The end of the evolutionary road on a far future earth

The question of how life first came to be on early earth  has been of fascination to scientists for decades. By stark contrast, the question of how life may end on Earth in the distant future has received little attention, probably because the question itself forces us to face the rather depressing reality that one day, our star will die, our planet’s center will cool, and Earth will no longer be capable of supporting life. But the end of the road for life on Earth will not dawn on us overnight. Just as it took billions of years for our earliest prokaryotic ancestors to evolve into complex, multicellular life (these early microbial communities, spread far and wide across the earth, had the big responsibility of oxidizing the atmosphere so that multicellular organisms could have something to breathe) it will likely take millions if not billions of years for life on Earth to devolve as the planetary systems that allow complex ecosystems to exist slowly shut down.

How will the end of life happen?  On present day Earth, the weathering of silicate rocks draws carbon dioxide out of the atmosphere and returns it to the continental and oceanic crusts. This process occurs quite slowly (millions – billions of year timescales), but ultimately if there were no source of carbon to replenish the CO2 drawn out of the atmosphere by weathering, it would eventually be depleted. This would make our planet inhospitable to plant life, which requires CO2 to do photosynthesis. Unfortunately this is exactly what will happen when our star begins to die. As the Sun ages, its luminosity will increase, meaning Earth’s surface is going to get hotter. This will speed up silicate weathering and the speed with which CO2 leaves our atmosphere. Normally, this CO2 would be replenished with carbon recycled through the crust and mantle due to tectonic activity. However, the heating up of the planet’s surface will also increase surface water loss due to evaporation, which will increase the friction between the plates until eventually plate tectonics shuts down entirely.

When atmospheric CO2 concentrations drop below a critical threshold (< 10 ppm), plants will begin to die off. With the plants will go both the food and oxygen source for animals. Large ectotherms (large mammals and birds) will likely go first, due to their high oxygen requirements, followed by smaller ectotherms (i.e., small birds, rodents). Reptiles, amphibians and fish will likely outlast the rest of us due to their lower oxygen requirements, with marine species lasting longer in general because the oceans will likely heat up less rapidly than the land. Eventually, within perhaps 100 million years of the end of plant life, the world will be returned to the microbes.

And what might the final microbes on Earth be? In all likelihood, they will be extremophiles capable of tolerating multiple environmental stressors (polyextremophiles)- low oxygen concentrations and high temperatures, for example. They will recede from the planet’s surface where high heat and rapid evaporation will be too stressful. In all likelihood, they will return to deep ocean vents and similar subsurface environments that are both buffered from surface temperatures and capable of supplying their own source of energy.

I’ve just described the devolving (or perhaps mass extinction occurring on along a trajectory of evolutionary complexity over geologic timescales) of life of Earth in broad brushstrokes. As with most scientific theories, there are details, nuances, unanswered questions and questionable assumptions. Of course, this is also assuming that some anthropogenic force doesn’t end life on Earth far sooner (nuclear war, chemical warfare, climate change, GMOs, encountering an unfriendly and technologically superior alien race, creation and subsequent uprising of AI, zombie apocalypse). And it’s also possible that I’m being far too pessimistic, and some future technological or geoengineering solution will be able to keep our planet liveable far beyond its natural lifetime.

I’ll leave you with a plug for SpaceRip, a really awesome astronomy youtube show that has several episodes focused on questions of planetary habitability. These guys know way more than I do.

The Search for Earth-Like Planets

Isotopes?

Isotopes?

A term thrown in so many different fields of science but never really successfully explained outside the realm of the super nerdy. They’re pretty simple, really- essentially just flavors of the same atom with different numbers of neutrons. More neutrons, heavier isotope. Too many neutrons, and your isotope becomes unstable, radioactively decaying over time to a different version of that element or perhaps another element entirely. With that brief introduction I’d like to explain one stable isotope system that is particularly interesting to me because it allows scientists to take a piece of the earth and reconstruct ancient environments.

C12 and C13 are stable isotopes of carbon- they both occur naturally in the environment and do not undergo any natural physical transformations over time. However, because of a small difference in their molecular weights due to the “extra” neutron in the C13 isotope, these two isotopes are processed quite differently in the environment.

