Guilford Harbor

More on natural variability vs. anthropogenic warming

March 11th, 2010

Matthew McDermot features another GRL article that comes to a similar conclusion as the last post:  The kinds of natural variability we have experienced in volcanoes and solar cycles over the past 1000 years has been relatively small compared to the temperature changes we face with anthropogenic greenhouse gases.

This paper by Thomas Crowley is an earlier example of how climate scientists compare the relative effects of natural vs. human factors on climate over the past 1000 years.

Natural climate factors unlikely to put the brakes on greenhouse-gas-driven sea level rise this century

March 10th, 2010

The IPCC 2007 report projected a conservative sea level rise of about 18-59 cm by the year 2100.

Why conservative?  Because it mainly accounted for things we know are happening and can measure well—like thermal expansion of the ocean and melting of land glaciers (see here for a discussion of the Kilimanjaro example).  What it doesn’t do so well is account for all of the potential ways that the big ice sheets (Greenland and Antarctica) can contribute to sea level rise.  Things like ice flow and mass loss are generally assumed to be constant, even though recent research papers discussed in previous posts (here and here) suggest they are accelerating.

Since the publication of the IPCC report in 2007, there have been several studies suggesting that sea level rise will be 1-2 meters or more by 2100 (one example here).  One study looked at geological evidence for sea level rise during the previous interglacial period 125,000 years ago, which was 1-2 degrees C warmer than today.  Their work indicated that there was a 95% chance that sea level rose by 6 meters (22 feet).

In a forthcoming issue of Geophysical Research Letters,  Svetlana Jevrejeva and colleagues used statistical models to project sea level rise by 2100.1 But they also did something else interesting.  They looked back several thousands of years to the most extreme events that could cause climate cooling—things like severe volcanic eruptions, which create stratospheric dust clouds that block sunlight.

If events like this were to happen again, they asked, would they cause enough cooling to be able to slow sea level rise caused by greenhouse gases?

The answer is no.  There appears to be no natural factors like vulcanism that will significantly slow greenhouse-gas-driven sea level rise that we are already committed to or future sea level rise that we may experience if we continue to emit fossil fuels.

Excerpts (emphasis mine):

  • With the assumption that sea level will continue to respond over the next 100 years to the same forcings that have influenced it during the past 1000 years, we estimate 0.6 -1.6 m of global sea level rise in the 21st century using a statistical model driven by projected natural and anthropogenic forcings.
  • In contrast to the 20th century sea level rise that was associated with a significant contribution of 25% from natural (solar and volcanic) forcing, 21st century sea level rise will be clearly dominated by the changes in CO2 and other greenhouse gases.
  • Alternative scenarios for solar forcing with a potential decrease in solar irradiance of 1W/m2 (using the lowest level recorded throughout the last 9300 years) only produce a 10-20 cm reduction in our estimate of 21st century sea level rise.
  • If we utilize the 13th century past volcanic forcing to estimate a possible (but unlikely) contribution from volcanic activity, then an almost negligible 8 cm decrease is projected in the estimated sea level rise.
  • The suggested reduction of radiative forcing by injections of SO2 into atmosphere (equivalent to a Pinatubo eruption every 4 years) would be equivalent to delaying sea level rise by 12 -20 years.
  • A “no changes in radiative forcing” scenario produced 16-22 cm (with lower limit of 10 cm and upper limit of 31 cm) sea level rise in the 21 century due to the inertia of the climate system, providing evidence that conditions established during the past centuries have already committed us to a considerable global sea level rise during the next 100 years.

1Jevrejeva, S., J. C. Moore, and A. Grinsted (2010). How will sea level respond to changes in natural and anthropogenic forcings by 2100? Geophysical Research Letters : 10.1029/2010GL042947

UPDATE:  RealClimate provides more explanation of the IPCC being too cautious about sea level rise.

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Photo Credit: http://www.flickr.com/photos/gsfc/ / CC BY 2.0

How much is a ton of CO2?

March 9th, 2010

One of the challenges of climate literacy is helping folks visualize fossil fuel emissions and their impacts.

Last year, Bowdoin College completed its emissions inventory and climate action plan.  We discovered that the campus emits a total of 24,000 tons of CO2 equivalents each year.   So how much is that really?

