Over the past 3.5 billion years, photosynthetic plant life has been on this planet. For the most part, it has consisted of single-celled algae and cyanobacteria, which have dominated for well over 3 billion years. But then, roughly 600 million years ago, plant life began to change. It started to become multicellular, like the newly evolved animals. It began in the oceans and slowly began to colonize the land roughly 450 million years ago. Today, plants make up most of the biomass on Earth, with millions of species. They produce life-sustaining oxygen and keep carbon dioxide levels in check. Without plants, animal life on this planet would be impossible. However, plant life doesn't have such a long history ahead of it. Plantlife will go extinct between 800 million and 1.2 billion years from now and its results will be catastrophic for the planet. By why and how?
There are three main driving forces that will bring about the end of complex life on Earth, the increasing brightness of the Sun, the carbon cycle and the cooling of the Earth's interior.
Since the formation of the solar system, our Sun has been fusing hydrogen into helium in its core. This incredible process releases a titanic amount of energy. This release of energy pushes against gravity and keeps the star in balance. However, at every second, the amount of hydrogen decreases and the core becomes less active. This causes gravity to temporarily win the battle and crush the core, making it smaller. This makes the core hotter and thus fuse the hydrogen even faster, rebalancing the star. This actually causes the Sun's outer layers to expand slightly, heating the entire star. On a human timescale, the Sun's brightness will change by an inconceivable amount, but over millions of years, it adds up. Currently, the Sun has been increasing in brightness by about 1% every 100 million years, though this rate will increase before the Sun dies.
When the Sun settled down, 4 billion years ago, it was about 30% dimmer than it is today, which would have placed Earth outside the habitable zone. Currently, the inner edge of the habitable zone is around 10 million miles toward the Sun (0.95 AU’s) and the outer edge is just beyond the orbit of Mars (~1.7 AU’s). In roughly 1.5 billion years from now, the habitable zone will move out of Earth’s orbit and the planet will begin to die.
The second problem that will take down plant life is the carbon-silicate cycle. This cycle is caused by the rate of erosion as well as plate tectonics. When the Earth gets warmer, the rate of weathering of rocks and minerals increases, causing carbon dioxide to bind with minerals in the soil and form carbonates (chalk, limestone, etc). These carbonates then wash off into the ocean and become deposited on the seafloor. Over time, these carbonates are pulled under the Earth by subduction, till the eventually melt and erupt carbon dioxide back into the atmosphere through volcanoes. This creates a steady stream of greenhouse gases that keep the Earth warm, but not too warm.
When the Earth first formed, 4.54 billion years ago, it was outside the habitable zone of the Sun and thus should have been frozen. But geologic evidence shows that there was lots of liquid water back then and the Earth was much warmer than it is now. This was because the Earth had much more carbon dioxide and methane in its atmosphere, up to 10,000 times higher than today. This was because volcanic activity was much higher, as the Earth’s interior was rich in radioactive elements and erosion couldn’t keep up. If Earth couldn’t get rid of its high carbon dioxide atmosphere, Earth would boil away like Venus, even while it was still in the habitable zone. However, as the Sun got brighter, the rate of volcanic activity decreased as the mantle cooled. The rate of erosion increased as the amount of land grew and the carbon dioxide began to drop. These systems were able to keep up with the brightening Sun and thus allowing Earth to stay habitable. This clever game of geology and temperature regulation allowed life to flourish and evolve. However, it’s a game that can only be played for so long. Eventually, carbon dioxide levels will become so low that plants won’t be able to breathe.
So how long do we have? That depends on what kind of plant, as each type absorbs carbon dioxide more efficiently than others. Most present-day plant species are “C3 photosynthetic”, and can tolerate a carbon dioxide level of around 150 parts per million (today it’s at around 410 parts per million and rising). It is expected that this level will be reached in around 500 million years. However, there are other plants that use C4 photosynthesis and can handle carbon dioxide levels as low as 10 ppm. They will live till around 800-900 million years, after which multicellular plant life will no longer be able to survive.
