While there's a chance our species can survive the effects of climate change, it looks increasingly likely that we and many other living things will go extinct, says JV Chamary
When would Mother Nature recover? Will humans be replaced by another dominant species? Previous mass extinctions provide clues to help answer these questions.
"Past episodes of warming often precipitated massive biodiversity loss, which is why there's serious concern regarding current anthropogenic climate change," says Professor Erin Saupe, a palaeobiologist at Oxford University.
But while human activity is causing a huge fall in biodiversity -- species are dying out an estimated 1000 times faster than the natural background rate -- as with previous mass extinctions, climate won't affect organisms equally.
"Past warming often differentially affected groups of organisms based on their traits," says Saupe, who studies interactions between life and environments over geological time scales. "Overall, species with small geographic ranges seem to be at higher risk of extinction, and this trait is one of the best predictors."
There are other general trends too. Mass extinctions favour smaller animals, for instance: the end-Cretaceous event that killed non-avian dinosaurs (every branch except birds) was worse for creatures with bigger bodies.
Will there be another ice age?
Warmer average global surface temperatures and more acidic oceans will continue to have negative effects on biological diversity thousands of years from today.
A temperature drop is overdue, however. Our planet is actually in a time of relatively cool climate, characterised by polar ice sheets and alpine glaciers, that alternates between glacial periods like the one that ended 11,700 years ago -- popularly known as the last 'Ice Age' -- and interglacial periods.
The shift between glacial and interglacial is caused by periodic changes in the tilt of Earth's axis and shape of its orbit around the Sun. Climate change may have postponed the next Ice Age, which could occur within 100,000 years.
A cooler world could even reduce the number of species – its 'richness'. "The higher the temperature, the more biodiversity," says Saupe. "This may mean that colder climates will reduce overall richness of animals and plants on Earth."
Lower temperatures tend to favour larger animals, but there are many exceptions. Although megafauna such as wooly mammoths and giant ground sloths evolved in the last Ice Age, modern elephants are enormous and the blue whale is the largest mammal to ever live – whether they survive climate change is another matter.
Could new species evolve to replace humans?
Following extinctions, natural selection will drive some populations to split into distinct species -- speciation -- to exploit vacant ecological niches. This 'adaptive radiation' is why mammals diversified after dinosaurs died out.
"Species evolve to take advantage of the newly-available resources," explains Professor Jonathan Losos, a biologist at Washington University in St Louis. "The new species that arise will be descended from those that survive the mass extinction." So which species might evolve once humans go extinct?
Using past trends to make predictions about the paths that life might take is called speculative evolution. In a pioneering 1981 book, After Man: A Zoology of the Future, geologist and author Douglas Dixon imagined creatures such as predatory rats the size of dogs and massive penguins that resemble whales.
While there's no way to know for sure, Losos can make an educated guess about future species. "It seems very likely to me that the evolutionary radiations that will ensue will be derived from the species that thrive around humans -- cats, rats, cockroaches, pigeons and the like," he says. These animals are common in habitats worldwide. For example, cats number up to 1 billion.
"Domestic cats may give rise to new species akin to lions, tigers and ocelots," says Losos. "Maybe sabre-toothed cats will evolve again, and new ways of making a feline living never previously seen, like aquatic otter-cats or gliding cats."
How about another dominant species? Losos thinks it's unlikely. "My intuition is that nothing will replace us -- humans have been unique in the history of life in the overwhelming impact that we've had on other species and the biosphere."
Can the Earth recover?
For adaptive radiation to occur, the environment must recover from whatever was destroying it, whether that's climate change, asteroid impact or volcanoes. But the current crisis is killing creatures that build the foundation of ecosystems.
"The loss of coral reefs, which are biodiversity hotspots today and over Earth history, depresses diversity until their eventual recovery," says Erin Saupe.
When will nature fully recover? One clue comes from an event with worrying similarities to climate change: the end-Permian mass extinction 251 million years ago, when extreme volcanic activity led to high temperatures, more acidic oceans and the death of up to 96 per cent of marine species. Corals, whose calcium-carbonate skeletons ultimately form reefs, were almost wiped out.
"Based on the fossil record, we often see biodiversity recovering after major mass extinctions within roughly 2-5 million years," says Saupe. "However, recovery can take longer for the most severe environmental perturbations, such as the end-Permian mass extinction -- ocean warming and acidification prompted the collapse of coral reefs, which were largely replaced by carbonate mounds for up to 6 million years."
So while it will take time, life will go on – even after we're long gone.