Being slightly heavier means that C13 is a bit more difficult for biological systems to process. Most biological processes are adapted to using the lighter isotope, which is far more abundant.  When air diffuses into plant leaves via stomates, the tiny pores that suck up carbon dioxide for photosynthesis, CO2 is tightly bound by an enzyme before it can diffuse out again. In most cases this enzyme is Rubisco (see post: enzymes in the environment) . It turns out that Rubisco preferentially binds to C12, causing C13-enriched air to be released back into the atmosphere. Since most plants take up CO2 via Rubisco (this is known as the C3 photosynthetic pathway), most plant tissue on Earth is depleted in C13 relative to the atmosphere.

However, when a plant keeps its stomata open to take up CO2, a problem emerges- transpiration. Water loss occurs primarily through these same pores that plants must keep open if they want to feed themselves. In hot, arid environments, this puts your normal C3 plants in a sticky situation. They must open their stomata to eat, but risk losing dangerous amounts of water when they do so.

Millions of years ago, a group of plants evolved a rather elegant solution to this problem, known as the C4 photosynthetic pathway. They co-opted an enzyme already present in mitochondria for cellular respiration and gave it Rubisco’s job. This enzyme, known as PEP-carboxylase (I’ll call it PEPC here for simplicity), has a much higher affinity for CO2 than Rubisco- in fact, it binds CO2 so tightly that leaf stomata only need to be open for a fraction of the time they would otherwise. The high affinity of PEPC for CO2 also means that it doesn’t “distinguish” C12 from C13- it grabs whatever CO2 molecule is closest and binds tightly.

What does it matter that two classes of plants fractionate C13 differently? Scientists now have the tools to analyze the molecular composition of plant tissue and can determine a plant’s specific C13/C12 ratio.  C3 and C4 plants have distinct C13/C12 ratios and are easy to distinguish once isotopic analysis has been performed. For living plants, this would not be a terribly illuminating exercise- there are other anatomical and taxonomical ways to distinguish C3 and C4 plants that would be much more straightforward and less expensive.

But what about dead plants? Soil organic matter is composed principally of decomposed plant material, but even the most knowledgeable soil scientists aren’t able to look at a soil and say exactly what plants produced it. If we could, however, the soil would tell us numerous things. Accumulating over hundreds to tens of thousands of years, the soil profile from bedrock to the surface essentially represents a continuum of accumulated material that represents different floral and faunal assemblages, climate regimes and major environmental disturbances.

However, a complex series of transformation processes take place as plant material is decomposed and moved down the soil profile, some of which lead to C13 accumulation while others lead to C13 depletion. Carbon compounds are sorbed to surfaces, eaten by microbes, recycled, taken up by plants, leached, oxidized, and protected, to name a few. Given the inherent complexity of these systems, how can scientists can’t always sample down a soil pit and accurately describe species assemblages at different times using carbon isotopes alone.

A more fruitful path has involved obtaining environmental  samples that have undergone relatively little decomposition, such as cores of sediment from the bottom of a lake, or a core of peat from an inundated field. The plant material within samples that have been buried or otherwise protected from decomposition will be relatively similar, at least at the molecular level, to the original plant tissue, and can thus provide meaningful information about a past environment. For example, a sudden switch from C3 to C4 dominated plant material could indicate a transition from a cooler, wetter climate to a warmer, drier one. Stable carbon isotopes have proved incredibly valuable in tracing the spread of human agriculture, which can often lead to rather dramatic changes in the isotopic signature of a sample.

Still don’t think isotopes are interesting? If you have a friend whose fidelity to vegetarianism is in question, sending a sample of their hair to an isotope lab should resolve the situation. Chances are, if you’re a vegetarian your C13 levels will be relatively high, indicating a more plant-rich diet (a disproportionate number of the world’s major crops are C4 plants).

Didmyo invades Patagonian rivers

A leathery brown slime is slowly creeping across what are widely considered to be the world’s most pristine rivers. Discovered first by William Horvath, a Patagonian kayaking guide, Didymosphenia geminata, or Didmyo, is a highly invasive alga that has mucked up numerous freshwater bodies across the world.  Didmyo forms large algal mats, secreting a thick layer of muck for which the popular nickname “rock snot” was given.

Aesthetics aside, Didmyo can be ecologically devastating. By coating river surfaces, it prevents sunlight from penetrating, thus killing off other photosynthetic organisms. Thick colonies rapidly deplete both river nutrients and oxygen; in much the same was that algal blooms have devastated the Gulf Coast and led to a massive anoxic dead zone.

