Plants and animals shrinking with climate change?

The common toad is one of many organisms shown to decrease in body size when the climate warms. Credit: T.J. Blackwell

Life on earth may be shrinking in size as a result of climate change, according to a recent study published in Nature Climate Change. From tortoises to sheep, to trees and grass, University of Singapore biologists Jennifer Sheridan and David Bickford have compiled fossil and experimental evidence showing that many species adapt to climate change by decreasing in body size.

This is not Alice in Wonderland – the adaptation evolves over the course of generations, not within given individuals. Still, species with short generation times can evolve quickly and some have already begun to show growth changes associated with climate trends. For example, over the course of a 23 year study of the common toad in southern England, British ecologist C.J. Reading showed that female body size decreased as winters became more mild from 1983 to 2005. Dr. Reading, who published his findings in Oecologia in 2007, argues that the mild winters disrupted the metabolic hibernation cycles within these cold-blooded creatures. As in all cold-blooded animals, the common toad’s metabolism speeds up in warm temperatures and slows down in cool temperatures. So, during mild winters, a toad burns through its fat reserves faster than during a colder winter, depleting its energy reserves available for springtime growth.

Some plant species may also experience stunted growth from indirect effects of climate warming, like changes in water and nutrient availability. Increased flooding, for example, washes nutrients from forest floors and prevents plants from growing to full capacity. Drought, on the other hand, decreases plant respiration and growth. In response, animals that feed on these shrinking plants will need to compensate either by eating larger quantities of the plant, or by succumbing to the shrinking trend by evolving to be smaller themselves. Since animals with small bodies often give birth to small offspring, Sheridan and Bickford argue that this trend may be exacerbated through time in a positive feedback loop.

How low will we go? The extent of growth stuntedness will vary across species and habitats, and will unfold in a web of complicated ecological adjustments. Fossil evidence from the last great warming period on earth, which occurred about 56 million years ago, indicates that beetles, ants, and cicadas shrank by 50 – 75% over the course of 20,000 years. This likely bares little on the fate of 21st century animals, since warming is happening at a faster rate today than it did back then. However, it does at least offer compelling evidence that warming-shrinking trends have unfolded in the past.

But how low will we go, as humans? Sheridan and Bickford argue that ecological shrinking could impact human nutrition by limiting important crop and protein sources. This may or may not ultimately affect the way human body size evolves. Meanwhile, if you do find yourself falling down a rabbit hole with the opportunity to eat size-reducing cake, it may be nutritionally beneficial to go for it. These things are difficult to predict.

Posted by Laura Poppick, Assistant Editor of Maine Climate News

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Sedimentary Bedrock Fertilizes Californian Forest, Improving its Capacity to Store Carbon

Sedimentary rocks fertilize the coniferous forest on South Fork Mountain, CA, increasing the forest's capacity to store carbon. Credit: S. Morford.

New findings from a team of U.C. Davis ecologists show that sedimentary bedrock fertilizes plant growth in a northern Californian forest, filling an ecological role previously thought to be reserved for bacteria. Graduate student Scott Morford and colleagues have found that the rock provides ecologically available nitrogen (N), an essential nutrient for plant growth. Since plants cannot use N in its most abundant form as an atmospheric gas, they rely largely on bacteria to ‘fix’ atmospheric N into a usable form in soil. Now, Morford’s team shows that plants can also use ancient N stored in rocks, unveiling a hitherto unaccounted for reservoir of fertilizer that promotes plant growth and carbon sequestration in forests.

The sedimentary rocks fertilizing Morford’s study site formed on ocean and lake floors roughly 140 million years ago during the Cretaceous period. As dinosaurs frolicked and battled around these bodies of water, layers of N-rich organic material slowly accumulated within bottom sediments. After millions of years of compaction, these sediments were exhumed as solid rock to form the modern forest floor. Over time, the rock has broken down enough to slowly release nitrogen into the soil.