One student decided to help illustrate this by creating an art installation, cordoning off a 27-ft x 27-ft x 27-ft cube in the student center with red ribbon.

Now imagine 24,000 of these cubes emanating from a college campus each year.   That helps show the magnitude of the challenge.

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Photo courtesy of Bowdoin College

The hidden global CO2 emissions of consumerism

March 8th, 2010

It’s been easy for citizens of the developed, industrialized world to criticize China and India over their rapidly growing greenhouse gas emissions.  This was one of the major reasons why the Kyoto Protocol was never ratified in the United States.

As many have  pointed out, however, there are several flaws with this argument:

  • The per-capita carbon emissions in China and India remain much lower (1/4 and 1/16, respectively) compared to the U.S..
  • Perhaps more importantly, some of the carbon emission in these countries is caused by the production of export goods to fuel consumer demand in wealthy nations.  Thus, we are responsible for “shadow carbon emissions” that get attributed to developing nations.

Until today, there haven’t been very good estimates of these kinds of shadow emissions.

In the Early Edition of the Proceedings of the National Academy of Sciences, Steven Davies and Ken Caldeira examine how much CO2 is embodied in the import and export of goods.1

Their results are interesting (excerpts below—If you can get a copy of the article, check out figures 1 and 2; they are terrific visuals for this information.  Alas, copyrights don’t allow me to post them):

  • Approximately 6.2 gigatonnes (Gt) of CO2, 23% of all CO2 emissions from fossil-fuel burning, were emitted during the production of goods that were ultimately consumed in a different country.
  • Emissions imported to the United States exceed those of any other country or region, primarily embodied in machinery (91 Mt), electronics (77 Mt), motor vehicles and parts (75 Mt), chemical, rubber, and plastic products (52 Mt), unclassified manufactured products (52 Mt), wearing apparel (42 Mt), and intermediate goods (654 Mt).
  • These imports are offset by considerable US exports of transport services (49 Mt CO2), machinery (42 Mt), electronics (26 Mt), chemical, rubber, and plastics products (25 Mt), motor vehicles (22 Mt), and intermediate goods (263 Mt).
  • [G]oods imported to Western Europe and Japan embody much more CO2 per US$ than do their exports, reflecting the import of energy-intensive products from elsewhere.
  • The carbon intensity of imports to China, Russia, India, and the Middle East is consistently far less than that of their exports.
  • China is by far the largest net exporter of emissions, followed by Russia, the Middle East, South Africa, Ukraine, and India and, to a lesser extent, Southeast Asia, Eastern Europe, and areas of South America.
  • The primary net importers of emissions are the United States, Japan, the United Kingdom, Germany, France, and Italy. Although the overall mass of emissions is much less, the other countries of Western Europe are all net importers, as are New Zealand, Mexico, Singapore, and many areas of Africa and South America. Similarly, Canada, Australia, Indonesia, the Czech Republic, and Egypt are among the countries whose net exports of emissions are small.
  • On a per-capita basis, net imports of emissions to the United States, Japan, and countries in Western Europe are disproportionately large, with each individual consumer associated with 2.4–10.3 tons of CO2 emitted elsewhere.

Their conclusion:

Consumption-based accounting reveals that substantial CO2 emissions are traded internationally and therefore not included in traditional production-based national emissions inventories. The net effect of trade is the export of emissions from China and other emerging markets to consumers in the United States, Japan, and Western Europe. In the large economies of Western Europe, net imported emissions are 20–50% of consumption emissions; the net imported emissions fall to 17.8% and 10.8% in Japan and the United States, respectively. In contrast, net exports represent 22.5% of emissions produced in China. Thus, to the extent that constraints on emissions in developing countries are the major impediment to effective international climate policy, allocating responsibility for some portion of these emissions to final consumers elsewhere may represent an opportunity for compromise.

1Steven J. Davis and Ken Caldeira (2010). Consumption-based accounting of CO2 emissions PNAS : 10.1073/pnas.0906974107

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Photo Credit: http://www.flickr.com/photos/deks/ / CC BY-NC 2.0

The rise of drug-resistent bacteria

March 6th, 2010

Nicholas Kristof has another column in the Sunday NY Times, The Spread of Superbugs, about bacteria that are increasingly difficult to kill with antibiotics and their links to the way we produce meat in modern agricultural systems.

