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Mostly environmental things.
And yes, it's sort of like dominoes, when one falls the rest can go with it or become considerably weaker. If you are sitting stool with 9 legs and three of them are cut 3/4 of the way through, the rest have to "take up the slack" which stresses them more.
And there are no dumb questions. Only people who refuse to ask questions and remain dumb.
Stratospheric ozone layer
The stratospheric ozone layer filters out ultraviolet radiation from the sun. If this layer decreases, increasing amounts of ultraviolet (UV) radiation will reach ground level and can cause a higher incidence of skin cancer in humans as well as damage to terrestrial and marine biological systems.
Biodiversity
The drivers of change that cause biodiversity loss and lead to changes in ecosystem services are either steady, show no evidence of declining over time, or are increasing in intensity.
Chemicals dispersion
Emissions of persistent toxic compounds such as metals, various organic compounds and radionuclides, represent some of the key human-driven changes to the planetary environment. There are a number of examples of additive and synergic effects from these compounds. These effects are potentially irreversible.
Climate Change
We have reached a point at which the loss of summer polar ice is almost certainly irreversible. From the perspective of the Earth as a complex system, this is one example of the sharp threshold above which large feedback mechanisms could drive the Earth system into a much warmer, greenhouse gas-rich state with sea levels metres higher than present.
Ocean acidification
Around a quarter of the CO2 humanity produces is dissolved in the oceans. Here it forms carbonic acid, altering ocean chemistry and decreasing the pH of the surface water. Increased acidity reduces the amount of available carbonate ions, an essential building block used for shell and skeleton formation in organisms such as corals, and some shellfish and plankton species. This will seriously change ocean ecology and potentially lead to drastic reductions in fish stocks. Compared to pre-industrial times, surface ocean acidity has increased by 30%.
Freshwater consumption and the global hydrological cycle
The freshwater cycle is both a major prerequisite for staying within the climate boundary, and is strongly affected by climate change. Human pressure is now the dominating driving force determining the function and distribution of global freshwater systems. Water is becoming increasingly scarce and by 2050 about half a billion people are likely to have moved into the water-stressed category.
Land system change
Forests, wetlands and other vegetation types are converted primarily to agricultural land. This land-use change is one driving force behind reduced biodiversity and has impacts on water flows as well as carbon and other cycles.
Nitrogen and phosphorus inputs to the biosphere and oceans
Human modification of the nitrogen cycle has been even greater than our modification of the carbon cycle. Human activities now convert more N2 from the atmosphere into reactive forms than all of the Earth´s terrestrial processes combined. Much of this new reactive nitrogen pollutes waterways and coastal zones, is emitted to the atmosphere in various forms, or accumulates in the terrestrial biosphere.
Atmospheric aerosol loading
This is considered a planetary boundary for two main reasons: (i) the influence of aerosols on the climate system and (ii) their adverse effects on human health at a regional and global scale. Without aerosol particles in the atmosphere, we would not have clouds. Most clouds and aerosol particles act to cool the planet by reflecting incoming sunlight back to space. Some particles (such as soot) or thin high clouds act like greenhouse gases to warm the planet. In addition, aerosols have been shown to affect monsoon circulations and global-scale circulation systems. Particles also have adverse effects on human health, causing roughly 800,000 premature deaths worldwide each year.
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