Not all bedrock has the same fertilizing effect as this Cretaceous sedimentary rock. Igneous rocks, for example, come directly from the Earth’s interior and contain only a small fraction of the N available in sedimentary rocks. Morford compared plant productivity in a forest underlain by sedimentary rock versus a forest underlain by igneous rock and found that the sedimentary forest was 50% more productive and stored more carbon than the igneous forest. Since sedimentary rocks are extremely prevalent, covering 75% of the Earth’s surface, these findings have global implications for the carbon cycle and the capacity for forests to slow climate change through carbon sequestration.

Morford has yet to identify the rate at which the bedrock releases nitrogen, which will be important in understanding the full potential of sedimentary forests to store carbon in the future. Regardless, he has discovered a new reservoir of N that fundamentally changes the way scientists understand the global N-cycle and the ecological role of rocks in forests.

Posted by Laura Poppick, Assistant Editor of Maine Climate News

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Tropical Rainforests: from carbon sink to source?

Tropical rainforests cover 6% of Earth’s surface, but their soils contain nearly 30% of the total global soil carbon stocks, making them one of the most important carbon sinks on Earth. Yet, ironically, tropical rainforests are also the largest natural source of carbon dioxide on Earth. Plant material grows, decays, and makes way for new growth so rapidly within these lush habitats that carbon stocks remain fairly unstable. Still, rates of carbon sequestration generally outpace carbon release in tropical rainforests, allowing them to maintain their crown in the carbon sink-dom.

Now, with preliminary results published in last week’s issue of Nature Climate Change, British ecologist Emma Sayer and colleagues are finding that climate change may dethrone tropical rainforests from their reign in the world of carbon sequestration. Will increasing atmospheric carbon dioxide levels transform this carbon sink into a carbon source?

World map showing tropical rainforests highlighted in yellow. Credit: National Geographic and World Wildlife Fund.

Atmospheric carbon dioxide naturally fertilizes plants, as plants use carbon dioxide to photosynthesize. With ever-increasing carbon dioxide emissions in the global atmosphere, plant production has already started to increase in some locations, particularly in tropical forests. Increased plant productivity amounts to larger and more abundant leaves. Since leaves suck carbon dioxide out of the atmosphere, increased leaf growth would presumably bolster a given tree’s ability to sequester carbon dioxide. Sayer and her colleagues, however, show that plant-atmosphere interactions are not this simple, and are further complicated by increased carbon-fertilization.

Yes, carbon-fertilized leaves serve as a carbon sink while they are still attached to their respective trees. But, upon dying, they fall to the forest the floor and contribute to a growing pile of dead plant material, or litter. Litter is quickly scavenged by decomposing microbes in the soil that release the once-sequestered carbon back into the atmosphere. This activity is normal, and explains why tropical rainforests are such a large natural carbon dioxide emitter. However, according to Sayer’s findings, increased leaf litter produced by carbon-fertilization may stimulate microbes to decompose more efficiently because leaves are more decomposable than woody material on the forest floor. This ‘priming’ of the microbes with an influx of easily decomposable material could disproportionally increase the net amount of carbon dioxide emitted from a given stand of trees.

A lizard resting on a leaf in El Yunque National Forest in Puerto, the only tropical rainforest in the U.S. National Forest system. Credit: Brian Stinga.

Sayer’s study is uniquely long term, with data spanning from 2005 to 2009 . Such long term studies are crucial in detecting plant-soil-atmosphere interactions that fluctuate from season to season and year to year. Similar long term studies have yet to be implemented on other forest types, but Sayer suggests that other forest types will likely experience a similar ‘priming’ effect induced by increased carbon-fertilization. If this is the case, climate models should be tweaked accordingly to better predict future oscillations in forest carbon stocks.

Posted by Laura Poppick, Assistant Editor of Maine Climate News.

