To properly explain this, I need to give some background in recent Earth history. The Pleistocene Epoch is a relatively short interval of time in Earth history which began about 2.588 million years ago and ended 11,700 years ago. It represents the time interval of the most recent ‘ice age’1. This ice age consisted of a series of glacial advances (glacials) and retreats (interglacials)2. At its numerous peaks, ice sheets covered large tracts of northern Eurasia and North America, extending as far south so as to cover all of Scandinavia and the Baltic, large parts of northern Russia, Poland and Germany, most the UK, as well as most of Canada and parts of the northern states of the US. Parts of these ice sheets were as much as 4km thick3. At the same time, sea level was about 120 metres lower than it is today4, and one could have walked from the island of New Guinea to northern Australia, and from mainland Australia to Tasmania.
At the end of the last glacial maximum, about 20,000 years ago, the ice began to recede and sea level began to rise, and by about 11,700 years ago, sea level was about half way to its present level (i.e. about 60 metres below). This is the beginning of what has been defined as the Holocene Epoch. This was the interval during which agriculture began in several different places around the world (about 10,000 years ago) and eventually led to the industrial revolution just over 200 years ago.
The Anthropocene Epoch has been proposed as a more recent epoch characterised by the impact of humans on the planet. A working group of experts are currently considering whether it is reasonable to formalise such a concept, as almost all of the other intervals (e.g. the Holocene) have been. The event marking the start of the Anthropocene is being debated, with an early date suggested being the commencement of intensive farming, while later dates being suggested are the commencement of the industrial revolution or the fallout from nuclear blasts, which can be detected in most sediments deposited from the early 1950s5. Here endeth the lesson on Earth History.
A new paper published in the US journal Proceedings of the National Academy of Sciences, has as first author Will Steffen of the Australian National University. This paper explores the risk that self-reinforcing feedbacks could push the Earth toward a threshold that, if crossed, could prevent stabilisation of the average surface temperature anywhere near where the Paris Accord requires. It could lead to a much higher average temperature than in any interglacial in the last 1.2 million years, and to sea levels significantly higher than at present. They conclude that Earth could have already reached a ‘fork in the road’ where we have left behind the glacial/interglacial cycle already, because of the present 1 degree C rise in the global average temperature. Current climate modelling is largely driven by the amount of greenhouse gases that human activities have already emitted and will continue to emit over the rest of this century and beyond, and that a near linear relationship exists between cumulative Carbon Dioxide emissions and global average temperature. However, Steffen et al. argue that strong nonlinear feedback processes could become an important, or even a dominant factor in determining the trajectory of the Earth’s climate6.
There are two types of feedback types; negative and positive. Negative feedbacks act against the driver of increasing Carbon Dioxide emissions, and positive feedbacks reinforce the effect of that driver. Negative feedbacks include carbon uptake by land and ocean systems, but these are relatively weak compared to the forcing by human activity, and suggest that positive feedbacks could play an increasingly important role in the climate trajectory of the planet. Among positive feedbacks, they can exhibit both continuous responses and tipping point behaviour. In the latter, the feedback becomes self-perpetuating after a critical threshold is reached6.
Steffen et al. list carbon cycle positive feedbacks that will likely accelerate global warming and they provide an estimate of how much, in degrees C of additional temperature rise, will occur by the year 2100 based on a ~2 degrees C modelled warming. These include:
- Permafrost thawing: 0.09 degrees C
- Relative weakening of land and ocean physiological Carbon sinks: 0.25 degrees C
- Increased bacterial respiration in the ocean: 0.02 degrees C
- Amazon forest dieback: 0.05 degrees C
- Boreal forest dieback: 0.06 degrees C
Together these add up to a total of 0.47 degrees C6.
Several of the feedbacks that will likely show a small or negligible effect by 2100 may generate significant feedbacks after that time, perhaps for centuries. These include:
- Permafrost thawing
- Decomposition of ocean methane hydrates*
- Increased marine bacterial respiration
- Loss of Antarctic and Greenland ice sheets
- Rise in sea levels
- Changes in ocean circulation
The last three of these are intimately linked6.
Steffen et al. then refer to tipping cascades, when a rise in global temperature reaches a level of a low temperature (~2 degrees C) tipping point (e.g. loss of the Greenland Ice Sheet or Arctic Sea Ice). These could push the temperature even higher, inducing higher temperature tipping points. For instance, loss of the Greenland ice sheet could trigger a critical transition in the Atlantic Ocean circulation, which could together cause sea level rise and heat accumulation in the Southern Ocean, thereby accelerating ice loss from the East Antarctic ice sheet on a timescale of centuries. While this may sound extreme, it suggests that warming into the range of the Paris Accord (~2 degrees C) target may reach such a threshold. This threshold could lock the planet into a continuing rapid pathway toward much hotter conditions – a Hothouse Earth. This could be a pathway that would be beyond the possibility of reversal. The impacts of this on human societies, and nations would likely be massive, sometimes abrupt, and disruptive6.
Humanity is facing a dire need for critical decisions and actions that must be taken. Politicians in the pockets of big coal and big gas seem incapable of even contemplating the risk we are taking, let alone doing anything about it, especially if that would interfere with their future donations. If they will not take these decisions, then we will have to get rid of them. If we do not, our future may well be beyond our control, and it will not be pretty.
*Methane hydrate, also called methane clathrate is essentially the trapping of molecules of methane (from decaying organic matter) within a crystal structure of water, forming a solid similar to ice. Its chemical formula is (CH4)4(H20)23, and it has been discovered in huge quantities within sediments on the ocean floor where temperatures are low (about 2 degrees C). It looks like ice, but can be made to burn, with a flame above and water dripping off below.
- Steffen, W., Rockström, J., Richardson, K., Lenton, T.M., Folke, C., Liverman, D., Summerhayes, C.P., Barnosky, A.D., Cornell, S.E., Crucifix, M., Donges, J.F., Fetzer, I., Lade, S.J., Scheffer, M., Winkelmann, R. & Schellnhuber, H.J., 2018. Trajectories of the Earth System in the Anthropocene. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1810141115