The NY Times published a lengthy article about the climate implications of the forest diebacks and fires. It is, on the whole, a great and all-too-rare example of longform science journalism.
The article does miss one important point about CO2 fertilization, the increase in plant growth thought to come from adding more CO2 to the air.
Climate-change contrarians tend to focus on this “fertilization effect,” hailing it as a boon for forests and the food supply. “The ongoing rise of the air’s CO2 content is causing a great greening of the Earth,” one advocate of this position, Craig D. Idso, said at a contrarian meeting in Washington in July.
Dr. Idso and others assert that this effect is likely to continue for the foreseeable future, ameliorating any negative impacts on plant growth from rising temperatures. More mainstream scientists, while stating that CO2 fertilization is real, are much less certain about the long-term effects, saying that the heat and water stress associated with climate change seem to be making forests vulnerable to insect attack, fires and many other problems.
The CO2 fertilization effect is limited, because plants require more than just CO2 to do their job: photosynthesis. Water is certainly a limiting factor, but nutrients are just as important. In experiment after experiment, scientists find that the CO2 fertilization effect is short-lived without additional inputs of nutrients, particularly nitrogen.
One of the reasons CO2 fertilization may have accelerated plant growth in parts of Europe and North America over the past few decades may be the fact that we've inadvertently been fertilizing the plants with nitrogen, as well as CO2. We'll actually be talking about this in GEOB400 in a couple weeks. For the 6.7 billion or so of you who were unable to register this semester - yes, yes, the class is too small, I hear that all the time - I wrote about this on Maribo a few years ago, in a cross-post with Eli and Tamino:
One culprit is carbon’s chemical sibling nitrogen, that’s #7 on your periodic table if you’re scoring at home. Like many siblings, carbon and nitrogen are quite co-dependent, and, one might argue, a bit resentful about the whole thing. Carbon fixation - photosynthesis, plant growth – is limited by the availability of nitrogen. Though only up to a point. If there’s too much nitrogen, things get saturated, and the carbon-based plants pout and refuse to grow more.
You might find it strange that nitrogen is limited, given that N2 or di-nitrogen gas makes up the majority of the atmosphere. However, N2 is unreactive. It only becomes available to plants when converted to reactive form by microbes. In the process of making fertilizer and burning fossil fuels, we not only have increased the rate at which this conversion happens, leaving more nitrogen in our soils and waterways, we've emitted nitrogen in other reactive, gaseous forms, like nitrogen oxides or NOx...
Sunday, October 02, 2011
Nutrient limitation missing from an otherwise good NY Times story on forests and climate change
Sunday, July 05, 2009
Abuse of science and logic by the National Corn Growers Association
The National Corn Growers Association released a report arguing that there is no connection between the use of nitrogen fertilizers on corn in the Midwestern US and the seasonal “Dead Zone” in the Gulf of Mexico.
There is no point mincing words about what this “analytical white paper”. It is the corn equivalent of irrational climate change skepticism. This is one truly shoddy piece of work. I encourage others in the scientific community to respond either independently or to append the critique offered here.
First, let’s review the actual science.
The “dead zone” in question, discussed many times before on this blog, is generated most summers on the continental shelf of the northern Gulf of Mexico. Nutrients originating in the Mississippi River Basin in the spring fuel the production of algae (primary production) in the surface waters along the continental shelf. The algae die and sink to the bottom, or something else eats the algae and the fecal matter from the something else sinks to the bottom. All that organic matter needs to decompose, and the process of decomposition (respiration) consumes oxygen. So the bottom waters on the continental shelf during the summer become very depleted in oxygen, or “hypoxic”.
Scientific research over the last few decades has shown that the increase in nitrogen flow from the Mississippi and neighbouring Atchafalaya Rivers since the 1950s has driven the development of these large seasonal periods of hypoxia. The evidence comes from basic ecological theory on nutrient limitation, lab experiments, tracking of the Mississippi River plume, long-term data analysis, sediment cores, isotopic analysis and mathematical modeling. While other nutrients like phosphorus and silica are important, nitrogen is the primary culprit.