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First Complete Map of Antarctic Ice Flow

On Thursday, NASA-funded scientists released the first complete map of Antarctic ice flow. Until now, Antarctic ice flow studies have focused on the outer fringes of the continent, leaving the frozen interior essentially uncharted. The new map illustrates the speed and direction of ice flow across the entire continent, dramatically improving the accuracy with which scientists will be able to track future ice sheet movement and sea-level rise related to climate change.

Map of Antarctica showing ice flow velocities. Credit: NASA/JPL-Caltech/UCI

Video of Antarctic ice flow (NASA, 2011).

The map reveals a major mountain ridge in East Antarctica that bisects the continent from east to west. In a video of the findings, you can track tributary glaciers weaving dendritically from the ridge across thousands of miles of the continental interior, eventually feeding outlet glaciers that flow into the sea. The video illustrates the true nature of glaciers as rivers of ice, like tributaries of the Mississippi River feeding from the Rocky Mountains into the Gulf of Mexico.

But how can ice flow like a river of water? While ice is solid under certain conditions, it is actually fairly deformable under high pressure because pressure exerts heat that induces melting and flow. Given that the Antarctic ice sheet is more than one mile thick on average, it exerts plenty of pressure to initiate flow.

A geology professor of mine demonstrated the concept of glacial flow to my class by relating glaciers to pancakes (yes, he pulled out a griddle and started cooking glacier pancakes for us — yes, we got to eat them). Like pancake batter spreading laterally as you pile it on the griddle, glaciers spread laterally as snow and ice accumulates at some topographic high. In both cases, the pressure exerted by the vertical accumulation of material forces that material outward. Theoretically, if a glacier were to build on a uniformly flat and sturdy surface like a pancake griddle, it would form the  circular shape of a pancake. This rarely happens. The elongated tributary glaciers on Antarctica have conformed to existing valleys or have carved out pathways through erodible rocks.

Aside from glacial shape, the direction and speed of glacial flow also varies greatly across Antarctica because of its varied landscape. For example, ice flows more quickly in narrow valleys than in broad valleys because narrow valleys channelize thick ice. Thick ice increases friction and heat, creating meltwater along the valley floor. Meltwater lubricates ice movement and increases the speed of glacial flow.

Such concepts in glaciology are well established but, until now, they have never been documented in detail across Antarctica. Now, with velocity data from hundreds of tributary glaciers and cutting-edge evidence that ice flow initiates thousands of miles from the sea near topographic divides, the new map offers a vibrant future for research in ice flow dynamics and global sea-level rise. And, underneath the ice flow velocities, the map reveals an Antarctic landscape of mountains and valleys that has hitherto been a frozen enigma. Yet again, climate change research allows us to become better acquainted with our planet.

Posted by Laura Poppick, Assistant Editor of Maine Climate News.

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Human Carbon Dioxide Emissions Dwarf Annual Volcanic Emissions

Human Carbon Dioxide Emissions Dwarf Annual Volcanic Emissions

Volcanic eruption in the Aleutian Islands of Alaska. Credit: International Space Station, ISS013-E-24184

Yes, aerosols from volcanic eruptions cool the climate, but don’t carbon dioxide emissions from the same eruptions counteract this
cooling? No, at least not significantly.

Terrance Gerlach, scientist with the U.S. Geological Survey, untangles this commonly misconstrued reality in the American Geophysical Union’s EOS June 2011 issue. The amount of carbon dioxide emitted by volcanoes, Gerlach affirms, is inconsequential when compared to human emissions. He calculates that humans emit the equivalent amount of carbon dioxide as the 1991 Mt. Pinatubo eruption every 12.5 hours. Every 2.7 days, we emit the equivalent amount of carbon dioxide released by volcanoes during an average year. Our ubiquitous and unabated emissions easily dwarf volcanic emissions, and the cooling effect of volcanic aerosols from a single catastrophic event is much larger than the average annual warming effect of volcanoes worldwide.

