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Year-Specific Predictions of Climate Change

And now for some good news: Despite headlines, we can't be sure when our goose will be cooked.
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And now for some good news: Despite headlines, we can't be sure when our goose will be cooked.

The recent release of the Working Group 1 report of the Intergovernmental Panel on Climate Change (IPCC) documents the steady gains we have made in understanding the climate system in general and in our own role as a species in driving climate change in the 20th and 21st centuries.

But the report also made note of some areas of climate science that remain poorly understood. One of these is what causes climate to vary on time scales of a year to a decade or two. Especially relevant in this regard is the apparent slowdown in the rate of warming of the Earth's surface temperature over the last 10-15 years. There are any number of hypotheses that have been proposed to explain this -- the leading one appears to involve extra heat going into the ocean -- but a good deal of research will be needed before we nail that down and are able to reliably predict when such phenomena happen.

These two contrasting aspects of our understanding of the climate -- on the one hand the near certainty that human activities are driving rising atmospheric temperatures, and on the other uncertainty on what is driving (or not driving) warming on an interannual-decadal time scale -- are the basis for the discordant notes sounded by three recently published papers. Two use climate models to make precise predictions about how climate change will play out in the 21st century, and a third points out that the very same climate models are unable to simulate global temperatures over the past 15 years let alone temperatures 30 years from now.

Prediction 1: The Year the Climate Becomes Out of Bounds

The first of these papers was published in the journal Nature by Camilo Mora of the University of Hawaii and co-authors. Mora et al. used "39 Earth System Models developed for the Coupled Model Intercomparison Project phase 5 (CMIP5)" run under two emissions scenarios -- one a business-as-usual scenario with essentially no restraint on carbon emissions and the other with modest carbon policies in place -- to attempt to determine when the climate will exceed all temperatures seen in our recent historical past.

"The bounds of climate variability were quantified as the minimum and maximum values yielded by the Earth System Models with the CMIP5 'historical' experiment, which for all models included the period from 1860 to 2005." The climate was deemed to be "out of historical bounds" when the mean annual temperature calculated in the model permanently (i.e., year after year) exceeded the historical maximum temperature. In other words, try to recall the hottest year you have ever experienced where you live. Call that “X.” Now imagine a year (call this one “Y”) when the mean annual temperature exceeds the year X's temperature and does so for all years thereafter. In the Mora et al. lexicon, Y is the year the climate where you live goes “out of bounds.”

Mora et al.'s results are sobering as they find that the "out of bounds" date is not all that far off. Under the business-as-usual scenario, the authors estimate that the global mean temperature goes "out of bounds" by 2047 with an uncertainty range of plus or minus 14 years. (The uncertainty range is determined by the spread in the results from the 39 models used.)

Of course some areas of the globe are worse off than others. The authors report that “unprecedented climates will occur earliest in the tropics and among low-income countries.” Much of the tropics begin to exceed historical climate bounds in the 2020s and 2030s, while most Americans (we really are exceptional!) won't begin to experience an “out of bounds” climate until the late 2040s and in parts of the Midwest, not until the late 2060s. By about 2050 some 5 billion will be living in areas that exceed the historical climate bounds.

Prediction 2: Seeing the Effect of Climate Mitigation

Claudia Tebaldi of the National Center for Atmospheric Research and Pierre Friedlingstein of the University of Exeter also used the CMIP5 models to make predictions about climate change in the 21st century in their paper published in Proceedings of the National Academy of Sciences but with a different twist. Instead of asking when the climate will get out of bounds if we do little to forestall warming, these authors start with the premise that global policies are put in place to slow global warming and then ask the question: how long would it take for us to observe the effect of those policies. To answer that question, they ran the CMIP5 models with an aggressive restraint on emissions and compared it to the same two emission scenarios used by Mora et al.: a business-as-usual scenario with essentially no restraint and one with modest carbon policies in place and compared the results to determine at what point the effects of mitigation can be statistically distinguished.

These results were also a bit sobering. While it only took about 10 years in their simulation world to discern a difference in carbon dioxide (CO2) concentrations, a discernible difference in global mean surface temperatures does not appear for some 25-30 years, and at regional scales for 35-40 years.

This long time gap between policy implementation and observable effects arises because of: 1. The inertia in the climate system due to the so-called warming in the pipeline; and 2. The noisiness of the temperature signal due to interannual and decadal variability.

Why do I think this is sobering? Because, as the authors note, the "[t]imes to detection appear long under a naive expectation [most likely held by much of the public] that climate impact of mitigation action would be discernible immediately." This suggests that it may be a challenge to keep carbon mitigation policies in place (assuming they do go in place) for decades with no measurable effect. I can well imagine a public outcry against policies that cost consumers money (even if it's a little) when the benefits remain obscure for decades.

The results also suggest that climate policy advocates need to be very careful to not overpromise the benefits of climate policies -- those of us on the other side of 50, will likely never see the climate benefits of these policies even if they are instituted today and so the over-50 set shouldn't be given the impression that they will.

CMIP5 Models Critique

The two studies discussed above use climate models to suss out how things will play out climate-wise in the coming decades. That begs the question: how good are the models anyway?

In a commentary published in the journal Nature Climate Change John Fyfe of the Canadian Centre for Climate Modelling and Analysis, Environment Canada and co-authors address that issue by examining how well the models perform in replicating the global temperature trend over the past 20 years. Such a test of model performance is especially challenging because this has been, as discussed above, a period when the rise in global temperatures has slowed significantly, some even say there has been a “hiatus*” in warming -- for reasons that are not well understood. Because they are not well understood, it is not surprising that the authors find that the CMIP5 models systematically overestimate the rate of warming over this period.

I would not, and suggest that you not, interpret this to mean that climate models are completely unreliable or that greenhouse gases such as CO2 may no longer be warming the atmosphere. They certainly are, and will continue to do so.

It is also important to keep in mind that this so-called hiatus is not unprecedented; there are several instances in the 20th century when global temperatures remained fairly constant over a period of 10 years or so and then resumed their upward climb. (See here and here.) It's a good bet that this will happen this time as well.

But what the Fyfe et al. commentary does suggest is that our ability to simulate climate variability on decadal time scales is not quite ready for prime time. The warming rate in the coming decades may continue to be slower than current models predict -- it could also be faster as the atmosphere catches up from the slow warming of recent years. I don't think we know. And so while the studies of Mora et al. and Tebaldi et al. are really interesting, they should be taken with a grain of salt.

While I am willing to bet that further warming is inevitable, I would not bet on the temporal specificity of these studies. Some time this century, maybe well after 2047 and maybe well before 2047, the climate will heat up beyond historical bounds. In other words, we don't know when our goose will be cooked, but we do know that the planetary "oven" is fully preheated.


End Note

* While atmospheric temperatures have flattened out over the past 10-20 years, it would not be correct to say that global warming has stopped. Observations show that the oceans continue to heat up, and are likely taking up the heat that otherwise would have gone into the atmosphere.

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