There are many possible explanations for the increased flow of nitrogen out of the Mississippi-Atchafalaya River Basin (MARB) including fertilizer use, manure use, NOx emissions from cars and sewage. A simple nutrient budget shows nitrogen fertilizer use in the MARB has increased 20-fold since the 1950s. And today, most of that nitrogen fertilizer is applied to corn fields. Measurements and mathematical modeling of nitrogen loss from corn fields show that corn production is a primary source of nitrogen to the Mississippi and Atchafalaya Rivers, and hence, a primary driver of the development of what's come to be called the “Dead Zone”.
The author of the NCGA report (from the consulting firm StrathKirn Inc.) attempt to counter the mass of scientific evidence with the following largely baseless and unscientific arguments. Basically, he throws a bunch of stuff at the wall to see if anything sticks. I’ll go one by one through the report's chain of five incorrect and comically inconsistent assertions:
Assertion #1: Oxygen levels on the continental shelf are not low in comparison to other parts of the ocean.
This is misleading and irrelevant. First, the large regions of upwelling in the open ocean have low oxygen concentration due to high primary production. There’s no sense in contrasting the naturally and persistently low oxygen levels in the eastern Pacific to the intermittent, seasonal hypoxia on the continental shelf of the Gulf of Mexico. Second, even if there were some sense in this comparison, the data resolution of these maps is far too poor to capture a hypoxia area, which, while among the largest in the world, is still at its largest on the order of 20,000 km2 [here’s a test – can you clearly delineate New Jersey on that map?]. The global map of marine nitrogen concentrations is even more ridiculous. The data is far too coarse to capture the plume of the Mississippi River.
Assertion #2: Hypoxia doesn’t affect the fishery (not there is any hypoxia).
The report shows no change in fish catch over the years. As Steve Carpenter of the University of Wisconsin mentioned in an e-mail, the problem is the report analyses data on fish landings, not fishing effort. The boats may come back with the same weight in fish – but it takes more time and money to get those fish.
Assertion #3: Nitrogen from the Mississippi and Atchafalaya doesn’t cause the hypoxia (not that the hypoxia affects the fishery, or that there is any hypoxia in the first place).
This argument is advanced through a series of graphs relating annual nitrogen export, annual river flow and the annual extent of the hypoxic zone. There are a number of problems here. The nitrogen and flow data are shown only since 1985, despite data existing back to the 1950s. If the graph went back thirty years, you’d see the 2-3fold increase in nitrogen export occurred between the 1950s and the 1980s. Instead, the author reports no evidence of a trend in nitrogen of hypoxia since 1993. That’s not the issue – the issue is the hypoxic zone began growing large in the 1980s because fertilizer use increased between the 1950s and the 1980s, and further increases in corn planting, say for ethanol production, may further increase the average annual extent of hypoxia.
The other glaring problem with this argument is that the report uses no statistics whatsoever. For example, after a chart of nitrogen export and hypoxia extent since 1985 is this unsupported passage:
Again, there appears to be an association between water flow and the amount of nitrite (NO2) plus nitrate (NO3), but these do not relate well to the size of the hypoxic zone (except that they are all low in the year 2000). Thus, many of the statements about the relationship between water flow, nitrogen, and the size of the hypoxic zone are inaccurate.
Some actual statistical analysis, or frankly, just eyeballing the graph, would suggest that there is a significant relationship between the annual nitrogen export from the MARB and the annual extent of the hypoxic zone. It is not a perfect one-to-one relationship between nitrogen and the extent of hypoxia because of how the weather effects mixing of oxygen in the Gulf, the load of other nutrients and a myriad of other mitigating factors. If the author had done any research, they’d find proper statistical analysis and explanations in dozens of published papers, including this one of from my own work, a 2007 paper in Limnology and Oceanography:
Between 1985 and 2004, there is a significant relationship (r2 > 0.61) between midsummer hypoxia area and the May + June nitrate flux (Fig. 1). The strength of this relationship is limited by a number of other variables, including the advection of sub-pycnoclinal waters on the continental shelf, summer tropical storms that increase vertical mixing, recycling of N sequestered in shelf sediments during previous years, and the input of other nutrients such as phosphorus (Rabalais et al. 2002; Scavia et al. 2003; Wawrik et al. 2004).
Assertion #4: Not very much nitrogen is applied to corn (not that nitrogen causes hypoxia, or that hypoxia affects the fishery, or that there is any hypoxia in the first place).
