The Biosphere: Global limits of human habitability

human habitability
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Here, Dr Steven Running dives into the questions of Net Primary Product and ongoing climate change, to illuminate what the global limits of the biosphere are

One of the foundational principles of biology is that a population cannot grow forever in a finite ecosystem, in this case the Earth. Systems ecology theory predicts when resource limits are exceeded, a progressive system feedback of starvation, predation, and disease limits uncontrolled population and consumption growth. The global human population has now nearly tripled since 1950, and economic activity increased tenfold, leading many to suggest that humanity is heading toward a population and consumption overshoot and correction this century. The global population, currently at 7.5 billion people, is projected to rise beyond 10 billion by 2100. Future limits become an urgent policy issue when one considers the expansion in living standards aspired to by the underdeveloped world. Is humanity smart enough to anticipate global overshoot, and shift to sustainable policies before these morally unacceptable systems feedbacks take over?

Net primary production, or NPP, usually in units of kg/ha (or equivalent) of plant biomass, is the core metric scientist use for quantifying total plant growth over broad areas without worrying about the complicating factors of species, yield fraction or other details. Because of these simplifications, NPP can be measured for entire regions, countries, and ultimately the whole biosphere. Feeding people is a moral imperative, yet global NPP data does not now show sufficient increases in crop production to meet these future needs.

Plant material is also the source of wood fibre for construction and paper products, and for bioenergy – a renewable energy source such as wood, agricultural waste and specifically-grown energy crops, which is burnt for heating, cooking or electricity production. In the future, bioenergy could meet a proportion of our energy needs, as the net carbon footprint is far lower than that of fossil fuels. However, our current estimates vary greatly due to unknown factors, such as the future availability and productive potential of land, which needs to be balanced with food production to feed the growing human population.

While food, fibre and fuel are obvious consumption of biospheric NPP, non-consumptive uses are also of prime importance, particularly climate stabilisation. Photosynthesis continually removes carbon dioxide from the atmosphere, land-based plants absorb around 30% of the carbon dioxide that human activity adds to the atmosphere, demonstrating the critical role of NPP in climate change mitigation. Accurate NPP values are vital for scientists who are trying to understand and predict the severity and effects of future climate change on the biosphere. If NPP is degraded, atmospheric carbon dioxide would increase faster, and global warming would accelerate. Conversely, if humanity can enhance NPP it slows global warming down.

Until recently, measuring NPP continuously on a global scale was not possible. Most previous estimates have been made by taking measurements of plant biomass from a small plot and roughly extrapolating them to calculate global NPP. To obtain a more accurate picture, I started to work closely with NASA in the early 1980s as a member of the development team of NASA’s Earth Observing System. This fleet of satellites has been monitoring the Earth’s land, oceans and atmosphere for 20 years. The Terra and Aqua satellites are part of the Earth Observing System and are equipped with a Moderate Resolution Imaging Spectroradiometer (MODIS). For my algorithm, MODIS measures the proportion of Earth’s land surface that is covered in vegetation and the reflected sunlight in special wavelengths.

My lab developed an algorithm to assesses the amount of sunlight that can be absorbed by vegetation, along with the local weather conditions, to calculate the rate of photosynthesis, and therefore the amount of carbon dioxide that is being absorbed, producing now a large dataset of continuous global NPP measurements. NASA releases new NPP data every eight days, where it is used by various groups, including NASA, government organisations, many other countries and private landowners to aid in land management.

During our early research, we identified various climate factors that limit the rate of photosynthesis and plant growth in different regions. The team found that temperature is limiting at high latitudes but overall, water availability is the most important factor. The team’s long-term monitoring of NPP has helped us to understand how changes in climate can affect global NPP. For instance, rising temperatures increase growing season length, but decrease water availability, and many of the NPP trends identified can be directly attributed to these effects. We showed how significant droughts between 2000 and 2009 caused the reduction in NPP in the Southern Hemisphere. Decreases in cloud cover increased sunlight over tropical areas causing the largest increases in NPP, particularly in the Amazon rainforest. It initially appeared that rising global temperatures were having a positive effect on the growth of plants, potentially increasing their ability to act as a sink for excess carbon dioxide produced by human activity. However, the reduction in NPP from 2000 to 2009 from drought effects raises serious issues. If rising global temperatures reduce plant growth, the ability of vegetation to act as a carbon sink will be reduced, accelerating climate change.

This NPP data is also vital for landscape management, optimisation and planning. Is the Amazon rainforest more valuable as a carbon sink mitigating climate change, or as a food production source? Are trees in a savannah more valuable as a fuel supply, or for carbon sinks and wildlife habitat? What is the most sustainable intensity of grazing on rangeland? The answers to these priorities are different for each tract of land that has differing potential for NPP, and local demands by humanity for consumptive and non-consumptive uses. NPP from a biodiverse ecosystem provides wildlife habitat, and aesthetic landscapes for human recreation, note the huge popularity of U.S. National Parks. There is clear potential for conflicting priorities between consumptive uses, and non-consumptive uses of our ecosystems.

An example of conflicting uses of NPP is whether land should grow crops for food, or for bioenergy. It has been suggested that biofuels could substitute for fossil fuels as part of stabilising climate, so we investigated the maximum global primary bioenergy potential. We calculated a potential bioenergy capacity that is four times lower than previous estimates and would require 55.6 million square kilometres of natural vegetation to be converted to bioenergy cropland – an area greater than Asia and Europe combined! The conversion of natural ecosystems to cropland is not only destructive to the environment, but would also significantly reduce the amount of carbon that can be sequestered, reducing climate stabilisation potentials. By 2050, the demand for global energy is predicted to double, but we suggest that bioenergy can only meet less than 15% of global energy needs in a sustainable manner.

As the Earth’s population continues to increase, and climate continues to change, consistent monitoring of NPP will become an even more essential tool for understanding and mitigating damage caused to the biosphere. It is essential for humanity to not reach catastrophic planetary limits risking collapse. There is no better and available global dataset than NPP, the foundation of food, fibre, biofuel and climate stabilisation, for this essential monitor of global habitability.




WK Smith, M Zhao, SW Running, Global bioenergy capacity as constrained by observed biospheric productivity rates, BioScience, 62, 2012, 911–922.

SW Running, A measurable planetary boundary for the biosphere, Science, 2012, 337, 1458–1459.


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