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Photo credit: http://www.flickr.com/photos/estherase/ / CC BY-NC-SA 2.0

Climate communication: Is fear + collective action a winning strategy?

March 6th, 2010

In a previous post from my series on why people don’t engage climate change, I described my interpretations of work by Susanne Moser and Lisa Dilling1, which suggested that the use of fear can be a poor way to motivate behavioral changes to deal with climate warming:

Challenge 6: Fear can change perception but not willingness to take action and can lead to counterintuitive behaviors (like the “SUV effect”)

2006 was a watershed year in public opinion on climate change.  Al Gore’s An Inconvenient Truth and Time Magazine’s famous polar bear cover had the world scared to death about climate change.  They grabbed people’s attention and raised awareness, but they didn’t do much to galvanize widespread action against climate warming.  As we’ll see in the next post, 82% of Americans have not engaged the issue of climate change personally.

Even worse, if people become fearful of climate change, it could encourage counterintuitive behaviors.  For example, people might think, if it really does get stormier or icier in my area, I will need the SUV because it has 4-wheel drive.  The irony is not lost, given that large vehicles and their greenhouse gas emissions are part of the reason why we have climate warming in the first place.

When I first saw the Time cover, I thought that mainstream media is finally getting climate change and that people would start demanding action.  Now I’m not so sure fear is an effective tactic for driving change.

I also noted in that post that when people are fearful but don’t know what to do in the face of complex problems like climate warming, there can be a tendency to do nothing.

New research by Martijn van Zomeren and colleagues in the Journal of Environmental Psychology2 is beginning to challenge these views (emphasis added):

An inconvenient truth, the book and documentary by Nobel-prize laureate and former US Vice-President Al Gore, is a real-life example of the presumed power of psychology to increase pro-environmental behavior by telling individuals what they could do, and by telling them what to fear if they fail to do this. Although many applauded Gore’s efforts to raise environmental awareness and action, there was a danger that the fear invoked by his message could be counter-productive. Raising fear about the consequences of smoking and safe sex, for example, is thought to undermine health behavior if individuals do not have a sufficient sense of efficacy to transform their fear into action. Without such a sense of self-efficacy, fear is thought to lead individuals to protect themselves against their fear (rather than to take action to reduce the cause for fear). A key aim of this paper is to challenge this pessimistic conclusion.

Although we believe concern for the counter-productive effects of fear appeals is warranted, we think that self-protective responses are most likely in the context of individual problems such as individual health behavior. When individuals perceive a problem as an individual problem, their individual action should be best predicted by their self-efficacy beliefs. Unlike smoking and safer sex, however, one can perceive the climate crisis as a collective problem that requires collective action. Collective action is aimed at promoting collective interests, even if it is pursued by individuals. When individuals perceive a problem as collective, their collective action should be best predicted by their group efficacy beliefs – the belief that group goals can be achieved through joint effort.

This team is arguing that fear of climate warming impacts needs to be coupled with a clear message that

  • climate warming is a collective-action problem;
  • people can work together effectively to deal with climate warming.

In a series of experiments with university students in the Netherlands, the researchers manipulated climate fear (fear vs. no fear) and collective action efficacy (group action can be effective vs. no information about group action) through the use of different sets of readings.

After completing the different sets of readings, the students ranked in the following order (highest to lowest) in terms of their intentions to take actions on climate warming:

  • fear + group action can be effective
  • no fear + group action can be effective
  • fear + no information on group action
  • no fear + no information on group action

What’s interesting about this is the apparent importance on providing information on how collective action can be important.  Their results suggested that even students who were not given fearful messages about climate warming were still willing to take action on warming if shown how to do so.

This brings us back to one of my points in the earlier post.

Challenge 3:  Specific warming impacts and solutions are seldom conveyed clearly

Rather than just telling people that warming will be bad and we should all be afraid, warming advocates should state examples of how the impacts will be experienced by people in a specific region and specific steps that people can take to help adapt to or mitigate them.  Empower people to become part of the solutions process rather than letting them sit on the sidelines. Climate warming is not a spectator sport.

To paraphrase FDR:  The only thing we have to fear is fear (when used by) itself.