The report displays a graph illustrating that non-crop uses of nitrogen fertilizer, like fertilizer used on lawns, is equal to or greater than the use of nitrogen fertilizer on corn. The problem, or I should say, the most glaring problem? It is national data. Over 90% of the corn grown in the US, and over 90% of the nitrogen fertilizer applied to corn in the US, is grown in the MARB. A 1999 EPA report estimated that non-agricultural fertilizer use is only 5% of total U.S fertilizer use - and that percentage of total fertilizer use in the major producing states of the Corn Belt.
Assertion #5: No nitrogen runs off of corn fields (not that much nitrogen is applied to corn, or that nitrogen causes hypoxia, or that hypoxia affects the fishery, or that there is any hypoxia in the first place).
The report proudly claims that the same amount of nitrogen is now removed during the corn harvest (i.e. in the grain) than is applied as fertilizer, so there can’t be any extra nitrogen left over to run off into the river. Fertilizer use efficiency has indeed increased over the years thanks to genetic technology and improved management. In other word, farmers are getting higher yields with the same amount of nitrogen fertilizer. That is positive news.
But the calculation in the paper is full of flaws. To name just one: the contention that fertilizer inputs = crop outputs = no nitrogen runoff only makes sense if fertilizer were the one and only source of nitrogen to the crops. For one, there is the mineralization of nitrogen in the soil – plant matter on the ground is naturally broken down by microbes, a process that released nitrogen from the plant matter to replenish the soil. This is a fundamental part of soil chemistry. The whole reason the Midwest is good land for growing corn is the high natural mineralization rates!
Final take-home message of the report: The US has a lot of golfers.
The report concludes that all other analyses are ignoring all the fertilizer applied to lawns and present maps and data to support this conclusion. The calculations are extremely suspect. First, the author assumes that the fraction of land devoted to lawns is greater in the MARB than in the rest of the country. Analysing the lawn data, eyeballing the national map, or simply reflecting about the fact that 4/5ths of the US population live outside the MARB, shows that this is a ridiculous assumption. Second, the report assumes that all the fertilizer not applied to corn, wheat, soybeans or cotton – which amounts to about 25% of annual fertilizer sales - is applied to lawns. This ignores all other crops grown in the United States, as well as all the fertilizer applied to rangelands and forests.
The report goes on to argue:
Since most lawns are cut and mulched there is relatively little removal of N, unlike the grain in corn. Consequently, a major portion of the N applied to lawns may be available for leaching… the net N available for leaching per acre is almost infinitely higher for lawns than from corn.
Not only does this argument incorrectly imply that no plant residue whatsoever is ever left behind after harvest to replenish the soil, it ignores the fact that unlike lawns, many corn fields are artificially drained by pipes or drainage tiles, such that excess nitrogen easily leaches to the nearest stream.
All told, the NCGA report is an embarrassment.
There are some legitimate outstanding questions about the nitrogen-hypoxia problem and definitely some legitimate critiques of the media coverage. In particular, the coverage often gives the mistaken impression that corn is the only source of nitrogen, that the hypoxic zone covers a large fraction of the Gulf of Mexico, that water at all depths is hypoxia, and that hypoxia is a permanent phenomena, rather than a seasonal occurrence. The NCGA could have issued on a report on those real concerns. Instead, it issued this dishonest mess of half-truths and pseudo-science.
Tuesday, June 17, 2008
A perfect storm for the Dead Zone
The massive floods in the Midwestern US are likely to fuel the largest Gulf of Mexico Dead Zone in recorded history. The image at right is from the NY Times and the National Weather Service.
Nitrogen applied to crops like corn in the Midwest is the major driver of the now famous Dead Zone, as I've described in a number of previous posts and this Google News commentary. The blame for the high nitrogen levels in the Mississippi and this year's record Dead Zone forecast is being placed on the production of more corn for ethanol. A more complete explanation would be that the surge in corn production, and, hence, fertilizer use, the past few years has made nitrogen pollution more sensitive to the climate than ever.
Nitrogen and hydrology are tightly linked in the Mississippi River Basin, and other agriculturally intensive river basins, thanks to nature and to humans. Several nitrogen 'species' like nitrate are highly soluble. What has exacerbates things in the Mississippi is activities like wetlands, installing artificial drainage under fields and channelizing rivers that reduce chances for nitrogen to be consumed before moving downstream. The result is the amount of nitrogen that the Mississippi sends to the Gulf can actually be predicted from the rainfall in the Corn Belt.
