- Dr. Short
A potentially boring but necessary post about climate feedbacks and tipping points: Part 1
Warning: this post is only for people who accept that scientific inquiry is a valid and useful tool for understanding our world. I would love to reach those who throw the climate science baby out with the Western Science bathwater, but I’ve found that effort to be rather futile. For anyone still here reading, let’s explore the science of positive feedbacks and tipping points in systems, particularly in Earth’s climate system.
Systems science is a way of studying complex things by dividing them into subsystems or components so that the processes that occur within them, and the connections between different subsystems, can be better understood. Systems, including the Earth system, are self-regulating, meaning that small changes to one part of the Earth system will automatically result in changes in other parts.
When one part of the Earth system is pushed out of equilibrium to the point where positive feedbacks begin and are maintained, a tipping point in the system has been passed. It is important to understand how these things work, because a lot of debate about what to do about our present climate emergency is centered around whether tipping points have been passed and/or positive feedbacks started, and what it means for life on Earth once they do.
Users can represent the very basic functions of a system by breaking it down into components (in boxes, below), which can be defined as a reservoir of matter, energy, an attribute of those two things, or even a sub-system. A coupling defines a relationship between two components, and is unidirectional; i.e., the change must follow the arrow. Positive coupling = a change (+ or -) in one component leads to a change in the same direction (both increase or both decrease) in the linked component (shown with an arrow). Negative coupling = a change in one component leads to a change in the opposite direction (ex: an increase in one leads to a decrease in the other) in the linked component (shown with a line with a circle on the end).
If two or more coupling relationships can be drawn between components in a system, then a feedback loop is formed (commonly shortened to just a 'feedback'). For example, photosynthetic activity has an effect on atmospheric CO2 content and vice versa (see above). Feedbacks can be either positive (see below), which amplify the initial change to the components, or negative, which diminish the initial change (example above). Positive feedbacks are destabilizing- a vicious cycle- whereas negative feedbacks are dampening. Unfortunately, most feedbacks in Earth’s climate system are positive.
A forcing in systems science is defined as a persistent disturbance applied to the system that forces it to change. A perturbation, for example, would be a large, ash-type volcanic eruption sending tons of volcanic ash into the atmosphere, cooling the climate slightly, until the ash settles to the surface and the climate goes back to ‘normal’. A forcing, however, causes much longer-term changes to the climate system because the disturbance, such as human CO2 emissions, is applied over a longer period of time. This causes positive feedbacks to begin in the climate system that amplify the initial heating.
There are several important positive feedbacks in the climate system, some more famous, and some more worrisome, than others. We’ll get into specific positive feedbacks in the following posts. We know from studying Earth’s past climates that amplifying feedbacks have been triggered before, that they have contributed to the rapid heating and cooling of Earth’s atmosphere, and that they can be associated with mass extinctions of varying degrees. But the details are sparse and it is very difficult to nail them down as causes of the events. It is however helpful to know their qualitative origin stories.
An example from climate past
To give you an example of rapid climate change caused by positive feedbacks in Earth’s past, we’ll look at Antarctica 35 million years ago. The climate was much warmer than today, and there was no glacial ice on the continent. Sea level was hundreds of feet higher because all the water that today is locked up in glacial ice caps was melted and in the oceans. Then things started to change, slowly at first, then quickly. As CO2 was drawn out of the atmosphere through high rates of photosynthesis in a warm world, the climate cooled slowly, over millions of years, until snow and ice began to stay on Antarctica year-round.
Snow is white, so it has a high albedo, or reflectivity, that reflects solar radiation back out into space quickly, and actually cools the local climate. So as snow and ice increased, albedo increased, and the air temperature cooled, causing more snow and ice to stay longer, higher albedo, more cooling, until we ended up with the 3-kilometre thick ice sheet that we have today.
But why did I give you an example of a positive feedback that produced cooling? Because the relationships between the components stay the same, and if Earth's air temperature warms to the point where the snow and ice melts, exposing darker land and water, we will have started a positive warming feedback that could result in an ice-free world, as long as the forcing is not removed quickly. That is a very different world than the one that we live in today, and at our present rate of emissions we could reach this tipping point by the year 2030.
A recent paper that looked at the genetic lineage of Antarctic octopus during the Last Interglacial period, which was roughly equivalent to today’s climate, demonstrated that the West Antarctic Ice Sheet likely did not exist at that time - it had collapsed due to warming caused by an increase of solar insolation from Milankovitch cycles. So we may be closer than we think to some tipping points, but ice sheets are notoriously slow to respond, and the best estimates now predict a committed one foot global average sea level rise over the next 30 years.
In the next post, we'll examine how humanity could avoid passing tipping points in our climate system.