The Model Case

This one is from a forthcoming paper by Meehl et al, and was shown by Jerry Meehl in his talk at the Annecy workshop this week. It shows the results for just a single model, CCSM4, so it shouldn't be taken as representative yet. The IPCC assessment will use graphs taken from ensembles of many models, as model ensembles have been shown to be consistently more reliable than any single model (the models tend to compensate for each other's idiosyncrasies).

But as a first glimpse of the results going into IPCC AR5, I find this graph fascinating:

The extension of a higher emissions scenario out to three centuries shows much more dramatically how the choices we make in the next few decades can profoundly change the planet for centuries to come. For IPCC AR4, only the lower scenarios were run beyond 2100. Here, we see that a scenario that gives us 5 degrees of warming by the end of the century is likely to give us that much again (well over 9 degrees) over the next three centuries. In the past, people talked too much about temperature change at the end of this century, without considering that the warming is likely to continue well beyond that.

The explicit inclusion of two mitigation scenarios (RCP2.6 and RCP4.5) give good reason for optimism about what can be achieved through a concerted global strategy to reduce emissions. It is still possible to keep emissions below 2 degrees of warming. But, as I discuss below, the optimism is bounded by some hard truths about how much adaptation will still be necessary - even in this wildly optimistic case, the temperature drops only slowly over the three centuries, and still ends up warmer than today, even at the year 2300.

As the approach to these model runs has changed so much since AR4, a few words of explanation might be needed.

First, note that the zero point on the temperature scale is the global average temperature for 1986-2005. That's different from the baseline used in the previous IPCC assessment, so you have to be careful with comparisons. I'd much prefer they used a pre-industrial baseline - to get that, you have to add 1 (roughly!) to the numbers on the y-axis on this graph. I'll do that throughout this discussion.

I introduced the RCPs ("Representative Concentration Pathways") a little in my previous post. Remember, these RCPs were carefully selected from the work of the integrated assessment modelling community, who analyze interactions between socio-economic conditions, climate policy, and energy use. They are representative in the sense that they were selected to span the range of plausible emissions paths discussed in the literature, both with and without a coordinated global emissions policy. They are pathways, as they specify in detail how emissions of greenhouse gases and other pollutants would change, year by year, under each set of assumptions. The pathways matters a lot, because it is cumulative emissions (and the relative amounts of different types of emissions) that determine how much warming we get, rather than the actual emissions level in any given year. (See this graph for details on the emissions and concentrations in each RCP).

By the way, you can safely ignore the meaning of the numbers used to label the RCPs - they're really just to remind the scientists which pathway is which. Briefly, the numbers represent the approximate anthropogenic forcing, in W/m², at the year 2100.

RCP8.5 and RCP6 represent two different pathways for a world with no explicit climate policy. RCP8.5 is at about the 90th percentile of the full set of non-mitigation scenarios described in the literature. So it's not quite a worse case scenario, but emissions much higher than this are unlikely. One scenario that follows this path is a world in which renewable power supply grows only slowly (to about 20% of the global power mix by 2070) while most of a growing demand for energy is still met from fossil fuels. Emissions continue to grow strongly, and don't peak before the end of the century. Incidentally, RCP8.5 ends up in the year 2100 with a similar atmospheric concentration to the old A1FI scenario in AR4, at around 900ppm CO2.

RCP6 (which is only shown to the year 2100 in this graph) is in the lower quartile of likely non-mitigation scenarios. Here, emissions peak by mid-century and then stabilize at a little below double current annual emissions. This is possible without an explicit climate policy because under some socio-economic conditions, the world still shifts (slowly) towards cleaner energy sources, presumably because the price of renewables continues to fall while oil starts to run out.

The two mitigation pathways, RCP2.6 and RCP4.5 bracket a range of likely scenarios for a concerted global carbon emissions policy. RCP2.6 was explicitly picked as one of the most optimistic possible pathways - note that it's outside the 90% confidence interval for mitigation scenarios. The expert group were cautious about selecting it, and

...