In coverage of our recent paper on corn and the Dead Zone, the prediction that the US Energy Policy would increase average nitrogen loading by 10-34% drew most of the attention. What might be missed is that the nitrogen loading could be much higher if the conditions are wetter.
The reason this matters is the the continental shelf of the Gulf of Mexico has a memory. The usual tale is that the Dead Zone grows each spring and summer when the big flood of Mississippi nitrogen arrives weather and water conditions are ripe for algae growth (it breaks up in the fall when the waters cool and mix, reintroducing oxygen to the bottom waters). However, nitrogen from previous years that is deposited in the sediments can also be recycled and feed algae growth. In other words, the system remembers a big flood of nitrogen. For example, during the 1993 Mississippi floods, the Dead Zone grew to a then-record 17,600 km2; the next year, it grew to an almost equal 16,600 km2, despite 31% less nitrate flowing down the Mississippi. That's just one reason why it is critical to consider climate and climate variability in ecological management and policy.
This year, the Dead Zone is projected to reach over 25,000 km2 in size, 20% greater than the previous maximum. What will that mean for 2009? For 2010? The longer you wait, the harder problems like the Dead Zone are to solve.
Monday, March 17, 2008
Hypoxic zones around the world
The World Resources Institute and scientist Bob Diaz of Virginia Marine Institute have compiled a new map of the world's coastal hypoxia zones like the famous Gulf of Mexico "Dead Zone" we discuss in the recent PNAS paper on nitrogen pollution and corn production for ethanol. The new map includes 169 documented hypoxic areas, 233 are areas of concern and 13 areas in recovery.
Bottom-water hypoxia can develop when high input of nutrients like nitrogen promote the excessive algae growth. When algae eventually dies and sinks to the bottom, it decomposes, and that process depletes oxygen from the water.
It is worth noting hypoxia will not arise anywhere simply because nutrients are added. To get things started, you still need to feed the algae, and nutrient pollution does the trick. But certain
coastal areas are more naturally prone to hypoxia.
If, for example, the water column is highly "stratified", by that I mean less dense water lying above more dense water there is little mixing between the surface and the bottom waters (think of making a simple oil and vinegar salad dressing). It is then difficult for oxygen from the air to diffuse to the bottom and replace the oxygen consumed by decomposition.
The outlet of big rivers like the Mississippi can be ideal for hypoxia development because the fresh and therefore lighter water introduced by the river creates a stratified water column. That explains some of the year-to-year dynamics of the hypoxic zones like the Gulf Dead Zone. First, hypoxia development can be much worse in a wet or flood year because of the addition of more nutrients and the increased stratification. Second, if a hurricane blows through the Gulf, it encourages mixing just like you do by shaking that bottle of salad dressing, and can break-up the Dead Zone.
The new map supposedly includes only human-driven cases of hypoxia. Which raise the question, what is the cause of the zone between very sparsely populated Somerset and Cornwallis Islands in Nunavut, in the Canadian Arctic? If you have an answer, let me know.
Monday, March 10, 2008
Corn ethanol production will worsen the Dead Zone
A new paper by my colleague Chris Kucharik and I looks at the new US Energy Policy, will calls for growing more corn to produce ethanol, will affect the "Dead Zone" in the Gulf of Mexico. For a quick summary, see Reuters, the CBC or AFP (or my 15 Minuten ruhm on German ARD). Wired and Scientific American go into more detail.
The Mississippi dumps a massive amount of nitrogen, largely in the form of the soluble ion nitrate, into the Gulf each spring. It promotes the growth of a lot of algae, which eventually sinks to the bottom and decomposes. This consumes much of the oxygen in the bottom waters, making life tough for bottom-dwelling fish and creatures like shrimp. The Dead Zone has reached over 20,000 km2 in recent years.
The primary source of all that nitrogen is fertilizer applied to corn grown in the Midwest and Central US. Reducing the Dead Zone to less than 5000 km2 in size, as is suggested in US policy, will require up to a 55% decrease in nitrogen levels in the Mississippi.
The new US Energy Policy calls for 36 billion gallons of renewable fuels by the year 2022. Of that, 15 billion can be produced from corn starch. Our study found meeting those would cause a 10-34% increase in nitrogen loading to the Gulf of Mexico.
