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Climate and Energy Blog


Planetary Boundaries: Global Guardrails for Humanity

Mon, 01/11/2016 - 1:25pm -- Steve Carpenter

If climate change was the star of the recently concluded Paris Climate Conference (COP21), ecology played a key supporting role. At COP21, 195 nations agreed to hold global warming below 2 degrees Celsius (3.6 degrees Fahrenheit) and, by 2050, to reduce emissions of heat-trapping greenhouse gases to a level that land and ocean ecosystems can absorb and thus keep them out of the atmosphere. A deep understanding of ecology—the relations of organisms to one another and to their physical surroundings—is key to implementing COP21's ecosystem-based strategy. But ecology also plays a more complex role for life on earth by setting our planetary boundaries—the environmental limits within which humanity can safely operate.

A concept introduced in 2009, planetary boundaries can be thought of as the global guardrails within which humanity can continue to develop and thrive for many more generations. Scientists have estimated planetary boundaries for climate warming, land use, freshwater use, ozone, nutrients (namely nitrogen and phosphorus), and other key characteristics of the earth that determine the quality of our lives and our survival. By crossing these boundaries, we may risk creating large-scale environmental changes that are difficult to reverse and could forever alter life on earth.

The warming boundary of 2°C was chosen with ecology in mind. As temperatures rise higher than 2°C, changes in the biosphere (the living parts of the planet) can accelerate warming. In that scenario, we’ll go from worrying about how Earth’s climate impacts its ecosystems to worrying about how these changed ecosystems drive even more warming.

For example, the northern tundra and taiga forests of Canada and Russia contain vast amounts of organic carbon frozen in permafrost. As the planet warms, the permafrost could melt and release this carbon into the atmosphere. Such a big infusion of greenhouse gases will rapidly drive warming well above 2°C. 

In an effort to counter this potential release, Russian scientists are conducting a large-scale experiment to convert the tundra and taiga to grassland. Ecologist Sergey Zimov and his son Nikita are re-establishing large herbivores—elk, bison, moose, horses and reindeer—in a 53-square-mile reserve in Siberia known as Pleistocene Park. The idea is that these animals’ eating habits—grazing—may turn the existing ecosystem into grassland. The hope is that the resulting grassland will insulate the permafrost, stave off melting, and thereby prevent the emission of greenhouse gases as the planet warms.

However, it is not certain whether the big grazers will restore the grassland or if the grassland will prevent permafrost melting. Moreover, if the scheme does work, millions of square miles of taiga and tundra would have to be converted to grassland in the next few decades—a large undertaking to say the least.

Warming could also affect the other planetary boundaries. For example, the phosphorus boundary depends on the amount of runoff from land to surface waters that occurs. Too much phosphorus runoff triggers toxic algal blooms, oxygen loss, and fish kills in rivers, lakes and coastal ocean waters. 

The current estimated planetary boundary for phosphorus assumes that the amount of runoff will stay about the same. However, climate change could test this boundary. It is already causing wetter conditions, bigger storms and more runoff in some regions of the planet, such as here in Wisconsin.

And as the planet continues to warm, scientists predict more frequent and intense rainfall events and thus more runoff, which would give us less room to work with to stay within the phosphorus boundary. This shrinking wiggle room will require that we drastically reduce our use of phosphorus fertilizer and develop advanced solutions for reducing or recycling livestock manure, a huge contributor to the phosphorus problem, in order to prevent further damage to the lakes, rivers and reservoirs that are essential for human needs.

Recognizing the risks of more intensive storms, the COP21 agreement noted the potential for loss and damage associated with climate-linked disasters. However, a footnote states that nations that emit greenhouse gases are not liable for such loss and damage, nor does it mean these emitters must compensate nations that become victims of climate disasters. Thus, who bears the responsibility for more intensive storms, increased pollution and changes in the phosphorus boundary is far from clear.

The 2°C target is more than just a number. If we can succeed in limiting warming to 2°C or less, we have a chance to maintain a climate that we can moderate to some extent and more flexibility to operate within the planetary boundaries. In other words, we can still have some control in shaping the future of our climate, ecosystems and communities.

But if we surpass this target, we’ll find ourselves with the increased risk that runaway biospheric feedbacks will accelerate warming even more and in ways that may be surprising and difficult for us to manage. That would leave us with a more uncertain future, one where ecology may become the star of conferences about climate change.

A version of this post originally appeared in Yahara in Situ on December 17, 2015.


Stephen R. Carpenter is the S.A. Forbes Professor of Zoology and Director of the Center for Limnology at the University of Wisconsin-Madison. His research on lake ecology led to fundamental concepts of trophic cascades, ecological regime shifts, and resilience of ecosystems and the societies that depend on them.

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