Climate Change: The world is "perilously close to tipping points that could lead to cascading and irreversible" consequences.

Climate Change: The world's scientists say the crisis is upon us, and unless we act now, multiple crucial planetary systems are on the cusp of permanent damage.

Here are five tipping points scientists say could start to teeter soon:

1. Amazon rainforest becomes a savanna

n most immediate peril is the Amazon rainforest.

The 2.5 million square mile rainforest is so vast it creates its own rainfall and is home to 10% of the world's species.

But rising temperatures and increasing drought are bringing it ever closer to crossing the threshold from lush rainforest to arid savannah.

In part because of the increased heat and lack of rain, the Amazon is seeing more wildfires. These destroy large areas that grow back not as rainforest but as grasslands with few trees. Illegal logging to grow grass or soybeans to feed cattle exacerbates the problem.

2. Coral reefs die

Coral are vital to the health of the oceans. Although they cover only 0.2% of the ocean floor, they are home to at least a quarter of all marine species. They provide safety for juvenile fish and are home to the small organisms and fish which provide food for larger fish. Scientists estimate that the reefs account for 25% of fish caught in developing countries.

Coral reefs can survive within only a relatively narrow temperature band. The coral that build them get much of their food from algae living in their tissues. When the seawater is too warm, the coral's stress response is to expel algae, causing the coral to turn white. The process is called coral bleaching, and if it lasts too long, the coral can starve—turning a thriving ecosystem into a cemetery of dead shells.

A report released last year showed that almost 15% of the planet's reefs have vanished since 2009, primarily because of climate change.

They're being cooked to death. The frequency at which we're seeing these bleaching events is astounding to those of us who study them. It's going to have a huge domino effect on marine systems and on humans.

3. Ice sheets melting

Both the Antarctic and Greenland ice sheets are melting, and the Antarctic is believed to be the most unstable.

If they melt entirely, it would cause catastrophic sea level rise around the globe. Loss of the Antarctic sheet could result in as high as 11 feet of rise. Loss of the Greenland sheet could be 23 feet.

Now scientists are thinking about  having to move many coastal megacities in the next 100 or 150 years.

4. Atlantic circulation stops

The circulation of the Atlantic is at risk.

The official name of this danger is Atlantic Thermohaline Circulation Collapse. If it were to happen, it could bring about an ice age in Europe and sea level rise in coastal  cities.

What's known as the Atlantic Meridional Overturning Circulation (AMOC) keeps warmer water from the tropics flowing north along the coast of northern Europe to the Arctic, where it cools and sinks to the bottom of the ocean. That cooler water is then pulled back southward along the coast of North America as part of a circular pattern.

This cycle keeps northern Europe several degrees warmer than it would otherwise be and brings colder water to the coast of North America.

There is some indication the system has experienced a gradual weakening over the past few decades, and it may be critically unstable.

New research suggests that if global temperatures continue to rise, the AMOC could collapse in 50 to 250 years.

5. The 'snow forest' disappears

The vast boreal forests of the north face a future as treeless grasslands.

Cold weather forests that run across the Western United States, Canada and Alaska are estimated to store more than 30% of all forest carbon on the planet. Without them, huge amounts of greenhouse gases would be released into the atmosphere, worsening global warming.

A combination of three things are destroying it: heat, fire and bark beetles. Rising temperatures cause droughts and make forest fires more likely. Heat also boosts the population of bark beetles devastating the forests.

Forests can tolerate heat and drought up to a point, and then there's a point where they can't tolerate anymore. There's evidence that we're hitting that point or close to it.

The time is now to act

Scientists and many of the world's political leaders are unequivocal: The time for action is now. Not next year, not a decade from now.

The stakes are clear. Complacency will be met by irreversible and unthinkable impacts from climate change.

We're approaching thresholds we really don't want to walk through. We're near the zone where the Earth is getting back at us.

Source: The United Nations' panel on climate change report