Tag Archives: global warming



A draft of the national climate report has been released for review by the American people. Please take a moment to look it over. It’s a lengthy report, but the message is clear from the opening letter: our climate is warming, faster than we anticipated even three years ago. We have many lines of evidence pointing to this conclusion. To name a few, hotter summers with periods of extreme heat lasting longer than any living American has ever experienced. More frequent, extreme weather events, such as superstorm Sandy that devastated coastal regions of the northeast this past year. Global sea level has risen approximately 8 inches since the end of the 19th century, and is projected to rise another 1-4 feet by the end of this one. The Greenland ice sheet is melting more rapidly than scientists have anticipated, and the north pole is expected to be completely ice free in the summer by mid-century. Massive die-offs of coral reefs are being observed, species distributions are shifting in time and space.

In deciding whether to try to massively reduce our carbon emissions and prevent some of the most dramatic consequences of climate change from being born out, or implementing a comprehensive global adaptation strategy, I believe that humanity faces its single greatest challenge yet as a species. The decisions we make now and in the coming decades will alter the environment we experience and the fundamental way our planet functions for thousands, if not hundreds of thousands of years to come.

Whats up with nitrogen at Hubbard Brook?

Buried deep in the White Mountains of New Hampshire, the Hubbard Brook experimental forest has provided ecologists with questions and answers for over four decades.  And more questions.

Hubbard Brook was founded on the principle that it was the ideal location for “small watershed” experiments: it is a forest ecosystem whose energetic inputs and outputs could be fully accounted for and directly measured. Ecologists have been able to ask many broad and profound questions at Hubbard Brook, and the forest has told them a story from which many fundamental ecological principles have been distilled. What causes forests to grow? Where do they get their nutrients? What sort of energy enters and leaves an ecosystem? How are human activities impacting forests? Are they resistant to change, and are they resilient in the wake of disturbance?

Before tackling any of these big questions, you may well wonder about my first statement: that the energetic inputs and outputs of the forest can be fully accounted for and measured. How can one possibly measure all the energy that enters and leaves a forest? In the case of Hubbard Brook, there is a deceptively simple answer: it’s all in the streams. Keep a record of what enters and leaves the streams, and you’ll have a good idea of what is entering and leaving the forest as a whole.

Plants need three things to grow: sunlight, nutrients and water. Of the plant essential nutrients, nitrogen is the most important in temperate deciduous forests, but phosphorous, calcium, magnesium and sulfur, among others, are also essential.  In describing where  nutrients and energy come from, ecologists like to talk about autochthonous and allochthonous inputs. Autochthonous inputs come from within the system. Allochthonous inputs come from elsewhere. In the case of nitrogen, an autochthonous source would be a leguminous plant, or a plant that can convert atmospheric nitrogen into a plant useable form through the biochemical process known as nitrogen fixation. An allochthonous input might be a nitrogen fertilizer sprayed on a field.

Hubbard Brook has plentiful rain and sunlight to support a healthy forest. The first unknown that ecologists tackled was nitrogen, and I’ll focus on it from here on out because understanding nitrogen at Hubbard Brook has proved more challenging than anyone ever imagined.

Where are the plants getting their nitrogen? It turns out that for the last 14,000 years (since the retreat of the North American glacier that allowed the northeastern US to re-vegetate), all nitrogen that has entered and fed Hubbard Brook has come from the atmosphere. From the rain. No nitrogen fixing plants, no weatherable rock-derived nutrients. All that plant nourishment has entered the forest silently, dissolved in the summer rains or the winter snow.

It also turns out that all nitrogen eventually leaves  Hubbard Brook by water as well, in a readily soluble form known as nitrate, or NO3-. Streams cut across the forest, forming discrete, independent watersheds over a relatively small space. (A watershed simply refers to a region whose hydrologic inputs and outputs are well defined and accounted for). Streams are an integral part of the Hubbard Brook forest. Much as your bloodstream carries essential nutrients to your tissue and allows your body to flush out toxins, streams transport nutrients to different parts of the forest and flush away excess chemicals that the system doesn’t use. Through long term monitoring of the stream chemistry at Hubbard Brook, including measurements of headwater inputs and lowland outputs, ecologists have developed an accurate record of how much nitrogen enters and leaves the system annually.

