Wetlands are hard to define since they are so complex and always changing. Merriam-Webster says that wetlands are "land or areas that are covered often intermittently with shallow water or have soil saturated with moisture." They are also distinct ecosystems and animal habitats with their own vegetation and are important for water purification, shoreline stability and flood control.

Although wetlands cover less than 4% of the Earth's surface, 40% of all animal species live or reproduce in them. Wetlands filter, store and supply the planet with water and food — more than a billion people worldwide depend on them for sustenance. They also play a key role in regulating the planet's climate. And they store twice as much carbon as all the world's forests combined.

Wetlands come in all shapes and sizes. When they support forests they are called swamps. These natural phenomenon most often form next to large rivers or lakes and support diverse flora and fauna. Swamps are found all over the globe; some of the largest straddle the Amazon, Mississippi or Congo rivers. But warmer temperatures and changes in precipitation are drying out many swamps across the word.

The Sundarbans is the world's largest coastal mangrove ecosystem and spans an area between the Indian and Bangladeshi coasts. These forests create habitats for many types of fish, crab and shrimp. They also protect coastal communies, who rely on the forest for a living, from dangerous cyclones. But the climate crisis is threatening this habitat and its resources, which are running out.

Nestled along the coast of Denmark, Germany and the Netherlands is a unique intertidal zone called the Wadden Sea. In a constant state of flux, it stretches nearly 500 kilometers (310 miles) and includes the tidal flats and wetlands between the mainland and the assorted North Sea islands. But climate change could cause entire islands to disappear.

Wetlands that gradually accumulate peat are called bogs. Peat is a deposit of dead plant material — usually moss — and can be several meters deep. Bogs, otherwise known as mires or quagmires, are usually found in the northern hemisphere. But even there, record temperatures have caused huge fires on peat soils, which releases 10 to 100 times more CO2 than when regular trees burn.

Spread over Brazil, Bolivia and Paraguay, the Pantanal is one of the biggest tropical wetlands in the world. It's only a seasonal wetland since it experiences periods of both flooding and desiccation. During the rainy seasons much of its area is under water only to later dry out. But in the past years, a lack of rainfall during summer has lead to an extreme drought and habitat loss through fires.

The Volga delta stretches from Russia to Kazakhstan, where the Volga River enters the Caspian Sea. Over the last century the delta has grown enormously because of changing sea levels and is now nearly 160 kilometers wide. That doesn't come without its problems though: sea level changes have outpaced aggradation to such an extent that Volga sediment is now spread over the whole Caspian plain.

Though wetlands can seem huge and indestructible, they are very susceptible to climate change and defenseless against human destruction. They are important cogs in the ecosystem and need to be better cared for. Once a bog dries out it is hard to restore since amassing peat takes decades. Planting mangroves in dry soil is pointless and revitalizing a drained swamp is nearly impossible.

Unfortunaltely, wetlands are being lost across the world at alarming rates. With 35% lost globally in the last 50 years, wetlands are our most threatened ecosystem, disappearing three times faster than forests. Wetlands protect communities against the impact of climate change and their degradation puts human lives at risk.

The devastating flood in Germany that killed 220 people and caused an estimated $40 billion in damages is one example of many. Upland ecosystems in Germany used to store large volumes of water from heavy rain, releasing it into rivers gradually over days and weeks. But last summer, the small rivers at the German-Belgian border quicky turned into a raging flood.

That's because the drainage for agriculture and commercial forestry, the reclamation of wetlands and construction of buildings and impermeable surfaces such as asphalt has radically diminished the absorption capacity of these sponges. When country lanes flood, they fast-track water into rivers, quickly turning them into violent torrents.

Experts believe the solution to flooding lies not in pouring concrete in a vain attempt to keep rivers within their banks, but in blocking drains, reducing intensive cropping, restoring soils and wetlands, and reconnecting rivers with their floodplains. Restored sponges and wetlands protect biodiversity, capture carbon, and stay wet for longer, maintaining river flows during droughts.