Meeting the hypoxia reduction goal was already a difficult challenge. If the US pursues this biofuels strategy, it will be impossible to shrink the Dead Zone without radically changing the US food production system. The one option would be to dramatically reduce the non-ethanol uses of corn. Since the majority of corn grain is used as animal feed, a trade-off between using corn to fuel animals and using corn to fuel cars could emerge.
Wednesday, October 03, 2007
Greenhouse gases from biofuels and the reporting of science
You may have read that a study by Nobel Prize winner Paul Crutzen and colleagues currently under review reports that biofuels, like corn-base ethanol and rapeseed biodiesel, emit more greenhouse gases than the fossil fuels they replace. It was covered by Reuters, the Times (UK) and the magazine Chemistry World (update: podcasts available on Scitizen). The news then bounced all over the internet, appearing in Grist, Green Car Congress, and a number of blogs like Alternapower, Green Diary, Biofuels Digest, Climateer Investing, Earth2Tech, Big Biofuels Blog, Classically liberal, After Gutenberg, Digital Journal, I could go on.
The paper, submitted to Atmospheric Chemistry and Physics, uses a global budget of the sources and sinks of atmospheric greenhouse gas nitrous oxide (N2O), which originates in part from nitrogen fertilizer use, to estimate a rate at which N2O is emitted in the production of biofuel crops like corn, rapeseed (canola) and sugarcane. N2O is a much less common greenhouse gas than CO2, but has each molecule has a greater “warming” effect. The authors then contrast the global warming potential of N2O in growing the biofuel crop with that of the CO2 saved by replacing gasoline use. Their results suggest that corn ethanol and rapeseed biodiesel would lead to a net “warming” from the N2O emissions alone. The problem is, as I was quoted as saying in the Chemistry World article, is that their method and their results are probably wrong. The paper is still under review and many of the comments on the paper, including my own, disagree with both their methods and their conclusions about a net “warming” from N2O emissions. Let me be clear before continuing: this is not to say that the production and use of biofuels like corn ethanol will certainly emit far fewer greenhouse gases than the production and use of regular gasoline. A number of studies (Farrell et al, 2005) have concluded that when you include the entire production cycle, corn-based ethanol is either a small “win” or possibly a small loss. Either way, these other studies are including all sources of emissions during production, including operating the machinery, producing the fertilizer, processing the grain, and the N2O from fertilizer use. The Crutzen et al. paper reports that the N2O emissions from fertilizer alone makes corn-based ethanol and rapeseed net losers in the emissions reduction game. Here’s the central science problem: The paper’s global budget analysis leads to the conclusion that 3-5% of nitrogen fertilizer is eventually emitted to the atmosphere as N2O, more than twice the rate of 1-2% found in all previous research. There is no actual physical evidence for the 3-5% result. The budget analysis used to determine that figure is interesting and clever, but it is fraught with problems (for more, I refer you to the comments). The take home message is that if you substitute the lower value, as most of us would, the basic conclusions of the study change. The larger problem with this story has less to do with the science than with the reporting of science. You no doubt noticed one key phrase in the leading paragraph of this post. Nobel Prize Winner. That jumps out, no? The revered status implies this is research we should trust.There is one other phrase you are unlikely to have noticed above. Under review.
You see, the study in question has not been accepted for publication. It is under review with the journal Atmospheric Chemistry and Physics. That is important. It means that the journal and the reviewers, ostensibly assigned as representatives the scientific community, are deciding whether the methods are appropriate, whether the results can be falsified, whether the conclusions are sensible and whether the article as a whole is worthy of presentation in its submitted form. The review process is key to science. It filters out fatally flawed research and helps authors improve a questionable paper by providing the judgment of outside experts. There’s no shame in reviews. All of us, whether graduate student or Nobel Prize winner, need them. Safe to say every paper I’ve published is better because of the input of anonymous peer reviewers.In this case, that process is still underway. But that's where things get screwy. Unlike other journals, where submitted articles are sent to individual expert reviewers selected by the editors, APC includes an open review process. That means in addition to the normal peer review, anyone in the community can read the submitted paper online and offer comments. Personally, I like the system, as it gives authors a wider array of reviews and helps eliminate the chances that one irrational reviewer will derail a good piece of research.
The unintended consequence of the open discussion, however, is that an unpublished, and hence, unfinished paper is there for anyone to read. The paper can be reported in the media and the public can get the mistaken that the findings are accepted by the scientific community.
