In 2007, my colleagues and I published a study examining of the likelihood of the 2005 “hot spot” occurring with and without human influence on the climate system. The study contrasted model simulations of the Caribbean with historical data and then computed the statistics of extreme ocean temperature events. The second slide summarizes some of the key results of from study. In a nutshell, our best analysis concluded the 2005 Caribbean “heat wave” would likely be on the order of a once in a thousand year event, had there been no human-generated greenhouse gas or aerosol emissions since the Industrial Revolution (“natural forcing”). By the 1990s, the human forcings increased the odds to once in 10-50 years. And continued warming under “business as usual” would make such heat waves happen in three out of every four years.
Five years later, a Caribbean “heat wave” has happened again. I’ve been writing for months that there was a strong likelihood of extensive coral bleaching in the Caribbean this fall according to NOAA’s advance forecast of sea surface temperatures (in fact, we had a good inkling of this last summer). Now we’re getting reports of bleaching from observers in the Caribbean. Add this to the observations (following predictions, once again!) from Southeast Asia and the Equatorial Pacific, and we have what may be the most, or second most, extensive “global” coral bleaching event in recorded history.
For all those writing about this event, keep in mind the predictions. This is what the scientific community predicted was likely to happen. An event which we calculated would be a once in a millennium occurrence without human impact on the climate, happened again five years later.
Low oxygen anoxic zones due to excess nutrient runoff from agriculture and are increasingly common worldwide. On Maribo Simon Donner writes about how the ongoing floods in the upper Mississippi are likely to produce the largest ever ‘dead zone’ in the Gulf of Mexico. Simon writes:
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 problem like the Dead Zone are to solve.
Simon Donner writes about his new paper Corn-based ethanol production compromises goal of reducing nitrogen export by the Mississippi River (Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0708300105) on his weblog maribo:
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.
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.
In a commentary Shifting Baselines, Local Impacts, and Global Change on Coral Reefs in PLoS Biology coral reef ecologists Nancy Knowlton and Jeremy Jackson write:
Imagine trying to understand the ecology of tropical rainforests by studying environmental changes and interactions among the surviving plants and animals on a vast cattle ranch in the center of a deforested Amazon, without any basic data on how the forest worked before it was cleared and burned. The soil would be baked dry or eroded away and the amount of rainfall would be greatly decreased. Most of the fantastic biodiversity would be gone. The trees would be replaced by grasses or soybeans, the major grazers would be leaf-cutter ants and cattle, and the major predators would be insects, rodents, and hawks. Ecologists could do experiments on the importance of cattle for the maintenance of plant species diversity, but the results would be meaningless for understanding the rainforest that used to be or how to restore it in the future.
This lack of a baseline for pristine marine ecosystems is particularly acute for coral reefs, the so-called rainforests of the sea, which are the most diverse marine ecosystems and among the most threatened [4–8]. Most of the world’s tropical coastal oceans are so heavily degraded locally that “pristine” reefs are essentially gone, even if one ignores changes associated with already rising temperatures and acidity . Most modern (post-SCUBA) ecological studies have focused on reef ecosystems that are moderately to severely degraded, and we have a much better understanding of transitions between human-dominated and collapsed reefs than between human-dominated and quasi-pristine reefs.
Knowlton and Jackson’s essay is a comment on an article in PLoS One Baselines and Degradation of Coral Reefs in the Northern Line Islands by Stuart Sandin and others that describes a large scale marine community assessment across a gradient of human dominated to relatively little impacted reefs in the Pacific. The study found that large predatory fish and reef-building organisms dominated the reefs around unpopulated islands, but around populated islands the reefs were dominated by small planktivorous fishes and fleshy algae. The reefs around populated islands exhibited more coral disease and less coral recruitment, suggesting that protection from overfishing and pollution may increase the resilience of coral reefs. The authors write:
Thus, local protection from overfishing and pollution may enhance ecosystem resilience to warm episodes and coral bleaching that result from global warming. To test this we need to determine how do coral recruitment, growth, and survivorship respond to changes in local community structure due to fishing, and how do these responses interact with episodes of warming measured by DHW. We also need to determine how fish productivity, i.e., the key currency of fisheries management, varies with changes in food web structure such as those observed between Kingman and Kiritimati. The only way to answer these questions is by investigation of reefs like the northern Line Islands that have remained remarkably intact in comparison to the global norm. They are among the only baselines that remain.