the loop itself. One or more other processes would instead have taken the carbon cycle into unstable territory.

He then determined that the critical rate applies only beyond the timescale at which the marine carbon cycle can re-establish its equilibrium after it is disturbed. Today, this timescale is about 10,000 years. For much shorter events, the critical threshold is no longer tied to the rate at which carbon is added to the oceans but instead to the carbon's total mass. Both scenarios would leave an excess of carbon circulating through the oceans and atmosphere, likely resulting in global warming and ocean acidification.

The century's the limit

From the critical rate and the equilibrium timescale, Rothman calculated the critical mass of carbon for the modern day to be about 310 gigatons.

He then compared his prediction to the total amount of carbon added to the Earth's oceans by the year 2100, as projected in the most recent report of the Intergovernmental Panel on Climate Change. The IPCC projections consider four possible pathways for carbon dioxide emissions, ranging from one associated with stringent policies to limit carbon dioxide emissions, to another related to the high range of scenarios with no limitations.

The best-case scenario projects that humans will add 300 gigatons of carbon to the oceans by 2100, while more than 500 gigatons will be added under the worst-case scenario, far exceeding the critical threshold. In all scenarios, Rothman shows that by 2100, the carbon cycle will either be close to or well beyond the threshold for catastrophe.

"There should be ways of pulling back [emissions of carbon dioxide]," Rothman says. "But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present."

Author: Jennifer Chu | Source: Massachusetts Institute of Technology [September 20, 2017]

Read More

New climate risk classification created to account for potential 'existential' threats

A new study evaluating models of future climate scenarios has led to the creation of the new risk categories “catastrophic” and “unknown” to characterize the range of threats posed by rapid global warming. Researchers propose that unknown risks imply existential threats to the survival of humanity.

Researchers projected warming scenarios that vary based on what societal actions are taken to reduce emissions 
[Credit: Scripps Institution of Oceanography at UC San Diego]

These categories describe two low-probability but statistically significant scenarios that could play out by century’s end, in a new study by Veerabhadran Ramanathan, a distinguished professor of climate and atmospheric sciences at Scripps Institution of Oceanography at the University of California San Diego, and his former Scripps graduate student Yangyang Xu, now an assistant professor at Texas A&M University.

The risk assessment stems from the objective stated in the 2015 Paris Agreement regarding climate change that society keep average global temperatures “well below” a 2°C (3.6°F) increase from what they were before the Industrial Revolution.

Even if that objective is met, a global temperature increase of 1.5°C (2.7°F) is still categorized as “dangerous,” meaning it could create substantial damage to human and natural systems. A temperature increase greater than 3°C (5.4°F) could lead to what the researchers term “catastrophic” effects, and an increase greater than 5°C (9°F) could lead to “unknown” consequences which they describe as beyond catastrophic including potentially existential threats. The specter of existential threats is raised to reflect the grave risks to human health and species extinction from warming beyond 5°C, which has not been experienced for at least the past 20 million years.

The scientists term warming probability of five percent or less as a “low-probability high-impact” scenario and assess such scenarios in the analysis “Well Below 2°C: Mitigation strategies for avoiding dangerous to catastrophic climate changes,” which appears today in the journal Proceedings of the National Academy of Sciences.

Ramanathan and Xu also describe three strategies for preventing the gravest threats from taking place.

“When we say 5 percent-probability high-impact event, people may dismiss it as small but it is equivalent to a one-in-20 chance the plane you are about to board will crash,” said Ramanathan. “We would never get on that plane with a one-in-20 chance of it coming down but we are willing to send our children and grandchildren on that plane.”

The researchers defined the risk categories based on guidelines established by the Intergovernmental Panel on Climate Change (IPCC) and previous independent studies. “Dangerous” global warming includes consequences such as increased risk of extreme weather and climate events ranging from more intense heat waves, hurricanes, and floods, to prolonged droughts. Planetary warming between 3°C and 5°C could trigger what scientists term “tipping points” such as the collapse of the West Antarctic Ice Sheet and subsequent global sea-level rise, and the dieback of the Amazon rainforest. In human systems, catastrophic climate change is marked by deadly heat waves becoming commonplace, exposing over 7 billion people to heat related mortalities and famine becoming widespread. Furthermore, the changes will be too rapid for most to adapt to, particularly the less affluent, said Ramanathan.

Risk assessments of global temperature rise greater than 5°C have not been undertaken by the IPCC.  Ramanathan and Xu named this category “unknown??” with the question marks acknowledging the “subjective nature of our deduction.” The existential threats could include species extinctions and major threats to human water and food supplies in addition to the health risks posed by exposing over 7 billion people worldwide to deadly heat.

