Oceanographer Stefan Rahmstorf sheds light on the potential catastrophic consequences of a breakdown in the Atlantic Meridional Overturning Circulation (AMOC), emphasizing that the risks have been severely underestimated. In an open letter signed by 44 experts from 15 countries, Rahmstorf, who leads the Earth System Analysis Department at the Potsdam Institute for Climate Impact Research in Germany, outlines how climate change is affecting AMOC and the implications for regions like Britain and Europe.

**What exactly is AMOC?**
AMOC, or the Atlantic meridional overturning circulation, is a network of ocean currents crucial for transporting heat across the northern Atlantic. It draws warm surface water from the tropics, which travels north, releasing heat in subpolar areas, specifically south of Greenland and near Britain and Ireland. As this water cools, it sinks to depths of 2,000 to 3,000 meters before flowing back south as a cold current. Essentially, AMOC plays a vital role in regulating Europe’s climate and influences carbon dioxide absorption and oxygen supply in the oceans.

**How does AMOC relate to the Gulf Stream?**
AMOC and the Gulf Stream are interconnected. The northward flow of AMOC feeds into the Gulf Stream—a fast, warm current originating in the Gulf of Mexico. Although AMOC contributes only about 20% of the Gulf Stream’s water flow, it is responsible for much of the heat transport, akin to a central heating system for the entire region.

**What changes are we seeing in AMOC?**
Indicators suggest that AMOC has been slowing down over the past 60 to 70 years due to global warming. A significant warning sign is the “cold blob” over the northern Atlantic; it is one of the few regions that has cooled in the last two decades while the rest of the planet has experienced warming. This pattern aligns with predictions from climate models anticipating reduced heat transport due to AMOC’s decline.

**Are there other signs of weakening?**
Absolutely. Along the eastern coast of North America, we see excessive heating predicted by climate models, caused by a slowing AMOC that draws the Gulf Stream closer to shore. Furthermore, there is a notable decrease in seawater salinity in this cold blob region—the lowest it has been in 120 years—which is likely related to AMOC’s reduced salt and heat transport.

**Why is the salt content of seawater so important?**
Lower salinity leads to less dense water, making it more difficult to sink—the sinking process is what drives AMOC. Consequently, fresher water causes AMOC to weaken further, creating a feedback loop that could result in its eventual collapse.

**What factors are affecting salinity?**
Global warming plays a direct role in altering salinity levels due to increased evaporation in subtropical areas and excessive precipitation in subpolar regions. Additionally, melting sea ice and the loss of ice from Greenland add more freshwater to the oceans, further decreasing salinity.

**When might we reach a tipping point for AMOC?**
The timing remains uncertain. The tipping point is challenging to pinpoint due to the non-linear dynamics of the system, which hinges on subtle changes in salinity driven by rainfall and cloud cover. Given the complexity of these interactions, modeling precise timelines is fraught with uncertainty.

**What do current forecasts look like?**
Recently, climate research has shifted the consensus. Previously, the Intergovernmental Panel on Climate Change (IPCC) assessed that the chance of reaching this tipping point within the century was below 10%. However, new studies indicate a potential collapse could occur within the next few decades. Rahmstorf’s assessment now suggests a 50/50 chance of crossing this tipping point this century.

**Could we have already passed it?**
While it’s difficult to rule out entirely, observing that nothing drastic happens at the tipping point means it could take 50 to 100 years for AMOC to die off slowly. Therefore, while there is no clear certitude, it is hopeful that we have not yet reached the tipping point.

**What warning signs should we watch for?**
Continued monitoring of Atlantic water flow is crucial. Initiatives like the Rapid project help track this. If we observe a significant decline in deep winter mixing in the northern Atlantic, it could be an early warning of approaching tipping points.

**What would a collapse of AMOC mean?**
Historically, similar collapses have led to severe climate upheavals. These include significant cooling in the northern hemisphere—especially northwestern Europe—and shifts in rainfall patterns that could result in droughts in some regions and floods in others. Additionally, we could see sea levels rising by up to half a meter in the northern Atlantic, alongside reduced CO₂ absorption by the oceans.

**Are these changes happening already?**
Yes, to some extent, and research indicates that similar patterns may be emerging with the Antarctic bottom water formation, suggesting broader impacts as the AMOC weakens.

**Would AMOC collapse counterbalance human-induced warming?**
There are no regions that would emerge unscathed. Weather patterns would become more erratic, leading to extremes that disrupt agriculture and contribute to more frequent storms. The overarching issue with climate change is that ecosystems and human infrastructures are adapted to historical climates; any shift can lead to maladaptation and resultant disasters.

**How certain is the science regarding AMOC collapse?**
While it is well-established that AMOC is weakening and that a tipping point exists, predicting the exact timing remains uncertain. Risk assessment is crucial; for instance, a 10% chance of a plane crash would likely deter most from boarding.

**Why hasn’t the IPCC focused more on AMOC risks?**
The IPCC tends to prioritize the most probable scenarios for climate impacts. Some experts fear that discussing extreme risks could divert attention from imminent issues caused by climate change. Rahmstorf, however, contends that these extreme risks should be part of a comprehensive understanding of climate challenges affecting responsible decision-making.

**What would the duration and survival implications of an AMOC collapse be?**
In past instances, recovery took about 1,000 years. This time, though, the high levels of CO₂ complicate predictions. While humanity is unlikely to face extinction, certain regions, such as Norway and Scotland, could face existential risks, prompting questions about the viability of continued habitation.

**How does AMOC’s threat compare to other climate tipping points?**
It’s challenging to quantify, given the trade-offs between long-term and immediate concerns. Many coral reefs have crossed a tipping point and are experiencing significant die-offs, and deforestation threatens the Amazon rainforest. Ice sheet tipping points in Greenland and Antarctica also pose long-term risks for sea-level rise.

**What is your overall concern?**
My worry spans all these issues. The consensus remains clear: fossil fuel emissions and deforestation are the primary drivers of climate change, and we must adhere to the Paris Agreement to limit global warming as closely as possible to 1.5°C. As a parent, I find myself increasingly concerned about the future my children will inherit, recognizing that even physicists, often seen as detached, deeply care about the world we are bequeathing.