Tipping points in the climate system

The Earth can change in big ways.

Sometimes a small change leads to a huge shift. This is called a tipping point. One example is melting ice. As ice melts, the land gets warmer. This heat makes even more ice melt.

This can happen to forests and oceans too. These changes can be very hard to stop. It is important to watch how our world changes.

We can learn how to help our planet.

The Earth has many parts that work together. Sometimes, a small change can lead to a huge shift. Scientists call this a tipping point.

A tipping point is a limit. Once we cross it, the system changes to a new state. These changes are often hard to stop.

One example is the Greenland ice sheet. As it melts, it gets thinner. This makes the surface lower. Air is warmer at lower heights. So, the ice meets warmer air and melts even faster. This is a feedback loop.

Other tipping points happen in the ocean or forests. If one part tips, it can cause a cascade. This means one change triggers many others. For example, melting ice can change how ocean water moves. This might then cause permafrost to thaw. Permafrost is ground that stays frozen. When it thaws, it lets out methane. Methane is a gas that warms the planet.

A tipping point is a critical threshold in the climate system. When this limit is crossed, it leads to large and often irreversible changes. These shifts happen because a small disturbance causes a much larger change in the system. Once a system moves past its tipping point, it reorganizes into a new state. This new state can be very different from the one before. These changes can happen quickly or over many years.

Many parts of our world have these tipping points. Some are found in the cryosphere, which includes ice and snow. Others are in ocean currents or terrestrial systems like forests. For example, the Greenland ice sheet has a special way it works. As it melts, the ice becomes thinner and its surface gets lower. Air is warmer at lower altitudes, so the ice meets warmer air. This causes even more melting, which is a self-reinforcing feedback loop.

Scientists have studied these limits for a long time. In the early 2000s, the IPCC began looking at these large-scale changes. At first, they thought tipping points might only happen with much higher warming. However, estimates for these thresholds have fallen over time. By 2016, some thought they could happen within the Paris Agreement range. Today, scientists see a significant probability of tipping points at current warming levels.

There are many specific elements that scientists watch closely. As of September 2022, nine global core tipping elements were known. Seven regional impact tipping elements were also identified. If global warming reaches 1.5 °C, several things may happen. The Greenland and West Antarctic ice sheets could collapse. Tropical coral reefs might die off, and permafrost could thaw abruptly.

One big worry is a tipping cascade. This happens when crossing one threshold triggers another one. For instance, losing ice in Greenland can change how ocean currents move. This change in the ocean could then cause permafrost to thaw. This process can link many different parts of the Earth together. It shows how everything in our climate system is connected.

A tipping point is a critical threshold within the Earth's climate system. When this specific limit is crossed, it triggers large, accelerating, and often irreversible changes. The Intergovernmental Panel on Climate Change (IPCC) defines a tipping point as a threshold where a system reorganizes, often in a non-linear manner. This means the system does not return to its original state even if the initial cause of change is removed.

These shifts occur because a small disturbance can cause a disproportionately large reaction. This process is often driven by self-reinforcing feedbacks. In a feedback loop, the result of a change becomes the cause of even more change. For example, melting ice reduces the Earth's albedo, which is its ability to reflect sunlight. As less sunlight is reflected, the planet warms faster, which leads to even more ice melting. This cycle can move a system from one stable state to a completely different one.

Scientists categorize these tipping elements into different groups based on their location. The cryosphere, which includes ice and snow, contains many elements like the Greenland ice sheet and mountain glaciers. Ocean systems include changes to circulation patterns, such as the Atlantic Meridional Overturning Circulation (AMOC). Terrestrial systems include ecosystems like the Amazon rainforest and boreal forests.

One specific mechanism involves the Greenland ice sheet and its melt-elevation feedback. As the surface of the ice sheet melts, the total height of the ice decreases. This lowers the surface to a lower altitude where the air is warmer. This warmer air then causes the ice to melt even faster. This process could lead to the disintegration of the ice sheet. If the Greenland ice sheet melted completely, it would raise global sea levels by 7.2 metres.

Another critical area is the West Antarctic Ice Sheet (WAIS). This ice sheet sits on bedrock that is mostly below sea level. It has formed a deep subglacial basin due to the weight of the ice over millions of years. Because of this, the ice is in contact with warm ocean heat. This makes the WAIS vulnerable to fast and irreversible ice loss. The point where ice stops sitting on rock and begins floating is called the grounding line.

Research into these thresholds has changed significantly over the decades. In the early 2000s, the IPCC originally referred to these events as large-scale discontinuities. At that time, scientists believed tipping points would only occur with much higher global warming. However, as more data became available, estimated thresholds began to fall. By 2016, some scientists believed tipping points were possible within the Paris Agreement warming targets.

As of September 2022, researchers identified nine global core tipping elements and seven regional impact elements. If global warming reaches 1.5 °C, several major shifts are likely. These include the collapse of the Greenland and West Antarctic ice sheets, the die-off of tropical coral reefs, and the abrupt thaw of boreal permafrost.

Thawing permafrost is a major concern because it holds roughly twice as much carbon as is currently in the atmosphere. If it thaws, it releases methane, which is a powerful greenhouse gas.

A major danger in climate science is the tipping cascade. This happens when crossing a threshold in one system triggers another system to tip. For example, ice loss in Greenland and West Antarctica can significantly alter ocean circulation. This change in the ocean could then cause sustained warming in northern latitudes. Such warming might then trigger permafrost degradation or the dieback of boreal forests. This shows how different parts of the Earth are deeply connected.