From Mike Beckett
The extensive damage humans have inflicted on our planet could easily be detected by an inquisitive alien even in the very distant future, in ways that Bob Holmes does not mention when he imagines an Earth without people (14 October, p 36).
In mediaeval times rich mineral deposits could be found at or near the surface of the Earth, accessible with hand tools. Even a hundred years ago commercial copper mines were extracting ore containing 25 per cent metal. These deposits have now been worked out and today a yield of 0.5 per cent is considered satisfactory. The situation is similar with many other metals. The continued supply of minerals that we take for granted is dependent on ever larger, deeper mines and continually improved extraction techniques.
These high-grade deposits formed over tens or hundreds of millions of years and presumably would re-accumulate very slowly if at all. It is most improbable that a large-scale industrialised civilisation could have existed before us and left these resources untouched.
The alien visitor in a million years time may well find an empty sterile planet, but she, he or it will have to dig deep if looking for mineral wealth.
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From Bob Muirhead
Bob Holmes implied that anthropogenic CO2 emissions would cease almost at once if human beings suddenly disappeared from the planet. This assumption is unwarranted.
Existing large inventories of gaseous and liquid hydrocarbons in refineries, chemical plants and storage facilities would be released to atmosphere as those untended facilities failed due to fire, accidents and operational upsets. We could expect such releases to occur over weeks and months, possibly extending to a few years as all equipment progressively failed.
That would not be the end – or even the worst – of hydrocarbon releases. Larger releases would follow over succeeding decades, as equipment controlling all the oil and gas wells on land and under the sea failed due to corrosion, fire or other accidents. Essentially all the hydrocarbons in all the producing fields would be released to atmosphere. This would occur rapidly at first until field pressures fell to atmospheric, but even afterwards residual evaporation would still occur.
On longer timescales, the wells themselves would probably collapse, but cracks and fissures would still allow continuing seepage of hydrocarbons.
All the hydrocarbons thus released would form CO2 either by combustion in the inevitable fires or by other chemical reactions in the atmosphere and oceans.
Port Melbourne, Victoria, Australia
From Alex Saragosa
I believe other things may survive us, giving future alien archaeologists clues about our civilisation.
For instance, the tens of tons of gold stored in place like Fort Knox or the Federal Reserve of New York could survive intact for many millions of years. Maybe some would even preserve the symbols engraved on them. Even if not, the fact that so rare a metal is so heavily concentrated in a few places would clearly be a clue of artificial activity.
Maybe an alien archaeologist would scratch their head (if they had one) trying to understand why this strange civilisation hoarded gold. It could be as puzzling as our probes on the moon.
Terranuova, Italy
From Tony Smullin
Holmes states that “…left once more to its own devices, Nature would begin to reclaim the planet…” thus implying, as so many supposedly scientific articles do, that humankind is separate from nature. Also mentioned is human “interference” with ecosystems. How can we interfere with an ecosystem that we are vitally a part of?
Though mostly written by people who would argue against intelligent design or special creation, these types of statements tend to reinforce the idea. If natural evolution is true, then the works and waste of mankind are as natural as any anthill or beaver dam and should never be referred to in any way as being outside of nature.
Everett, Washington, US
London, UK