With these scenarios in mind, the researchers identified what measures can be taken to slow the rate of global warming to avoid the worst consequences, particularly the low-probability high-impact events. Aggressive measures to curtail the use of fossil fuels and emissions of so-called short-lived climate pollutants such as soot, methane and HFCs would need to be accompanied by active efforts to extract CO2 from the air and sequester it before it can be emitted.  It would take all three efforts to meet the Paris Agreement goal to which countries agreed at a landmark United Nations climate conference in Nov 2015.

"This report shines a bright light on the existential threat that climate change presents to all humanity," said Calif. Governor Edmund G. Brown Jr., who has collaborated with Ramanathan on carbon neutrality measures in the state. "Scientists have many ideas about how to reduce emissions, but they all agree on the urgency of strong and decisive action to remove carbon from the economy."

Xu and Ramanathan point out that the goal is attainable. Global CO2 emissions had grown at a rate of 2.9 percent per year between 2000 and 2011, but had slowed to a near-zero growth rate by 2015.  They credited drops in CO2 emissions from the United States and China as the primary drivers of the trend. Increases in production of renewable energy, especially wind and solar power, have also bent the curve of emissions trends downward. Other studies have estimated that there was by 2015 enough renewable energy capacity to meet nearly 24 percent of global electricity demand.

Short-lived climate pollutants are so called because even though they warm the planet more efficiently than carbon dioxide, they only remain in the atmosphere for a period of weeks to roughly a decade whereas carbon dioxide molecules remain in the atmosphere for a century or more. The authors also note that most of the technologies needed to drastically curb emissions of short-lived climate pollutants already exist and are in use in much of the developed world.  They range from cleaner diesel engines to methane-capture infrastructure.

“While these are encouraging signs, aggressive policies will still be required to achieve carbon neutrality and climate stability,” the authors wrote.

The release of the study coincides with the start of Climate Week NYC in New York, a summit of business and government leaders to highlight global climate action. Ramanathan and colleagues will deliver a complementary report detailing the “three-lever” mitigation strategy of emissions control and carbon sequestration on Sept. 18 at the United Nations. That report was produced by the Committee to Prevent Extreme Climate Change, chaired by Ramanathan, Nobel Prize winner Mario Molina of UC San Diego, and Durwood Zaelke, who leads an advocacy organization, the Institute for Governance and Sustainable Development, with 30 experts from around the world including China and India.

Source: University of California - San Diego [September 14, 2017]

Read More

Earth's oldest trees in climate-induced race up the tree line

Bristlecone pine and limber pine trees in the Great Basin region are like two very gnarled, old men in a slow-motion race up the mountaintop, and climate change is the starting gun, according to a study from the University of California, Davis.

Gnarled, dead bristlecone pine trees, which can live more than 5,000 years, stand where young limber pine grow 
around them. Limber pine is beginning to colonize areas of the Great Basin once dominated by bristlecones 
[Credit: Brian Smithers/UC Davis]

The study, published in the journal Global Change Biology, shows that the tree line has been steadily moving upslope over the past 50 years in the Great Basin. The region extends from California's Sierra Nevada, across Nevada to Utah's Uinta Mountains. Its north and south are framed by the Columbia and Colorado rivers' watersheds.

The study also found that limber pine is successfully "leapfrogging" over bristlecone pine. They are growing in soils once almost completely dominated by bristlecone pine, and they are moving upslope at a faster rate than the bristlecone pine.

Charging upslope

"We are seeing very little regeneration anywhere in bristlecone ranges except in the tree line and, there, limber pine is taking all the good spots," said the study's corresponding author Brian Smithers, a Ph.D. candidate in the Department of Plant Sciences at UC Davis. "It's jarring because limber pine is a species you normally see further downslope, not at tree line. So it's very odd to see it charging upslope and not see bristlecone charging upslope ahead of limber pine, or at least with it."

Dead bristlecone pines stand among limber pine trees on the California side of the White Mountains, 
part of the Great Basin region [Credit: Brian Smithers/UC Davis]

The study concludes that if bristlecone pine trees are unable to advance upslope because they are blocked by limber pine, bristlecones could face a reduction of their range and possibly local extinctions.

Earth’s oldest living trees

Bristlecone pine trees are Earth's oldest individual trees and can live for more than 5,000 years. No spring chicken, limber pine trees can live 2,000 years or more.

Both tree species have seen many climate changes during their time on Earth -- from extremely warm periods to ice ages -- and have slowly advanced across the landscape. Over millennia, bristlecone pine trees have moved from the lowlands of the Great Basin up to the current tree line. But, the study notes, neither bristlecone nor limber pine have ever experienced climate change and temperature increases as rapidly as what has been occurring in recent decades.

Bristlecone pine trees grow on soils and in conditions where few other species can live. But limber pines in the 
Great Basin region, such as California’s White Mountains, are beginning to give them some competition 
[Credit: Brian Smithers/UC Davis]

Legacy effects

Smithers said he doesn't expect bristlecone pine adult trees to be impacted much by current climatic shifts, as those trees are well-established. But how, if and where new bristlecone pine trees will regenerate is less certain, particularly as other species like limber pine take up valuable space for them to germinate.

