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A less boring post on climate feedbacks & tipping points, Part 2: where should humanity aim?

In the previous post, I covered the very basics of systems science, positive feedbacks, and tipping points in the climate system. The onset of positive feedbacks could greatly accelerate global heating, among other things, cause by human greenhouse gas emissions - and we really want to avoid them. The example of the Antarctic ice-albedo-warming feedback has some world-altering consequences, so the obvious question is:

How might we avoid passing that tipping point?

Point A = just before Antarctica was glaciated; Point C = maximum of recent ice ages; Point B = halfway in-between in terms of global temperature and climate forcings.

This graph is from one of the first studies to attempt to incorporate long-term climate feedbacks into climate models (Hansen, et al., 2008), and it resulted in an estimate of the atmospheric CO2 tipping point for the melting of the Antarctic ice cap, i.e., an ice-free world. The right-hand side of the graph is the measured relative temperature on Earth’s surface; the left side is modeled, with the grey area representing uncertainty. The authors of this study concluded that an atmospheric CO2 concentration of ~ 450ppm +/- 100ppm existed when Antarctica began to grow an ice sheet 35 million years ago.

So because of the uncertainty inherent in reconstructing past climate and climate models, the tipping point that we want to avoid in order to keep ice on the poles has a large error margin, +/- 100ppm CO2 in the atmosphere. The tipping level (350ppm, as in can be temporarily exceeded because ocean and ice sheet inertia permit some overshoot of atmospheric CO2 levels. The tipping level is easier to assess, whereas the tipping point is difficult to define because the processes involved are non-linear and complex. This is the reason why most positive feedbacks that we know to exist in the climate system are not included in climate models used by the IPCC and others. There is just too much uncertainty about their atmospheric CO2 thresholds and internal dynamics to make conclusive statements about how and when they might affect global heating.

Nevertheless, paleoclimate data provide a known response to greenhouse gas forcing:

“It is a measure of the long-term climate forcing that humanity must aim to stay beneath to avoid large climate impacts.” - Hansen, et al., 2008

But the paper does not define the magnitude or duration of a tolerable CO2 forcing ‘overshoot’. The longer the tipping level is exceeded, the less likely that climate recovery without dramatic effects will be. The world just passed 420 ppm atmospheric CO2 last year.

Basically, we should stop burning fossil fuels as quickly as possible.

What does the IPCC say about all this?

The IPCC strongly advises that the world stay below 1.5 degrees C heating above pre-industrial levels in order to avoid the worst impacts of climate breakdown. This would entail reducing global CO2 emissions by at least 45% from 2010 levels by the year 2030, minimum (remember- those positive feedbacks are not included). 1.5 degrees C is roughly equivalent to 450ppm in the atmosphere.

We are presently INCREASING CO2 emissions globally, and Canada is leading the way. If we continue to increase emissions at the rate that we are today, then we could experience a 3.4 degrees C global average surface air temperature heating above pre-industrial levels by 2100 (not including the onset of positive feedbacks). The last time that the Earth's climate was that warm, sea level was at least 25 metres higher, and there were not yet any members of the genus Homo roaming the beaches. Global heating of 3.4 deg C within a few decades is what climate scientist Kevin Anderson describes as:

"Incompatible with a globally organized society"

Remember that 1.5 degrees C heating is the bare minimum of what we should aim for- because past climate change suggests that the 450ppm atmospheric CO2 required to get there is possibly also our tipping point for an ice-free world. Additionally, the IPCC ‘carbon budget’ for staying below 1.5 degrees C heating only has a 66% chance of succeeding – meaning that if the world did manage to reduce CO2 emissions in line with their recommendations, there is only a 2/3rds chance that it would work. Oh, and the missing positive feedbacks.

Basically, we should stop burning fossil fuels as quickly as possible.

Not ‘net-zero’ by 2050. Zero, ASAP.


At this point, if you are still reading, you’re probably thinking ‘this is really scary', and 'reducing emissions that quickly is impossible’, or something like it. I find it interesting that when most people are faced with the science-based prescription for the appropriate response to the greatest existential crisis of humanity, their first reaction is ‘No- not possible’, or ‘it’s too late’. I hear this from a wide range of people, with different political views and socioeconomic backgrounds, but almost exclusively folks over the age of 30 and from the global north. To me, it is simply what needs to be done if we want to avoid immense suffering and untimely death across the human and more-then-human populations over the coming decades. Simple, not easy. The big-picture objective is straightforward; the details are much more complicated.

There are a few key points from the scientific literature that should be understood by anyone who thinks that the task before us is impossible or not worth pursuing because ‘it’s too late’. If you need to feel like the necessary efforts and sacrifices required of the privileged portion of humanity that has agency in this, is worthwhile, then on to the next post!



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