Didmyo has appeared in numerous northern hemisphere rivers, and its first southern hemisphere appearance was in New Zealand five years ago. Researchers at the National Fisheries Service have identified several common factors in Didmyo blooms. Didmyo apparently thrive in stable water bodies, downstream of nutrient sources such as farming operations and wastewater treatment plants. Why it spreads so rapidly, and particularly in remote, unpolluted regions of southern Chile and New Zealand, remains a mystery.

Though scientists are not necessarily optimistic, the Chilean government is now launching a huge effort to stop the spread of the algae.

“Solar salamander” crosses symbiotic boundaries

Scientists have recently discovered the first example of a plant living symbiotically within a vertebrate, broadening our understanding of what sorts of symbioses are possible.

The spotted salamander Ambystoma maculatum is known to engage in a symbiotic relationship with the algae Oophila amblystomatis during embryonic development. Developing salamander eggs are covered in an algae-filled slime. The embryos produce nitrogen-rich waste, which the algae use for nutrition. In turn, the algae increase the amount of oxygen available to A. maculatum through photosynthesis.

This mutualistic relationship has long been understood. Recently, transmission electron microscopy, a high resolution microscopic imaging technology, has revealed O. amblystomatis cells residing within salamander embryos. Time-lapse videos of embryonic development reveal a fluorescent green flash occurring within an embryo as its nervous system forms.

There is evidence that these symbiotic algae may be maternally derived- they are the same algae that occur in female oviducts, where the embryo-encompassing jelly sacs form.

This unique symbiosis may provide insight into the early formation of eukaryotic cells. Chloroplasts, the principle photosynthetic organelle in plant cells, are thought to be the product of an ancient symbiosis between a free-living photosynthetic prokaryote and a larger proto-cell.

Credit:  Petherick, A. A solar salamander. Nature News. Nature. 30 July 2010.

GeoChip: linking genetics with environmental processes

Over the past decade, environmental scientists have been casting a wider net in their attempts to understand complex environmental processes on a molecular scale. Once fascinating new line of research involves co-opting techniques developed by geneticists, largely for the biomedical industry, in order to understand how genes are important regulators of earth-scale processes as carbon and nitrogen cycling.

The GeoChip is a clear example of this search for new methods to answer old questions. Microbiologists  are working on remote Antarctic islands to understand some of the simplest nutrient cycling pathways in the world. The ecosystems they study are often composed of only a handful of fungal and microbial species. These simple food chains allow resarchers to contruct basic models of how energy and nutrients (such as carbon and nitrogen) are transferred.

This is where GeoChip comes in. GeoChip is a gene microarray chip designed to identify “functional genes” involved in important nutrient cycles. It allows the identification of genes in an environmental sample that regulate carbon fixation, decomposition, and atmospheric nitrogen fixation, to name a few.  Understanding what functional genes are available in a system allows scientists to both understand the potential of that system for cycling nutrients and better predict how that system will respond to environmental change.

Imagine a glass floor divided into hundreds of indentical squares. Each of these squares contains a different fragment of DNA, reconstructed by geneticists from known DNA sequences. When scientists want to probe an environmental sample for specific DNA sequences, they “wash” their sample over the floor. Fragments of DNA will stick to their complementary sequence on the floor, causing a square to light up. Scientists can “read” a GeoChip by identifying fluroescently lit spots where environmental DNA has attached. They use this information to develop a picture of the functional genes present in that system.

In Antarctica, GeoChip is already been used to answer important ecological questions. For example, scientists are finding that genes for nitrogen fixation, the crucial ecosystem process that produces plant-useable nitrogen in the soil, occur in lichen-rich areas. Lichens are believed to be among the earliest land colonizers, and the ability of lichen-dominated systems to add nitrogen to the soil may be an important finding in reconstructing the early colonization of terrestrial systems. Other findings include carbon-fixation genes in plots that lack vegetation, indicating microbial communities that are able to perform some sort of photosythesis in the absence of plants.

Citation:

Yergeau et al. 2007. Functional microarray analysis of nitrogen and carbon cycling genes across an Antarctic latitudinal transect. The ISME Journal 1: 163–179

Shade leaves: nature’s most efficient light harvesters

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Not all leaves are created equal. Within a single plant, a spectrum of light availability exists- some leaves are positioned to receive maximum sunlight every day, while others, by luck of the draw, are primarily shaded and experience full sun only in short, erratic bursts.  The sun leaves, by virtue of their superior position, are equipped with the capacity to capture the most light and perform the most photosynthesis. They tend to have more cell layers, larger numbers of the enzymes involved in capturing CO2 and converting it to sugar, and a larger volume of space to perform carbon-fixing reactions than their less-exposed neighbors.