There have been periods in Earth History, however, when volcanic carbon dioxide emissions may have been more significant in global climate. Roughly 640 million years ago, for example, volcanic emissions may have pulled the Earth out of a global glaciation. During this glaciation, called Snowball Earth, the Earth is thought to have been sealed in ice as thick as 1 kilometer in some regions, tapering down to less than than 2 meters at the equator. In such a scenario, the atmospheric-oceanic-terrestrial carbon cycle would have slowed down to a near standstill, seriously hindering the Earth’s natural thermostat. Volcanic eruptions, both sub-marine and sub-glacial, may have been the one source of carbon dioxide available to thaw the snowball.

According to studies led by Paul Hoffman, Professor Emeritus at Harvard University and a leader in Snowball Earth research, carbon dioxide from volcanic emissions could have leaked through weakly-frozen regions of the equatorial ocean and cracks in terrestrial glaciers, slowly warming the atmosphere. With essentially all carbon sinks sealed off, this carbon dioxide would have built up unabatedly in the atmosphere. Over the course of millions of years, enough could have accumulated to end Snowball Earth.

So, yes, volcanoes may contribute to significant climate warming, but only over the extended course of geologic time. Over the course of the 21st century, a mere blink in geologic time, we can safely assume that, if anything, volcanoes will cool global climate.

Posted by Laura Poppick, Assistant Editor of Maine Climate News.

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Small Volcanic Eruptions Cool the Climate

The sky in Edvard Munch’s “The Scream” is thought to have been inspired by the red sunsets generated by the eruption of Krakatoa, Indonesia.

Small Eruptions Cool the Climate

On August 27, 1883, civilians on the Indian Ocean island of Rodrigues heard an explosion. What they initially assumed was cannon fire from a neighboring ship turned out to be a more distant threat: the sound of a volcano erupting on Krakatoa, Indonesia, nearly 3,000 miles away.

Traveling 50 miles through the atmosphere into the stratosphere, aerosols from the volcano’s ash plume billowed over the Earth along air currents that tinted the global sky for several years to follow. This veil of aerosols reflected enough incoming solar radiation to lower global temperatures by 2.2°F, and cast a dramatic red glow on skies above the U.S., Europe, and Asia. The brilliant sunset in Edvard Munch’s The Scream was allegedly inspired by these Krakatoan skies.

Until recently, atmospheric scientists assumed that only such catastrophic eruptions could significantly alter Earth’s climate. Other eruptions of similar scale have been documented to have temporarily cooled global climate, with the 1815 eruption of Mount Tambora in Indonesia causing a ‘year without a summer’, and the 1991 eruption of Pinatubo in the Phillippines cooling global temperatures by nearly 1.0°F.

Now, NOAA scientists say that even small volcanic eruptions can manipulate global climate. In a paper published last month in Science, Susan Solomon and her team report that ‘background’ levels of global stratospheric aerosols have nearly doubled over the past decade and have slowed the trajectory of climate warming. Given that no catastrophic volcanic eruptions have occurred since the 1991 Pinatubo event, other sources must account for this recent increase in aerosols. Some argue for coal burning power plants in China, but NASA scientist John Vernier and his team point to a series of moderate low-latitude tropical volcanic eruptions.

Low-latitude volcanic plumes rise above the equator and spread across both the Northern and Southern Hemispheres like tape in a cassette. High-latitude volcanic plumes, on the other hand, generally remain contained within their hemisphere of origin. Vernier shows that a series of increasingly more intense but still relatively moderate tropical eruptions have substantially contributed to the steady rise in global stratospheric aerosols over the past decade.

While the cooling effect of stratospheric aerosols is relatively fleeting, lasting up to 2 or 3 years, it is still significant in global climate. And yet models that predict future climate change do not generally account for volcanic aerosols. Solomon and Vernier both encourage climatologists to place more weight on volcanic aerosols to improve the accuracy of climate models.

Posted by Laura Poppick, Assistant Editor of Maine Climate News.