In an unpolluted forest, how much nitrogen enters, and how much nitrate leaves, tells ecologists a lot about the forest’s growth and nutrient requirements. At Hubbard Brook, scientists have found an annual cycle of stream nitrate that reflects a strategy the trees have evolved in response to a predictable environment.

When leaves  drop in autumn, nitrate export in streams peaks. Leaves fall both directly into the stream and onto the surrounding soil, where they are decomposed and their nutrients can be leached away. Over time, this process would lead to a dramatic reduction in the nitrogen held in the system, if not for the annual winter replenishment of nitrogen through snowfall. Snow contains nitrogen just like rain does, and as it builds up on the forest floor during the winter, a stockpile is created for the next growing season. Trees have timed their first buds to break in the spring just as this snow is melting and releasing a pulse of nitrogen into the earth. This is essentially how things work at Hubbard Brook, in the absence of human influence.

However, Hubbard Brook, situated just north of the Washington-New York-Boston urban corridor, is by no means isolated from human pollution. Nitrous oxide gases emitted from fossil fuel combustion dissolve in the atmosphere and are rained out across the world, often at great distances from the source. Since at least the 1960’s, excess nitrogen has been entering Hubbard Brook as nitrate through this process. As a negatively charged anion, nitrate often picks up a hydrogen cation, H+, to balance its charge. “Hydrogen cation” is basically a fancy term for acid (remember ocean acidification?) The greater the concentration of hydrogen in solution, the more acid a solution is. Nitrogen pollution, ecologists realized, was doing something very strange to rainwater chemistry at Hubbard Brook. The concept of acid rain was born.

Thus is should not be surprising that another more powerful trend overlies the annual nitrogen cycle at Hubbard Brook. From first measurements in the late 1950’s until 1970, scientists observed a steady, annual rise in stream water nitrogen export. Since 1970, however, nitrate concentrations have been dropping. This has been attributed in large part to the passage of the Clean Air Act and the subsequent reduction in nitrous oxide emissions from automobiles.

Here’s what is causing ecologists to scratch their heads. Nitrogen levels in Hubbard Brook streams have now been dropping steadily for forty years. They are approximately at their 1960 level, and this downward trend shows no indication of leveling off. This reduction is dramatic, unexpected, and not accounted for by the emissions reductions enforced through the Clean Air Act. It is becoming increasingly clear that something else about the ecosystem has fundamentally changed.

What exactly is happening to cause reduced nitrogen outflow from Hubbard Brook forests concerns ecologists for a variety of reasons. The main reason ecologists are concerned is because the forest has stopped growing. In fact, it is now thought that Hubbard Brook may be losing biomass.

Could reduced nitrogen loss reflect increased nutrient limitation in the forest, and a need for trees to hold tightly to the nutrients they have? Could it mean rainfall is no longer supplying the nitrogen the forest needs? Could the soil microbial community no longer be decomposing and transforming nitrogen in the manner that the forest has depended on for thousands of years? Could it possibly reflect some other fundamental hydrologic shift, perhaps induced by climate change?

The most compelling theories in my view are that the phenomenon of “missing nitrate” is climate change driven, or driven by changes in soil chemistry and loss of important soil cations. I’ll explain both of these ideas briefly and leave you to ponder.

Climate change is certainly causing other weird stuff to happen at Hubbard Brook. The forest typically experiences sub-freezing temperatures all winter and builds up a healthy snowpack. Snow insulates the soil and streams alike, preventing them from freezing. This allows frogs, salamanders and some fish to hibernate over winter without freezing. Recently, warmer and more varied winter weather patterns have lead to reduced snow cover, with the consequence that the soil and stream water are now freezing instead. (The number of frozen salamander bodies discovered in the winter of 2006 was apparently unprecendented). Loss of snow cover is also disrupting the annual nitrogen cycle, which could well be leading to a springtime nitrogen limitation for growing trees.

Calcium, which typically takes the form Ca2+ in soils, turns out have a high affinity for nitrate. The two ions balance each other’s charge, and when nitrate leaches out of soil, it often brings a calcium ion along with it. Actually, it often requires the charge-balancing association provided by calcium to leave the soil in the first place. Well, calcium turns out to be another very important plant nutrient, and unfortunately is not as quickly replenished through rain as nitrogen. Declining soil calcium levels will, over time, produce soil that leaches nitrate less readily. Nitrate loss in streams could thus be indicative of a serious calcium deficiency. Incidentally, sugar maple, a common species in New Hampshire forests and a calcium-accumulating tree, is dying out and may be entirely lost these forests within the next several decades.