"The things we're doing today have legacy effects for thousands of years in the Great Basin," Smithers said. "When those trees do start to die, they won't likely be replaced because it's just too hot and dry."

The study suggests that land managers identify the specific bottlenecks for a species to live long enough to reproduce, and focus on that stage. For long-lived trees like bristlecone and limber pines, the bottleneck is at the time of their initial establishment, not hundreds and thousands of years into their adulthoods.

Author: Kat Kerlin | Source: University of California - Davis [September 14, 2017]

Read More

Climate change challenges the survival of fish across the world

Climate change will force many amphibians, mammals and birds to move to cooler areas outside their normal ranges, provided they can find space and a clear trajectory among our urban developments and growing cities. But what are the chances for fish to survive as climate change continues to warm waters around the world?

Fish species in many river systems, including the John Day River pictured, will face the challenge of coping 
with warmer waters in the future [Credit: University of Washington]

University of Washington researchers are tackling this question in the first analysis of how vulnerable the world's freshwater and marine fishes are to climate change. Their paper, appearing online in Nature Climate Change, used physiological data to predict how nearly 3,000 fish species living in oceans and rivers will respond to warming water temperatures in different regions.

"Climate change is happening. We need tools to try to identify areas that are going to be the most at risk and try to develop plans to conserve these areas," said lead author Lise Comte, a postdoctoral researcher in the UW's School of Aquatic and Fishery Sciences. "It's important to look at the organisms themselves as we cannot just assume they will all be equally sensitive to these changes."

The researchers compiled data from lab experiments involving nearly 500 fish species, conducted over the past 80 years by researchers around the world. These standardized experiments measure the highest temperatures fish are able to tolerate before they die. This analysis is the first time these disparate data from lab experiments have been combined and translated to predict how fish will respond in the wild.

The researchers found that overall, sensitivity to temperature changes varied greatly between ocean-dwelling and freshwater fish. In general, marine fish in the tropics and freshwater fish in higher latitudes of the Northern Hemisphere were the most at risk when water temperatures warmed, the analysis showed.

This figure shows the risk that freshwater fish (top) and marine fish (bottom) could exceed their thermal limits 
by the year 2070. Blue indicates a low risk and red shows a high risk [Credit: University of Washington]

"Nowhere on Earth are fish spared from having to cope with climate change," said senior author Julian Olden, a UW professor of aquatic and fishery sciences. "Fish have unique challenges -- they either have to make rapid movements to track their temperature requirements, or they will be forced to adapt quickly."

Using years of data -- and relying on the fact that many fish species are taxonomically related and tend to share the same thermal limits -- the researchers were able to predict the breaking-point temperature for close to 3,000 species. Regional patterns then emerged when those data were paired with climate-model data predicting temperature increases under climate change.

For example, fish in the tropical oceans are already living in water that is approaching the upper range of their tolerance. They might not have much wiggle room when temperatures increase slightly. By contrast, in freshwater streams in the far north, fish are accustomed to cooler water temperatures but have much less tolerance for warming waters. Since the effects of climate change are acutely felt in high latitudes, this doesn't bode well for fish in those streams that have a small window for survivable temperatures.

Fish will either migrate, adapt or die off as temperatures continue to warm, the researchers explained. Given past evolutionary rates of critical thermal limits, it's unlikely that fish will be able to keep up with the rate at which temperatures are increasing, Olden said. The ability to move, then, is imperative for fish that live in the most critical areas identified in this analysis.

Currently, dams and other infrastructure may block fish from getting where they might need to be in the future; fish ladders and other means to allow fish to circumvent these barriers could be more readily used, although the effectiveness of these structures is highly variable.

Additionally, actions to restore vegetation along the edges of streams and lakes can help shade and reduce water temperature for the benefit of fish.

"Fishes across the world face mounting challenges associated with climate change," Olden said. "Looking forward, continued efforts to support conservation strategies that allow species to respond to these rapid changes are needed."

Author: Michelle Ma | Source: University of Washington [September 13, 2017]

Read More

Study: Asia's glaciers face massive melt from global warming

Scientists say one-third of the ice stored in Asia's glaciers will be lost by the end of the century even if the world manages to meet its ambitious goal of keeping global warming below 1.5 degrees Celsius, affecting water supplies for millions of people on the continent.

International trekkers pass through a glacier at the Mount Everest base camp, Nepal. Scientists say a third of the ice stored 
in Asia’s glaciers will be lost by the end of the century even if global warming stays below 1.5 degrees Celsius 
[Credit: AP/Tashi Sherpa]

In a paper published in the journal Nature, researchers in the Netherlands also examined what would happen if average global temperatures rise beyond 1.5 degrees Celsius (2.7 degrees Fahrenheit) by the end of the century. They concluded that almost two-thirds of the ice in Asia's glaciers could vanish, if no effort is made to curb climate change.