Though it may seem that shade leaves get a raw deal, they are in fact uniquely adapted to a low light environment. Seemingly minor biochemical changes allow shade leaves to be far more efficient light harvesters, maximizing the amount of sunlight captured during those rare moments of exposure.

Chlorophyll is the main light-harvesting pigment in most plants, capturing light radiation primarily in the red and blue parts of the visible spectrum. These light waves are between 400 and 700 nanometers in length and are known as photosynthetically active raditaion (PAR). When PAR is absorbed by chlorophyllous tissue, radiation of longer wavelengths in the 680-760 nanometer range is re-emitted in a process known as fluroescence.

Measuring the kinetics of chlorophyll fluorescence has proved a useful tool in discerning the different photosynthetic capabilities of sun and shade leaves. To do so, a plant physiologist may incubate identical leaves in contrasting light conditions and measured fluorescence levels when a “saturating” pulse of white light is applied in each case. It turns out that the dark incubated leaves respond far more dramatically to a brief pulse of light than that already light-exposed leaves.

The pulses of light used in such laboratory experiments are in fact comparable to the erratic “sun flecks” that understory leaves may experience over the course of a day. Much as a starving animal will metabolize and release energy more slowly than a well-fed one, shade leaves take the limited light resources available and make far more out of these brief, shining moments than sun leaves would ever be able to.

ocean color may affect hurricane activity

a recently study suggests that the color of the ocean, determined largely by the concentration of phytoplankton at the surface, may affect the development of tropical cyclones. “greener” sea surfaces with higher concentrations of photosynthetic plankton absorb more sunlight, causing light to scatter at the surface. in parts of the ocean that are devoid of phytoplankton, sunlight penetrates deeper and sea surface temperatures (SSTs) tend to be lower.

phytoplankton are a diverse group of photosynthetic microorganisms that form the basis of marine food chains

what do SST differences mean for tropical storm formation? higher SSTs lead to the formation of more energetic storms, providing both thermal energy and moisture required for storm formation. a new study suggests that if the north pacific subtropical gyre (an ocean circulation cell that comprises most of the north pacific) were entirely devoid of “light scattering particles” such as phytoplankton, the number of  cyclones forming in this region may be reduced by up to 2/3.

enzymes in the environment

enzymes are the catalysts of life. they are the link between higher forms of biological structure- cells, organisms, ecosystems- and the physical universe. they form such links by allowing incredible reactions to occur, reactions that strip complex molecules down into simple components that our cells can harvest energy from, reactions that detoxify harmful substances, reactions that take nonliving compounds and turn them into something organic. they have ugly names. ribulose-1,5-bisphosphate carboxylase oxygenase is a name that most eyes would glaze over while reading, but what if i told you that RuBisCO (it has a nickname!) is the only thing on earth that can add electrons to carbon dioxide? if that doesn’t seem to impressive, look out your window. not a single tree, flower, blade of grass, animal or human being (or man-made structure, for that matter) would exist if RuBisCO had not evolved to turn carbon dioxide into sugars.

there is a less appreciated truth about enzymes that i find to be equally intriguing, almost poetic. enzymes not only build and maintain life, they destroy it. or, to be a bit more accurate, they recycle its components. enzymes are largely responsible for decomposing organic matter, breaking down trees and blades of grass and human beings into the tiny carbon-rich compounds that RuBisCO created. in fact, if you take a small handful of soil from your garden, you are holding billions of free floating enzymes. they have been constructed by plants and microbes and were released into the environment to acquire something that their creator needs (i hate to use the word “creator”, when writing about science, if you have a better word, please do share). most often, this is an essential nutrient or a small sugar that can be used for energy. imagine if you could take your stomach out, and send it off to wendy’s to eat a chicken sandwich for you. not the prettiest analogy, perhaps, but this is in essence this is what microbes and plants do in the soil.

while intellectually it may be somewhat interesting to imagine billions of microbial exo-stomachs scouring the earth for their lunch, why should anyone really care about enzymes in the environment? well, truth be told, very few people do. but i’m going to tell you why an increasing number of environmental scientists are taking an interest in enzymes, not only in order to understand a process, but with the growing realization that understanding how enzymes shape our planet may be essential to averting looming environmental catastrophes.

