Pace of Climate Change Threatens Atlantic Ocean Currents
New research suggests that the rate of global warming, rather than the overall heat, may determine the fate of the Atlantic Meridional Overturning Circulation (AMOC)
The Atlantic Meridional Overturning Circulation (AMOC) is a critical system of ocean currents that regulates the global climate. However, new research suggests that the pace of climate change, rather than the overall heat, may determine whether the AMOC will collapse in the next few decades.
Introduction to the AMOC
The AMOC is a giant network of ocean currents that includes the Gulf Stream and carries heat to the Northern Hemisphere. It is vulnerable to global warming because the circulation relies on surface waters in the North Atlantic sinking to form bottom currents that propel the system. Surface waters sink if they are denser - typically, colder and saltier - than the layers underneath. But rising heat in the North Atlantic is warming surface waters, and Arctic meltwater is reducing their salt concentration, so these essential bottom currents are forming less reliably than they were before.
Impact of Climate Change on the AMOC
Climate models show that when global temperatures rise slowly due to small year-over-year increases in atmospheric carbon dioxide (CO2), evaporation and sea ice feedbacks maintain Atlantic currents' strength at temperatures exceeding 9 degrees Fahrenheit (5 degrees Celsius) of warming above preindustrial levels. By contrast, a rapid rise in global temperatures driven by yearly greenhouse gas emissions equal to or higher than today's may trigger a tipping point at just 3.6 F (2 C) of warming. The study investigated how different rates of climate change could affect the stability of the AMOC, and the results suggest that the pace of warming, rather than the extra heat on its own, will determine whether the AMOC persists under climate change.
Comparing Warming Scenarios
The researchers conducted model experiments known as CO2 ramp simulations, in which it is possible to simulate increases in atmospheric CO2 levels over time. The results of these simulations can be compared in the following table:
| Warming Scenario | CO2 Increase | Warming Threshold |
|---|---|---|
| Slow CO2 ramp | 0.5 ppm/year | 9.9 F (5.5 C) |
| Fast CO2 ramp | 2.5 ppm/year | 3.6 F (2 C) |
| Fast CO2 ramp | 5 ppm/year | 3.6 F (2 C) |
The slow CO2 ramp simulated a much slower increase than the current rise in atmospheric CO2, which is between 2.4 and 2.5 ppm per year.
Implications for Climate Policy
The research has implications for climate policy, as current targets and risk assessments are based on temperature thresholds. Policy agendas should also consider the rate of warming, the researchers argued. Additionally, climate strategies involving a temporary overshoot - meaning we temporarily exceed a warming target before dialing temperatures down again with technologies such as carbon capture - can trigger irreversible tipping points in the AMOC and other Earth systems. The new study shows that an AMOC collapse can be avoided if we slow the pace of global warming, but the exact limit is still unclear.