Science reporting is often a game broken telephone – I’ve argued about this before. As stories move from one medium to the next, the context fades away, the caveats are dropped, the uncertainty disappears, and we are left with just a headline. If you look at the comments on the paper, you’ll see that several scientists have produced well-referenced arguments questioning the methods and the conclusions. The reporter from Chemistry World spent the time to speak with the critics. So the subsequent article presented not only the results of the study, but the controversy over the methods and the specific critiques of the scientific community.But Chemistry World begat the Reuters and the Times. There, the context was reduced and the criticism of other scientists was gone.Reuters and the Times begat many, many more children, which spread like good little soldiers over the internet. In many cases, now even the context was gone, leaving us with just a headline: Biofuels are worse than gasoline. This is also an example of what New York Times science reporter Andrew Revkin famously calls the tyranny of the peg (see Matt Nisbet’s discussion in reference to hurricanes and climate change). The peg is the “new study”, in this case a big strong peg, as it is a “new study by Nobel Prize winner”. The tyranny is that instead of a more democratic story about greenhouse gas emissions from biofuel production, including a full reading of the research, we get a linear story just about the latest study.
Think about this.
Anyone writing about biofuels and GHG emissions, whether for a science publication, a general interest publication or even a small blog, knows that there have been a number of published studies comparing emissions from biofuel production with emissions from gasoline since those studies also received media attention. So before pounding this particular peg into the ground, there was one obvious question that should have been asked:How could the N2O from fertilizer offset the GHG reductions from cutting gasoline use, if other studies accounting for GHG from all aspects of biofuel production, including N2O from fertilizer, found that biofuels were, at the worst, a wash with traditional gas? What, then, has this study done to find such a dramatically different result? And how is the scientific community responding?
Monday, July 30, 2007
Dead Zone in Gulf is third largest since 1985
Though I'm reticent to link to any news article under the cable-TV-news-ish heading "Planet in Peril", CNN reports that this summer's Gulf of Mexico dead zone has been measured at about 20, 460 km2 in size. That makes it the third largest since measurement began in 1985.
The hypoxic zone was expected to be unusually large this year because of the high flux of nitrogen -- the nutrient that fuels the high productivity on the continental shelf that leads to the consumption of oxygen from the bottom waters -- from the Mississippi River this spring. The blame can likely be placed on the weather and possibly even the increase in corn planting (due to high prices / ethanol demand).
Now off to grab my cape. The planet is in peril.
Monday, June 25, 2007
Where does all that carbon go? Part II
Last week, Tamino at Open Mind, Eli at Rabbet Run and I began an experiment in mob-blogging’ about the carbon cycle. Following on our initial posts, profilic Eli has posted a couple interesting CO2 concentrations maps that highlight forest fires and fossil fuel emissions.
For a refresher on where all the carbon goes, the graph at right shows the IPCC's breakdown of the resting place, for now, of fossil fuel emissions over the past 25 years. The atmospheric build-up is measured (see Tamino's post) and the ocean uptake in well-constrained by measurement: that allows us to back-out the land uptake.
The drawback to this logic is that the land is both a prominent anthropogenic carbon dioxide source (e.g., deforestation, biomass burning) and a prominent carbon dioxide sink (e.g., net regrowth of vegetation). The positive uptake by land means that the sink is greater than the source. That, however, could change in the future, which would mean a larger fraction of carbon emissions would remain in the atmosphere. To answer that, it helps to study where the net carbon uptake occurring on land, and why?
One culprit is carbon’s chemical sibling nitrogen, that’s #7 on your periodic table if you’re scoring at home. Like many siblings, carbon and nitrogen are quite co-dependent, and, one might argue, a bit resentful about the whole thing. Carbon fixation - photosynthesis, plant growth – is limited by the availability of nitrogen. Though only up to a point. If there’s too much nitrogen, things get saturated, and the carbon-based plants pout and refuse to grow more.
You might find it strange that nitrogen is limited, given that N2 or di-nitrogen gas makes up the majority of the atmosphere. However, N2 is unreactive. It only becomes available to plants when converted to reactive form by microbes. In the process of making fertilizer and burning fossil fuels, we not only have increased the rate at which this conversion happens, leaving more nitrogen in our soils and waterways, we've emitted nitrogen in other reactive, gaseous forms, like nitrogen oxides or NOx. (eli, thanks for the suggestion - ed)
The IPCC map to the right shows nitrogen oxide (NOx) concentrations in the lower atmosphere. Notice the high levels above and downstream of North American, Europe and China. Deposition of this nitrogen could be increasing carbon fixation in forests.