What’s up with nitrogen at Hubbard Brook? No one really knows yet, but we’d sure like to figure it out.

In trying to paint a picture of the dynamic nature of nitrogen at Hubbard Brook, I’ve left quite a bit of detail out and haven’t discussed some of the fascinating experimental manipulations that have been conducted at the forest. Hopefully I’ve touched on enough of the important ideas that this ramble provides some sense of the many layers of complexity that ecologists must piece through to understand a system or solve a problem.


The title and much of the content of this article I’m crediting to Dr. Gene Likens, a senior ecologist at the Hubbard Brook experimental forest who provided fascinating information on the forest during a conversation and a guest lecture at the University of Pennsylvania.

Carbon sequestration in soils: sources, sinks and pitfalls

When we talk about “sequestering” carbon in the soil to mitigate anthropogenic climate change, what are we really saying as scientists, and what is the public getting from it?  And perhaps even more importantly, what implicit assumptions are we as scientists making that may affect the validity of the widely held belief that increased C storage in soils is a good thing, and can offset our CO2 emissions?

In a recent soil carbon series in the European Journal of Soil Science, one review paper attempts to address these issues. “Carbon sequestration” has become a popular idea among policy makers and scientists alike. As anyone who keeps up on ecosystem science literature will know, C sequestration has also become an explicit motivation for numerous studies of soil and ecosystem properties.

Current best estimates claim that worldwide, soils store 684-720 Pg of C within the upper 30 cm, and 1462-1548 Pg to a depth of 1m. (For anyone not familiar with these terms, 1Pg = 1×10ˆ15 gram). The upper 30% of the soil profile to 1m clearly stores a disproportionate amount of C; this ecologists know to be the zone where continuous inputs from roots and organic matter replenish C. To put these numbers in an ecosystem context, the amount of C stored globally in soils to 30cm is twice the amount  of C stored as CO2 in the atmosphere and three times the amount of C stored in above ground vegetation. Clearly, soils represent a huge pool in global C budgets and should be managed with the knowledge that changes may represent a powerful feedback on the global C cycle. This vast storage potential is what has popularized the idea of sequestering even more C in soils and has spurred a myriad of different research approaches, from ecosystem-scale reforestation efforts to molecular studies of C-mineral binding interactions.

Given the amount of hope that has already been invested in this powerful idea, it is important for scientists interested in soil C to keep several caveats in mind when planning and conducting their research, and when communicating that research to a broader audience at a science or policy conference.

Caveat #1) Carbon sequestration does not necessarily mean climate change mitigation.

Increasing the amount of C stored in a particular ecosystem’s soil does not by default decrease the amount of CO2 released to the atmosphere. This is partially an issue of scale. If policy makers choose to set aside a certain amount of pasture land for reforestation (with the assumption that this will increase the net C storage in these systems), it is often the case that new land will have to be cleared somewhere else to compensate for the lost agricultural production.  The clearest way around this problem is to be selective in land use changes: land that is more fertile should be exploited for farming, and land that is less fertile should be reforested to boost C storage. However, an increasing global population poses obvious constraints to this line of reasoning. Ultimately, the fact remains that in the upcoming decades, we will need to boost our agricultural yields to meet global demand, and furthermore, climate change is predicted to decrease the fertility of some of the world’s most productive regions.

Moreover, allowing land to go back to its “natural” state does not necessarily lead to a net accumulation of C. A forest respires far more CO2 per acre than a wheat field. Due to the high water demands of forests relative to crops, it can sometimes be the case that trees dry up otherwise inundated subsurface soils, and in doing so create a soil environment that accelerates the decomposition of C.  One must have a refined understanding of the sources and sinks of CO2 in any particular ecosystem to claim that it will or will not “sequester” C.

Finally, management changes leading to increased C storage may increase or decrease the flux of other greenhouse gases, most notably N2O and methane. Given that these gases have 298 and 25 times the global warming potential of CO2, respectively, they are not a trivial consideration. Forests often hold less nitrogen than pastures (due to the different C:N demands of woody vs. non-woody tissue), and consequently release more N2O through the microbial process known as denitrification.

