Ecosystems and the biosphere, the entire living collection of ecosystems, are fundamental components of life on Earth. In the 20th century, the scientific community began to recognise that life isn't simply a passive recipient of the climate and Earth’s conditions. Rather, scientists realised that life actively shapes and influences the nature of the atmosphere and the oceans.

Along these lines, as ecosystems begin to change, fundamental aspects of Earth are also shifting, the most obvious being the atmosphere. As ecosystems are degraded and lost, carbon dioxide is being emitted into the atmosphere. This change is causing climate change and a further release of CO₂.

This observation is important because ecosystems have widespread effects. For example, water circulates from the ground back through trees, evaporates back into the atmosphere and forms rain. This process depends on the existence of trees to circulate this water back into the atmosphere. As trees vanish, so do forests, and the hydrological cycle weakens, which affects rainfall patterns and cloud formation, as well as Earth's reflectivity and brightness on a large scale.

Other similar processes are also being affected. Soil ecosystems, for instance, are critical in retaining carbon and providing nutrients that support life. Nevertheless, soils are being degraded and overexploited in many regions. The climate change then warms soils in colder, more northern areas, causing them to release carbon into the atmosphere, which further affects the global climate. Overall, these are some examples of how ecosystems matter beyond the ecosystem itself.

Forests are an incredibly important and significant type of ecosystem. This is true for all kinds of forests from the North's great boreal forest to the broadleaf temperate woodlands of Europe, North America or China.

However, the equatorial regions' tropical forests are particularly important because they account for around half of the land surface's productivity. More specifically, half of all metabolism – the activity of the land biosphere – is found within tropical forests and the tropical savannas. Over half of the biodiversity is found within the canopies and the soil of tropical forests.

Furthermore, a lot of the functioning of the biosphere is determined by what happens in the tropics. Tropics contain much of the carbon stored in living biomass globally, and a lot of water and rainfall circulates through tropical forests. The roots of the trees pick up that water. It then evaporates through the leaves and is pushed back into the atmosphere, where it condenses into clouds and forms rain.

This circle of water in the tropics drives global weather patterns and is an engine of the entire global circulation of water. The degradation of tropical ecosystems is incredibly impactful because they are so influential, not only on local weather but also on a much larger scale.

Finally, when we lose a part of the tropics, we lose great museums of biodiversity. These reservoirs of diversity have evolved over millions or billions of years. As such, tropical forests are incredibly valuable treasure houses of the richness of life on Earth.

Still, it is crucial to recognise that there is a long history of deforestation. While we are concerned about what is happening in the Amazon, we need to acknowledge that Europe and North America are the most deforested landscapes on Earth. We need to restore and bring back some of these natural ecosystems in these highly degraded landscapes.

Simultaneously, we also need to be concerned about not losing the vast treasure houses in the tropics where agricultural frontiers are expanding, and most of the deforestation is currently taking place. These frontier regions often have relatively poor governance without much regulation. A lot of the habitat in these areas isn't just detrimental to the natural world and biodiversity, but also harmful to the cultures that live in these tropical frontier regions.

The Amazon is one of the most iconic hot spots of the interaction between deforestation and climate change. The Amazon matters disproportionately because it seems to be a potential tipping point. In other words, if human activities put enough pressure on the Amazon, it may fundamentally flip into an altered state, something like a degraded savanna or a fire-prone shrub. This type of negative change would lead to a massive loss of biodiversity and affect the climate of South America and beyond.

Computer simulations first predicted this type of tipping point in the Amazon around 20 years ago. While these simulations were quite sophisticated, they likely miss some of the complexity of real-world ecosystems. Therefore, over the last few decades, other scientists and I have been trying to understand the actual risk of a tipping point in the Amazon and what could drive the Amazon to change. We’re mostly concerned about deforestation and drought.

Unfortunately, we see increasingly frequent and intense droughts in the Amazon. For context, a massive drought in 2005 was called the "drought of the century." However, a similarly intense drought occurred in 2010, which was followed by even stronger droughts in 2015 and 2016. The frequency of severe droughts is increasing in the Amazon, which is likely linked to global climate change.

At the same time, deforestation has also been expanding. Although it slowed for a period, it has picked up again over the last year or two. Deforestation further affects the weather system of the region.

