You guys, lemmy splain you a thing. I'm not going to cite sources here and take the time to find all of the exact numbers and names of specific volatile organic compounds; I just want to get the general message across for anyone who is unaware, and you can research the details (and I've previously posted some useful links for that) if you are interested in doing that. I am not an "expert" on this, so excuse the informal or imprecise language.
The Atlantic Ocean has this thing called the AMOC – the Atlantic Meridional Overturning Circulation. It distributes the warm water from the equator to the northern latitudes, and it brings the cold water from the north toward the equator. That's important. If that didn't happen, then the ocean at the equator would continually heat up, and the ocean around Greenland and Scandinavia would stay cold all year (even in summer due to the melting ice entering the ocean).
The AMOC results in a mixing of the water, and that mixing facilitates evaporation. Evaporation over the ocean means a moist air mass that has the potential to move over the land, and if the moisture then condenses, the result is life-giving rain. (AMOC isn't strictly necessary for evaporation, of course, but it helps.) More importantly, AMOC regulates the temperature of the water in the ocean, and by extension, the temperature of these moist air masses that form over the ocean. Warmer conditions lead to more evaporation (up to a certain temperature), and warmer air holds more moisture. The accepted figure is a 7% increase in the moisture-holding capacity of the air for every 1°C increase in temperature. This relationship between temperature and atmospheric moisture is one major reason that tropical regions tend to get more rain than higher latitudes. The warming effect of AMOC on the northern latitude Atlantic Ocean therefore increases rainfall in this region, aiding agriculture in western Europe.
At the equator, there is no prevailing wind direction in the absence of biological influence; there are no "trade winds" at the equator itself. The warm and moist air over the Atlantic Ocean could go west toward South America, or it could go east toward Africa. Which way this moist air mass moves is determined by the difference in atmospheric pressure between these regions. Weather systems move from areas of high pressure to areas of low pressure. When the weather over the ocean is sunny and warm, that is a high pressure system, and there is plenty of evaporation, and the warm air mass forms, and then whichever direction has the lowest atmospheric pressure is the direction that it will move, all else being equal. Forests are the largest influence of atmospheric pressure in the equatorial regions of West Africa and South America.
The trees in a forest are not just photosynthesis machines. They do MUCH more than suck in carbon dioxide and output oxygen. A forest is a complex network of interconnected systems that work in tandem with each other and amplify each other in positive feedback loops, and one of the many functions of this forest super-system is regulation of humidity and precipitation. The forest canopy of course provides shade and retains moisture within the forest itself by shading the ground and lower layers of vegetation from the direct sunlight, but the leaves of the forest canopy also emit many substances into the air – again, not only oxygen. The multi-layered canopy emits a variety of volatile organic compounds (VOCs), some of which are well-known to scientists and some of which have not been thoroughly studied. These small organic molecules serve as condensation nuclei, increasing the proportion of atmospheric moisture over the forest that condenses and falls as rain. This is obviously of great benefit to the forest, as all life in the forest requires water in order to live, but the profundity of the forest's influence on local rainfall is often underappreciated.
What happens when the moisture condenses out of the air and falls as rain? This creates what is called a vapour pressure deficit. This is a difference between the amount of moisture (water vapour) that the air could contain at its current temperature and the amount that it actually contains. Normally, the air would need to cool down in order for the moisture to begin to condense and fall as rain, but the forest has just bypassed this requirement and made rain anyway. The result is a reduction in the air pressure over the forest. This effect scales with the extent and density of the forest.
Think about it. If the weather on land is sunny and without rain, then there is clearly more evaporation than precipitation, and there is maximal vapour pressure. Warm, sunny weather means that there is a high-pressure system; anyone familiar with reading weather forecasts can tell you that. Without a prevailing wind direction, the moist air mass over the equatorial ocean has nothing to drive it inland. If anything, the air over the land is going to be warmer than the air over the ocean during dry and sunny times, creating an area of higher pressure, and moist air is more likely to travel away from the land and out over the ocean. We see this in coastal deserts in the tropics such as in Peru and Somalia. Without a forest to pull the moist air inland, the landmass will dry out.
So which way does the moist air over the Atlantic go? It depends on which forest is able to achieve the lower atmospheric pressure. (Of course other large-scale weather systems can also affect this, but all else being equal, at the equator, the forest is the major deciding influence.) As we might imagine, the immense Amazon Rainforest usually wins this tug of war, and most of the rain falls over South America. In the northern hemisphere summer, the northern Atlantic is warmer, there is more evaporation, and there is more rain to go around, and West Africa's vapour pressure deficit is sufficient that it gets a monsoon season and an abundance of rain.
