Saturday, March 5, 2016

Biogeochemical Planetary Boundary: Beyond the zone of uncertainty? (Part II)

I think of scientists as having two jobs.

One is to create intellectual tension.

The other is to resolve it.

Creating intellectual tension is generating hypotheses. Hypotheses that we do not know whether they are true or false represents intellectual tension. Competing hypotheses about how the world works are also intellectual tension. We do not know which is true. This is the tension.

Resolving intellectual tension can sometimes occur by identifying logical flaws in one hypothesis. Generally, intellectual tension is resolved by collecting data. It is a fair question about whether a hypothesis can ever be proven or disproven and therefore whether intellectual tension is ever fully resolved, but the process of science works to reduce intellectual by favoring hypotheses.

In the previous post, I identified some important intellectual tension in the scientific world.

There is the hypothesis that the planet has exceeded a biogeochemical "planetary boundary". Too much nitrogen is being fixed and entering ecosystems. This is the hypothesis.

Yet, it is unclear whether this is causing planetary-scale eutrophication of terrestrial ecosystems or  aquatic ecosystems.

On the one hand, we have a hypothesis where the world is awash in nitrogen. We fix more nitrogen than ever and apply it to ecosystems on a massive scale. As a result, nitrogen is leaking out into waterways creating dead zones in the oceans. Nitrogen is also entering the atmosphere and raining down on even the most remote ecosystems on earth. As a result, terrestrial ecosystems are becoming eutrophied. Species adapted to low nitrogen availability are being crowded out by faster growing plants. Biodiversity is plummeting. Productivity is increasing unsustainably. With all this extra nitrogen, we have exceeded a biogeochemical planetary boundary. Civilization as we know it is threatened.

Yet, the intellectual tension on this hypothesis actually takes the form of a competing hypothesis. It is possible that not only have we not exceeded a planetary boundary for nitrogen, but ecosystems might be becoming more nitrogen limited over time. As temperatures warm and atmospheric CO2 builds up, this might stimulate the demand for N more than it is being supplied. Plants and microbes become more limited by N. Plant N concentrations decline. Photosynthesis declines. Plants that compete well for N become more dominant. Less N leaks out of ecosystems into streams. Productivity becomes more and more constrained by the lack of nitrogen. Vegetation sequesters less and less carbon than they could be, all because there is not enough nitrogen. As a result, more CO2 accumulates in the atmosphere than could be if forests had more nitrogen. Climates warm even faster. Civilization as we know it is threatened.

Intellectual tension like this could not be as stark.

If you reduce the world to one pixel, there is either too much nitrogen. Or there is too little.

Resolving this tension requires data. On the one hand, we know that N is being fixed in ever greater amounts. On the other hand, CO2 continues to increase which shifts demand for N even higher. Back again, N is raining down on ecosystems still at an elevated rate. Yet, the NO3- concentrations of water in streams is so low, stream water is approaching the NO3- concentrations of distilled water.

The only way to resolve this tension is to collect data on N availability.

Yet we need long-term measurements of N availability to know for sure whether N is becoming more or less limiting.

We don't have these.

We could use the species composition of plant communities in conjunction with indices of what plants represent low or high N availability, but again we have not invested in long-term monitoring of our plant communities.

The tension of whether the world is becoming more eutrophic or more oligotrophic has existed for a long time now.

It probably is not a bad thing to think that civilization is threatened. But we should at least know whether it is because there is too much nitrogen or too little before we try to fix it. Or else our remedies might exacerbate the situation.

Without the right data, we cannot resolve this tension. That means we start monitoring key indices like N availability and species composition now and try to answer the question in 10 years.

Or we find a different dataset that allows us to reconstruct N availability on broad spatial scales far enough back in time to discern the trajectory of N availability.

Do we have the data to resolve this tension?

I think we might...

Let's see what reviewers say.

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