A recent paper in Nature found just that: nitrogen fertilization, not forest regrowth after logging, may explain the majority of the net carbon sink in northern forests. The authors used chronosequences – yes, that’s a real word, not some star trek science word referring to data taken from a forest with trees of varying age that can be used to represent different stages of tree growth – to estimate mean carbon uptake at sites across the northern hemisphere.
By integrating uptake over entire rotations (from planting to forest replacement), the authors were able to get a more complete representation of carbon uptake by forests. Using that data, they found a strong relationship between nitrogen deposition and carbon sequestration, implying nitrogen fertilization may be driving the land carbon sink.
Nitrogen oxide emissions and nitrogen deposition are expected to increase in the future without tougher air pollution policies here and especially in Asia (see this paper). That could increase the carbon sequestration in northern forests, presuming those forests do not become N-saturated. Of course, hopefully the world will reduce NOx emissions and improve air quality. Unfortunately, that could also reduce carbon uptake and thus allow a larger fraction of carbon emissions to stay in the atmosphere.
Friday, April 20, 2007
The truth on greenhouse gases and meat consumption
Last week, Washington Post George Will wrote a sadly uniformed column attacking the public campaigns to reduce greenhouse gas emissions. It drew the usual array of responses and partisan blustering (left, right, ridiculous).
At heart was Time Magazine’s “Global Warming Survival Guide” which featured advice like 51 Tips on Saving the Environment, never mind that global warming is not merely an environmental problem, and that many of the tips have nothing to do with the environment, rather with improving human health.
Tip #22 -- Skip the Steak – claims that livestock is responsible for around 18% of the world’s greenhouse gas emissions. That’s not a mistake. The number comes from a prominent UN report released last year.
What Time, what George Will and what the vast majority of commentators on this subject get wrong is why livestock is responsible for such a large proportion of the world’s GHG emissions.
Columnists and pundits love to joke about cow farts and manure – producing methane and nitrous oxide, respectively – like they’re in bad Adam Sandler movie. That is an important source of GHG. But, in reality, the majority of the emissions attributable to livestock are not coming out the back end, but coming from all the energy used to grow the grain that is fed to livestock.
The United States alone grows almost half of the world’s corn and soybeans. And more than two-thirds of that production is used to manufacture animal feed. It requires an enormous volume of oil, to produce fertilizer and run farm machinery, and an enormous area of land. In turn, it is responsible in part for a number of ecological problems, like the “Dead Zone” in the Gulf of Mexico.
Will seems to mistakenly stumble upon this point, in the midst of some sarcasm, but :
Ben & Jerry's ice cream might be even more sinister [than a steak]: A gallon of it requires electricity-guzzling refrigeration and four gallons of milk produced by cows that simultaneously produce eight gallons of manure and flatulence with eight gallons of methane. The cows do this while consuming lots of grain and hay, which are cultivated by using tractor fuel, chemical fertilizers, herbicides and insecticides, and transported by fuel-consuming trains and trucks.
The concept is right, the comparison is flat wrong. Producing a gram of dairy protein requires only a fraction of the energy of producing a gram of meat protein, especially beef (for the simple reason you don’t kill the cow every time you milk it).Of the feed produced in the United States, only 12% is devoted to dairy cattle (see here). The rest goes to beef cattle, poultry and pork production. That’s why you often hear claims that we should all eat less meat, but not less dairy.
In essence, this problem is not about meat consumption. It is about devoting a significant proportion of our energy and our land to produce meat. One of the biggest obstacles to reducing greenhouse gas emissions in the future will be diet. We may or may not be exporting democracy to the world, but we certainly are exporting our meat-rich diets. As meat consumption rises in China and other parts of the developing world, the challenge of reducing oil consumption and reducing greenhouse gas emissions will grow. More on that later.
Monday, March 05, 2007
Climate and the Gulf of Mexico "dead zone"
My colleague Don Scavia and I have an article in the latest issue of Limnology and Oceanography about the effect of climate on the development of the seasonal “dead zone” in the Gulf of Mexico.
