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Climate Risks and ‘Hothouse Earth’

A new paper by researchers from a number of institutions, but including the Stockholm Resilience Centre and the Potsdam Institute for Climate Impact Research, has made headlines in many on-line and print media outlets in recent days.

The paper warns of the possibility of a ‘hothouse earth’ condition, which arises due to current warming forcing the existing Glacial-Inter-glacial Cycle to flip to a new cycle characterized by much warmer temperatures. In the opinion of the authors, the flip could occur because of known but uncertain feedback effects in the climate cycle, such as permafrost thawing, rain-forest die-back and increased bacterial respiration in the ocean.

A similar story in The Atlantic notes that the hothouse conditions of the Eocene, which experienced average global temperatures 9-14°C higher than current conditions, were characterized by carbon dioxide (CO2) levels of at least 1000 ppm, a level that could conceivably be reached early next century if no mitigation actions are forthcoming. The author of The Atlantic article argues that if we do, in fact, push CO2 up to around 1,000 ppm by the end of the century, the warming will persist and the earth will continue to change for what, to humans, is a practical eternity. And when the earth system finally does arrive at its equilibrium, it will most likely be in a climate state with no analogue in our short evolutionary history.

Based on an analysis of the Eocene and other periods (A, Mid-Holocene; B, Eemian; C, Mid-Pliocene; and D, Mid-Miocene are shown in the illustration above), it would appear that high temperature outcomes are possible. It’s not unreasonable to explore such a proposition, although the media reporting of it had few caveats in terms of understanding this risk.

That risk can be visualized (chart below) by looking at work done in 2009 by the MIT Joint Program on the Science and Policy of Global Change which I discussed in a recent post. Their analysis demonstrated that even a modest attempt to mitigate emissions could profoundly affect the risk profile for equilibrium surface temperature. They looked at five mitigation scenarios, from a ‘no policy’ approach to a very stringent climate regime (Level 1, akin to a 2°C case). In the ‘no policy’ approach, mid-range warming by the end of the century is some 5°C compared to the late 20th century, but with a wide distribution, which means that there is a small probability of warming up to 8°C or more – an unacceptably high outcome even when accounting for the small probability that it might occur. In probability distribution terms, this is known as a ‘long tail’. It is shown in black on the right-hand side of the chart below and stretches out to 10°C, or Eocene levels of warming.

But even modest mitigation efforts, while not shifting the mid-range sufficiently for an outcome close to 2°C, nevertheless radically change the shape of the distribution curve such that the spread narrows considerably, with the highest impact outcome dropping by some 3-4°C. As mitigation effort increases and the mid-range approaches 2°C, the distribution narrows further such that the highest possible outcome is limited to 3°C.

While low risk, high impact outcomes might seem overstated by some and certainly cause a few to retreat to a position of defense (e.g. climate skepticism), even a very small chance of a very large impact event is not something that can be ignored. Rather, that risk must be managed. As surface temperature rise is largely a function of cumulative carbon added to the atmosphere, then risk management should focus on the goal of net zero emissions, otherwise carbon dioxide levels in the atmosphere will continue to rise, even if emissions are much lower than today. In that case the risk persists.

The level of mitigation, as shown in the chart above, is then largely a function of when net-zero emissions is reached. Level 1 could be associated with net-zero emissions in 2070, which is achieved in the Shell Sky Scenario, released earlier this year. Level 4 might see net-zero emissions reached in the first half of the 22nd century.

A question to consider when reading the Hothouse Earth paper, is whether we are now beyond the ‘no policy’ outcome, which means that the long tail risk associated with a no mitigation scenario has been managed to some extent at least. With energy technology development proceeding at the pace it is, in combination with the Paris Agreement’s nationally determined contributions and emission targets, policies and goals implemented sporadically by cities, states, countries and regions, there is reason to consider that a path towards net-zero emissions is now unfolding, albeit gradually (see below, the UNFCCC analysis of INDCs, published in May 2016). That pathway is still far from one which equates to well below 2°C, but it could be one that mitigates the long tail impact.

The authors of the Hothouse Earth paper argue for rapid and significant mitigation action, in combination with other measures to improve the stewardship of the entire Earth System. While that call should not be ignored, it is also important not to lambaste early but incomplete progress; it represents an important starting point for the eventual destination. It also helps mitigate long term risk, a critical requirement that is often overlooked in the search for the perfect solution.

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