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Loops of Change: the Positive Feedback Loops that Drive Climate Change (Part IV)

 Permafrost Thaw and Bogland Loss

Tolland Man. Credit: Sven Rosborn

You may or may not be familiar with the bog man named Tolland Man. Tolland Man lived during the 4th century BCE and is believed to have been ritualistically sacrificed and buried in a bog in what is now Tolland, Denmark. The relatively low temperature, acidic, and oxygen-poor conditions of the waterlogged bog inhibited decay of his body, allowing his skin and other soft tissues to be remarkably well preserved. So much so that the Danish woman who discovered him in 1950 while harvesting peat to burn in her stove assumed that she had come across a recent murder victim. Only after the local police hired an archaeologist to investigate the site was Tolland Man discovered to have lived more than 2,000 years prior, during the Iron Age. His body now sits in the Silkeborg Museum in Denmark.

Fascinating from an archaeological standpoint, the preservation capacity of bogs is also an important component of the global climate system. That is, the same chemical and physical properties that slowed decomposition of Tolland Man’s body also do so for dead plant material. Since decomposition releases greenhouse gases like carbon dioxide and methane, the limited decay rates in bogs allows them to serve as a significant terrestrial carbon sink.

Current distribution of peat bogs in North America. Credit: *Cai and Yu, 2011.

For a terrestrial habitat to be a carbon sink, plants need to grow and photosynthesize (absorb carbon) at a faster rate than they respire or decay (release carbon). The waterlogged nature of bogs creates an anoxic, or oxygen-depleted, environment that greatly slows decomposition and allows carbon absorption to outpace release. Over time, accumulated layers of partially decayed plant matter, or peat, form a secure bank of sequestered carbon similar to the sedimentary deposits created by marine snow on the sea floor.

Climate change may drain bogs of their characteristic waterlogged conditions, thereby threatening their role as a global carbon sink. Bogs are susceptible to warming because they rely on permafrost, or frozen soil, to maintain their structure. Generally occurring in northern boreal forests, bogs form when seasonal snowmelt pools at the soil surface above a barrier of permafrost below. As permafrost thaws with climate change, this barrier weakens and bog water drains from the surface. Soils that were once waterlogged and anoxic become aerated, thereby increasing decomposition rates and carbon dioxide emissions. As permafrost continues to thaw in this positive feedback loop, drained bogs become carbon sources.

Still, while permafrost drives bog hydrology, other factors affect bog health and carbon sequestration capacity. For example, in a paper published last month in Nature Geoscience, University of Victoria professor Christopher Avis and his research team suggest that increased precipitation with climate change could help bogs maintain waterlogged conditions as permafrost degrades. On the other hand, they point out, warmer conditions will increase both aerobic and anaerobic decay rates. Anaerobic decomposition produces methane, a stronger greenhouse gas than carbon dioxide. Since bog decay occurs primarily anaerobically, methane emissions are expected to increase, thereby expediting further warming.

Tolland Man would likely be surprised to know of the global climatic implications of the bog conditions that preserved his skin and allowed him to gain fame more than 2,000 years after his death. Now imagine yourself waking up 2,000 years from today. What will the global climatic landscape look like then? Though we are in an era of rapid change now, the Earth system will eventually reclaim equilibrium over the course of geologic time. We can’t know if this will occur 200, 2,000 or more years from now.

As chaotic as 21st century climate change may seem, it at least provides us a chance to understand the Earth as a unified system in a way that Tolland Man could never have conceived. Perceiving the Earth as a unified system, and understanding the strengths and vulnerabilities of this system, may help ground us through a future of change.

Posted by Laura Poppick, Assistant Editor of Maine Climate News.

Loops of Change is a weekly series through July exploring the major positive feedback loops that drive climate change.

*Cai, S., and Yu, Z. 2011. Response of a warm temperate peatland to Holocene climate change in northeastern Pennsylvania. Quaternary Research: 75. 531 – 540.

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Loops of Change: the Positive Feedback Loops that Drive Climate Change (Part III)

NASA satellite image (with inset) from July 15, 2011 showing sea ice extent and concentration. Purple identifies areas of high sea ice concentration, while red and yellow show areas of low concentration.

