Man-made climate warming could lead to some impacts that are abrupt or irreversible

The IPCC assessment is that climate change is likely to lead to some irreversible impacts. Abrupt changes cannot be excluded, but they are not currently considered likely. Such changes will depend on both the rate and the magnitude of climate change.

What is abrupt climate change?

When scientists talk about climate change, they are usually referring to "gradual climate change." In other words, if the planet warms steadily, the climate changes steadily. But there is evidence that some parts of the climate system work more like a switch than a dial; if a certain temperature level is reached, there may be abrupt climate change.
Some scientists have pointed out the risk of catastrophic climate change caused by gradual increases in greenhouse gas concentrations - like the breakup of the West Antarctic ice sheet, or the collapse of the thermohaline circulation (THC).

Could abrupt climate change really happen?

Scientists have only recently begun to study the possibility of abrupt climate change in more detail. Our knowledge is still too limited to know at what stage abrupt climate change could be triggered by increasing greenhouse gas concentrations, but it is generally considered very unlikely to happen within the next several decades. However, the probability of an abrupt change in some part of  the climate system is expected to increase if the rate, or the magnitude, or the duration of climate change from greenhouse gas emissions increase.
Thermohaline circulation (THC) is often called the "Ocean Conveyor". This system consists of water sinking at high latitudes in the North Atlantic, then flowing south at depth through the North and South Atlantic Oceans, around Antarctica in the circumpolar current and then flowing north into the Indian and Pacific Oceans. The core of deep water is continuously eroded by mixing with waters above it, and finally peters out in the North Pacific. The conveyor is completed by warm water flowing back towards the North Atlantic, via the Indonesian Throughflow from the Pacific Ocean, around the Cape of Good Hope from the Indian Ocean and then northwards through the South and North Atlantic Ocean. This return flow of warm water reinforces the Gulf Stream in the North Atlantic and warms the North Atlantic region by ~10°C. The sinking in the North Atlantic is critically dependent on temperature and on the salinity of the water.

Could climate change shut down the thermohaline circulation?

Climate change is expected to increase ocean temperatures particularly in polar regions and to increase the flow of freshwater into the ocean through rain, run-off, and melting of glaciers. The increased surface ocean temperatures and reduced salinity (saltiness of the water) are very likely to weaken the thermohaline circulation during the 21st century. However current thinking is that it is very unlikely there will be an abrupt change this century

What would happen to New Zealand and the Pacific if the thermohaline circulation shuts down?

Current studies suggest a shut down of the THC would lead to an extra 1-2 degree celsius increase in temperature in the New Zealand region, on top of that caused by other global warming processes. This would reinforce the effects of global warming. We do not know what effect this could have on our day-to-day weather as scientists are only beginning to study this phenomenon. The additional warming effect of a shut-down of the THC in the Pacific region is the opposite to the expected effects in the northern hemisphere, where a reduction in the THC is expected to lead to a cooling, or at least a reduced rate of warming in northern regions of Europe and eastern Canada and the United States.

Should we be really worried?

The scientific consensus is that we are not on the brink of a shutdown of the THC. Many scientists believe that it cannot happen or is unlikely to happen for at least another century. However, the THC system is pivotal to climate stability, and if it shuts down this would have an enormous impact likely to last for several centuries.
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