Hello, I'm Sarah Cornell and I'm a coordinator of the International Planetary Boundaries
Research Network at Stockholm Resilience Centre. My own research is about human changes to
global environmental processes. I'm going to talk with you about human impacts on the
global nitrogen and phosphorus cycles. The global biogeochemical cycles link the living
and the non-living parts of the Earth system. Chemical relationships control the general
patterns of interaction between geological and biological processes. It's the way that
the nutrient elements flow through land, ocean and atmosphere that ultimately regulate all
of life on the planet. And human activities are changing all of these basic cycles. As
we explore this topic, I'll make reference to very simplified diagrams like this one,
that show the present day cycles and the human changes. The arrows just show the overall
annual flows of the elements between the Earth's crust, atmosphere, oceans and the biosphere,
that is all the living organisms on Earth. It's important to keep in mind that these
flows are the result of many different Earth system processes, and at the same time it's
also important to remember that in the boxes, atmosphere, ocean, biosphere, and so on, we
see many processes and feedbacks happening all the time. The Earth system is a very dynamic
system, and the simple diagram is a very simple representation. Because all of these boxes
are connected to each other through these different processes, human activities in one
will play out in all of the others. It's these consequences that mean that we need a planetary
boundary for nutrient elements. Piecing together the global budgets, in other words putting
the numbers on the up and down arrows in those diagrams, is one of the major achievements
of the last 50 years. It takes international and interdisciplinary research, and it involves
scientists working in all kinds of environments around the world. People are most familiar
with the global carbon cycle because changes in carbon are so tightly linked to the climate
system. The figure shows some of the processes that are happening at the moment. And here
is the first of the simple flow diagrams I talked about. The numbers show the annual
flows between the different stocks of carbon in the Earth system, and the red arrow shows
the human disturbance. Every year the biggest single change that we make to the carbon cycle
is that we're moving carbon from underground fossil fuel stocks into the atmosphere. We
also contribute carbon dioxide into the atmosphere through deforestation and agriculture. But
our main concern is that this carbon dioxide is accumulating in the atmosphere where it
acts as a greenhouse gas. Climate is changing as the concentration increases. But carbon
isn't the only element that we're changing. People are much less familiar with the other
elemental cycles, but they're just as important, and actually they're tightly linked to carbon
and the climate system too. In this picture you can see the main changes in the nitrogen
cycle. Nitrogen is one of the essential elements that sustains all of life on Earth. We use
it in our proteins that make up our muscles, for example. The flow diagram shows the natural
changes in white and the red arrows show the major changes that humans are causing in the
system. The biggest change is the transformation of atmospheric nitrogen into reactive forms,
like nitrate and ammonia. We do this because we need that nitrogen as fertilizer for the
food production. We also fix nitrogen from the atmosphere through many other processes,
industrial processes and transport as well. Right now the red arrow of nitrogen fixation
is bigger than the natural arrow of natural biogenic fixation of nitrogen. We are currently
more than doubling the natural rate of drawing down of nitrogen from the atmosphere into
the biosphere, but you can see that that isn't all that we're doing. This nitrogen is not
all taken up by crops. A large amount is being released back into the atmosphere where it
causes air pollution problems and acid rain, and is a climate greenhouse gas, and the rest
is released into rivers and oceans where we have problems of nitrogen enrichment and eutrophication.
I'll explain what that is in a little while. This is happening all around the world. When
rivers, lakes, and coastal zones have very high concentrations of nitrogen, and other
nutrient elements, the most responsive organisms draw up this nutrient most quickly. We end
up with algal blooms, microorganisms are thriving in these water conditions. The image shows
areas where there are very large amounts of marine algae, reflecting the highest concentrations
of nutrient elements. In the nitrogen cycle we have another particular challenge, which
is that a large amount of the nitrogen is transported all around the world, very large
distances, in the atmosphere. We see biogeochemical and ecological tipping points in area far
away from the most intense sources. The densely populated areas have the most intense nitrogen
pollution problems, but you can see from the figure that in this century of industrial
activity, nitrogen really has become a global problem. The issue is not just a regional
one. And nitrogen is not the only element that's causing us concern at the moment. The
phosphorus cycle is also being changed very substantially by human activity, and phosphorus
is another one of these essential nutrient elements. The figure again shows the processes
that are happening in the Earth system, and the simple diagram shows the natural flows
in white and the major human perturbation in red. Unlike nitrogen and carbon, phosphorus
isn't really affecting the atmosphere. The biggest change is that human activities are
mining it from the ground and applying it to land surfaces for agriculture, 25 million
tons a year. But a large amount of this phosphorus is being mobilized in the Earth system through
the watercourses. It goes into our rivers and receiving oceans. It also contributes
to nutrient enrichment. Nutrient enrichment doesn't really sound like a problem, but it
is a very serious one. Locally it's a major problem for public health and environment,
and it's an economic issue as well. In nutrient-enriched water systems biodiversity typically decreases.
And when the algal blooms die and rot away they starve the water of oxygen. We end up
with dead zones, and this is another example of tipping points that we see in the environment.
It's the risk of large and irreversible dead zones in the oceans that really calls for
a planetary boundary for phosphorus. The changes in both the nitrogen and phosphorus cycles
are largely driven by the same human activity. In other words, fertilizer application for
food production. The global impact arises from regional activities, so a really major
area for research and policy at the moment is to improve the global assessment of both
these nutrient elements. The planetary boundaries for human interference in the global nutrient
cycles now reflect these regional differences. The ecological impacts of the eutrophication
of surface waters, and increasingly we're working on trying to improve the links between
the elemental cycles because they're linked in nature.
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