"In regions where glacier melt water is an important part of the river flow, the retreating glaciers can become a problem," Philip Kraaijenbrink, a University of Utrecht geographer who led the study, said.

"There are many people living in basins that have their rivers originating in Asia's high mountains, such as the Indus, Ganges and Brahmaputra," Kraaijenbrink said. "In these basins, the river water is used for irrigation of cropland, drinking water and for hydropower dams."

The 1.5-degree target was set at the international climate conference in Paris two years ago, but experts say it would require a massive shift to the world economy.

In total, the researchers compared 110 climate simulations and found that high mountain glaciers in Asia tended to experience greater levels of warming than the global average. All glaciers analyzed already are losing mass except those in the Kunlun Mountains of western China.

A Kashmiri nomad tends to his heard of sheep and goats as he crosses a glacier near Dubgan, 70 kilometers (43 miles) 
south of Srinagar, India. Scientists say a third of the ice stored in Asia’s glaciers will be lost by the end of the century
 even if global warming stays below 1.5 degrees Celsius. Bakarwals are nomadic herders of India's Jammu-Kashmir 
state who wander in search of good pastures for their cattle. Every year in April-May more than one hundred thousand 
people from the nomadic Bakarwal tribes arrive in the meadows of Kashmir and parts of Ladakh from areas of the
 Jammu region with their flocks of cattle and sheep. After crossing snowy mountains with their cattle and belongings, 
Kashmir valley's lush green meadows become their home from April to September, after which they begin their 
return journey. This seasonal shifting of "homes" ensures a regular flow of income for the families 
[Credit: AP/Dar Yasin]

Taking into account the effect on melting levels of rubble covering some of the glaciers, they concluded that the amount of ice lost from Asian glaciers is almost proportional to the amount of warming they experience, though with some regional variations.

"Even if temperatures stabilize at their current level, mass loss will continue for decades to come until a new equilibrium is reached," the researchers said.

Kraaijenbrink acknowledged that a scenario in which global warming remains under 1.5 degrees Celsius is optimistic.

"We are aware that more extreme, business-as-usual scenarios are possibly a more likely future," he said.

In a comment published along with the study, J. Graham Cogley of Trent University in Canada said the researchers' glacier model "has some innovative features that might raise eyebrows among glaciologists, but it is difficult to find fault with it as a pioneering effort."

"The authors have shown that achieving the 1.5 Celsius target will conserve a substantial fraction of Asia's water resources and that, if we fail in this regard, we will pay in direct proportion to the extent of the failure," Cogley said.

Author: Frank Jordans | Source: The Associated Press [September 13, 2017]

Read More

Rapid climate changes across northern hemisphere in the earliest Middle Pleistocene

By studying climate changes that took place thousands of years ago, we can better understand the global climate system and predict Earth's future climate. A multi-organization research team led by Professor HYODO Masayuki (Research Center for Inland Seas, Kobe University) has discovered evidence of rapid climate changes on a millennial-to-centennial scale that occurred 780 to 760 thousand years ago. The findings were published in Scientific Reports.

Figure 1: Sample locations (a) Chiba section, Osaka Bay, North Pacific mid-latitude point (U1313). (b)(c) show
 the Chiba section location. (d) Location of core TB2 near the Chiba section along the Yoro River
[Credit: Kobe University]

During the 2.6 million year Quaternary Period, the climate repeated a glacial and interglacial cycle, caused by changes in the geographical distribution of solar radiation due to orbital changes including those of Earth's orbit and the tilt of its axis. These changes are regarded as "Milankovitch cycles," over 20,000 years in period. But in the Holocene and last glacial periods, a number of millennial-to-centennial scale climate changes have been observed. Such rapid climate changes have scarcely been reported before the last glacial period.

In the interglacial period between 780 and 760 thousand years ago, Earth's orbital patterns were quite similar to the current (Holocene) era, so this interglacial climate could be useful in predicting Earth's future climate.

Figure 2: Records of climate and environment between 790 and 750 thousand years ago in three areas 
[Credit: Kobe University]

The research team focused on the Kazusa Group (Chiba prefecture, Japan), which has the fastest sedimentation rate in the world for strata of that era, and obtained high resolution paleoceanic environmental records every 10 years. When combined with records from Osaka Bay and the North Atlantic, they found evidence of multiple instances of rapid warming and cooling across all three regions at the same time. The data includes the unusual phenomenon of a rapid temperature rise with cyclicity suddenly finishing with a cold event. The cold events occurred at the same time as the great iceberg flow reached mid-latitudes in the North Atlantic, so they are thought to be caused by meltwater that covered the North Atlantic Ocean.