The Biological Pump

Earlier this week, the National Snow and Ice Data Center reported that Arctic sea ice extent declined at an average rate of 46,000 square miles per day during the first two weeks of July. You can track daily images of sea ice extent here.

While sea ice naturally melts every summer, this summer’s melt area already surpasses the 1979 – 2000 average by 865,000 square miles. This, in part, results from this year’s early onset of melting, which occurred two weeks to two months earlier than the 1979 – 2000 average in certain regions. Early spring melting produces colonies of melt water ponds on top of the ice that decrease ice albedo and induce further melting. While sea ice extent is currently at record low levels, predictions for the duration of the summer suggest that ice extent will not ultimately surpass the record minimum levels of September 2007.

The direct climatic effects of increased melting, such as decreased net albedo,  are relatively easy to identify. The indirect effects, on the other hand, can be more elusive. For example, what are the implications of melting sea ice on the phytoplankton community of the ocean? Why does this question concern climate scientists?

Marine phytoplankton drive the biological pump, a carbon sequestration mechanism that, along with the solubility pump, makes the ocean a major global carbon sink. Phytoplankton suck carbon dioxide out of the atmosphere during photosynthesis. When they die, or are released as fecal pellets of zooplankton, they aggregate and sink out of reach of the atmosphere. The resulting mass of ‘marine snow’ showers the sea floor with sequestered carbon dioxide, and slowly accumulates over geologic time as a layer of sedimentary rock. The White Cliffs of Dover are an example of an exhumed phytoplankton graveyard and vault of ancient atmospheric carbon dioxide.

A photo of 'marine snow' falling at 55 meters in Monterey Bay. Credit: Woods Hole Oceanographic Institution.

Global climate both manipulates, and is manipulated by, the efficiency of the biological pump. As sea ice melts, the ocean surface freshens and absorbs more heat. This fresh, warm water is more buoyant than the cold, saline water below, and forms a floating layer that stratifies the water column. The resulting density barrier near the surface of the ocean can prevent deep, nutrient rich bottom waters from upwelling and fertilizing plankton blooms. With a limited nutrient supply, plankton blooms atrophy and the efficiency of the biological pump decreases.

Given the wide variability in temperature, salinity, and currents across the global ocean, melting sea ice will not have a uniform effect on the biological pump. In some regions, its efficiency may remain unaltered, or may improve for unrelated reasons. Ultimately, climate-induced changes in the solubility pump will likely have a larger impact on the role of the ocean as a carbon sink.  Still, the biological pump remains an active area of research for climate scientists.

As oceanic carbon sinks begin to weaken, will terrestrial sinks compensate? Stay tuned for next week’s post on Permafrost Thaw and Wetland Decay, where we will leave the oceans to trek through a terrestrial loop of change.

Posted by Laura Poppick, Assistant Editor of Maine Climate News.

Loops of Change is a weekly series through July exploring the major positive feedback loops that drive climate change.

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Loops of Change: the Positive Feedback Loops that Drive Climate Change (Part II)

The Solubility Pump: An Oceanic Cooling System

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In a paper published earlier this week in Nature Geoscience, oceanographer Galen McKinley and her research team reveal that rising North Atlantic ocean temperatures are dampening the ocean’s ability to absorb atmospheric carbon dioxide. They present data from the past 29 years, long enough to overcome natural oscillations in the ocean system and expose a real trend. Since the ocean is one of the most important global carbon sinks, absorbing a quarter of all atmospheric carbon emissions, these findings have significant implications for the Earth’s carbon cycle and future climatic system.

But why does a warmer ocean reject carbon dioxide? For the same reason that drinking a lukewarm carbonated beverage is generally underwhelming. Lacking the crispness that we crave in such refreshments, these drinks have been subjected to gas laws that make it hard for warm water to keep gas in solution. As a result, they lose their bubbles to the atmosphere and become ‘flat’.

