The end-Holocene Climatic Optimum

6,000 years hence, Latest Holocene

The Holocene, while it could be considered the true 'Age of Man', did not end when the humans disappeared. This extremely short geological epoch would continue for several thousand years after their disappearance. During these late Holocene years, the previous dominion of man is still very noticeable. The most obvious reminders are the millions of relics that remain scattered across the world, corroding and breaking down over time. Where there were once cities there are now fields and forests filled with rubble. Given a few thousand more years, most of these physical reminders of humans will be nothing but dust.

While less obvious, a far more influential reminder of man's past domination is the climate. It is currently the warmest it has been in the past 15 million years, with temperatures similar to those of the mid-Miocene, a global average temperature 4 degrees (Celsius) higher than it was in the 21st century. It is a period known as a climatic optimum, the highest prevailing temperatures after a period of cooling. The early Holocene also had a climatic optimum roughly between 10,000 and 6,000 years before man's extinction in the 31st century, but it was much cooler than this one. This warm period is entirely caused by anthropogenic climate change, but could have been a lot worse. Had humans continued along the same path at their height of carbon emissions in the early 21st century, the world would have warmed up to Eocene-levels and beyond, but over 200 times quicker, enough to cause a mass extinction near comparable to the Great Dying at the end of the Permian. Luckily, humans were smart enough to lower their emissions, instead leading to a more mild climate which warmed over a period of a thousand years, giving most terrestrial ecosystems time to adapt. Oceans are much more vulnerable to changes in temperature and thus suffered much harder from the warming, but again, it could have been a lot worse.

Due to the warming, many (though not all) icecaps and glaciers have melted and sea levels worldwide have risen. The continent of Australia is still easily recognisable, but the ocean has encroached several kilometres inland in some areas, changing the shape of parts of the coastline and inundating low lying areas into saltmarshes and estuaries. A small inland sea has sprung up in what was once southern South Australia, like the one that existed there back in those Miocene days. Looking at Australia from an ecological perspective however, the warming climate had a far greater affect.

Back in the 21st century, scientist first noticed how the rising temperature had begun to affect Australia's climate, with changing weather suggesting that in general the south of the continent was getting dryer, while the north was getting wetter. Looking at the distribution of habitats during the climatic optimum, it is clear that their predictions were right.

In southern Australia, where there were once tall forests and wet grasslands there are now drought-resistant woodlands, dry scrub and arid plains. The warming climate has caused rainfall to become much more erratic and scarce. The wet forests of southern Australia still survive in Tasmania and in pockets along the mountains of the south-east and south-west, but overall they have been replaced by much drier habitats. Despite the drying of this part of the continent, it nonetheless remains a biodiverse area. Australia's complex desert ecology (probably inherited from the inland's rainforest heritage) has been completely reformed, which allows a heightened number of species to thrive in such a harsh environment. As well as species from the dry areas of the south, aridland species from the north have also migrated south to exploit the new preferable conditions.

The landscape of Gariwerd (commonly known as the Grampians) is a good example of the drying out of southern Australia due to global warming. The first image on the left shows the mountain range during the 21st century before significant warming, while the image on the right shows the landscape during the climatic optimum. The wet forests have been replaced by more drought-resistant woodlands of dry-country eucalypts and cypress pines.

These northern species have good reason to move south, as the north of the continent has also changed, perhaps even more radically. The further north you go, the woodlands turn into forest, and then eventually lush green rainforest. Fuelled by the wetter conditions brought here by the warming, the rainforests have spread from the small pockets on the east coast right across the north in one long unbroken band, which stretches from one end of the continent to the other. This is one of the most rich environments on earth, and species that were once confined to the tiny pockets along the coast now have distributions spanning more than a million square kilometres right across the north. Species of birds, bats and insects from New Guinea have also made their way south to take advantage of these vast jungles. As well as rainforest, the north also possesses huge areas of tropical wetlands, the home of millions of waterbirds and a haven for crocodiles, turtles and frogs. Most of the south may be as dry as a bone, but here there's plenty of water.

Despite the huge variation in climate from north to south, there is a powerful force brought alongside humans that is almost completely absent here in this hothouse. Although habitat clearing was an enormous threat, invasive species were perhaps the greatest hazard to Australia's biodiversity after the arrival of European settlers. Their populations rose to unnatural levels and devastated the local ecosystems through degradation and predation, upsetting the natural order and fragile ecology of this ancient land. However, searching through these diverse environments, the absence of these destructive animals is extremely noticeable. No lagomorphs nibble the vegetation, no ungulates compact and erode the soil, no toads poison predators, and no foxes or cats decimate the populations of small animals. Humans brought these creatures here, but despite their abundance they also had the power to take them away. And that they did. And because of their effort, lineages once thought doomed like potoroids, bilbies and numbats still shuffle quietly through the leaf litter, just like their ancestors did millions of years before them. These creatures have been given a second chance.

Not every introduced species was wiped out by man. Eradicating abundant species, while it had been done before, was a process that took a huge amount of time, effort and money, not to mention the ethics of it. Thus, only species that had a large, negative affect were chosen to be subjected to this genocide. Cats, foxes, lagomorphs, deer, bovids, equines, pigs, camels and toads were the species chosen for eradication, which was done in the most humane ways possible for obvious reasons. Invasive birds, geckoes and rodents were all left alone, for while many of these did have an effect on the local diversity of similar animals they did not cause the same level of destruction as the aforementioned species, and therefore their eradication was not worth the effort. Nonetheless, most of these less destructive species relied on humans create suitable habitat, only being prominent in modified environments. So when humans disappeared, they disappeared to. Some did have the ability to survive in natural habitats, but here they were outnumbered and outcompeted by native species. By the climatic optimum, only two species of tetrapods introduced by man remain - the adaptable House Mouse, which despite its name thrives in Australia's arid regions, and the ecologically vital native dog, the Dingo. A third species, the Mallard, survived as a hybridised species with the native Pacific Black Duck.

Invasive, destructive species were not exterminated everywhere however. The island of New Guinea in the north of the continent was under different governments than the rest of the continent, and did not eradicate their invasive species like the Commonwealth of Australia did. Cats, deer, pigs, buffalo and cane toads all still survive in Papua's dense, steamy jungles. They've now been here for several thousand years, and in that time they've adapted to the local ecosystem, and the ecosystem has adapted to them.

This climatic optimum seems the perfect setting for life to adapt and diversify, but nonetheless, this hothouse is only temporary. The climate is on the verge of changing radically once again, but in an almost opposite way.