This cyclic warming and rapid cooling repeated twice just after a geomagnetic reversal, a key event for the Early/Middle Pleistocene boundary, and a third time about 10 thousand years later. All occurred after Earth had recovered its geomagnetic strength. This shows that the second half of this interglacial period, namely the earliest stage of the Middle Pleistocene, was a time of extreme climate change when ice sheets expanded and shrunk causing changes of several meters in sea levels, repeating every 500 to 2000 years.

Figure 3: Close-up of events A,B and G,H [Credit: Kobe University]

The phenomenon of rapid temperature rises modulated by bi-centennial cycles ending with a sudden freeze only occurred during a very brief portion of this interglacial period, during the two warmest periods. There is a high possibility that this 200 year period marks the de Vries Cycle (205 years), when the climate was particularly sensitive to solar activity.

Researchers will now verify whether the same phenomenon can be observed in other regions. Evidence from the southern hemisphere will be the key to showing whether it was a global phenomenon. This discovery is very unusual among the climate warming that occurred in the past, as well as being an important key to learning about the diversity of temperature rises and understanding current global warming.

Additionally, this discovery was made in the Chiba Section (Japan), a candidate section for the Early/Middle Pleistocene era global boundary stratotype sections and points (GSSP), currently under review by the International Union of Geological Sciences. These findings provide further evidence for the academic value of the Chiba Section.

Source: Kobe University [September 12, 2017]

Read More

Ancient tree reveals cause of spike in Arctic temperature

A kauri tree preserved in a New Zealand peat swamp for 30,000 years has revealed a new mechanism that may explain how temperatures in the Northern Hemisphere spiked several degrees centigrade in just a few decades during the last global ice age.

It's only when a human stands beside the tree stump of an ancient kauri, that you can get 
a clear sense of the size of these ancient trees [Credit: www.ancientkauriproject.com]

Unexpectedly, according to new research led by scientists from UNSW Sydney and published in Nature Communications, it looks like the origin of this warming may lie half-a-world away, in Antarctica.

Rapid warming spikes of this kind during glacial periods, called Dansgaard-Oeschger events, are well known to climate researchers. They are linked to a phenomenon known as the "bipolar seesaw," where increasing temperatures in the Arctic happen at the same time as cooling over the Antarctic, and vice versa.

Until now, these divergences in temperature at the opposite ends of Earth were believed to have been driven by changes in the North Atlantic, causing deep ocean currents, often referred to as the ocean "conveyor belt," to shut down. This led to warming in the Northern Hemisphere and cooling in the south.

But the study, which examines a specific Dansgaard-Oeschger event that occurred around 30,000 years ago, suggests Antarctica plays a role too.

The paper describes how the researchers used a detailed sequence of radiocarbon dates from an ancient New Zealand kauri tree to precisely align ice, marine and sediment records across a period of greatly changing climate.

"Intriguingly, we found that the spike in temperature preserved in the Greenland ice core corresponded with a 400-year-long surface cooling period in the Southern Ocean and a major retreat of Antarctic ice," said lead author and UNSW scientist Professor Chris Turney.

Summary of CSIRO Mk3L ensemble simulations showing the impact of a 338-year duration freshwater flux 
of 0.54 Sv into the Weddell and Ross Seas [Credit: Keele University]

"As we looked more closely for the cause of this opposite response we found that there were no changes to the global ocean circulation during the Antarctic cooling event that could explain the warming in the North Atlantic. There had to be another cause."

A clue to what might be going on if the oceans weren't involved appeared in lake sediments from the Atherton Tableland, Queensland. The sediments showed a simultaneous collapse of rain-bearing trade winds over tropical northeast Australia.

It was a curious change, so the researchers turned to climate models to see if these climate events might somehow be linked.

They started by modelling the release of large volumes of freshwater into the Southern Ocean, exactly as would happen with rapid ice retreat around the Antarctic.

Consistent with the data, they found that there was cooling in the Southern Ocean but no change in the global ocean circulation.

They also found that the freshwater pulse caused rapid warming in the tropical Pacific. This in turn led to changes to the atmospheric circulation that went on to trigger sharply higher temperatures over the North Atlantic and the collapse of rain-bearing winds over tropical Australia.

Essentially, the model showed the formation of a 20,000 km long "atmospheric bridge" that linked melting ice in Antarctica to rapid atmospheric warming in the North Atlantic.

"Our study shows just how important Antarctica's ice is to the climate of the rest of the world and reveals how rapid melting of the ice here can affect us all. This is something we need to be acutely aware of in a warming world," Professor Turney said.

It also showed how deeply the climate was linked across great distances said fellow author and climate modeller from the University of Tasmania, Dr Steven Phipps.

"Our research has revealed yet another remarkable example of the interconnections that are so much a part of our climate system," Dr Phipps said.

"By combining past records of past events with climate modelling, we see how a change in one region can have major climatic impacts at the opposite ends of Earth."