Even if kept cool, beverages eventually lose their fizz, and their appeal, to the atmosphere. This is because, according to Henry’s Law, gases constantly try to attain equilibrium with their surroundings. Beverages maintain carbonation over some time because manufacturers pump beverage containers to the brim with carbon dioxide. Upon opening, gas bubbles immediately begin to escape and equilibrate with the atmosphere.

The ocean, unlike carbonated beverages, naturally contains less carbon dioxide than the ambient atmosphere and thus absorbs atmospheric carbon dioxide. This natural carbon sequestration mechanism is called the Oceanic Solubility Pump, and is an important cooling knob on the global thermostat discussed in last week’s post. But, as McKinley’s team reveals, this pump weakens in a warming climate system and, in some regions, may begin to hose carbon back into the atmosphere.

Famously, carbon dioxide warms our atmosphere. As the carbon-grabbing tendrils of the ocean begin to loosen with warming waters, the following positive feedback loop takes hold:

More carbon dioxide accumulates in the atmosphere, thereby

1. increasing atmospheric and ocean temperatures,
2. further weakening the ocean’s grip on atmospheric carbon dioxide,
3. further raising atmospheric carbon dioxide concentrations,
4. leading to further warming.

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Loops of Change: the Positive Feedback Loops that Drive Climate Change (Part I)

Sea Ice-Albedo: Earth’s Chilling Reflection

Last month, the European Space Agency (ESA) released the first ever map of polar sea ice thickness. While two-dimensional maps of sea ice extent have been available since 1979, this is the first project to present total ice thickness in addition to extent. The ESA plans to continue updating the map in the future, providing climate scientists with an invaluable record of sea ice response to climate change.

(European Space Agency, 2011)

Climate-induced changes in polar sea ice extent depend largely on ice thickness for 2 reasons, the first more obvious than the second: 1) thin ice melts away faster than thick ice, and 2) the absence of ice promotes further melting in a positive feedback loop called the sea ice-albedo climate feedback mechanism.

The sea ice-albedo mechanism is rooted in the simple concept that light-colored surfaces reflect light and dark surfaces absorb light. When you choose to wear light-colored rather than dark-colored clothing on a warm summer day, you are increasing your body’s albedo, or its reflectivity power, and allowing yourself to feel cooler. The same is true on a local scale of dark pavement absorbing more heat and feeling hotter to walk on than the grass around it, and on a global scale of Earth’s surface and atmosphere reflecting more sunlight in light regions (polar ice caps) than in dark regions (open ocean). As the hues of your clothing affect your personal climate, Earth’s surface features collectively manipulate global climate.

Earth’s net albedo, or the percentage of incoming solar radiation reflected back into space, changes with shifts in surface features. It is susceptible to cloud cover, vegetation growth, and even your wardrobe choices. My particularly pale geologist friend jokes that she increases Earth’s albedo when she bares her skin outside — and she’s right.

While everything under the sun does contribute to Earth’s net albedo, only vast expanses can significantly manipulate Earth’s climate. Polar sea ice, for example, is an especially important component of net albedo because it is far reaching, highly reflective, and dynamically involved in global climate as follows:

When sea ice melts, it exposes dark, heat-absorbent open ocean which, in turn,

1. decreases net albedo,
2. increases regional heat absorption,
3. induces further melting,
4. further decreases net albedo,
5. further increases regional heat absorption,
6. and ultimately warms global climate.

This is the sea ice-albedo positive feedback loop.

So why doesn’t sea ice melt away completely during polar summers? Because albedo is just one knob on Earth’s natural thermostat, which hosts many other knobs working in tandem to maintain a relatively stable climate system. ESA’s new ice thickness mapping program will help determine the future stability of this thermostat. Meanwhile, stay tuned for next week’s post on the Solubility Pump: an Oceanic Cooling System to learn how ocean chemistry balances global climate.

Posted by Laura Poppick, Assistant Editor of Maine Climate News.

Loops of Change is a weekly series through July exploring the major positive feedback loops that drive climate change.

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