|MadSci Network: Earth Sciences|
This may be the most difficult Mad Scientist's Network question I've ever received. I think most scientists would say that it's impossible to predict what Earth's climate will be like in four million years. We're not sure we understand Earth's past climate change over the last few million years, and predicting the future is even harder. So I will tell you what factors we believe influence climate over this time range, describe what climate has been doing for the past four (or so) million years, and I'll conclude with a wild guess about what might happen the future.
The two major factors controlling Earth's climate on timescales longer than a million years are probably plate tectonics and changes caused by living organisms. As you may know, plate tectonics describes the slow movement of the continents around the globe. This might change climate for two reasons. First, when continents collide to form mountains, fresh broken rock is exposed to the air. Water, carbon dioxide, and fresh silicate rock can react together to form carbonate rock in a process called "silicate weathering": this process takes carbon dioxide from the atmosphere and locks it in solid mineral form. In the atmosphere, carbon dioxide is a "greenhouse gas", which traps heat, keeping Earth's surface warm. So continent collision causes mountain-building, which causes silicate weathering, which reduces carbon dioxide, which makes the planet colder. You should note that this is a theoretical hypothesis, and we haven't yet found solid evidence to prove that it really happens. The second thing plate tectonics can do is to open or close ocean passages between continents. This can allow or prevent ocean currents to flow through these passages. Ocean currents carry heat from one part of the planet to another, so large parts of the planet can be made warmer or colder when the currents change.
Living things can also affect climate. When green plants first evolved, billions of years ago, Earth's atmosphere had much more carbon dioxide than it does now. The green plants gradually, over a billion years or so, replaced the carbon dioxide with oxygen. This probably had a huge effect on climate via the greenhouse effect. But that sort of change takes much longer than 4 million years. On shorter timescales, the evolution or spread of a new sort of plant for example, which is lighter or darker in color than existing species, could absorb more or less sunlight, thus warming or cooling the planet. Changes in the size of forests, or the nutrients available to ocean plants, might increase the removal of carbon dioxide from the atmosphere by a small (but important) amount.
Both plate tectonics and evolution mostly take place on time scales longer than four million years. Four million years ago, during the "Pliocene Epoch", the continents were pretty much in the same places as they were today, and most of the major types of plants and animals would be familiar to you (grasses, flowers, birds, mammals, etc.), though many of the individual species would be unusual.
What has changed since the early Pliocene? The Earth has gotten about six degrees Celsius (11 F) colder during this period. This change is especially strong in the middle latitudes where we live. Along with this, the middles of the continents have generally become dryer. There has been a small amount of important plate tectonics in this period. North America and South America, which were once separated by an ocean channel, have joined together at Panama. Some people think this may have changed ocean currents to cool the planet, but I'm skeptical about this. Most important, I think, is that India crashed into Asia, forming the huge Himalayan Mountains. As I described earlier, mountain-building could lead to a cooling of the planet by removing carbon dioxide. I suspect that the formation of the Himalayas is responsible for the cooling, but some scientists would disagree with this.
What will happen in the next four million years? Nobody knows. But I'll take a guess. If the cooling is caused by weathering of the Himalayas, then since the Himalayas aren't going away any time soon, the weathering will continue. Looking at animations of plate tectonics, I don't see any other major continental collisions about to happen. I therefore suspect Earth will remain relatively cold, or perhaps get slightly cooler.
This assumes humans aren't having an effect. Four million years is longer than our species has existed, and if we're still around during the next four million years, all bets are off. Humans can be a huge monkey wrench in the climate machine: if our understanding of human-caused global warming is correct, humans could cancel out four million years of gradual natural cooling in the next few hundred years!!! If we develop a civilization in outer space (and there's plenty of time for that), we could do even more. A spacefaring society could re-shape Earth's climate to be almost anything, from a steamy tropical jungle planet to a frozen ball of ice, in four million years. In guessing that natural processes will continue to control Earth's climate, I'm assuming that we will have either killed ourselves off by war, gone extinct naturally, left Earth to live in space, or decided to shape our civilization to avoid changing Earth's climate.
I want to close by reminding you that this whole discussion is very uncertain. We're pretty sure that the Pliocene was warmer than today, but we don't really know why, and my "prediction" that Earth will remain cool for the next four million years is ONLY A GUESS.
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