The Ski Slope Curve and the IPCC report

The reason there is an increasingly urgent need to curb carbon emissions...

This past week, the IPCC released its last report of this year, detailing the need to reduce carbon emissions, and outlining possible policies necessary to do so. I don’t want to comment here on the policy issues, which they discuss in detail. Rather I thought I would briefly outline a simple argument that I reviewed in my book, The Physics of Climate Change, which underlies why it gets increasingly harder, and increasingly more urgent, to act to reduce carbon emissions in order to achieve the goal of limiting the global temperature rise to something like 1.5-2 degrees C since the pre-industrial era.

In 2009 a group called the Copenhagen Diagnosis produced a diagram that the climate scientist Richard Somerville has dubbed The Ski Slope Curve. It arises from the fact that much of the additional CO2 that humanity emits into the atmosphere will effectively remain in the atmosphere for a millennium. Humanity currently emits about 10 Gigatons (Gt) of carbon into the atmosphere (in the form of CO2), and about 50% of that is then exchanged with the ocean, leaving about 5 Gt per year increase in atmospheric carbon. That additional amount persisting for centuries. I have jokingly called this the Las Vegas Effect: when it comes to CO2 abundance, what goes in atmosphere stays in the atmosphere.

As CO2 concentration in the atmosphere increases, additional radiative forcing, due to increased absorption of outgoing IR radiation, will lead to additional global warming. If we assume a goal of limiting probable global warming to some value, say 2 degrees, above pre-industrial levels with a 66% likelihood, one can estimate the associated maximum amount of additional carbon that can put in the atmosphere. The pre-industrial level of carbon in the atmosphere was about 600 Gt. We have dumped an additional 500 or so Gt in the intervening 150 years, about 250 Gt of which has remained in the atmosphere. Dumping another 75-100 or so Gt into the atmosphere will increase temperatures, resulting an overall 2 degree rise since the pre-industrial era.

This means that every year we don’t reduce our emissions makes the need to do so greater in subsequent years in order to achieve the same end result. Had we started to reduce in 2011, as shown below, the world would have had to follow what Somerville calls the bunny slope, with 3.7% reduction needed per year to reach the goal. Waiting till 2020, having dumped another 100 Gt or so of carbon into the atmosphere in the intervening years, meant that the world would have had to follow the black diamond slope to achieve the goal, with approximately 9% reductions in carbon emissions needed per year.

Needless to say, the world has not begun to significantly reduce its carbon emissions (although the growth has slowed). As a result we have a new set of ski slope curves, illustrated here in a plot from The World in DATA

As you can see, the curves get progressively steeper, until, by 2035 or so, they are essentially vertical, meaning that almost complete reduction would be necessary over the course of a few years.

So that is why, while the impacts of human-induced climate change may take decades or centuries to fully manifest themselves, if we are to head off these possible impacts then every year we wait makes the task more challenging.