Some of those global alterations made by humans may be approaching tipping points—thresholds—that could destabilize the whole Earth system. Drawing on a landmark paper in Nature in 2009 (“A Safe Operating Space for Humanity,” by Johan Rockström et al.) Lynas outlined the nine boundaries we should stay within, starting with three we’ve already crossed. 1. Loss of biodiversity reduces every form of ecological resilience. The boundary is 10 species going extinct per million per year. Currently we lose over 100 species per million per year. 2. Global warming is the most overwhelming boundary. Long-term stability requires 350 parts per million (ppm) of carbon dioxide in the atmosphere; we’re currently at 391 ppm and rising fast. “The entire human economy must become carbon neutral by 2050 and carbon negative thereafter.” 3. Nitrogen pollution. With the invention a century ago of the Haber-Bosch process for creating nitrogen fertilizer, we doubled the terrestrial nitrogen cycle. We need to reduce the amount of atmospheric nitrogen we fix per year to 35 million tons; we’re currently at 121 million tons.
Other quantifiable boundaries have yet to be exceeded, but we’re close. 4. Land use. Every bit of natural landscape lost threatens ecosystem services like clean water and air and atmospheric carbon balance. “Already 85% of the Earth’s ice-free land is fragmented or substantially affected by human activity.” The danger point is 15% of land being used for row crops; we’re currently at 12%. 5. Fresh water scarcity. Increasing droughts from global warming will make the problem ever worse. In the world’s rivers, “the blue arteries of the living planet,” there are 800,000 dams with two new large ones built every day. The numeric limit is thought to be 4,000 cubic kilometers of runoff water consumed per year; the current number is 2,600. 6. Ocean acidification from excess atmospheric carbon dioxide is increasingly lethal to ocean life such as coral reefs. The measure here is “aragonite saturation level.” Before the industrial revolution it was 3.44; the limit is 2.75; we’re already down to 2.90. 7. The ozone layer protects the Earth from ultraviolet radiation. One man (Thomas Midgley) invented the chlorofluorocarbon coolant that rapidly reduced stratospheric ozone, and one remarkable agreement (Montreal Protocol, 1987) cut back on CFCs and began restoring the ozone layer. (In Dobson units the limit is 276; before Midgley it was 290; we’re now back up to 283.)
Two boundaries are so far unquantifiable. 8. Chemical pollution. Rachel Carson was right. Human toxics are showing up everywhere and causing harm. Coal-fired power plants are one of the worst offenders in this category. (Lynas added that nuclear waste belongs in this category but “the supposedly unsolved problem of nuclear waste hasn’t so far harmed a single living thing.” 9. Atmospheric aerosols—airborne dust and smoke. It kills hundreds of thousands of people annually, the soot causes ice to melt faster, and everyone wants to get rid of it. But one beneficial effect it has is cooling, so Lynas proposes “we could move this pollution from the troposphere where people have to breathe it up to the stratosphere where it can still cool the Earth and no one has to breathe it. That’s called geoengineering.”