Sir

Intelligence may be a deadly pathogen that, given sufficient time, will infect all habitable planets. It rapidly learns to obtain energy by reversing the geochemical fluxes that hitherto maintained the surface conditions of the planet stable and habitable.

Such changes may be very rapid. At the current incremental rate, forced by intelligent biota, atmospheric carbon dioxide will be 600 parts per million by the year 2050. This is a non-trivial increase over the 180 and 280 p.p.m. indicated by ice-core data for glacial and interglacial periods respectively over the previous 160,000 years1.

In his Review Article in Nature, Timothy Lenton2 did not discuss this most interesting aspect of Gaia — its current manifestation. I think it is now agreed that self-regulating feedback by the pre-sentient biosphere maintained, and may have produced, an atmospheric composition and a mean global surface temperature conducive to the evolution of biota likely to develop intelligence. What is not clear is whether the sentient part of the biosphere will develop sufficient self-control to restore geochemical fluxes to the state necessary to maintain the surface of the Earth habitable.

In short, is intelligence a pathogen to which the biosphere can adjust, or is it terminal? Perhaps it is as well that we cannot yet observe other habitable planets, to discover what became of their sentient life!

Sir — Biology, geochemical cycles and climate are intimately linked. There are many feedbacks, both stabilizing and destabilizing, involving biology, geochemical cycles and climate. Obviously, systems with strong stabilizing feedbacks are more likely to persist than systems lacking such stabilization. And species that are components of stable systems are more likely to persist than species that are components of unstable systems. These simple observations can explain the prevalence of important stabilizing feedbacks on Earth involving biology, geochemistry and climate.

Is there such a thing as a negative feedback involving biology, geochemistry and climate that is not “Gaian regulation”2? If all such feedbacks are ipso facto Gaian, then “Gaian regulation” is but old wine in a new bottle.