It’s an interesting idea, although I’m not yet convinced for several reasons:

  • I’d like to see how well this resonates with working-class America rather than students from a country like the Netherlands where people are much more likely to be socially conditioned to take action on climate warming.   Put another way, in certain cultural contexts, fear + solutions may work.  In others, fear + anything may turn people off.
  • As I mentioned above, research suggests that 82% of Americans have not engaged the issue of climate change personally.   Is there really a lack of information on what we can do collectively to deal with climate warming, or is there a lack of interest in taking collective action?  I think it’s both.  So long as climate warming solutions are framed in terms of national or international policy (cap and trade or C taxes) or matters of renewable energy innovation and investment, people will feel like they have little leverage to engage either of these approaches—either individually or collectively.
  • One might argue that collective action can be promulgated as political action at local, state, and federal levels.   However, the deep cynicism about the efficacy of federal institutions may dissuade people from investing the effort.  And when the world continues to struggle with the recession and unemployment, jobs will usually trump collective action on climate.
  • It’s a complex story with multiple layers of cultural and economic confounding factors.  Just as scientific evidence seldom speaks for itself, clear pathways for collective action may not translate to greater action either.

1Susanne Moser and Lisa Dilling (2004) Making Climate Hot: Communicating the Urgency and Challenge of Global Climate Change. Environment

2Martijn van Zomeren, Russell Spears, Colin Wayne Leach (2010). Experimental evidence for a dual pathway model analysis of coping with the climate crisis Journal of Environmental Psychology : 10.1016/j.jenvp.2010.02.006

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Photo Credit:  http://www.flickr.com/photos/lunadirimmel/ / CC BY-NC-ND 2.0

This week’s good ideas in campus sustainability: 3/1/10

March 4th, 2010

This week’s showcase features Beloit College, Central College, and Iowa State University.  LEED Platinum is not easy to achieve, and it’s even more impressive with projects this large.

1. Beloit College’s Science Center gets LEED Platinum Nod

“The success of our new science center reflects the phenomenal collaboration of creative architects, talented engineers, professional construction firms and the finest faculty and staff who were, and are, committed to the best outcome for our students,” said Beloit College president Scott Bierman. “We are, of course, thrilled to have gotten LEED platinum status; but even more important is that we have a building that works terrifically well­—as well as any I have ever seen—as an integrated set of learning spaces.”

2.  Central receives platinum LEED rating for new building

“This special recognition from the USGBC brings great joy to the whole Central College community and reflects continuing success of our pursuit of a sustainable future as a long-term goal adopted by Central’s board of trustees,” said Central College President David Roe. “The achievement was made possible through the concerted efforts of the professionals on Central’s staff led by Mike Lubberden and a large team of amazing corporate partners including Weitz Corporation as our general contractor, RDG Planning and Design, MEP and Associates, and Pella Corporation.”

3. ISU’s King Pavilion first education building in Iowa to earn LEED Platinum certification

Located on the north side of the College of Design building, the $6.6 million, 23,735 gross-square-foot King Pavilion features a central, two-story “forum” surrounded by instructional studios used by all freshmen in the college, as well as sophomores in architecture, landscape architecture and interior design. “We are delighted to have the King Pavilion receive LEED Platinum certification,” said ISU President Gregory Geoffroy. “The King Pavilion stands as a testament to the commitment that Iowa State University has made to becoming a model ‘green’ university, in our daily operations as well as in our teaching, research and outreach programs.”

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For more information:  AASHE bulletin 3/1/10

East Siberian Arctic Ocean discovered to be venting a lot of methane

March 4th, 2010

Methane (CH4) release from ocean sediments has long intrigued scientists.  There is an event that happened 54 million years ago called the Paleocene-Eocene Thermal Maximum (PETM), when 3,000-4,500 gigatons of carbon were released from the oceans, possibly as large methane burps caused by underwater landslides.

That’s a lot of carbon—more than 10 times the total amount we have burned as fossil fuels since the Industrial Revolution began.   Researchers think that it could have caused ocean temperatures to rise by as much as 5 degrees C and the atmosphere to warm by 5-9 degrees C. And when all of that methane carbon in the atmosphere oxidized to CO2, it dissolved back into the ocean and it reacted with water to form a weak acid

H2O + CO2 –> H2CO3 (carbonic acid)

which caused the ocean to acidify, melting the calcium carbonate shells of marine organisms and leading to one of the largest known marine extinction events of all time.