Caveat #2) The amount of carbon that can be locked up in soil in finite

Our understanding of how C is actually “stabilized” in soil on a molecular scale is still evolving. Scientists are using powerful technologies such as x-ray crystallography to understand in detail the chemical interactions between soil minerals and carbon-rich compounds that cause C to be tightly bound and inaccessible to microbes. As a general rule, the more mineral surface area available for C binding, the more tightly C is bound. In subsurface soils lower levels of C mean higher (mineral surface area: C) ratios and thus tighter binding of the C that is present.

However, mineral stabilization has its limits. Laboratory experiments are finding that as C is added to soils, “steady states” are sequentially reached. Careful manipulation of the  chemical and thermodynamic parameters of a soil may bump that soil from a lower steady state to a higher one, but whether such techniques can be applied on a whole-ecosystem scale remains to be seen.

To repeat, caveat #2 is that the amount of carbon that can be locked up in soil is finite, which leads to caveat #3.
At present, the amount of CO2 we put in the atmosphere does not seem to be.

Powlson et al. 2011. Soil carbon sequestration to mitigate climate change: a critical re-examination to identify the true and the false. European Journal of Soil Science 62: 42-55.

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.

in a warmer world, polar bears will lose out to grizzlies

More bad news for polar bears on the climate change front- a new study by evolutionary biologists at the Univeristy of California, Los Angeles, predicts that as northern climates warm and the habitat ranges of polar bears and grizzly bears overlap more, polar bears will be outcompeted for food.

Milder temperatures in the high Canadian boreal forest and tundra are already causing grizzlier bears to push further north. At the same time, the Arctic sea ice that polar bears depend on to hunt fish and seals is shrinking, which is causing the bears to migrate further south in search of food.

The study, which used 3-D modeling to compare the skull and jaw strength of the closely related species, found that polar bears are ill-suited for the tougher chewing demands imposed by a more vegetarian diet they will likely need to adopt. Grizzly bears, whose jaws are adapted for breaking up tough grasses, bark and berries, will not be required to alter their diet much as they migrate north.

The species range of grizzlies and polar bears has already begun to overlap. In 2006, the first confirmed hybrid of a grizzly bear and a polar bear was shot by a Canadian hunter.

This study was also reported by Facts About Climate Change

Global warming racks up heating bills

There has been an absolute flurry of research, news articles and blog posts over the past several weeks concerning what scientists are calling an unexpected consequence of global warming- some of the heaviest snowfalls on record across eastern North America. Scientists now believe that this phenomenon on domino effect in an interconnected series of warming-related changes that are occurring globally.

Briefly, the Arctic ice cap is melting at an alarming rate. This has already and will continue to produce an excess of moisture. Coupled with this is the warming of sea surfaces globally, which is causing more evaporation and moisture input to the atmosphere. This has resulted in a steady increase in snowfall across Siberia.

Snow changes the thermal properties of landscapes due to its high reflectivity. Just as we feel cooler in the summer if we’re outside in a white shirt,  a snow covered meadow will absorb less incoming solar radiation, and produce a region of lower air temperatures. This effect known as the “albedo effect”, and it has been leading to abnormally low winter temperatures in Siberia despite globally warmer winters.

Global temperature anomaly predictions for January 2011. Blue areas are predicted to experience a colder-than-average January, red areas a warmer-than average month.

The jet stream refers to a “virtual river” of 100-250 mph winds that circle the earth at mid-latitudes. It is known to be a dominant force controlling global weather patterns. As the jet stream pushes across Siberia, it is now encountering a colder dome of air. This causes its waves of air to scatter and for its west-east trajectory to become more erratic. In particular, waves of air are now being deflected north and then south, causing cold Arctic air to push south over eastern North America.

In December 2009, northern hemisphere snow cover was the second largest extent on record. North American snow cover was the largest extent ever recorded.

I won’t go any more into the details of this discussion here, but I’ll point you to a few articles that do a good job summarizing the science:

Bundle Up, It’s Global Warming

–This is a recent New York Times article by Judah Cohen, director of seasonal forecasting at environmental research agency

Climate Change Needs: Boats and Snowshoes

–By Robert Conners, environmental journalist

Predicting Seasonal Weather

— A special report by the National Science Foundation on temperature anomaly predictions for North America

Happy 2011, stay warm!!