Source: University of New South Wales [September 12, 2017]

Read More

Rising CO2 leading to changes in land plant photosynthesis

Researchers led by Scripps Institution of Oceanography at the University of California San Diego have determined that major changes in plant behavior have occurred over the past 40 years, using measurements of subtle changes in the carbon dioxide (CO2) currently found in the atmosphere.

Photo: MistikaS/iStock

The two main isotopes, or atomic forms, of carbon are carbon-12 (12C) and carbon-13 (13C). As CO2 has risen since the late 19th century, the ratio of 13C to 12C in atmospheric CO2 has decreased. That's in part because the CO2 produced by the combustion of fossil fuels has a low 13C/12C ratio. There are other factors in nature as well, however, that have influenced the rate of decrease in the isotopic ratio. The measured rate of decrease in the isotopic ratio turns out to be different than what scientists previously expected.

The Scripps-led team updated the record of CO2 isotopic ratios that has been made at Scripps since 1978 using air samples collected at Hawaii's Mauna Loa and the South Pole. The researchers confirmed that the discrepancy exists and considered several reasons for it. They concluded that no combination of factors could plausibly explain the changes in the CO2 isotopic ratio unless plant behavior was changing in a way that influences how much water plants need for growth.

The work helps to understand the details of how leaves are responding to changes in CO2. Prior to this study, it was already clear that plants behave differently when they are exposed to higher atmospheric CO2 levels because CO2 influences the behavior of stomata, the microscopic holes in leaves that allow a leaf to take up CO2. These holes also allow water to evaporate from the leaf, which must be replenished by water supplied to the roots to avoid drying out. With more CO2 in the atmosphere, a plant can afford to have smaller or fewer stomata, thus allowing more photosynthesis for the same amount of water.

But measuring exactly how much more efficient plants have become at using water has not been easy. This study provides a new method for measuring this effect, because as a leaf becomes more efficient at using water, this also influences how it takes up the different carbon isotopes in CO2. When that factor is included as a variable, the ratio of the two forms of CO2 conforms much more closely to expectations. The National Science Foundation, the Department of Energy, NASA, and the Eric and Wendy Schmidt Fund for Strategic Innovation supported the study, "Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis," which appears in the journal Proceedings of the National Academy of Sciences.

The research supports a long-standing hypothesis introduced by plant biologists, that posits plants will achieve an optimum response to rising CO2 levels in the atmosphere.

"This optimal model predicts nearly proportional scaling between water-use efficiency and CO2 itself," said study lead author and Scripps scientist Ralph Keeling, who also maintains the internationally renowned Keeling Curve data set measuring atmospheric CO2 since 1958. "Optimal or near optimal behavior has been found in smaller studies on individual plants, but this paper is the first to show that it may be evident at the scale of the entire planet."

The increase in the efficiency of photosynthesis documented in this study has likely helped plants offset a portion of human-induced climate change by removing more CO2 from the atmosphere than they would have otherwise.

"The full implications are still far from clear, however, and any benefits may be more than offset by other negative changes, such as heat waves and extreme weather, biodiversity loss, sea level rise, and so on," said Keeling.

Source: University of California - San Diego [September 12, 2017]

Read More

How openings in Antarctic sea ice affect worldwide climate

In 1974, images acquired from NOAA satellites revealed a puzzling phenomenon: a 250,000 square kilometer opening in the winter sea ice in the Weddell Sea, south of South America. The opening, known as a polynya, persisted over three winters. Such expansive ice-free areas in the ocean surrounding Antarctica have not been seen since, though a small polynya was seen last year.

A polynya, or an opening in the sea ice, was present in the Southern Ocean in the 1970s. This image shows the sea ice 
concentration averaged over three September months 1974-1976 during the Weddell Polynya, made with data from 
the NIMBUS-V satellite from the National Snow Ice Data Center [Credit: University of Pennsylvania]

In a new analysis of climate models, researchers from the University of Pennslyvania, Spain's Institute of Marine Sciences and Johns Hopkins University reveal the significant global effects that these seemingly anomalous polynyas can have. Their findings indicate that heat escaping from the ocean through these openings impacts sea and atmospheric temperatures and wind patterns around the globe and even rainfall around the tropics. Though this process is part of a natural pattern of climate variability, it has implications for how the global climate will respond to future anthropogenic warming.

"This small, isolated opening in the sea ice in the Southern Ocean can have significant, large-scale climate implications," said Irina Marinov, a study author and assistant professor in Penn's Department of Earth and Enviromental Science in the School of Arts & Sciences. "Climate models suggest that, in years and decades with a large polynya, the entire atmosphere warms globally, and we see changes in the winds in the Southern Hemisphere and a southward shift in the equatorial rain belt. This is attributable to the polynya."

The study appears in the Journal of Climate. Marinov coauthored the work with Anna Cabre, a former postdoc in Marinov's lab and now an oceanographer with the Institue of Marine Sciences in Barcelona, and Anand Gnanadesikan, a professor in the Department of Earth and Planetary Science at Johns Hopkins.