A new study by Natalia Shakhova and colleagues1 in this week’s issue of Science indicates that the coastal marine shelf in eastern Siberia may now be venting as much methane as was previously thought for all of Earth’s oceans combined.

In their words:

These findings do change our view of the vulnerability of the large sub-sea permafrost carbon reservoir on the [East Siberian Arctic Shelf] ESAS; the permafrost “lid” is clearly perforated, and sedimentary CH4 is escaping to the atmosphere.

For a cool visual of what methane release from ocean sediments looks like, check out the images in this article at Science Daily.

Whether or not the thawing of sub-sea permafrost will release enough methane to cause another PETM-type warming/extinction event is an active area of investigation.  Nobody knows for sure yet.  There is a lot of uncertainty in determining the size of the frozen methane pool in global marine sediments (possibly 500 – 2,500 gigatons of carbon), and the potential rate of release with warming is poorly known.  Clearly, there’s more work to do.

Even if the methane release is not as catastrophic as a PETM-type event, accelerated release will likely lead to a positive feedback on current warming, meaning that all associated impacts will happen faster than originally expected.  As I’ve said before, that becomes a nightmare scenario for policy makers.

1Shakhova, N., Semiletov, I., Salyuk, A., Yusupov, V., Kosmach, D., & Gustafsson, O. (2010). Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf Science, 327 (5970), 1246-1250 DOI: 10.1126/science.1182221

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Photo credit:  http://www.flickr.com/photos/artic/ / CC BY-SA 2.0

Land consumption and open space loss across U.S. cities

March 2nd, 2010

The issue of land use change is a complex, with many factors being important historically, such as

  • population growth (more land required for more people)
  • technology (e.g., automobiles made suburban expansion feasible)
  • economics (cheaper land and rents in suburbs compared to cities)
  • policy (things like 30-yr mortgages, mortgage insurance, and FHA loans had a large impact on urban sprawl because they often made it cheaper to own rather than rent)
  • cultural values (the romanticized notion of a detached home in a safe, pollution-free neighborhood with good schools)

In this week’s PLoS One, Robert McDonald and colleagues1 examined land use change for 274 metro areas (figure 1) in the U.S. to determine tends across cities.

Their results were interesting (excerpts):

  • 1.4 million ha of open space was lost, and the amount lost in a given city was correlated with population growth.
  • American cities vary by more than an order of magnitude in their MSA-wide per capita land consumption. Generally large cities have small per capita land consumption, with the five smallest in 2000 being New York (459 m2/person), Miami (476 m2/person), Philadelphia (519 m2/person), Los Angeles (535 m2/person), and Washington, DC (536 m2/person). Conversely, many small cities have large per capita land consumption, with the five biggest in 2000 being Grand Forks, ND (5394 m2/person), Bismark, ND (3913 m2/person), Flagstaff, AZ (3381 m2/person), Enid, OK (3249 m2/person), and Cheyenne, WY (3073 m2/person).
  • The per capita land consumption (m2/person) of most cities decreased on average over the decade from 1,564 to 1,454 m 2/person, but there was substantial regional variation and some cities even increased.
  • Cities with greater conservation funding or more reform-minded zoning tended to decrease in per capita land consumption (scroll to table 1) more than other cities.
  • The inequality of land consumption varied geographically, with less inequality on the East Coast compared to the West Coast (scroll to figure 4).

They provide a simplified snapshot of how development changes with history and geography (for a more-thorough yet readable treatment of land use in the U.S., check out Crabgrass Frontier by Kenneth Jackson):

The process of development plays out differently in cities with different socioeconomic histories. Moreover, cultural differences exist among and within many U.S. cities, leading to varying spatial patterns of development. However, a general historical pattern exists. In many U.S. cities, an urban core existed in the decades or centuries prior to the widespread use of the automobile, and these neighborhoods have high population density and small amounts of developed area per capita. The surrounding suburban and exurban areas, created predominately after WWII, contain residents living at lower population density and consume more land per capita. There are substantial economic links between these two zones, and in contemporary U.S. cities commuting occurs in both directions. Northeast U.S. cities that developed before the automobile typically follow this narrative. Many have a relatively dense urban core, but have adopted zoning policies that ensure contemporary suburban settlements occur at lower density. While they remain dense compared to other U.S. cities, they are getting less dense over time, as proportionally more of the population is in suburban areas. The declining manufacturing cities of the Rust Belt and the Southern Appalachians are an extreme example of this spreading out of population.