Another vital factor is fire. There's very little natural fire in the Amazon due to its high humidity. As such, lightning strikes may kill an individual tree, but they don't tend to burn out large forest areas. However, during the dry season, farmers in the Amazon often burn their pastures to clear up shrubs or dead wood. Under the right conditions, like a dry season with exceptional drought, the normally-humid forest becomes temporarily flammable, albeit to a minor degree.

Fires can then spread from farms to the forest's dry and crisp portion. While these individual fires are small — for example, you can walk over them or stomp them out — we find that they collectively kill half the trees in a given area over the coming few years. Moreover, if half the trees in the forest die, the microclimate of the forest changes: more sunlight gets through because of the thinner canopy, making the forest litter layer even drier and the climate more stressful.

Under these conditions, it’s more likely that a fire will sweep through during the next dry season. After a region experiences a second or third fire, it risks switching from a humid tropical forest to a fire-prone, degraded ecosystem. This is a tipping point, and this type of tipping point occurs at very local levels inside a forest.

Nevertheless, there's a different sort of tipping point that occurs at a much larger scale. This will happen when large portions of the Amazon are lost or degraded by either deforestation or fire. If a significant enough part of the Amazon is affected, the amount of water and rainfall circulating throughout the atmosphere will decline, which also means there will be less rainfall downwind.

More significantly, however, once the winds from the Atlantic move over the Amazon, they hit the Andes Mountains and tend to curl down to the south. They then pass over São Paulo and northern Argentinian regions, which are the breadbaskets of South America.

As such, the patterns of vegetation covering the Amazon likely influence the rainfall patterns in some of the most productive agricultural lands in South America. If there is sufficient forest loss, the climate of the Amazon will start to dry, which will cause further loss of forest, further drying of the climate downwind and negatively impacting some of the major food-producing regions of South America.

The real danger lies in the permanent nature of this type of change. If the Amazon becomes sufficiently dry, even if deforestation halted, the forest won't come back because there will be too little rainfall. Overall, this is one definition of a tipping point where you push the system over a cliff into a different state. Even if you stop pushing, it can't climb back over that cliff to the state that it was in before.

In other words, a tipping point is a change where you tip the ecosystem into a fundamentally different state, and that can have effects on biodiversity, as well as the function of ecosystems and the resources they provide. Tipping points are important because they signify dangerous levels of climate change.

At the moment, we're experiencing significant amounts of climate change. A certain degree of carbon dioxide from fossil fuel emissions in the atmosphere is causing some warming. Currently, the Earth is about one degree Celsius warmer than it was in the pre-industrial era.

We have a reasonably good understanding that tipping points matter when the behaviour of the biosphere itself starts affecting the rate of climate change. Currently, the land biosphere absorbs around a quarter of the carbon dioxide from fossil fuel emissions, and it, therefore, acts as a moderate brake on the rate of climate change. However, if the biosphere weren't doing what it's doing now, the rate of climate change would be around 15-20% faster.

The real danger with tipping points occurs when the biosphere stops acting as a brake on climate change and becomes an accelerator of climate change. For instance, if the Amazon starts dying in substantial amounts, all the carbon that's locked up in its trees and soil will begin pouring into the atmosphere. Similarly, suppose the permafrost regions of the high Arctic and the frozen soils of the boreal regions start warming and melting. In that case, they will also release carbon into the atmosphere causing further warming.

When we cross these ecosystem tipping points, the ecosystems themselves become accelerators of climate change and push us further and further into a warmer and more precarious climate. This would continue even if we were to stabilise fossil fuel emissions or absorb carbon dioxide from the atmosphere. The biosphere's tipping point may have pushed us to a dangerous state where the natural momentum alone moves us into a warmer planet.

Naturally, tipping points are things that we really want to avoid and cannot get anywhere near, which is why there is so much interest in and concern about what is happening in the Amazon, the tundra and the high Arctic regions. These tipping points matter not only for the ecosystems themselves but also for determining the dangerous level of climate change that we don't want to surpass.

At the moment, there's a broad agreement that we want to avoid 1.5 or 2.0 degrees of climate change. While that's an important figure, there’s a little bit of guesswork involved. We don't know what the feedbacks are in the biosphere and when they kick in. By staying within two degrees of global warming, we avoid these possible tipping points in the biosphere where we may trip into a dangerous climate.