Tag Archives: environment

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

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deep sea carbon cycling: more dynamic than we thought?

For years, scientists have speculated that deep sea carbon may have played an essential role in past climate change episodes. Specifically, it has been suggested that the C bound in seafloor sediment has undergone thermodynamic alterations in the past to due upwellings of molten magma from the mantle. Magma may have triggered the release of CO2 and methane into the upper ocean and eventually the atmosphere. Evidence suggests that the Paleocene-Eocene thermal maximum, which lasted approximately 100,000 years, may have been triggered in part by the release of greenhouse gases from the seaflooor.

Despite these speculations about deep sea carbon influencing past climate, little research has been done on the role of seafloor carbon in the present day C cycle. In some ways, this is surprising given the enormous amount of attention being payed to global C budgets and possible means of C sequestration. It is generally assumed that the deep sea represents a huge “carbon sink”, to which organic C from the upper ocean enters and does not emerge again for thousands to millions of years. This would suggest that whatever carbon-cycling processes are occuring at the seafloor are not powerful enough to cause a net carbon release.

Recent research published in Nature Geoscience suggests otherwise. Several case studies have demonstrated dynamic processes occurring on the ocean floor can in fact lead to a net release of greenhouse gases. Spreading seafloor centers- regions where oceanic plates pull apart-  are a site of magma activity and hydrothermal venting. Hydrothermal vents release a variety of hot, mineral-rich fluids that can support a diverse microbial and invertebrate community. At one such spreading center in the Gulf of California, magma is intruding into thick organic basin sediments. These sediments have long been thought to sequester C, however, it now appears tht their heating is causing the release of methane into the upper ocean.

In the Northeast Pacific, another intriguing deep ocean C cycling system has been discovered. Here, microbes are converting ancient inorganic C into dissolved organic C, which is subsequently released to the overlying ocean. This discovery contradicts the general belief that ancient deep-sea C is highly stable and not accessible to microbes.

Other distinct seafloor C sources are rapidly emerging around the world, as improved technology and a heightened interest in seafloor processes are accelerate the pace of discovery. However, the contribution of such “point sources” to global C budgets is still highly uncertain and far more research is needed to come up with even a rough estimate of global deep sea C sources. Nonetheless, it would seem that we can no longer consider the deep ocean a black box of C sequestration, and that we should think carefully about the ramifications of introducing more carbon- either accidentally through the introduction of dissolved greenhouse gases to the ocean, or intentionally as part of a climate change mitigation strategy- to a system that is clearly more dynamic than we once thought.,

Reference : “Deep Sea Discoveries.” 2011. Nature Geoscience: Letters. Volume 4, Page 1.

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.

Saharan dust fertilizes new world rainforests

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As winds sweep eastward into the Atlantic off the northwest African coast, a remarkable thing happens: plumes of aeolian dust particles are swept off the surface of the Sahara. They will meander along the varied paths of the easterly trade winds, only to settle again in places as remote from each other as they are from the source: North America, the Caribbean, the Amazon Basin, the southern Mediterranean, eastern Europe, and occasionally even the chilly southern shores of Scandinavia.

A major Saharan dust plume event, November 1988

Where does the dust come from? Total Ozone Mapping Spectrometer (TOMS) suggest two major source areas: the Bodélé depression at the southern edge of the Sahara and an area covering eastern Mauritania, Western Mali and southern Algeria.

A little bit of dust blowing around shouldn’t be anything for meteorologists to bother about.  This dust, however, is anything but inconsequential- in the Caribbean alone, an estimated 20 million tones are deposited annually. It is the primary source of several essential trace elements, such as calcium and magnesium, to island rain forests whose soils have been leached through tens of thousands of years of erosion. Saharan dust enters the Amazon basin in bursts accompanying major wet season rains, feeding the soil with nutrients that the forest depends on. In fact, scientists now believe the Amazon to be so dependent on aeolian dust inputs that efforts are underway to model long-term expansions and contractions of the world’s largest rain forest in relation to the size of the Sahara over geologic time.

Dust not only nourishes the forests, it moderates their climates. African mineral dust is now considered the dominant light scattering aerosol throughout the tropical and subtropical Atlantic. The ability of airborne dust particles to scatter light decreases the amount of direct solar radiation hitting earth’s surface around the equator.

The dependence of major ecosystems across the world on Saharan dust underscores the deep connectivity of the biosphere, atmosphere, lithosphere and hydrosphere.

The world’s largest rainforest is nourished by mineral dust blown from across the Atlantic

1.Goudie, A. & Middleton, N. Saharan dust storms: nature and consequences. EARTH-SCIENCE REVIEWS 56, 179-204 (2001).

2.    SWAP, R., GARSTANG, M., GRECO, S., TALBOT, R. & KALLBERG, P. SAHARAN DUST IN THE AMAZON BASIN. TELLUS SERIES B-CHEMICAL AND PHYSICAL METEOROLOGY 44, 133-149 (1992).

Ice crystal formation in clouds stimulated by marine diatoms

The formation of ice crystals in the atmosphere is often facilitated by the presence of small, airborne particles that serve as a “nucleation” site for the growing crystal. Ice nucleation, with or without airborne particles, plays a large role in cirrus cloud formation. However, airborne particles allow ice crystals form in warmer, mixed-phase clouds that would otherwise have been ice-free.

A recent study published in Nature Geoscience reports that a common planktonic diatom, Thalassiosira pseudonana, can actually serve as a nucleation site for ice crystals. Diatoms are single-celled, marine photosynthetic organisms that are most famous for their often beautiful, glassy, silica-rich shells. They are found worldwide and are particularly abundant in cold, nutrient-rich ocean waters, such as the northern Pacific and Antarctic. Samples of T. pseudonana were exposed to water vapor and a supercooled salt solution under “typical tropospheric conditions” (ie, conditions that diatoms would be exposed to in the region of the atmosphere where cirrus-cloud formation takes place). The researchers found that the presence of diatoms in water allowed ice to form at substantially higher temperatures, and that the rate of ice nucleation in the presence of diatoms was generally rapid.

Thalassiosira pseudonana, a planktonic diatom

Small organisms that they are, the ability of diatoms and possibly other phytoplankton to initiate ice nucleation in clouds may have profound effects on climate. Increased ice crystal production due to diatoms could mean more incoming solar radiation reflected away from the earth by clouds (remember albedo effects?). Thus, diatom fragments in clouds may in fact increase the cooling potential of clouds (clouds are also important climate warmers, the water vapor contained within them is a powerful greenhouse gas).

A warming climate has been linked to changes in diatom populations. Warming is expected to lead to selection for smaller species of diatoms, which could be more easily aerosolized. Furthermore, warming may increase diatom populations due to enhanced ocean nutrient availability and decreased Arctic sea ice cover. These processes would both result in an increased concentration of diatomaceous aerosol material in clouds, leading to increased ice-crystal formation. Tiny glass cells, swept up unwittingly and unwillingly from their oceanic homes, may prove an important climate driver as they build icy shelters in the clouds.

Knopf et al. 2010. Stimulation of ice nucleation by marine diatoms. Nature Geoscience 2: 1037.

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.