Forests produce the VOCs that act as condensation nuclei, but their leaves also transpire water vapour into the air, allowing the water from the soil/river/groundwater to cycle up through the tree and return to the atmosphere where it then condenses and falls as rain again. We can therefore say that forests recycle much of the rain that falls. If there is any sort of pressure gradient (e.g. there is an elevation gradient, a slope in the landmass), then the air mass above the forest, containing the moisture transpired, will travel to areas of lower pressure (e.g. farther inland, to higher elevations, uphill) and the rain will fall there. This effect is what people mean when they refer to the "flying rivers" of Amazonia.
We have two simultaneous issues right now.
The first is that the AMOC is weakening and is predicted to cease to function entirely in the coming decades (and possibly quite soon). What this means for western Europe is that the ocean will be cold. Cold wind from the ocean keeps the summer from getting too hot, at least in coastal areas, but it also means that spring will come later and tender young crops may freeze. It means less rainfall. It means a return to brutal winters in precisely the coastal areas where one would expect the winters to become increasingly mild. It means a decrease in agricultural production and an increase in energy costs. But most relevant to the treehuggers among us, it means that any reforestation effort that has been thinking ahead to a warmer world and planting trees that will survive in a warmer environment will fail as the sudden reversion to harsh winter cold kills these young trees.
In a world where AMOC has collapsed and the equatorial Atlantic Ocean is warmer and the air above the ocean is warmer, there is going to be a major high-pressure system, and the air is going to hold more moisture (potential rain) than ever before. But if the air can hold more moisture because the temperature is higher, then rain is not going to fall until a greater amount of evaporation has occurred (less frequent, more intense rains) or the temperature decreases (due to the air mass moving to a higher altitude, for example). If the warm air simply rises straight up to where the atmosphere is cooler and the rain eventually falls over the ocean, okay, but that doesn't water the plants, does it? All of us living on land need some of that rain to come over here... which requires the forest to produce those VOCs as condensation nuclei in order to create a vapour pressure deficit and pull that moist air mass over the continent where we live.
The second issue is the massive and unrelenting deforestation of the Amazon and the West African rainforest. Do you like to eat? Do you like to have water to drink that isn't stagnant and full of mosquito larvae and parasites? Then you probably require rain, sooner or later. For countless beings who inhabit West Africa and eastern South America, the rain depends on the forest's ability to pull moist air inland by creating the vapour pressure deficit.
People are cutting down the forest. Make no mistake: forests are not passively dying. This isn't a tragic accident brought on by natural cycles or unforeseen events. The forest is being killed by human beings, mostly in order to plant grass for cows and goats and other enslaved animals. The animal exploitation industry is destroying the world's forests and disrupting the climate.
Given everything that I've just described in the previous paragraphs, you the reader can probably predict as well as I can what is going to happen as a result of continued deforestation in the coastal regions of equatorial South America and West Africa.
When deforestation reaches a critical tipping point, the production of VOCs will be insufficient to create a vapour pressure deficit strong enough to counter the increasingly high temperatures over land. The moist air mass from the Atlantic Ocean will move over the land less and less frequently. When it does, it will be warmer, with a higher water content, and a larger proportion of rain events will be torrential. Dry seasons will get longer and more severe. Crops will fail. Drought-sensitive rainforest trees will die, worsening the deforestation problem, which further worsens the drought problem, and the rainforest will begin to self-destruct. This is what people mean when they refer to the tipping point of the Amazon Rainforest, though it could happen to other forests also.
In most of coastal West Africa, if this happens, that's it. That's the end. Everyone is dead. The low-lying areas will be underwater from sea-level rise anyway, but the forested hills that are home to chimpanzees, elephants, and countless other beings will dry out, the occasional torrential rains will wash away the topsoil, and the land will turn to savanna at best, desert at worst. If the forest dies, its inhabitants die with it. Could the Gulf of Guinea escape the worst of it, the dry season less severe due to its greater proximity to the Intertropical Convergence Zone and its greater distance from the Sahara? I don't know the answer, but I'd rather be in Cameroon or Gabon than in Sierra Leone. Unfortunately, the forest is already so fragmented that most non-avian beings don't have the luxury of migrating.
In South America, I don't know how extensive the collapse of the Amazon would be. If the eastern slopes of the Andes remain forested, that may or may not be enough to draw moisture across the continent. The higher elevation (higher than anything present at such a scale in West Africa) would result in lower pressure, and this combined with the forest's influence would be the greatest chance of success, especially if the elevation-adjusted temperatures remain lower than those of West Africa. The most severe deforestation is in the eastern Amazon, the part which is already drier and likely the first to self-destruct, but this is also the area closest to the coast which initiates the entire process of moisture transfer from ocean to land. If what is now the eastern Amazon Rainforest turns to savanna (or worse), then the rain may reverse direction, at least seasonally, sucking the forest dry all the way to the Andes. Deforestation in Pará and Maranhão may doom the forest as far away as Ecuador.
