Where has All the Carbon Gone

The abyssal plain of Apterra's oceans is, by and large, a dead place. A few scavengers and detritivores perform the most rudimentary of cleaning functions, ferrying bits and pieces of biomass back to the surface. Despite their efforts, though, material continues to build up on the seabed year after year. In some areas, deep below the open sea where algal blooms are rare, this carbon-rich sediment is only a few meters thick. In regions near the shore, or at the bottom of shallow seas, there can be hundreds of meters of accumulated, rotting algae. At the surface, the solid red tide continuously grows, dividing to produce new cells in its upper layers. Meanwhile, the lower parts of the algal mat die, sloughing off and sinking to become part of the organic sludge that smothers the seabed. As they fall, they take with them all the carbon they'd removed from the atmosphere through photosynthesis over their lifetimes.

This carbon remains locked away for millions of years, never returning to the atmosphere in the form of CO2. As the millennia wear on, this produces a reverse greenhouse effect, cooling the planet. As of 2.5 million years Post-Abandonment, this has resulted in a global drop of around eight degrees Celsius worldwide, with the poles changing faster than the equator. This is just the beginning; the planet will eventually reach a tipping point where the lack of atmospheric carbon hinders plant growth, resulting in die-offs that accelerate the collapse of the planet's carbon cycle. For now, the planet continues steadily decreasing by a degree every couple hundred thousand years, but this feedback loop will eventually result in a runaway icehouse effect, causing Apterra's first ever mass extinction.

For the time being, most of the planet is still livable - indeed, it's currently at its peak level of diversity so far. Cold-adapted species flourish as temperate and subarctic zones expand to lower latitudes, while the dominant organisms of bygone warm periods have, so far, been able to roll with the changes. The Late Muricene is a connected world; falling sea levels have resulted in the islands of Sub-Abeli reconnecting to the mainland at multiple points, while the formerly isolated Northern Isles now have a land bridge tying them to the tip of Loxodia. Even the large island chain in the western Perithalassic sea now sees visits from Panapterran fauna. Animals reach the remote landmasses by island-hopping across a newly-exposed archipelago that crosses the gap to Ailuropia. In the Early and Middle Muricene, this group of massive islands (known as Choeropica after the pygmy hippopotamuses that would've been preserved there) was entirely tropical, consisting of a single, massive rain-pseudoforest. Today, though, nearly the entire subcontinent is a temperate zone, similar in climate to the interconnected northern pseudoforests of Middle Muricene Ailuropia and Loxodia (a habitat that has now become fragmented due to advancing glaciers and the spread of less-hospitable taiga woodlands). It should be noted, though, that some other coastlines display surprisingly little change. Millions of years of pre-terraforming erosion and tidal forces created sheer cliffs along many shores. Consequently, areas like the Gecko Isles have changed little, with their steep beaches simply expanding a few dozen meters as the ocean falls around them.

The lower sea levels of the Late Muricene have had effects beyond simply linking lands together. The inland sea between Loxodia and Abeli is now high and dry across much of its former extent, though this is also in part due to the massive quantities of algal muck that raised the seabed as the waterline was falling. The ten-year desert has significantly increased in size, encompassing most of this formerly marine territory. Meanwhile, isolated pockets of ocean that got left behind as the waters receded have now become expansive freshwater lakes. Nowhere is this more obvious than in Unciolis, where a large marine inlet has become disconnected from the sea, creating a lake of immense proportions. Other regions, like eastern equatorial Loxodia, have seen their river-valley estuaries drain away, with coastal rain-pseudoforests now finding themselves high and dry. In Ailuropia, glaciation has altered the courses of many rivers; streams of meltwater now bring much higher moisture levels into the massive endorrheic basin between the two ridges. These rivers never make their way to any ocean, spreading across the dry lowlands and evaporating. This results in inland deltas that serve as hubs of biodiversity, outpacing the productivity of other temperate grasslands.

The life of Apterra has changed just as much as the physical landscape. Species once restricted to small pockets of cold in a tropical world now have free reign over most of the planet's area, diversifying from hardy generalists into a slew of niches optimized for the conditions of this incipient ice age. With the decline of tropical pseudoforests, competition is fierce. Gone are the days when the tropics were a haven for nearly every lineage, providing an abundant habitat with space for any who adapted to live there. Today, the ever-shrinking rain-pseudoforests are a battleground, a race over who can hold out the longest in the small pockets that remain of their old home. This competition is just as powerful a driver of evolution as any climatic change, and jungle grasses are rapidly becoming hyper-specialized to gain the slightest edge over other species. In the end, precious few warm-loving plants will make it out of the end-Muricene extinction event. Many animals, too, won't live to see the ice recede many generations in the future. Those that succeed will inherit the planet, rapidly radiating across the post-Muricene world in a wave of diversification unseen since the beginning of Apterra. But before the freeze begins in earnest, it's time to meet some of the species vying for their place among the survivors.