Climate change and biodiversity loss are the two greatest environmental challenges of our time. The 2015 Paris climate agreement states that global warming must be limited to a rise in temperature of less than 2˚C above pre-industrial levels to avoid the greatest impacts of climate change. This goal has served as a rallying point for global efforts to limit carbon emissions.

However, a comparably clear, agreed target for the amount of natural space that should be conserved to address the biodiversity crisis has been much more elusive.

The idea of securing at least half of our planet for nature conservation has recently been gathering momentum. Various studies indicate that achieving this “Half-Earth” goal—the name given to this endeavor by pre-eminent biologist E. O. Wilson—would help to avoid widespread biodiversity declines and prevent the collapse of vital services provided by ecosystems, such as carbon sequestration and climate regulation.

Unfortunately, the goal of 50 percent protection of all terrestrial ecosystems far exceeds current global conservation commitments. For example, the plan that is currently accepted by the United Nations Convention on Biological Diversity has a target of protecting 17 percent of land and freshwater regions and 10 percent of marine areas by 2020.

The much greater scale of conservation needed for Half-Earth protection has left many people questioning whether it is even possible given the unrelenting rate of habitat conversion eroding opportunities for large-scale conservation on a daily basis.

Eric Dinerstein and his colleagues have made a first attempt to answer this key ques­tion for land areas, and their findings are both encouraging and disheartening. Writing in BioScience not long ago, they analyzed the current level of ecosystem protection on a global scale. The authors’ data clearly demonstrate that achieving Half-Earth conservation is indeed still possible in many places, but not everywhere. 

They began by determining which of the earth’s 846 terrestrial ecoregions had at least 50 percent of their natural habitat remaining by studying satellite-derived maps of forest cover, land use patterns, and human occupancy. They found that nearly half of the world’s ecoregions have the remaining habitat necessary to achieve a Half-Earth protection target. Roughly one-eighth of these are already 50 percent protected.

About a quarter of the ecoregions with less than 50 percent of natural habitat remaining (requiring substantial restoration) were described by the Dinerstein team as “in peril,” meaning that they have no more than 20 percent remaining. 

But by assessing the possibility of achieving Half-Earth conservation, Dinerstein and his colleagues have helped to take the first step towards a road map for implementing such a goal. The authors outline the idea of a “global deal for nature,” whereby nations agree to achieving Half-Earth conservation, where possible, by 2050. Such an agreement would offer a biodiversity-focused counterpart to the Paris climate accord.

What is needed now is a clear, science-based plan for the next steps to making Half-Earth conservation a reality. We propose that this plan and the science agenda should be based around three fundamental questions.

The first question is in many ways the key question in conservation biology: Which are the most important places to conserve? The answer must take into account both the scale of the Half-Earth undertaking and the immense diversity of conservation objectives being considered. As a start, to halt imminent biodiversity loss, places that are home to the last remaining populations of a species and the last samples of any ecosystem type should be prioritized.

To complement these irreplaceable sites, we must also identify and protect places that are still large, intact, and functioning in ways that are unimpeded by large-scale human activities. Such areas represent the places best prepared to allow species and ecosystems to self-regenerate and to adapt to stressors that arise from human activity.

Second, what are the scope, severity, and trajectory of the threats to local biodiversity? And what are the processes that sustain those threats? To address these questions, we must understand not only the drivers of habitat loss and degradation, but also the nebulous effects that other factors can have on a habitat—things like invasive species, over-harvesting, disease, climate change, and shifts in the occurrence of natural fires.

All of these threats (which can be further influenced by social and economic forces) tend to reduce the quality of a given habitat without necessarily affecting its size. Understanding them helps us assess where action is most urgently needed to conserve and restore habitats. It can further allow conservation efforts to focus on minimizing the number of ecoregions that dip below retention of half of their natural habitat.

And finally, in places that require intervention, what measures will be needed to ensure that they maintain their natural integrity? Although protected areas have been the cornerstone of conservation action so far, they would probably have a reduced role in a conservation network that scales up to the level of Half-Earth.

Completing the research required to answer these three questions should provide an effective framework for reaching Half-Earth by 2050. The analysis produced by Dinerstein and colleagues has shown that this goal is possible in many places, and has helped to embolden this call for action. Half-Earth is about thinking big, and we urge conservation scientists to play their part in this process.