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

The rise and fall of human society with climate change

Scientists are becoming increasingly skilled at reconstructing past climates through use of a variety of “climate proxies”, including ice cores, lake sediments, fossils and tree pollen assemblages. Dendochronology, the analysis of tree rings, is now providing powerful evidence for the connection between human welfare and climate. Climate variations have influenced agricultural productivity, warfare and health of preindustrial peoples. A recent study in Science reports a high-resolution reconstruction of Central European summer precipitation and temperature for the past 2500 years, providing direct evidence that periods of social stability correspond with climactic stability, while periods of social upheaval, famine and even plague correspond with dramatic climate shifts and increased climactic variation.

To reconstruct a long-term climate record, researchers examined nearly 9000 pieces of wood from living and dead trees. Over 7000 were oak samples from France and Germany. Many of these were collected from historic buildings, or rivers and bogs that preserve ancient wood. To obtain the earliest dates possible, samples were obtained from archaeological sites. Researchers separately collected 1500 stone pine and larch wood samples from high altitudes in Austria.

To make a continuous record from the present to the past, dendochronologists first examine tree rings from live wood samples to provide a baseline for dating. From there they work back to older and older samples. The width of spacing between consecutive rings corresponds to the amount of growth experienced that year. In particularly bad years, a ring may be broken, fuzzy, or barely present. The isotopic composition of wood samples can also be taken and several important climate metrics can be extracted from it. O18, for example, is a heavy isotope of oxygen. In rainy years, paleoclimatologists expect a sample to be relatively enriched in O18. Taken together, these two sources of data provide strong evidence for both temperature and moisture conditions in a particular year.

To calibrate such a climate record, human records are extremely valuable. Weather records providing temperature and moisture data over the past 200 years were collected for Central Europe. These records allow scientists to precisely determine how weather affects ring growth in that year.

The result?  A continuous, 2500-year climate record that, when compared with archaeological and historical data, showed a stark pattern. Times of social stability and prosperity corresponded with warm, wet summers that led to high agricultural yields. Warm, stable climates coincide with the rise of the Roman Empire and peak years of medieval Europe. The opposite was also true. For example, a dramatic cold snap around ~1300 A.D. occurred directly prior to the famines and plague that spread across Europe half a century later. The decline of the Holy Roman Empire from AD ~250-600 coincides with a marked increase in climactic variability.

Though ancient peoples were clearly more susceptible to the affects of a bad harvest year than modern societies are, the powerful and fundamental connection between human welfare and climate still manifests itself today. Many scientists believe that the Holocene, the geological era of relatively mild, stable climates that led to the global dominance of the human species, is now ending. The Anthropocene, an era in which human actions are the primary climate driver, has just begun. We may do well to consider the effect of climate on human society throughout a climactically stable era when making decisions that will affect our climate in the future.

 

Buntgen et al. 2011. 2500 Years of European Climate Variability and Human Susceptibility. Science. In press.

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.