Tag Archives: Nature Geoscience

microbes improve carbon cycle models

Microorganisms are key drivers of the global carbon cycle both on the land and in the ocean. Through a diverse array of metabolic strategies, microbes decompose organic and recycle organic carbon. Just as we respire a fraction of the carbon we consume as CO2, so do microbes. Globally, vast quantities of organic carbon are funneled through many billions of microbes every year: to be decomposed, recycled, and respired into the atmosphere. In spite of this, microbial activity has historically been ignored in attempts to model the global carbon cycle and predict climate change-related feedbacks. Instead, our models rely on untested assumptions that microbes and their carbon cycle activities will respond in uniform, predictable manners to increases in temperature, and as such, can essentially be ignored.

A recent paper in Nature Geoscience paper challenges this assumption by explicitly integrating microbial physiology into a new model of the soil carbon cycle. Compared with traditional models, the new model more accurately matches current observations of carbon stocks and fluxes across ecosystems. Regarding future carbon cycling in a warming world, the model produces several widely different scenarios that vary due to the potential response of microbes to rising temperatures. In short, if microbes respond negatively to warming by decreasing their “growth efficiency”; that is, if warming slows down their growth rates, no additional carbon is released to the atmosphere as the soil warms. But if microbes are able to adapt to higher temperatures and maintain their current growth efficiencies, higher temperatures will accelerate carbon decomposition rates and lead to potentially huge additional losses of carbon dioxide from soils.

The integration of biological mechanisms into earth system models is an important step forward in our ability to forecast future climates. Future research that empirically measures microbial community responses to long-term warming is desperately needed in order to accurately model and predict this potentially huge feedback to the global carbon cycle.



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.

Mountain glaciers become sea water, frozen tombs uncovered

Over the past several decades, the rapid melting of mountain glaicers has been a primary contributor to rising sea levels. Estimates of the long-term contribution of non-polar glacial melting to sea level rise vary substantially, but most experts agree that the contribution will fall somewhere between a tenth and a third of a meter. A recent Nature Geoscience report used the World Glacier Inventory, a repository of information on >120,000 glaciers, to predict changes this century in all 19 non-polar regions containing mountain glaciers and ice caps. This study predicts that total glacial volume will reduce by 21 +/- 6% by 2100, though in certain areas the reduction may be as high as 75%. This will lead to dramatic changes in regional hydrology and serious water problems for people who depend on seasonal glacial melting for freshwater and irrigation (see my December post, “Fog harvesting for a thirstier world”).

Water shortages, sea level rise, and erosion and hydrologic changes resulting from mountain glacial melting all pose real and very apparent problems for human populations. Another fascinating result of glacial melting will not incite new environmental dangers, but is already leading to social unrest and conflict between scientists and indigenous populations. It turns out that in certain regions, tombs, bodies and ruins from ancient civilizations, once buried deep beneath the ice, are now thawing. The most prominent example of this is in the Central Asian Altai mountains, where over 700 tombs have been preserved for 2,500 years by ice or permafrost. Increasing ground surface temperatures are causing these tombs to thaw. Another example is the huge coastal cemetery near Barrow, Alaska, where sea ice loss is causing the coastline to erode at rates of up to 20 m/year, exposing generations of human remains. It is becoming apparent that glacial thawing will impact frozen archaeology worldwide, and will potentially lead to both great discoveries and great unrest.

Globally, some of the most fascinating human archaelogical discoveries have involved frozen remains. Freezing allows preservation of human tissue that would otherwise decay in several decades, and archaelogists are now using advanced molecular techniques to date such tissue and even extract ancient DNA samples. Archaeologists across the world are now clamoring to take advantage of newley exposed human remains that may only be valuable for a short period of time. This has already stirred anger amongst many indigenous populations, who do not wish to see their ancestor’s remains and a part of their cultural heritage uprooted and shipped off to a lab thousands of miles away for chemical analysis.

The problem essentially arises from the fact that there is currently no standard legal framework to mediate the interests of scientists, governments and indigenous people with respect to these precious archaeological repositories. Glacial retreat necessitates the creation of new laws and policies to address these concerns- and soon, if our mountain melting rate predictions are at all accurate.

Radick and Hock. 2011. Regionally differentiated contribution of mountain glaciers and ice caps to future sea-level rise. Nature Geoscience. In press.

Molyneaux and Reay 2011. Frozen archaeology meltdown. Correspondence . Nature Geoscience. In press.

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.