Typically, the Southern Ocean is covered in ice during the Southern Hemisphere's

winter. Polynyas occur when warm subsurface waters of North Atlantic and equatorial origin mix locally with cold surface waters, a process known as open-ocean convection.

Until recently, climate scientists and oceanographers believed that atmospheric and ocean conditions around the tropics were the primary drivers in affecting conditions outside the tropics. But in the last few years, Marinov and collaborators and others have shown that the opposite is also true: the Southern Ocean has an important role in affecting tropical and Northern Hemisphere climates.

Marinov's team uncovered a natural climate pattern originating from openings in the sea ice of the Weddell Sea. This 
diagram shows the Weddell Sea surface temperature (in black) and Southern Ocean sea ice cover percentage (red) for 
500 years in the researchers' climate model simulation. At bottom: the temperature in the Weddell Sea is shown over 
sea depth for the same 500 years of the simulation. In non-convective years the heat is stored in the Southern Ocean 
subsurface, the surface is cold and there is extensive sea-ice. In convective years the subsurface heat is released 
from the deep ocean, heating the surface ocean and melting the sea ice, resulting in polynyas 
[Credit: University of Pennsylvania]

In the current work, Marinov and colleagues used powerful models that simulate past and future climate to determine how the effects of polynya ripple out around the globe.

Their model indicated that polynyas and accompanying open-ocean convection occur roughly every 75 years. When they occur, the researchers observed, they act as a release valve for the ocean's heat. Not only does the immediate area warm, but there are also increases in overall sea-surface and atmospheric temperatures of the entire Southern Hemisphere and, to a lesser extent, the Northern Hemisphere, as well.

Changes in north-south temperature gradients lead to changes in wind patterns as well.

"We are seeing a decrease in what we call the Southern Hemisphere westerlies and changes in trade winds," Marinov said. "And these winds affect storms, precipitation and clouds."

Among these changes in precipitation is a shift in the Intertropical Convergence Zone, an equatorial belt where trade winds converge, resulting in intense precipitation. When a polynya occurs, this rain belt moves south a few degrees and stays there for 20 to 30 years before shifting back.

"This affects water resources in, for example, Indonesia, South America and sub-Saharan Africa," said Marinov. "We have a natural variation in climate that may be, among other effects, impacting agricultural production in heavily populated regions of the world."

Given these broad-scale implications of a Southern Ocean phenomenon, Marinov underscores the need to increase monitoring in the region. She is part of an effort called SOCCOM, for Southern Ocean Carbon and Climate Observations and Modeling, placing robotic floats in the Southern Ocean to collect data on ocean temperature, salinity, carbon, nutrients and oxygen.

"We're also urging people to keep a close eye on the satellites to look for other polynyas, this year and going forward," Marinov said.

Earlier research by Marinov's group and collaborators suggested that, under climate change, polynyas may become less frequent. As sea ice melts it freshens the top layer of the sea surface, making it lighter and less likely to mix with the heavier bottom waters. Marinov notes that the fact that no significant polynyas opened up from the mid-1970s until last year may have contributed to the so-called "climate hiatus" in the late 1990s and early 2000s, when global average surface temperatures appeared to stall in their otherwise persistent upward climb.

"During this hiatus period abnormal amounts of heat were stored in the subsurface ocean waters" Marinov said. "Most research has attributed this hiatus to a prolonged La Nina period, resulting in a storage of heat in the low-latitude Pacific. But I think that a lack of a Weddell Sea polynya also contributed, storing more heat in the Southern Ocean and preventing the additional release of heat to the atmosphere."

The work raises many new questions, such as how a decreasing sea ice extent, including the recent breaking off of a massive chunk of the Antarctic peninsula, will affect the frequency of polynyas and how the presence or absence of polynyas will affect how much atmospheric temperatures warm in response to anthropogenic climate change.

"This investigating into polynyas and Southern Ocean convection turned out to be a very important and interesting story for the global climate that we think a lot of people will be studying in the next decade," Marinov said.

Source: University of Pennsylvania [September 11, 2017]

Read More

Cold region 'tipping point' now inevitable

The decline of cold regions called periglacial zones is now inevitable due to climate change, researchers say.

Intense soil frost churning at Kilpisjarvi, northwestern Finland, at 800 metres above sea level 
[Credit: uha Aalto]

Periglacial zones, where there is often a layer of frozen ground known as permafrost, make up about a quarter of Earth's land surface and are mostly found in the far north and south, and at high altitudes.

Scientists from the universities of Exeter and Helsinki and the Finnish Meteorological Institute examined natural processes caused by frost and snow which take place in these zones.

Their findings suggest that -- even with optimistic estimates of future carbon emissions -- areas covered by periglacial zones will reduce dramatically by 2050, and they will "almost disappear" by 2100.