Let us travel into the future, from today to around 800 million years from now. As we travel, we see the landscape change but the tempo of flora stays mostly the same. There are tropical rainforests, woodlands, grasslands, taiga, swamps, savanna, tundra and many of the flora groups we know of today. Then, around 100 million years from now, things begin to change. The Sun-sensitive species, like grasses and flowers, begin to disappear, replaced by shrubs, cacti, bamboos, palms, and ferns. Woodlands spread. Around the time of the new supercontinent, the forests begin to change. The deciduous rainforests and oaks decline, replaced with vast evergreens and pines. Once the supercontinent breaks apart, the forests eventually disappear into vast shrublands and cacti. This persists for a while till eventually, the green hills give way to bare soil and rock. Dust storms lash the planet. There was a while of the land flipping from green to brown as the level of carbon dioxide seesawed right at the limit of what plants could tolerate. Soon, we stop the machine, left with a truly depressing world. A world without plants.
How plant life copes with the low carbon world will be quite interesting. It is likely that plants will begin to evolve new traits that require less photosynthesis. Plants may become more carnivorous and we could see the spread of venus fly traps or pitcher plants. However, such adaptations will likely be short-term, Carnivorous plants will likely struggle as the amount of animal life will likely decrease along with plant life, making it not effective in the long-term. The same goes for associating with fungi, as the plants that do consume fungi usually rely on nutrients in the soil that are produced by C3 photosynthetic plants, which won't survive long in the carbon starved world. As the amount of plant life decreases, global temperatures will begin to rise as the Earth's albedo increases. This may cause plants to become more tolerant of desiccation (more adapt to dry climates), which may be the best long term adaptation, as most desiccation plants are C4 photosynthetic.
If you were to stand in this far-off world, say 600-700 million years in the future. Most of the land would be dry desert covered in dunes of sand and dirt. Winds would last the planet. The deserts would be covered in lichens and mushrooms as well as a layer of sticky, oozing bacteria tolerant to the high heat. The Sun would be murderous in the sky, it's deadly breath burning any exposed skin. Plants during this time would concentrate around pools of water. There would no more snow or ice. It does snow sometimes on the tallest mountains, but the snow never sticks. The only land creatures left would be some species of lizards and insects as well as the hibernating creatures. Gone are the mammals, amphibians or anything else that hasn't adapted to hot environments. Though, this isn't a world of arid dryness, far from it. The high heat has saturated the atmosphere with water vapor, making it extremely humid. Although the air is full of water, only in the cooler regions can the water pool, making most of the ground bone-dry. The high water vapor concentration makes storms extremely large. Hypercanes last the planet, dumping tons of rain onto the land. However, the air is so hot at the equator, that even the heaviest of rain just falls as virga. The oceans are a different story. The seas hold most remaining plants and animals. However, even life in the oceans is struggling, as the low carbon dioxide level in the water has made the water less acidic. Gone are the coral reefs, seaweed forests or any other ecosystems that are sensitive to temperature and pH change. Most creatures are concentrated in cooler waters just like on land.
As plants decline and temperatures rise, the land itself will begin to change. The climate will become hotter and dryer. The topsoil, no longer held up the roots of plants will dry out and blow away into the ocean, leaving bare rock and the oceans themselves will turn a gray or light brown color. Dust storms will lash the planet. The shape of the rivers will also change. They will become more stredded and fork-like. Instead of the gentle, bending river we know of, rivers will become more like a branching tree, more jagged and sharp. The situation will likely be different in the oceans. Complex sea plants will follow a similar trend to plants on land as dissolved carbon dioxide in the water declines.
However, photosynthetic microorganisms like cyanobacteria and phytoplankton will continue to thrive, as they can conduct photosynthesis at concentrations as low as 1 ppm. These organisms will likely sustain most of the animal life on Earth. But what about oxygen in the air? Without plant life on land, models show that oxygen levels will fall to less than 1% in just 15 million years. This will not only suffocate animals but deteriorate the planet's ozone layer. What a world the Earth has become. From space, Earth would look like a blue Mars. A planet with stunning oceans, but barren and dry land. But, things have only just begun for the surviving animals...

This is part two of my series. I will have a much shorter part three that will explore what will happen to animals after plants are gone...