I’ve written a bit about this issue before on Maribo. The intensification of agriculture in the central US since the 1950s – huge increases in nitrogen fertilizer use, planting of more nitrogen-fixing soybeans, drainage of wetlands, installation of artificial drains under fields – caused a 2-3 fold increase in the amount of nitrogen the Mississippi River delivers to the Gulf of Mexico. The large influx of nitrogen now promotes the growth of a seasonal low oxygen or hypoxic zone each summer on the continental shelf of the northern Gulf of Mexico.
From the 1980s until quite recently, however, agricultural land use and land cover were relatively stable in contrast to the more dramatic changes in the previous three decades (the surge in interest for ethanol may end the relative stability). The one factor that changed the most, year to year, is the weather. Our study examines how this year-to-year variability in rainfall influences the amount of nitrogen flooding down the Mississippi in the spring and the extent of hypoxia in the Gulf.
The study finds that, absent any major changes in land use and land cover, the year-to-year variability in precipitation across the “Corn Belt” (in the previous November and December and in March, April and May) is the primary driver of the year-to-year variability in amount of nitrogen delivered by the Mississippi during the late spring (in May and June). Using this relationship, the study then examines how climate variability affects the potential size of the hypoxic zone and the implications for reducing nitrogen losses and the size of the hypoxic zone. During very wet years, a nitrogen reduction of 50-60% – close to twice the original recommended target – is necessary to reach the goal of minimizing the size of the hypoxic zone (
The results are a reminder of the importance of factoring climate variability into water quality or aquatic ecosystem policy, particularly given the changes in climate expected to occur in the coming decades.
Sunday, November 12, 2006
As the corn turns
Last week, I was at an EPA symposium about nutrient pollution in the Mississippi River Basin. Although the seminars had titles like “Nitrogen Processing in Flow-Controlled Backwater Systems of the Upper Mississippi River” and “Nitrogen Removal Capacity of Entire River Networks—Interactions of Geomorphic, Hydraulic and Biological Factors”, the same subject kept cropping up:
Ethanol
In 2004, the production of corn-based ethanol reached 3.4 billion gallons – or 2% of all U.S. gasoline by volume – by far the highest in history. The Energy Policy Act calls for ethanol production to more than double, to 7.5 billion gallons, by the year 2012. Since energy independence is likely to be one of the only areas of agreement between the Bush Administration and the newly Democratic Congress and Senate, it would not be surprising to see an even more aggressive policy emerge in the next couple years.
Every passing mention of the inevitable expansion of corn-based ethanol production brought sighs from many of the participants.
Why? First, most of the people I spoke with agree with the conclusion that the energy derived from corn-based ethanol is, at best, only slightly greater than the energy required in production. It may be net energy loss. Second, the participants of the Symposium have for the most part been working on the difficult challenge of reducing nitrogen pollution in the Mississippi River Basin. Increasing the production of the fertilizer-intensive crop will make it even more difficult to goal of shrinking the nitrogen-fuelled “dead zone” in the Gulf of Mexico.
To meet the 2012 ethanol goal, corn production is bound to increase [the only other option is to meet the goal purely by diverting corn grain away from feed or exports – not impossible, but less likely given the financial incentive to expand production]. That will require either the conversion of existing croplands to corn or the cultivation of existing croplands to corn.
The total area of croplands is unlikely to change significantly – it hasn’t in the past century. The best croplands were identified long ago. The change over the century has been in what crops are grown on those lands. Right now, around 2/3s to 3/4s of US croplands are devoted to just three crops: corn, soybeans and wheat.
So the thought it is that the extra corn production will come either at the expense of some other crop or at the expense of croplands currently left uncultivated. Some at the meeting suggested that farmers will replace soybeans with corn. Others, myself included, dismiss that notion: soybeans have been expanding for fifty years in the US and are too valuable crop to abandon (for ecological and economic reasons). It is more likely that either land devoted to other crops or lands contained within US Conservation Reserve Program – essentially farms are paid to leave some croplands fallow – will be used to expand corn production. Unless there is a major change in the production practices, the addition of more corn cultivation does not bode well for the nitrogen cycle.
The one reasonable argument for expanding corn-based ethanol production is that creating a market for biofuels will spur research on more efficient fuels. Thanks to market forces, corn-based ethanol may pave the way for a sensible form of biofuel production: either the “cellulosic” ethanol from high yielding grasses like switchgrass (that require no fertilizer) or biodiesel from oil-crops like soybeans, rapeseed or canola. If so, let’s hope the transition does not take too long.
