As we consume fossil fuels, the concentration of carbon dioxide in Earth’s atmosphere is steadily increasing, along with the planet’s average temperature. The velocity at which atmospheric carbon dioxide levels — and consequently, temperatures — rise is significant for the capacity of humans and ecosystems to adapt. A more gradual rise allows humans to evacuate low-lying regions and gives wildlife the opportunity to migrate to new environments.

Interestingly, the pace of this increase is crucial for non-living systems as well. A recent investigation by the University of Washington examined how a prominent current in the Atlantic Ocean, which includes the Gulf Stream, will react to a doubling of carbon dioxide from preindustrial benchmarks. The research, published in the Proceedings of the National Academy of Sciences, revealed that when carbon dioxide concentrations rise more slowly, they exert a diminished effect on ocean circulation.

UW News interviewed author Camille Hankel, a postdoctoral researcher at the Cooperative Institute for Climate, Ocean and Ecosystem Studies, to gain further insights into her research.

What inspired you to investigate how the rate of increasing CO2 impacts the climate system?

Camille Hankel: During my PhD, part of my research involved “climate tipping points,” stemming from the notion that there could exist critical thresholds of warming or CO2 variation that might trigger sudden and irreversible changes in certain aspects of the climate system. Through that work, I encountered literature on “rate-induced tipping points,” positing that rather than merely passing a critical threshold, specific rates of CO2 alteration might hold significant importance for the climate system.

In particular, I came across a study analyzing this concept in relation to the AMOC, the Atlantic Meridional Overturning Circulation, which represents a vast ocean current system. That research utilized a simplified mathematical model, referred to as a box model, to represent ocean circulation. I thought, why not employ these global models, which provide a much more authentic depiction of Earth’s climate — encompassing ocean, atmosphere, land, and sea ice — to test whether the rate of CO2 change truly matters.

What is the Atlantic Meridional Overturning Circulation, which encompasses the Gulf Stream ocean current, and why is it vital for Earth’s climate?

CH: It represents one of the critical large-scale elements within the climate system. Specifically, it transports a significant amount of heat from lower latitudes in the South Atlantic towards higher latitudes near the North Pole. Therefore, it primarily delivers a great deal of heat to Northern Europe. Furthermore, it redistributes nutrients through a characteristic sinking motion — pulling surface waters down into the deep ocean while recirculating deep waters back to the surface. It plays a crucial role in the climate system, particularly in the North Atlantic, but also on a global scale.

There have been talks about a possible deceleration of the Gulf Stream current that could impact European weather. This concept was sensationalized (perhaps inaccurately) in the 2004 disaster film ‘The Day After Tomorrow.’ Are we genuinely observing a slowdown in Atlantic Ocean circulation?

CH: Our observational record of the AMOC current is relatively brief and limited. One must consider that this exists as a three-dimensional circulation spanning the entire Atlantic basin, while we only possess a few fragmented data pieces from certain areas in the Atlantic. Thus far, we are witnessing a slight deceleration, but the observational data is quite noisy and uncertain, making definitive conclusions difficult.

Nevertheless, the current slowdown we observe is consistent with model predictions for future deceleration. Additionally, there exists a temperature pattern wherein, while the majority of the globe is experiencing warming due to increased CO2, there’s what some refer to as a “warming hole” over the North Atlantic, indicating less warming in that region compared to the rest of the planet. While it’s challenging to definitively attribute this occurrence to specific causes within Earth’s climate, the notion is that the minor slow down of the AMOC we have observed so far could be one contributing factor to the reduced warming in the North Atlantic.

Thus, observations suggest some deceleration, although far less dramatic than what was portrayed in that film.

Why is the AMOC predicted to slow down because of climate change?

CH: A fundamental way to understand what drives this significant ocean current is through variations in ocean density. There exists a critical zone in the North Atlantic where surface waters sink due to their higher density compared to the water beneath. When CO2 levels change, two main effects occur: the surface ocean begins to warm, and the melting of glaciers, along with alterations in sea ice, introduces freshwater to the otherwise salty surface waters of the ocean. Both the warming and the addition of freshwater diminish the density of the upper ocean water, potentially hindering or disrupting that essential sinking motion.

There are various approaches to consider this, but my focus in the study is on how these changes in density occur within a higher-CO2 environment and how they influence the AMOC’s sensitivity to variations in CO2 change.

Your research indicates that if atmospheric carbon dioxide doubles from pre-industrial amounts at a more gradual rate, the Atlantic Ocean experiences less deceleration compared to a quicker doubling of CO2. Is this because the processes are occurring at a slower pace?

CH: Absolutely, that plays a significant role. The various components of the climate system — ocean, atmosphere, and ice — have distinct response times to CO2 alterations, meaning they react to disturbances with varied lag times. Consequently, how these components interact under either slower or faster CO2 changes can lead to vastly different results, which, in this regard, influences ocean circulation.

In particular, I discovered what is referred to as positive feedback — a sort of self-reinforcing cycle — that elucidates why the extent of AMOC weakening depends on the rate of CO2 change. In this feedback loop, the initial minor slowdown of the AMOC reduces heat transport into the Arctic, consequently cooling that area and leading to a short-lived period of Arctic sea ice growth. This expansion of sea ice results in additional ice being transported to the North Atlantic, where it melts and introduces freshwater to the ocean, further contributing to the AMOC’s slowdown: hence this self-reinforcing cycle. It appears that this feedback loop is far more impactful.

efficacious at enhancing AMOC reduction during more rapid CO2 fluctuations compared to gradual CO2 variations.

Why is this research significant?

CH: We have extensive knowledge regarding AMOC slowdowns — there exists a vast body of research on this topic and the factors that contribute to such a slowdown. However, what is innovative is the responsiveness of circulation alterations to the speed of CO2 escalation, independent of the overall increase in CO2 concentrations.

When we contemplate policy and fundamental science, our emphasis often lies on how the degree of global warming can influence the climate system. I am striving to introduce a fresh viewpoint by considering the rate of escalation as a significant factor in itself, which could yield numerous ramifications.

One could envisage that if multiple distinct climates are feasible for the same warming level, then constraining ourselves to 1.5°C or 2°C could signify different implications, correct? I firmly believe that the primary concern for the climate system remains the total amount of CO2 emitted into the atmosphere, but the speed at which we reach that threshold is evidently also of great importance.

 

For additional details, reach out to Hankel at [email protected].