Southeastern U.S. cities, excluding Florida, are often newer and have less of a legacy of a dense urban core. They do not appear to be getting markedly denser, and the relatively fast population growth of these cities implies that their total impact on natural habitat in coming decades will be large. In contrast to the Southeast, Western cities appear to be getting denser, including those that do not have a historical legacy of a dense urban core such as Phoenix. These Western cities are often still growing quickly and consuming a great deal of land, but contemporary development is making these cities denser than they were previously. Many of these Western cities have a strong conservation culture, and the degree of conservation funding and reform-minded zoning correlates with how much denser they are getting. However, it should be noted that contemporary development in Western cities is still well below the densities found in the dense urban core of Northeastern U.S. cities, posing problems for designing effective public transit systems.

1McDonald, R., Forman, R., & Kareiva, P. (2010). Open Space Loss and Land Inequality in United States’ Cities, 1990–2000 PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009509

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Photo Credit:  http://www.flickr.com/photos/bobjagendorf/ / CC BY-NC 2.0

Herbicide exposure causes male frogs to turn into female frogs

March 1st, 2010

How do you turn a male frog into a female frog that breeds with other male frogs?   Expose them to herbicides that are routinely sprayed on agricultural fields worldwide.

Last year, Tyrone Hayes from UC Berkeley gave a talk at Bowdoin about his career’s work studying the impacts of endocrine-disrupting chemicals on amphibian development.

This week’s Early Edition of the Proceedings of the National Academy of Sciences features some of this research.1

Excerpts:

Atrazine is one of the most widely used pesticides in the world. Approximately 80 million pounds are applied annually in the United States alone, and atrazine is the most common pesticide contaminant of ground and surface water. Atrazine can be transported more than 1,000 km from the point of application via rainfall and, as a result, contaminates otherwise pristine habitats, even in remote areas where it is not used.  In fact, more than a half million pounds of atrazine are precipitated in rainfall each year in the United States.

In addition to its persistence, mobility, and widespread contamination of water, atrazine is also a concern because several studies have shown that atrazine is a potent endocrine disruptor active in the ppb (parts per billion) range in fish, amphibians, reptiles, and human cell lines, and at higher doses (ppm) in reptiles, birds, and laboratory rodents. Atrazine seems to be most potent in amphibians, where it is active at levels as low as 0.1 ppb.  Although a few studies suggest that atrazine has no effect on amphibians under certain laboratory conditions, in other studies, atrazine reduces testicular volume; reduces germ cell and Sertoli cell numbers; induces hermaphroditism; reduces testosterone; and induces testicular oogenesis. Furthermore, atrazine contamination is associated with demasculinization and feminization of amphibians in agricultural areas where atrazine is used and directly correlated with atrazine contamination in the wild.

Using an experiment where his team exposed frogs to a 2.5 parts per billion atrizine solution, here’s what they found:

Atrazine-exposed males were both demasculinized (chemically castrated) and completely feminized as adults. Ten percent of the exposed genetic males developed into functional females that copulated with unexposed males and produced viable eggs. Atrazine exposed males suffered from depressed testosterone, decreased breeding gland size, demasculinized/feminized laryngeal development, suppressed mating behavior, reduced spermatogenesis, and decreased fertility. These data are consistent with effects of atrazine observed in other vertebrate classes. The present findings exemplify the role that atrazine and other endocrine-disrupting pesticides likely play in global amphibian declines.

The main implication of this chemically induced sex switching is that it has the potential to disrupt breeding and contribute to the amphibian declines observed worldwide:

Although many studies have focused on death from disease and its role in global amphibian declines and sudden enigmatic disappearances of populations, virtually no attention has been paid to the slow gradual loss of amphibian populations due to failed recruitment. The present study suggests several ways that exposure to endocrine disruptors such as atrazine may lead to population level effects in the wild and contribute to amphibian declines. Certainly, the inability to compete for females and the significant decline in fertility in exposed males, as reported in the present study, will have a direct impact on exposed populations.

1Hayes, T. et al (2010) Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis). Proceedings of the National Academy of Sciences. doi:10.1073/pnas.0909519107

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Photo credit:  http://www.flickr.com/photos/arte/ / CC BY-NC 2.0

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