So what do we do about it?
It may or may not already be too late to prevent tipping points from being crossed. We have no way of knowing that for sure until well after the fact. All that we can do right now is assume that there is still a chance to fix things and do everything within our power to be a part of the solution rather than contribute to the problem. The problem is multifaceted, as climate change and deforestation exacerbate each other, and human "economic" interests often clash with forest protection and climate stability. We cannot control the laws of nature, and we cannot regulate away the environmental impact of inherently destructive industries, but we can control our daily choices.
Many people, environmentalist types, want to blame billionaire oil magnates and corrupt politicians for the current mass extinction and for environmental devastation in general. People take comfort in portraying themselves as innocent victims of a system that is destroying the Earth against their wishes and ignoring any objections that they may raise. This framing of the issue can be useful for uniting people against a common enemy, and it can certainly motivate people to donate money to fight "the bad guys" doing horrible things somewhere else, and to a degree, it is true. The capitalist/industrial system exploits the average, poor, working-class human to the advantage of the wealthy owning/ruling class who have (both in modern times and especially in the past) contributed disproportionately to the current multivariate environmental crises. But pointing the finger and saying "He started it!" does not solve the problem. Pointing to the rich, or the politicians, or people in a far-away land, and saying "Their mess is much bigger than mine!" does nothing to begin to clean up the mess.
All that we can do is choose to be part of the solution rather than part of the problem. When it comes to deforestation itself, we can obviously choose to not cut down trees. We can choose to plant trees instead. We can avoid practices that could start a forest fire. We can protect and clean up rivers and other bodies of water that sustain the forests, and we can protect or re-plant especially trees and other vegetation growing on slopes, as these prevent soil erosion. We can choose to avoid purchasing products of deforestation (cacao, coffee, palm oil, tropical hardwoods, etc), and if we have the means, we can grow our own food forests that produce fruit and nuts and fibre and wood and dyes without disturbing the sanctity of old-growth forest somewhere else. We can minimise the use of minerals obtained through destructive mining practices in forested regions (gold, nickel, lithium) and reuse and recycle such materials as much as possible. Most importantly, we can live vegan, as the animal exploitation industry is the #1 driver of deforestation.
In order to address the climate change side of the problem, which is also very much a threat to forests, the often-repeated solution remains applicable: we can reduce our greenhouse gas emissions. Fortunately, we are living in the era of peak petroleum, so this and other fossil fuels will become less and less economical to extract, and that industry will cease to be a viable business model, alternative energy technologies will become more lucrative, and the energy transition will occur as a matter of economics and high-level business decisions, and we won't need to do anything. What we can do that has an immediate impact on our greenhouse gas emissions is reduce our overall resource consumption, as all stages of production and transport of material resources require energy, and currently this energy is often supplied by burning hydrocarbon fuels. In other words, buy less. Cancel subscriptions. Mend and reuse what has already been produced. Stop making babies. (There are what, almost 200 million orphans in the world now? Let's take care of the kids that are already here.) Share resources with your neighbours so that not everyone needs to buy their own hammer/scythe/bicycle/[insert non-single-use manufactured item here].
More than this, we can reduce our land use, as any land that can be freed up and allowed to regrow into forest will effectively act as negative carbon dioxide emissions. This is perhaps more effective than reducing our emissions directly, as there is a certain minimal amount of carbon dioxide that we must emit as animals on this Earth; we need to burn glucose and oxygen and exhale carbon dioxide in order to live. If the forests of the world take in more greenhouse gases (in the form of carbon dioxide) annually than what is emitted, then climate change, in terms of heating of the planet, is solved. Most of us do not have our own private golf courses, airstrips, tennis courts, or other land-intensive useless status symbols, and so our land-use "footprint" is mainly determined by the amount of land required to produce the items that we consume; see above.
The #1 driver of climate change, when we account for both greenhouse gas emissions and land-use change, is animal agriculture. Fortunately for us, the capitalist "free market" system prides itself on being governed by supply and demand; if the people don't demand it, then the system won't supply it. If we want to address the climate crisis and stop deforestation before it is too late, then it is imperative that we live vegan and stop contributing to the problem. If all of the land used by the animal exploitation industry were allowed to regrow into forest, then that forest could sequester all of the greenhouse gases that humans have emitted since the start of the industrial revolution, and then some.
After we've done all that we can do in order to stop contributing to the problem ourselves, then of course we can and probably should start looking at ways to "eat the rich" in order to dismantle all forms of hierarchy and oppression, but getting other working-class folks on board with at least saving enough of the forest to prevent catastrophic climate change that would kill off nearly all life on Earth seems like a prerequisite for all of that...