The astronomer Carl Sagan famously described Earth as a pale blue dot when viewed from far away. Tiny and insignificant it may look, but this blue dot has provided us with a remarkably stable environment for almost 12,000 years.
As we have multiplied, we have pushed Earth close to 鈥� and possibly beyond 鈥� its ability to support human society. On several fronts, we have exceeded what scientists regard as 鈥渓imits鈥� for some of Earth鈥檚 most vital life support systems, such as the nitrogen and phosphorus cycles and biodiversity.
On other fronts, we are fast approaching the danger zone. Climate change is the most pressing, not least because it has the ability to drive Earth into a new state that鈥檚 inconsistent with our well-being. To stay safe, we need to limit warming to 2掳C 鈥� and preferably 1.5掳C 鈥� above pre-industrial levels.
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With emissions still growing, without radical technical intervention. And so, the prevailing assumption 鈥� even at the Intergovernmental Panel on Climate Change 鈥� is that within decades we will suck large amounts of carbon dioxide out of the air, using a technology called bioenergy with carbon capture and storage (BECCS).
In practical terms, this means devoting millions of hectares to plants like maize, which pull CO2 out of the air as they grow. They are then used to make fuel, with the carbon released during combustion captured and buried deep underground for millennia.
Plan B
There is a Plan B, too. It comprises measures to counter climate change by injecting cooling aerosols high into the stratosphere to reflect sunlight, for example, or by seeding the ocean with iron to encourage the growth of carbon-consuming plankton. Needless to say there are worries about how these techno-fixes will impinge upon Earth鈥檚 other vital processes. Separate strands of new evidence add weight to these concerns.
Typically, BECCS is seen as the safer bet. Not so. A team of German scientists has taken a fresh look at how using it on a large scale 鈥� to store up to 10 billion tonnes of carbon annually 鈥� , including climate change, biodiversity, land use, freshwater availability and the nitrogen and phosphorus cycles.
The research finds that relying on BECCS as a key means to keep a lid on warming would be highly risky, compromising Earth鈥檚 well-being in other ways. If plantations were grown in areas low in biodiversity, such as on grasslands, they would need twice as much irrigation as used in modern agriculture, threatening the world鈥檚 freshwater supplies. However, if conserving water was the priority, the same biomass yield could be achieved by planting in the tropics 鈥� but that would mean clearing forests, which creates a new source of emissions and harms already fragile biodiversity.
Should we opt for Plan B then? Not so fast. takes a look at how using aerosols to reflect sunlight would affect biodiversity, both at the outset and also if it was suddenly stopped after 50 years for some reason. As it turns out, few species could cope with the rapid warming that would follow abrupt termination, with many corals, land mammals, amphibians and mangroves becoming extinct.
Last week, the influential American Geophysical Union . This is sound advice; it is unfair to write off such approaches based on a handful of studies. Still, the recent conclusions don鈥檛 bode well for the most popular strategy for climate change: sitting tight and praying for a technological solution.