as the agents responsible for the breakdown of organic, carbon containing compounds (and this is true in soils and aquatic ecosystems), enzymes are gatekeepers. they regulate how quickly carbon is broken down and taken up anew by living organisms. if you want to think realistically about any form of carbon sequestration in soils (an idea that has exploded in popularity in the last several years), or understand how global warming is altering ecosystems and the balance of carbon and nutrients within them, you simply cannot ignore enzymes.

the fact is, much as we would like to find a way to store the huge amounts of  carbon our activities are releasing into the atmosphere back in the earth, adding carbon feeds the soil. and just as human populations increase during times of food surplus, microbial populations explode, produce more enzymes and cycle that carbon at a faster rate.

another aspect of enzyme behavior that makes global climate change scenarios even stickier is that enzymes are very, very sensitive to changes in their environment. the activity and efficiency of enzymes in the environment is closely linked to temperature, moisture, and pH conditions. my own research on soil enzymes from northeastern forests is showing that even a few degrees of temperature increase can cause a dramatic increase in the rate of the carbon-cycling reactions that these enzymes perform. droughts, on the other hand, can quickly kill demolish enzyme communities and cause carbon cycling in a system to drop off.

the behavior of enzymes in the environment, we are discovering, is far more complex and nuanced than the story i’ve outlined here. moreover, ecologists know that enzymes must be understood within a broad context. the plants, animals and environmental processes that interact to form complex ecosystems, which enzymes regulate on a very fundamental level, must be somehow integrated if we are to fully understand how these tiny reaction machines keep our earth running.

Maybe we should reconsider raking our leaves

I recently learned a fascinating fact about leaf raking that should be painfully obvious to a forest ecologist- it’s bad for trees! Every spring, deciduous trees produce leaves that they use throughout the growing season for photosynthesis and sugar production.  Plants concentrate essential nutrients such as nitrogen, potassium, calcium and magnesium in their leaves, as these nutrients are all required in relatively high amounts to perform photosynthesis.

As winter approaches and the growing season ends, trees withdraw many of the proteins and nutrients they have stockpiled in leaves back into their woody tissue, so that these nutrients can be recycled to make new leaves the following year. However, most trees are able to do even better than this- after their leaves have fallen, the nutrients that couldn’t be recaptured in time are decomposed into the surface soil surrounding the tree, and will be available for uptake through the roots several years later. This regular flux of plant essential nutrients back to the soil through leaf litter means that plants depend on those same nutrients, year after year, to grow new leaves.

In fact, if you look at the typical architecture of a deciduous tree, it is no accident that probably appears like two umbrellas attached together at their handles. The top umbrella is the above ground parts of a tree from the base of the trunk to its canopy. The bottom umbrella is inverted and planted into the ground. It is composed of a main taproot that drives straight down into the earth, and lateral roots that branch out horizontally. Of these lateral roots are branching networks of finer and finer “root hairs” and associated fungi that are able, through their enormous surface area, to mine the soil underneath a tree for nutrients. Everything that is dropped from the top umbrella should theoretically be recoverable by this root system.

I’d imagine most of you can already see where this is going, but I find that sometimes simple truths are quite elusive. When we rake our leaves in the fall to maintain our clean, grassy lawns, we are removing loads of nutrients that our trees are expecting to get back! We are creating an artificially open, leaky system, that trees have spent millions of evolutionary years refining. A recent paper in a relatively esoteric research journal, “Nutrient Cycling in Agroecosystems” (who reads that??) attempted to quantify the impact of historic leaf raking on old agricultural towns in central Europe. The fascinating bit of historical information in this paper is that centuries ago, medieval farmers actually knew that leaves were a great nutrient source- farmers removed leaves from nearby forests specifically to use as fertilizer on their fields. This paper claims that the result of historic leaf raking is that the “majority of central European forests were severely depleted of nutrients…when modern long-term rotation forestry became the dominant form of forest land use”.

So next fall, when you’re pulling out your rakes or enlisting your kids to do so for a few dollars, think carefully about your trees. In all likelihood, the average patch of suburban lawn is already so nutrient depauperate from numerous land use changes (deforestation, asphalt paving, over-fertilization, the cultivation of a monoculture of non-native grasses, to name a few) that removing a few leaves isn’t going to make a big difference. But if I’ve learned anything from Malcom Gladwell, it’s that little changes that add up to produce big effects, and if medieval Europeans were knowingly removing nutrients from their forests, I figured modern suburbanites should at least be aware.

