Toward a Half-Earth Future

However, no one to date has offered a particularly detailed proposal of what exactly would be conserved, nor a convincing explanation as to how we might move from the present — where competition between agriculture, cities, conservation, and other land uses is increasing — to a future in which human societies decide to set aside half of the earth’s land for conservation. How, in other words, would current land use patterns need to change in order to set aside half of the earth’s land area for conservation?

In a fascinating new paper, Zia Mehrabi, Erle Ellis, and Navin Ramankutty attempt to address this question. At first blush, the results are encouraging. Under the lowest trade-off global scenario, they find that the ‘Half-Earth’ target could be met without displacing any cropland or pasture at all. But dig a little deeper, and it becomes clear that given the likely trajectory of future food demand and the low present-day agricultural productivity of many regions of the world, difficult trade-offs will be required to achieve the vision of ‘Half-Earth’ advocates.

Difficult doesn’t mean impossible. We read what Mehrabi and his colleagues have done as a framework for quantifying the tradeoffs and thinking about what would be necessary to mitigate them. In our view, it is a sustained commitment to agricultural intensification that could make a future in which humans leave half the earth to rich, biodiverse ecosystems possible.

In order to gauge the prospects for achieving the ‘Half-Earth’ vision, Mehrabi et al. consider the proposition at multiple scales — globally, nationally, and within all of the world’s 846 ecoregions. As we mentioned above, they find that, under the least costly global scenario, setting aside half of the earth for nature entails little in the way of trade-offs. This finding is heartening, but it comes with several important caveats.

First, this low trade-off scenario wouldn’t actually require a significant shift from present global land use. Direct human land use accounts for less than 50% of the earth’s land area at present. Much of the land designated for conservation under this scenario would be deserts, tundra, and other areas that face little human development pressure anyway. This scenario would broadly comport with the basic template that has characterized much of the historic success of global conservation to date. Although the world has managed to place a significant amount of the earth’s land area in protected reserves of one sort or another over the last century, about 15% by the most recent estimates, much of that land has been characterized as “high and far,” land that would have been unlikely to see significant development whether it was formally protected or not.

Another important caveat to the low trade-off global scenario is that much of the land reserved for nature is not only not facing much in the way of development pressure, it also doesn’t preserve that much land in a number of the most biodiverse ecoregions of the world. So the benefits for biodiversity under this sort of global effort might be limited. Lots of land would be protected, but not necessarily a whole lot more biodiversity. Further, the low trade-off global scenario would also protect many of those areas in a fragmented fashion. The authors divide the planet into a grid of 8 X 8 kilometer pixels and then look into each pixel and aggregate the unused land, starting with the easiest areas to set aside (such as those that are already protected) before proceeding to non-agricultural areas. If this isn’t enough, the selection proceeds to increasingly high trade-off lands, like pasture and cropland.

So the low trade-off scenario protects a lot of land that is not under threat, much of which is not particularly high in biodiversity, and does so in a way that may turn out to be highly fragmented. By contrast, many of the leading advocates for the Nature Needs Half proposal envision setting aside large blocks of biodiverse ecoregions exclusively for nature, in order to allow for a broad representation of the world’s ecosystems and to ensure the survival of a large share of threatened species.

Realizing the vision of ‘Half-Earth’ advocates would therefore require much more difficult trade-offs. It would necessitate protecting more land in ecoregions that are under much more developmental pressure and displacing substantial areas of highly productive cropland and pasture for conservation. When the target is 50% in each ecoregion — the only scale that could ensure that much of the world’s biodiversity would be preserved — the trade-offs get much harder. According to Mehrabi et al.’s analysis, 15% of the world’s cropland and 10% of pasture would be displaced under this scenario. Under scenarios designed to ensure that large contiguous areas in most ecoregions are set aside in order to preserve an area’s full range of species, the task becomes harder still, requiring displacement as high as 31% of the world’s cropland and 45% of pasture.

A final and crucial qualification is that all of Mehrabi et al.’s scenarios are based on current land use patterns, which reflect global agricultural output that meets food demand — in a highly uneven and imperfect fashion — for a global population of over 7 billion people. That demand will likely grow by 50% by mid-century, when population is projected grow to over 9 billion, and substantially more than that by the end of the century, when population will likely exceed 10 or even 11 billion. With rising food demand, trade-offs will become yet more difficult.

So one potential conclusion from Mehrabi et al. might be that the notion of setting aside half the earth for nature in a way that protects a wide range of ecosystems may be more fantastical than aspirational. Another might be that global efforts to conserve biodiversity might be better served by focusing on areas rich in biodiversity, even if the absolute share of global land area is something significantly below 50%.

But we need to remember that ‘Half-Earth’ is not something that will be achieved overnight, as is the de facto assumption in the Mehrabi analysis. It is a multi-decadal ambition. This means that the trade-offs between conservation and food production are not fixed, and could be reduced.

How? We maintain that increasing agricultural productivity is the longest lever for mitigating, or even eliminating, the difficult trade-offs that Mehrabi et al. illuminate. If a scenario with current yields implies that 15% of cropland would be displaced, then a corresponding increase in yields could, at least in theory, mean no displacement would occur, allowing human societies to set aside half the earth exclusively for conservation without any loss of agricultural output. A shift from extensive to intensive meat production in most parts of the world would similarly allow displacement of large areas of pasture for conservation objectives without dramatically altering diets.

In this way, Mehrabi, Ellis, and Ramankutty’s findings provide a useful first-order estimate of just how much more efficient we will need to get at producing food to achieve the underlying objective that most conservationists desire: preserve our ecological inheritance in all of its majestic diversity.

What should be clear is that setting aside half the earth for conservation is a daunting but not impossible challenge. Ultimately, that is the point of this endeavor: to set a long-term, aspirational, and audacious goal around which the conservation community can mobilize. Obviously, the ‘Half-Earth’ vision will not be implemented top-down, globally, following some algorithm as in Mehrabi et al.’s thought experiment; growing the area of land set aside for nature will ultimately be the decision of national and local stakeholders. But whatever the governance process, if the objective is not just to set aside a fixed amount of land but to preserve a wide variety of biologically-rich ecosystems for future generations, it’s clear that our agricultural systems will have to become much more productive. That will bring significant benefits to human societies as well as conservation efforts.

The future of conservation and our global food system are inseparably tied to one another. In appreciating this, the conservation community has the opportunity to not only set ambitious goals, but to pursue a plausible course to their realization.