This would have a major impact on landscapes and biodiversity, and could trigger climate "feedbacks" -- processes that can amplify or diminish the effects of climate change.

"The results suggest that profound changes can be expected in current periglacial zones regardless of climate change mitigation policies," said Dr Juha Aalto, of the University of Helsinki and the Finnish Meteorological Institute.

"Unfortunately, it seems that many of the frost-driven processes we studied are already at the margin of the climate in which they can exist."

The scientists studied four processes which take place in periglacial zones, including snow accumulation sites and "frost churning" -- which refers to mixing of materials caused by freezing and thawing.

"Our results forecast a future tipping point in the operation of these processes, and predict fundamental changes in ground conditions and related atmospheric feedbacks," Dr Aalto added.

Dr Stephan Harrison, of the University of Exeter's Penryn Campus in Cornwall, said: "The project used very high-resolution climate and land surface models to demonstrate that geological processes and ecosystems in high latitudes (the far north and south) will be fundamentally altered by climate change during this century."

Even based on the optimistic RCP2.6 estimate for future carbon emissions, the researchers predict a 72% reduction in the current periglacial zone in the area of northern Europe they studied.

By 2100, periglacial zones in will only exist in high mountain regions, they say.

Professor Miska Luoto, of the University of Helsinki, said: "The anticipated changes in land surface processes can feedback to the regional climate system via alterations in carbon cycle and ground surface reflectance (light reflected by snow and ice) caused by the increase of shrub vegetation to alpine tundra.

"Our results indicate significant changes in Northern European plant life. Many rare species can only be sustained in areas of intense frost activity or late-lying snow packs, so the disappearance of such unique environments will reduce biodiversity."

The paper is published in the journal Nature Communications.

Source: University of Exeter [September 11, 2017]

Read More

Indicators of global climate change are detected in tropical oceans

Researchers from the University of California, Irvine and NASA's Jet Propulsion Laboratory have reported the first observation of sea level "fingerprints," tell-tale differences in sea level rise around the world in response to changes in continental water and ice sheet mass. The team's findings were published in the American Geophysical Union journal Geophysical Research Letters.

"Scientists have a solid understanding of the physics of sea level fingerprints, but we've never had a direct detection 
of the phenomenon until now," says study co-author Isabella Velicogna, a UCI professor of Earth system science
 and Jet Propulsion Laboratory research scientist, shown here on an expedition to Greenland 
[Credit: Maria Stenzel/UCI]

"Scientists have a solid understanding of the physics of sea level fingerprints, but we've never had a direct detection of the phenomenon until now," said co-author Isabella Velicogna, UCI professor of Earth system science and JPL research scientist.

As ice sheets and glaciers undergo climate-related melting, they alter Earth's gravity field, which causes nonuniform sea level change. Certain regions, particularly in the middle latitudes, are harder hit. For instance, Antarctica-generated sea level rise in California and Florida is as much as 52 percent greater than what's average in the rest of the world.

Cumulative sea level fingerprints calculated from observations of Greenland, Antarctica, glaciers 
and ice caps and land water storage mass changes observed with the GRACE satellites for the time
 period January 2003 to April 2014 [Credit: Isabella Velicogna & Chia-Wei Hsu/UCI]

The team calculated sea level fingerprints using time-variable gravity data collected by the twin satellites of NASA's Gravity Recovery & Climate Experiment between April 2002 and October 2014. During that time, according to the study, the global mean sea level grew by about 1.8 millimeters per year, with 43 percent of the increased water mass coming from Greenland, 16 percent from Antarctica, and 30 percent from mountain glaciers. The scientists verified their calculations of sea level fingerprints associated with these mass variations via ocean-bottom pressure readings from stations in the tropics.

Sea level fingerprints in millimeters per year calculated from observations of the mass loss of Greenland, 
Antarctica, glaciers and ice caps and changes in land water storage using time-variable gravity data collected 
by the twin satellites of the U.S./German Gravity Recovery and Climate Experiment mission between April 2002 
and October 2014. The blue contour (1.8 mm/year) is the average sea level rise if the total addition of mass 
of water to the ocean was spread uniformly over the world’s oceans. The SLF are not uniform and bulge
 around the equatorial regions [Credit: Isabella Velicogna & Chia-Wei Hsu/UCI]

"It was very exciting to observe the sea level fingerprints in the tropics, where they were not expected to be detectable," said lead author Chia-Wei Hsu, a graduate student researcher at UCI. "In the tropics, sea level fingerprint values are very close to global average sea level values, making them harder to detect."

Velicogna added: "We know that sea levels climb faster in the middle to low latitudes versus the high latitudes and that Greenland and Antarctica contribute differently to the process. With our improved understanding through GRACE data and other techniques, we're now able to take any point on the global ocean and determine how much the sea level there will rise as a result of glacier ice melt."

Source: University of California - Irvine [September 08, 2017]

Read More