Acid oceans and the next 800,000 years

Have you heard the term “ocean acidification” being thrown around in the popular media recently? If you have heard a bit about this phenomenon but you’re not a climate scientist, it’s likely that you’ve been left with the impression that ocean acidification is yet another in a long list of the potentially nasty consequences of climate change. Probably something for the scientists to be concerned about, but not nearly as pressing as the melting of polar ice caps or the possibility of a twenty foot rise in sea level. However, given the invaluable ecosystem services and role in climate regulation that our oceans provide, I find it shocking that so little attention has been paid to ocean acidification, a process that we understand much more precisely than the elusive changes in global climate associated with rising CO2 levels.

Ocean water is slightly basic, just around pH 8 (the water we drink is generally neutral or slightly acidic, around pH 7). When CO2 from the atmosphere dissolves at the ocean surface, it reacts with water to form a balance of different “carbonate species”. The most abundant form that this dissolved CO2 converts into is bicarbonate, HCO−3. The prevalence of bicarbonate in oceans is due to their basicity. However, as more CO2 dissolves in water, the balance of carbonate species shifts. Bicarbonate begins losing a hydrogen ion and becoming carbonate, CO-3. This results in a release of free hydrogen ions (H+) into the water, which increase its acidity.

If you’re not a chemist, why would you be remotely interested in the balance of carbonate and hydrogen in the water? The reason ocean pH is so important is that all organisms and biological processes that occur in the ocean are finely attuned to changes in water chemistry, much as we can feel the slightest changes in atmospheric chemistry  that cause the difference between a hot sticky day and a hot dry day. However, the acidification of oceans isn’t just a comfort problem for marine organisms- a slight attenuation of ocean pH can represent a lethal environmental change.

Many marine organisms produce exoskeletons made of calcium carbonate. Calcium carbonate, molecular formula CaCO3,  forms when a calcium atom and a carbonate ion bond together and precipitate out of solution. This is called calcification. It is essential to the survival of corals, a variety of shelled invertebrates, and single celled planktonic organisms that play an essential role in ocean photosynthesis and nutrient cycling. However, an increased abundance of H+ ions in the water interferes with the formation of calcium carbonate. In fact, calcium carbonate formation is so sensitive to pH that a fraction of a pH unit can make the difference between a habitable and a lethal environment.

So, a summary of why we should be worried about CO2 in the oceans: Increased CO2 alters the balance of carbonate in ocean water and causes the release of protons, which by definition increases acidity. This in turn prevents the formation of calcium carbonate, which is essential to the survival of a variety of marine organisms.

What kind of damage are we looking at if calcium carbonate stops forming? One of the largest challenge climate scientists face is giving accurate predictions of the ramifications of climate change (the other is communicating these predictions and their significance to the public, a task which has thus far been a resounding failure). The most useful resource we have in making these predictions is the past. Climates have been highly variable over geologic history, and the earth has experienced periods of much higher CO2 levels and correspondingly warmer climates. With regard to oceans, we know ocean CO2 levels increased dramatically about 251 million years ago, at the end of an era known as the Permian. The end of the Permian also marks the largest extinction event in Earth’s history, with up to 96% of all marine species disappearing. There is still much debate as to the cause of the Permian extinction, but there is growing evidence that rapid acidification may have been a primary driver in the oceans. But how fast is ocean pH dropping today compared to past acidification events?  William Howard of the Antarctic Climate and Ecosystems Cooperative Research Center in Hobart, Tasmania stated this past July that  “the current rate of ocean acidification is about a hundred times faster than the most rapid events” in the geologic past.

Maybe this is a little dramatic. But whether or not we believe that acidification caused a huge extinction event in the past, the fact remains that the functioning of many marine ecosystems today is entirely dependent on the ability of a select group of organisms to precipitate calcium carbonate. Coral reefs provide the a habitat for thousands of other species, but without their calcareous exoskeletons, their soft tissues would not be able to survive. Calcareous phytoplankton nourish huge regions of open ocean due to their ability to produce sugars from sunlight. Many human populations rely on calcifying shellfish as a valuable food and economic resource.

Globally, ocean pH has already dropped about 0.1 unit. A drop of another 0.3 to 0.5 pH units is predicted by 2100. In most surface oceans, this level of pH change will make the precipitation of solid calcium carbonate energetically impossible. In short, we are on a trajectory for change that we may only have a small window of opportunity to alter, if our chance has not already past. If we are worried about economic costs associated with dramatically reducing our CO2 emissions today, it might be valuable in our cost-benefit analysis to consider the reduction in quality of life our children will face in a world whose oceans are biologically impoverished.