Planetary Boundaries: A Review of the Evidence
The planetary boundaries hypothesis - embraced by United Nations bodies and leading nongovernmental organizations like Oxfam and WWF - has serious scientific flaws and is a misleading guide to global environmental management, according to a new report by the Breakthrough Institute. The hypothesis, which will be debated this month at the UN Earth Summit in Brazil, posits that there are nine global biophysical limits to human development. But after an extensive literature review and informal peer review by leading experts, the Breakthrough Institute has found the concept of "planetary boundaries" to be a poor basis for policy and for understanding local and global environmental challenges.
- Six of the "planetary boundaries" -- land-use change, biodiversity loss, nitrogen levels, freshwater use, aerosol loading, and chemical pollution -- do not have planetary biophysical boundaries in themselves. Real, global biophysical threshold elements exist in the global climate system, and partly also for ocean acidification (same driver as climate change, carbon dioxide), ozone depletion (regional tipping point averted), and phosphorous levels (tipping point extremely far off). But for all the remaining "boundaries," there are no global tipping points beyond which these ecological processes will begin to function in fundamentally different ways than they do at present or have historically. Hence the setting of boundaries for these mechanisms is an arbitrary exercise. A lax boundary may result in more degradation. A strict boundary less. But there is no evidence that exceeding the boundary will result in a fundamentally different magnitude of impacts associated with human activities.
- Aside from their impacts on the global climate, these non-threshold "boundaries" operate on local and regional, not global, levels. This means that no global boundary can be meaningfully determined. For example, freshwater use, land-use change, or nitrogen levels in one region are ecologically independent of these processes or their impacts in other regions.
- There is little evidence to support the claim that transgressing any of the six non-threshold boundaries would have a net negative effect on human material welfare. While there may be many reasons to limit degradation and constrain human activities that impact upon natural resources and ecosystems, impacts of environmental change on human material welfare are typically both positive and negative, and the net benefit or cost varies with place, socioeconomic conditions, and many other factors. Hence, the same type of environmental change may in one place result in a net benefit for human material welfare, and in a different locale, a net loss.
IMPLICATIONS FOR SCIENCE AND POLICY
1. Global limits may risk misleading local and regional policy choices.
For the six environmental processes that lack global biophysical thresholds, limits or boundaries cannot be set with reference to science alone. Changes in these systems necessarily entail continuous political and economic trade-offs between positive and negative impacts on human welfare, nearly all of which exist on local and regional, not global, scales. Specifying regional and local systems as global may in many cases result in misguided policies at the local and regional levels. Two cases illustrate this:
- Synthetic fertilizer -- the main source of human nitrogen additions to the environment -- boosts food production but may, if used excessively, cause groundwater pollution and "dead zones" in the coastal ocean. In some places, where nitrogen is used excessively, the negative impacts are substantial, and a reduction may be warranted. At the same time, there are other areas, notably many parts of Africa, where increased use of nitrogen in agriculture would yield very significant benefits for human welfare. Moreover, limiting nitrogen use in Africa would in no way mitigate the impacts of excessive nitrogen use in other regions. As such, the positing of a global boundary is of little use to policy makers in either those regions that underutilize nitrogen fertilizers, or those that overutilize them.
- Freshwater extraction meets direct human needs for consumption and irrigation, but may compromise riverine ecosystems. Just as there are places where water is scarce, and reducing human use may be preferred, in many rivers around the world, especially where population density is low, moderate increases in water extraction would not endanger ecosystem health. Furthermore, limiting human use of water in one river basin or aquifer does not ameliorate water scarcity elsewhere, thus making a global limit meaningless.
2. Environmental factors are among many that influence the climate.
Most of the non-threshold systems interact with climate change in one way or another, putting greenhouse gases at the center of all the planetary systems. For example:
- Nitrogen can increase growth rates in plants and thereby stimulate faster uptake of carbon from the atmosphere.
- Land-use change is the source of a large share of global greenhouse gas emissions.
- Freshwater levels influence the ability of the terrestrial biosphere to act as a carbon sink.
- Carbon dioxide is the key driver of ocean acidification.
While no climate strategy is complete without accounting for environmental factors such as nitrogen, land-use change, and freshwater use, assigning them global boundaries confuses means (factors that influence the level of greenhouse gases in the atmosphere) with ends (climate stability). The fact that environmental processes can affect the level of greenhouse gases in the atmosphere and therefore constitute means to climate change mitigation does not mean that there is any absolute boundary for them.
3. Ecological degradation has thus far shown little correlation with global measures of human material welfare.
The planetary boundaries hypothesis rests on the assumption that environmental variables are closely linked to human welfare, and that, consequently, loss of ecosystem services or natural capital implies declining human welfare. This assumption, however, has thus far not stood up well to observed trends with regard to both human welfare and ecological degradation. Over the last few decades, human welfare has improved significantly on a global level, even as a majority of ecosystem services have declined.
4. With the notable exception of climate, there is little reason to assume that other conditions that characterized the Holocene are particularly important to human material welfare.
The planetary boundaries hypothesis presupposes that the Holocene -- the geological epoch spanning from the end of the last ice age to the Industrial Revolution -- represents the most desirable state of the environment for human welfare. While there are of course very good reasons to prefer the climate of the Holocene, which was relatively warm and stable, there is little evidence that land cover, nitrogen levels, biodiversity, or any of the other non-climate systems had in themselves a stability or level that was particularly beneficial for human development. In many ways, the human population and the level of material welfare that exist today fundamentally depend on the fact that some of the non-climate systems do not remain at Holocene levels. This would suggest that it is not the environmental conditions of the Holocene that have enabled human development in the past two hundred years, but the environmental conditions of the Anthropocene. For example:
- Nitrogen, in the form of synthetic fertilizers, and increased freshwater withdrawals for irrigation were of critical importance to the enormous increase in food production over the past century.
- Land-use change has been fundamental to expanding agriculture and thus feeding the world.
- Until now, the net benefit in terms of human welfare of using fossil fuels and thus emitting carbon dioxide to the atmosphere has been immense.
5. The relationship between human material welfare and ecological systems is better explicated through trade-offs than boundaries.
The claim that the planetary boundaries represent "non-negotiable" limits upon human activities, development, or consumption that "exist irrespective of peoples' preferences, values, or compromises based on political and socioeconomic feasibility" is not supported by empirical evidence on either ecosystem functioning or the relationship between environmental change and human welfare. Instead, our review of the nine "planetary boundaries" suggests that there are multiple costs and benefits of human impacts on the environment, and that balancing these is an inherently political question -- not one that science alone can resolve. Suggesting otherwise may harm the policy process, as it precludes democratic and transparent resolution of these debates, and limits, rather than expands, the range of available choices. The important role of the earth sciences in informing management of environmental problems would be enhanced by shifting focus to identifying and explicating various possible courses of action and the trade-offs they entail, as well as exploring both negative and positive impacts of environmental change on human welfare.
Download the report here.